GB2356184A - Lighter-than-air craft using steam to provide buoyancy - Google Patents

Abstract

An envelope of an airship or balloon is filled with steam to render it buoyant in air. As steam condenses the resulting water collects at a low point in the envelope and is either drained to atmosphere (fig. 3) or to a boiler B for re-boiling and return to the envelope (figs. 6, 7A, 8,9, 10,11). The water may be drained via a float valve 3 (fig. 4) which prevents steam from escaping from the envelope. In the case of an airship, a steam engine SE (reciprocating engine or turbine) may be provided for driving a propeller P, the exhaust from the engine being supplied via a hose 7, to the envelope, and the boiler B providing variable quantities of steam to the engine and direct to the envelope. A water reservoir WR, fuel tank FT, and ballonets BA, BF, may be provided. The envelope may be covered with insulating slabs (S fig. 7A) and may be fabricated from material comprising woven elastic fibres (fig. 12C).

Description

1 2356184 STEAM LTA CRAFT

FIELD OF THE INVENTION

This invention relates to a lighter than air craft - balloon or airship in which steam is utilized as the lift gas; to such a craft in which the envelope is insulated; to such a craft in which means are provided for draining water condensed from the steam lift gas upon the inside of the envelope; to such a craft 'in which means are provided for reboiling this condensed water back into steam lift gas; to an airship in which steam is utilized as the lift gas, which is powered by a steam engine; and to an airship in which steam is utilized as the lift gas, not equipped with any ballonet.

BACKGROUND OF THE INVENTION

The term "lighter than air craft", hereinafter abbreviated herein as "LTA craft", is general use in reference to any aircraft for which a considerable proportion of its support in the atmosphere is provided, not by dynamic aerodynamic lift induced by motion relative to the air, but instead by buoyancy. This buoyancy is in practice generated by a large and usually flexible container (the so called "envelope") which is filled with a gas (the "lift gas") whose density is substantially less than that of normal atmospheric air. Such craft in fact often "fly heavy", so that they tend to sink to a certain extent in the atmosphere when stationary; but they are nevertheless conventionally termed "lighter than air" or LTA craft. Such LTA craft sometimes are provided with driving means for propelling them along a course through the atmosphere, in which case they are termed "airships" or (formally) "aeronefs", while on the other hand sometimes they are left to float freely in the atmosphere without any means for propulsion relative to the atmosphere, 'in which case they are termed "free balloons" or (formally) "aerostats".

In the prior art various lighter-than-air gases have been used as lift gases for fillig the envelope of such a craft. Specifically, hydrogen, helium, methane, ammomia, and heated air have been utilized. Each of these lift gases has its advantages and disadvantages, which will now be discussed, bearing in mind that the effective molecular weight of air is about 29 (air being composed of approximately 80% N2 with molecular weight of 28 and approximately 20% 02with molecular weight of 32, with some traces of C02which is heavier and lesser traces of noble gases which are lighter) and that the mass of a cubic meter of air at the temperature (15 - Q and pressure (10 13.2 mba) of the ISA (International Standard Atmosphere) at sea level is 1.225 kilograms, so that its weight is 12.02 newtons.

(1) HYDROGEN AS A LIFT GAS Hydrogen (H2) was the first gas other than heated air to be used as a lift gas. It is cheap and easy to make, even in the field, and its molecular weight of 2 means that it offers superb lifting performance of 11. 19 newton/M3 at sea level ISA, but it suffers from the great disadvantage of bemig very inflammable. Accordingly, although hydrogen was the mainstay for providing lift during the heyday of airships, nowadays hydrogen is no longer used in practice in airships as a lift gas, due to the risk of igmition from the propulsive means. Even for free balloons hydrogen is little used nowadays, again for reasons of safety.

(2) HELIUM AS A LIFT GAS Almost all airships nowadays use helium (He) for lift. Helium has a molecular weight of 4 and accordingly it provides 10.36 newton/m3of lift at sea level ISA almost as much as hydrogen - and it is completely safe because it is inert. However helium is so costly that it must be stringently conserved. The cost of a single fill of helium lift gas is nowadays a significant factor in the deployment of an airship. Furthermore, since helium is an miert element and does not form compounds, helium gas cannot be manufactured chemically from any solid or liquid precursor. Also the liquefaction of helium requires extremely low temperature. Accordingly it is difficult to provide helium in the field, since the only practicable way of handling it after production is to store it mi a compressed state in cylinders which are expensive, heavy, and unwieldy.

(3) METHANE AS A LIFT GAS Methane (CH4, or coal gas) was once used as the lift gas for an early airship, chiefly because of the convenience of obtaining it ftom the gas mamis at the time. However methane is inflammable and offers no real safety advantage as compared with hydrogen, and its molecular weight of 16 means that it provides 5.39 newton/m3 of lift at sea level ISA - less than half the lift of hydrogen. Methane has no merit nowadays as a lift gas.

(4) AMMONIA AS A LIFT GAS Ammonia (NH3) has been used as the lift gas for an experimental free balloon. Due to its molecular weight of 17 it provides lift of 4.97 newton/M3 at sea level ISA, i.e.

rather less than half the lift of hydrogen, and it is substantially nonexplosive. Furthermore it is quite easy to transport and supply in the field, because it can easily be liquefied under moderate pressure. Moreover, it is cheap. However ammonia is somewhat toxic and corrosive,, as well as being malodorous, and accordingly it has not found favor in practice as a lift gas.

(5) HEATED AIR AS A LIFT GAS The density of heated air at ambient pressure is of course reduced from that of the surrounding air in proportion to the ratio between its absolute temperature and that of the surrounding air, and accordingly heated air can be used for providing lift for an LTA craft. If hot air is to be used as the lift gas for an LTA craft, 'in practice, in order to maintain lift for any realistically long time period, this hot air must be continually reheated because it steadily cools down due to loss of heat through the envelope to the outside. The only practicable way of heating such a large volume of air is to project the flame from a gas bumer directly into the envelope. As a very important additional beneficial feature, buoyancy control for the craft can be exerted simply and convemiently by varying the rate of this reheating. Hot air is very cheap and easy to produce in the field, and it is completely safe. Hot air free balloons are extremely common nowadays, and hot air airships are also used. The chief disadvantage of hot air as a liffing gas, however, is the poor lift which it provides. In practice the average temperature of the hot air within the envelope varies between about 100oC and about 120oC, i.e. between about 373,K and about 393oK, and 120-C is the maximum operationally allowed lift air temperature. Since the outside air temperature at sea level ISA is 288oK, this means that the lift provided by one cubic meter of hot air varies from about 2.74 newtons to about 3. 19 newtons - about a quarter of the lift provided by hydrogen. This means that the envelope required for lifting a given payload needs to be comparatively large.

In the case of an airship, a subsidiary disadvantage to the use of heated air as a lift gas is that, since typically the reheating method employed is to project the flame from a gas burner directly into the envelope, it is impossible to keep the envelope at positive pressure relative to the atmosphere. (Of course the upper portion of the envelope is at slight positive pressure relative to the atmosphere outside it, but its lower portion proximate to its downwardly facing open mouth is not). Accordingly, if as is typical the envelope is not supported by any rigid stiffening members, then the envelope is necessarily very floppy, and it is impossible to sustain any high speed through the atmosphere. Furthermore, the mounting of fins and the like to the envelope, and the suspension of a car therefrom, becomes difficult.

SUMMARY OF PRIOR ART LIFT GAS PERFORMANCE

The advantages and disadvantages of these five prior art lift gases are summ ' -A anzed in the first six columns (i.e. in the non shaded portion) of the Table which is presented as Fig. I of the drawings.

From the above it will be understood that in the prior art no lift gas has been found to be ideal, and that in fact every lift gas suggested 'in the prior art has been subject to serious disadvantages of one type or another.

ENVELOPE PRESSURE MANAGEMENT FOR PRIOR ART AIRSHIPS

As mentioned above, a typical prior art airship in which heated air has been used as the lift gas has not been pressurized: its envelope has had an opening at its bottom, and the gas burner flame has been directed from below straight upwards into this opening in order to heat the air within the airship. As a result of not being positively pressurized, such a heated air airship does not function very satisfactorily, although it is usable for particular purposes such as sport.

The envelopes of all other non-rigid or semi-rigid airships which have utilized other prior art lift gases have been closed and pressurized to somewhat above ambient pressure. (So-called rigid airships, in which the membranous bags containing the lift gas make no contribution to structural rigidity, will not be discussed or considered herein). With such a pressurized airship, if the pressure differential between the pressure within the envelope and the pressure of the outside atmosphere becomes too great, the envelope will be overstretched and may be damaged or even may burst, while if this pressure differential becomes too small, the envelope will lose its rigidity and will become floppy, which not only makes it impossible to sustain adequate speed through the atmosphere, but also threatens the maintenance of the physical shape of the envelope even when stationary, because the weight of the fins and the car and so on hanging upon the envelope may cause distortion.

Now, this pressure differential is subject to much disturbance. The greatest source of disturbance is that the external atmospheric pressure naturally varies as the airship changes altitude. However, disturbance of the pressure differential can also be caused by temperature-induced changes in the pressure of the lift gas due to variation in the amount of sunshine falling upon the envelope (so-called "insolation"), or due to change of the outside air temperature. Furthermore, significant change in the pressure of the external atmosphere due to weather conditions can progressively occur, and this also disturbs the pressure Oferential. Accordingly, the requirement has arisen to provide a means for adjusting the pressure level within such an airship envelope so as to keep the pressure differential at a suitable level for the current operational conditions.

With prior art lift gases other than heated air as described above, there has been no practical possibility of carrying a store of additional lift gas on board the airship for supply into the envelope when required due to a drop of pressure differential, and conversely there has been no practical possibility of removing some of the lift: gas from the envelope and storing it when required due to a rise of pressure differential. In other words, if during flight the lift gas pressure inside the envelope starts to become insufficient, there has been no possibility of supplying ftu-ther additional lift gas in order to remedy the problem; and, conversely, if during flight the lift gas pressure inside the envelope starts to become excessive, there has been no possibility of temporarily withdrawing some of the lift gas in order to remedy the problem (although of course simple venting of the lift gas, in a non-reversible fashion, has been possible). Accordingly the system which has been virtually universally adopted in the past for airship envelope pressure management has been to provide one or a plurality of ballonets within the envelope. Such ballonets are air bags which are pressurized with atmospheric air from outside the envelope by means of fans or other suitable devices. This pressurized air supply can be managed so as to expand or contract the ballonets in order to adjust the pressure differential.

However the provision of such ballonets and the provision of means for appropriately pressurizing them increases the initial cost of the airship. Moreover, during flight, managing the supply of pressurized air to these ballonets complicates the operation of the airship.

PROPULSION FOR PRIOR ART AIRSHIPS

The first airship which ever took to the air, that of Henri Gfffard in 1852, was powered by a steam engine. The weight of the propulsive apparatus, however, proved to be excessive, and furthermore there was a great danger of igniting the lift gas, which was hydrogen, from the furnace of such an external combustion engine. Although the present applicant does not have access to Ul details, this steam engine certainly must have included a boiler, a reciprocating piston and crank, a fuel supply, and a supply of operating water, and probably ftu-ther included a condenser. The second airship which flew, that of Renard and Krebs in 1884, was electrically powered from batteries. This concept proved impracticable due to battery weight. Since then all airships have been powered by either gasoline or diesel internal combustion engines. These engines are effective, but require frequent maintenance when employed in aircraft applications. They are also very noisy unless severely silenced, and the required silencing system is heavy.

LIFT CONTROL FOR PRIOR ART AIRSHIPS

Practical airships nowadays use helium as lift gas, and this helium should not be vented except in emergency, due to cost considerations, and is of course at substantially ambient atmospheric pressure. This means that the gross lift of the airship (which is equal to the weight of the air displaced by the lift gas) is constant.

The problem therefore arises that during flight the total weight of the airship inevitably reduces steadily, due to consumption of fuel. It might be thought that lightness of an aircraft can be nothing but beneficial, but in fact the airship soon becomes so light as to make its control in the vertical direction very difficult; and yet routine venting of helium cannot be seriously contemplated.

Various means have been tried to overcome this problem. In some sophisticated airships condensers are provided for condensing water from the engine exhaust gases, and this can keep the weight of the airship approximately constant. However, such condensers are heavy and expensive. Some Zeppelins of the prewar years used to collect rainwater from the outside of the envelope during flight for ballast, and this expedient was helpful. Nowadays airships usually are ballasted for takeoff to as heavy a state as possible consistent with flight, and for a flight of short duration this often suffices. However, the problem remains. The lift of free balloons is sometimes varied by warming the lift gas as appropriate, and in this case the balloon is termed a "Roziere". The present writer does not know of this system being adopted for any powered airship (other than a hot air airship, of course).

OBJECTIVES OF THE INVENTION Accordingly, it is an objective of this invention to provide an LTA craft ftu-nished with a lift gas which overcomes the above described problems.

It is a Rirther objective of this invention to provide an LTA craft finnished with a lift gas which has good lifting performance.

It is a fiu-ther objective of this invention to provide an LTA craft furnished with a lift gas which is safe.

It is a far-ther objective of this invention to provide an LTA craft finnished with a lift gas which is cheap.

It is a further objective of this invention to provide an LTA craft finnished with a lift gas which is easy to deploy in the field.

It is a further objective of this invention to provide an LTA craft Runished with a lift gas which is non-corrosive.

It is a ftirther objective of this invention to provide an LTA craft furnished with a lift gas which is inoffensive.

It is a ftu-ther objective of this invention to provide an LTA craft the lift of which is easy to control.

It is a fiirther objective of this invention to provide such an LTA craft which is propelled by a simple and rehable propulsion means.

It is a ftirther objective of this invention to provide such an LTA craft, the propulsive means of which is efficient.

It is a ftirther objective of this invention to provide such an LTA craft which is propelled by the use of a cheap fuel.

It is a further objective of this invention to provide such an LTA craft which can easily be stored upon the ground.

It is a further objective of this 'invention to provide such an LTA craft which is quiet in operation.

- I I - It is a ftu-ther objective of this invention to provide such an LTA craft which is environmentally friendly in operation.

It is a ftu-ther objective of this invention to provide such an LTA craft, the propulsive means of which does not require frequent maintenance.

It is a ftuther objective of this invention to provide such an LTA craft which is operated at positive pressure differential and the construction of which is simple.

It is a firther objective of this invention to provide such an LTA craft which is operated at positive pressure differential and which is cheap to construct.

It is a ftirther objective of this invention to provide such an LTA craft which is operated at positive pressure differential and which is simple to operate.

It is a ftirther objective of this invention to provide such an LTA craft, for which during operation it is easy and simple to control the differential between the pressure within the envelope and the external atmospheric pressure.

SUMMARY OF THE INVENTION

The present inventor has conceived the concept of using steam as a lift gas.

Furthermore, the present inventor has conceived the concept that, if in an airship the lift gas used is steam, then it becomes practicable and indeed advantageous to utilize a steam engine for propulsion of this airship.

Yet fin-thermore,, the present inventor has conceived the concept that, with an airship having a pressurized envelope in which the lift gas used is steam, there is no absolute requirement for any ballonet to be provided.

GENERAL DISCUSSION OF STEAM AS A LIFT GAS Steam is the vapour phase of water or H20. The molecular weight of H20 is 18, and therefore if a mass of steam were at sea level ISA temperature and pressure (which is of course impossible because in those conditions it would be in the liquid phase) it would provide lift of about 4.56 newton/M3. However in fact, at the pressure of the sea level ISA, steam needs to be maintained at a minimum temperature of 100oC, i.e. 373oK, in order to remain in the gaseous phase. This implies that the lift which such steam provides in air which itself is at sea level ISA will be greater than the above in accordance with this higher steam temperature, and calculation in fact shows that tile lift will be about 6.26 newton/M3. This is about 60% of the lift provided by helium and twice or more the lift provided by hot air.

Steam is substantially non-corrosive, and it is non-poisonous and possesses no odour. It cannot ignite, and it can be easily produced anywhere simply by boiling water. As for the question of cost, assuming that the available supply of water is at I OoC, then 90 kCal of heat is required to heat a kilogram of this water to I OOoC and a further 540 kCal is required to convert it into vapour (steam) still at I OOoC - a total of 630 kCal. A typical heat value for fuel oil is 10,000 kCal/kg, and therefore, allowing for inefficiencies of the boiler etc., the combustion of one kilogram of fuel oil will provide sufficient energy to produce about 15 kilograms of steam, and rather more in the alternative case when the feed water is already at I OOoC. Since a typical cost for fuel oil is US$0.30 per kilogram, it will be seen that the cost per kilograrn of 13 producing steam is very low. Actually steam is in fact even cheaper than air that has been heated by the use of liquid petroleum gas such as propane or butane, as is typically the case with heated air LTA craft nowadays, because in terms of calorific value fuel oil is much cheaper than LPG.

The characteristics of steam as lift gas for an LTA craft are as summarized in the seventh column (the shaded column) of the Table of Fig. 1.

REMARKS RELATING TO THE PRIOR ART

The present inventor has made a search of prior art and has located U.S. Patents Serial Nos. 4,032,085, 5,090,637, and 5,-890,676 which may be considered relevant as prior art for the present invention. Howeve it is his contention that the inventive concept of the present invention is not disclosed in any of said prior art patents nor is obvious therefrom to a person of ordinary skill in the relevant aft.

STATEMENT OF THE INVENTION

According to one aspect of the present invention, there is proposed an LTA craft comprising an envelope which is substantially completely filled with steam or is intended to be so,, which may ftu-ther comprise means for boiling liquid water into steam and for supplying the steam to within the envelope, or means for collecting liquid water which has condensed upon the inside of the envelope and conducting the liquid water out from the envelope while preventing gas from escaping from the envelope. In the first of these twocases, the LTA craft may fin-ther comprise means for collecting liquid water which has condensed upon the inside of the envelope and supplying the liquid water to the boiling means while preventing gas from escaping from the envelope. In any of the above cases, there may fin-ther be included means for 'insulating the envelope.

According to another aspect of the present invention, there is proposed an LTA craft comprisig an envelope which is substantially completely filled with steam or is intended to be so, means for boiling liquid water into steam and for supplying the steam to within the envelope, means for collecting liquid water which has condensed upon the inside of the envelope and supplying the liquid water to the boiling means while preventing gas from escaping from the envelope, and means for propulsion through the atmosphere. As one possibility, this propulsion means may be a steam engine, which may be supplied with pressurized steam from the boiling means; and steam exhausted from the steam engine may be supplied into the envelope. A ballonet system may be further included; or no bAonet system may be included. The material of the envelope may be elastic, 'in which case the elasticity of the material of the envelope in the longitudinal direction of the envelope and the elasticity of the material of the envelope in the circumferential direction of the envelope may be different; in more detail, the elasticity of the material of the envelope in the longitudinal direction of the envelope and the elasticity of the material of the envelope in the circumferential direction of the envelope may be so proportioned to one another that the maximum over the entire envelope of the warp fiber strain is substantially equal to the maximum over the entire envelope of the weft fiber strain. As before, means may be provided for insulating the envelope.

40. A free balloon substantially as described and shown in the accompanying drawings.

BRIEF DESCRIEPTION OF THE DRAWINGS Fig. I is a table summarizing the characteristics of various lift gases; Fig. 2 is a schematic vertical sectional view of a free balloon which is a first preferred embodiment of the present invention, also schematically showing an envelope charging truck; Fig. 3 is a schematic vertical sectional view similar to Fig. 2 for the first preferred embodiment, showing a free balloon which is a second preferred embodiment of the present invention, and which incorporates a condensed water drain valve; Fig. 4 is a schematic enlarged vertical sectional view of this condensed water drain valve; Figs. 5A and 5B are schematic enlarged vertical sectional views similar to Fig. 5, showing the operation of this water drain valve; Fig. 6 is a schematic vertical sectional view similar to Figs. 2 and 3, showing a free balloon which is a third preferred embodiment of the present invention, and which incorporates a steam regeneration apparatus; Fig. 7A is a schematic vertical sectional view similar to Figs. 2, 3, and 6, showing a free balloon which is a fourth preferred embodiment of the present invention, and whose envelope 'incorporates insulation slabs; and Fig. 7B is a schematic cross sectional view of a possible material for these insulation slabs; Fig. 8 is a schematic vertical longitudinal sectional view, showmig an airship which is a fifth preferred embodiment of the present invention; Fig. 9 is a schematic vertical longitudinal sectional view similar to Fig. 8 for the fifth preferred embodiment, showing an airship which is a sixth preferred embodiment of the present 'invention, and which is propelled by a steam engine; Fig. 10 is a schematic enlarged vertical sectional view showmig a car of this sixth preferred embodiment airship; Fig. I I is a schematic vertical longitudinal sectional view similar to Figs. 8 and 9, showing an airship which is a seventh preferred embodiment of the present invention, and whose envelope incorporates no ballonets; and:

Figs. 12A, 12B, and 12C are schematic figures for explanation of this seventh preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

THE FIRST PREFERRED EMBODIMENT STRUCTURE Fig. 2 is a schematic sectional view, taken in a vertical plane, showing the first preferred embodiment of the LTA craft of this invention, along with a charging truck therefor. This first preferred embodiment LTA craft is a free balloon whose envelope has a generally conventional shape, and its distinctive feature is that the lift gas which is utilized is steam, i.e. water (H20) which has been vaporized by heat. This first preferred embodiment does not incorporate any means for reheating the steam lift gas, and accordingly is somewhat ideal, but it is advantageous for demonstrating the principle of the present invention in a pure form.

The balloon envelope is generally denoted by the reference numeral 1, and it comprises a globular upper portion I a and a generally conical lower portion I b which terminates 'in a pointed lower end portion I c. A basket 2 is suspended by lines 2a and 2b below the envelope 1. The envelope I is filled with steam (H20 in its gaseous phase).

OPERATION This first preferred embodiment free balloon is operated as will now be described.

For initial inflation on the ground from the collapsed condition there is provided a steam truck T, which is equipped with a water tank WT, a high capacity boll er system B, and a steam supply hose H. To inflate this balloon, the hose H is connected so as to communicate with the interior of the envelope 1, and then the boiler B is operated to boil water from the water tank WT, the resulting steam being supplied through the hose H into the envelope 1. When the envelope I is appropriately charged with steam, this balloon may be released to commence flight, of course leaving the steam truck T on the ground.

Optionally the envelope I may not be completely filled upon takeoff, so that some additional room remains still available within it. In such a case,, of course, the pressure within the envelope I (at the base thereof) will not be elevated above atmospheric. In any case there is no reason to operate this first preferred embodiment LTA craft at positive pressure, since it is a balloon and there is no question of it moving relative to the air in which it floats (apart from ascending and descending), and accordingly the envelope I is not required to be distended with positive pressure in order to provide rigidity.

As soon as steam is supplied into the envelope 1, heat inevitably starts to pass from this steam to the outside atmosphere through the material of the envelope, and accordmigly water droplets will start condensing upon the inner envelope surface. These liquid water droplets will soon start trickling downwards to the lowest point of the envelope 1, i.e. downwards to the pointed lower end envelope portion I c. Thus liquid water at a temperature of substantially I OOoC will start accumulatig this pointedlower end portion I c, and in this first preferred embodiment this liquid water will continue thus to accumulate. Moreover, in some operational circumstances, some of the steam within the envelope I may condense into floating liquid water droplets which do not immediately settle upon the inner envelope surface, but which remain as a sort of mist within the interior space of the envelope 1. With this first preferred embodiment the duration of flight will inevitably be limited, since no means are provided for maintaining the temperature of the steam within the envelope 1.

The advantages and disadvantages of this first preferred embodiment free balloon which uses steam as lift gas are as follows.

COMPARISON WITH HYDROGEN AND HELIUM BALLOONS The lifting performance provided by the steam used in this balloon is only about 60% that of hydrogen or helium, so that the present invention is markedly inferior in this regard. Moreover the duration of flight is limited by cooling down of the steam lift gas. However, this balloon avoids the excessive danger of hydrogen and the excessive cost of helium. Yet fin-ther., the great difficulty of providing helium in the field is completely eliminated, since in fact steam is even easier to produce locally than hydrogen.

COMPARISON WITH HEATED AIR BALLOONS Actually hydrogen and helium balloons are little used nowadays, and for a free balloon the really significant comparison must be with heated air. In order to focus the discussion, the case will be considered of a balloon according to this first preferred embodiment whose envelope has a radius of 10 m, so that its volume (including the conical lower portion lb) is about 4,500 m and its surface area is about 1,300 m2. This balloon is similar,, for example, to the balloon model N 140 currently being marketed by Cameron Balloons, which has a volume of about 3970 3 in In the conventional case of using heated air as the lift gas, as explained above, the maximum gross lift available even at the highest operationally acceptable lift air temperature of 1200C will be about 14, 355 newtons, so that the maximum gross mass that can be flown will be about 1,460 kg. In fact, usually the lift win be lower than this value, since this maximurn lift air temperature of 120oC is an extreme which is rarely attained in practice. On the other hand, with the use of steam as the lift gas, the gross lift is virtually doubled to about 28,170 newtons, which can fly a gross mass of about 2,870 kg. This is a great enhancement over the conventional art.

Moreover, with regard to the duration of flight, the temperature inside the envelope of this balloon according to the first preferred embodiment which uses steam as the lift gas is a uniform I OOoC, and at sea level ISA the temperature of the outside air is 15oC, so that the temperature differential between the inside and outside of the envelope is about 85oC. On the other hand, in the case of a similarly sized conventional balloon which uses heated air as the lift gas, as described above the average temperature within the envelope needs to be at the maximum rated value of 120oC even to obtain half the gross lift of this first preferred embodiment balloon, so that the temperature differential is about 105oC. Clearly, therefore, the rate of loss of heat in the case of the present invention will be substantially less than in the comparison case when heated air is used as the lift gas. Accordingly the duration of flight will be correspondingly prolonged.

THE SECOND PREFERRED EMBODIMENT STRUCTURE Fig. 3 is a schematic sectional view similar to Fig. 2 for the first preferred embodiment', showing a free balloon which is a second preferred embodiment of the present invention. In this figure, elements of the second preferred embodiment which correspond to elements of the first preferred embodiment shown in Fig. 2 and which have the same fiinctions are denoted by the same reference symbols, and the explanation of this second preferred embodiment will focus chiefly upon the features in which it differs from the first preferred embodiment described above, thus omitting repetitive matter. In this case, a condensed water drain valve 3 is provided at the bottom of the pointed lower end portion I c of the envelope 1, and the upper end of a condensed water drain hose 4 passes through the envelope I and is connected to this drain valve 3. Thus,, the condensed water drain valve 3 controls the communication between the condensed water drain hose 4 and the interior of the envelope 1. In this second preferred embodiment the lower end of the drain hose 4 is left open to the atmosphere below the basket 2.

A construction for the condensed water drain valve 3 is shown in Fig. 4, which is a schematic enlarged sectional view thereof. This drain valve 3 is fixed upon the upper inside surface of the pointed lower end envelope portion I c, and comprises a generally hollow cylindrical body 3 a within which is slidably received a float F, which is shaped as generally cylindrical with a pointed lower end E, and whose density is somewhat less than that-of water at 100oC. The upper end of the condensed water drain hose 4 projects inside the valve body 3a and is provided with a conical seat 3b which faces upwards and which receives the downwardly pointing lower pointed end E of the float F. Fill holes 3c and 3d for allowing the passage of fluid are pierced through the lower portion of the hollow cylindrical body 3a of the valve 3, at a level lower than that of the top edge of the conical seat 3b, and a vent hole 3e is pierced through the upper end face of the body 3a for venting air or steam, etc..

OPERATION When the envelope I is filled with steam, this drain valve 3 functions as will now be described with reference to Fig. 5, which consists of two schematic enlarged sectional views similar to Fig. 4. Due to heat loss through the material of the envelope I to the outside atmosphere, water droplets will condense out from the steam within the envelope I upon the inner envelope surface, and some of these droplets will trickle downwards and will accumulate as liquid water at a temperature of substantially I OOoC in the pointed lower end portion I c of the envelope 1.)A%le the level S of the surface of this accumulated water mass remains lower than a critical level which will cause the float F to float, then as shown in Fig. 5A the pointed lower end E of the float F will remain seated in the conical seat 3b and will close off the upper end of the hose 4 so as to interrupt communication between the hose 4 and the interior of the envelope 1. In this condition, there is no possibility that either any of the accumulated water in the pointed lower end envelope portion I c or any of the gaseous steam within the envelope I should pass downwards through the hose 4. On the other hand, when the level of the surface S of the accumulated liquid water mass in the pointed lower end portion I c of the envelope I rises above the critical level and causes the float F to float, then as shown 'in Fig. 5B the pointed lower end E of the float F rises out of the conical seat 3b and opens up the seat 3b so as to communicate the hose 4 with the interior of the envelope I via the holes 3c and 3d. In this condition, the level of the surface S of the accumulated water mass is definitely higher than the top of the conical seat 3b, so there is no danger that any of the steam from within the envelope I should escape downwards through the hose 4; on the contrary, only liquid water from the accumulated liquid water mass M the pointed lower end portion I c of the envelope I is allowed to pass downwards through the hose 4, from which it is discharged. As a result, it is seen that the valve 3 according to this construction allows accumulated liquid water in the lower portion of the envelope I to be drained out therefrom, while reliably preventing any of the steam in the interior of the envelope I from escaping via this path.

The consequence of the provision of this condensed water drain valve 3 in this second preferred embodiment of the present invention is to substantially prolong flight time, as compared with the first preferred embodiment described above, for the following reasons.

Let the characteristic time period required for 540 kCal of heat to pass through the material of the envelope 1, both in the case of the balloon according to the first preferred embodiment described above and in this case of the balloon according to this second preferred embodiment, be P seconds; this time period P will depend only upon the temperature of the external atmosphere, since the temperature inside the envelope I is in both cases a uniform 100 o C because it contains both water and steam in mutual thermodynamic equilibrium. This heat loss will cause I kg of steam (1.70 m3of steam at sea level ISA) to condense into liquid water, Accordingly mi both cases the loss of gross lift will be 10.66 newtons. However in the case of the first preferred embodiment the resulting one kilogram of condensed water is retained within the envelope I in the growing water pool in its pointed lower end portion I c, and accordingly the weight of this water pool, which is a useless parasitic load, will increase by 9.81 newtons. In the case of this first preferred embodiment, therefore, the loss of net lift is a total of 20.47 newtons per P seconds, while in the case of the second preferred embodiment the loss of net lift is only 10.66 newtons per P seconds, because the useless water condensed within the envelope I is continually draffied via the valve 3 and the hose 4 downwards and is discharged from the balloon. Accordingly the time of flight will be correspondingly prolonged.

THE THIRD PREFERRED EMBODIMENT STRUCTURE Fig. 6 is a schematic sectional view similar to Figs. 2 and 3, showing a free balloon which is a third preferred embodiment of the present 'invention. In this figure, elements of the third preferred embodiment which correspond to elements of the first and second preferred embodiments shown in Figs. 2 through 5 and which have the same fimctions are denoted by the same reference symbols, and the explanation of this fiftli preferred embodiment will focus chiefly upon the features in which it differs from the first through fourth preferred embodiments described above, thus omitting repetitive matter. The distinctive feature of this third preferred embodiment is that the steam lift gas is continually regenerated by the condensed water being reboiled.

In this third preferred embodiment, in substantially the same manner as 'in the second preferred embodiment described above, a condensed water drain valve 3 is provided at the bottom of the pointed lower end portion I c of the envelope 1, and the upper end of a condensed water drain hose 4 is connected to this drain valve 3.

However, in this third preferred embodiment, the lower end of the drain hose 4 is connected to and discharges drained water into a steam regeneration apparatus 5. This steam regeneration apparatus 5 is also communicated to the envelope I by a steam supply hose 6, which passes through the conical envelope portion I b at a position somewhat higher than the pointed lower end envelope portion I c and opens within the envelope 1.

The steam regeneration apparatus 5 may conveniently be powered by combustion of a fossil fuel such as fuel oil (although in principle it could be nuclear powered), and it comprises a water reservoir VVR, a fuel tank FT, a boiler B which comprises a burner not particularly shown, and ignition apparatus, control apparatus and the like none of which are particularly shown either. Various methods of embodying these components and their interactions will be evident to a person of ordinary skill in the relevant art without undue experimentation, based upon the functional descriptions in this specification. The function of the steam regeneration apparatus 5 is to receive condensed liquid water from within the pointed lower end envelope portion I c via the drain hose 4,, to accumulate a quantity of this liquid water in the water reservoir WR if the operator deems doing so is advantageous, and to boil water from this reservoir WR using the boiler B at a desired rate to produce steam, which it then supplies into the envelope I via the steam supply hose 6 at a pressure equal to or slightly greater than the external atmospheric pressure.

OPERATION For initial inflation on the ground from the collapsed condition it is not contemplated that the boiler B in the steam regeneration apparatus 5 will ordinarily be used, both because its boiling capacity will probably not be fully adequate for such a task, and also because it is desirable for this boiler B to be reserved only for use for water that has already been conditioned, i.e. has already been boiled and recondensed so that it has been purged of dissolved solids. Instead, steam for initial ground inflation of the envelope I will typically be provided from an external steam supply means (not particularly shown) like the steam truck T described above with regard to the operation of the first preferred embodiment. Optionally, some reserve water may be stored in the water reservoir WR before takeoff. As with the first preferred embodiment, optionally the envelope I may not be completely filled upon takeoff, so that some additional room remains still available within it. In any case, when the envelope I is appropriately charged with steam, this third preferred embodiment free balloon may be released to commence flight, of course leaving the steam truck T on the ground.

As soon as steam is supplied into the envelope 1, heat inevitably starts to pass from this steam to the outside atmosphere through the material of the envelope, and accordingly water droplets will start condensing upon the inner envelope surface. These liquid water droplets will soon start trickling downwards to the lowest point of the envelope 1, i.e. downwards to the pointed lower end envelope portion I c. Thus liquid water at a temperature of substantially 100 oC will start accumulating in this pointed lower end portion I c, and this hot water will drain via the condensed water drain valve 3 and the drain hose 4 into the water reservoir WR of the steam regeneration apparatus 5.

In this third preferred embodiment balloon, this drained water in the reservoir WR is reboiled in order to maintain lift. The operator does this by operating the boiler B, which combusts an appropriate amount of fuel from the fuel tank FT to convert the desired amount of the hot water in the reservoir WR into steam. This steam is supplied via the steam supply hose 6 back to the interior space within the envelope I.

Now, in some circumstances some of the steam within the envelope I may condense into floating liquid water droplets which do not immediately settle upon the inner envelope surface, but which remain as a sort of mist within the interior space of the envelope 1. In order to reconvert these floating liquid water droplets into steam, it may in some circumstances be advantageous for the operator so to control the steam regeneration apparatus 5 as to supply steam to within the envelope I at a temperature higher than the boiling point of water at the current ambient atmospheric pressure, i.e. so as to supply superheated steam. The extra heat present within such steam (which is superheated but is not at substantially higher pressure than current ambient atmospheric pressure) will diffuse within the interior space of the envelope and will reboil the floating droplets.

Obviously in order to maintain the lift of this free balloon at a constant value the rate of reboiling of condensed hot water must be equal to the rate of condensation of water upon the inner skin of the envelope 1. However it is possible for the operator to vary the lift of this balloon in a manner analogous to the practice with a hot air balloon of the current conventional type: if the operator allows the reboiling rate of the boiler B to drop, so that the current rate of condensation of water is higher than the current rate of reboiling of water by the boiler B, then liquid water will start to progressively accumulate in the water reservoir WR and the balloon will start to progressively lose lift; while on the other hand, if the operator raises the current reboiling rate of the boiler B above the current rate of condensation of water, then the balloon will start to progressively gain lift, and this increasing of lift can continue while a sufficient quantity of water is available in the reservoir VVTR, and while sufficient room is available within the envelope 1.

The advantages and disadvantages of this third preferred embodiment free balloon which uses steam as lift gas are as follows.

COMPARISON WITH HYDROGEN AND HELIUM BALLOONS The liftmig performance provided by the steam used 'in this balloon is only about 60% that of hydrogen or helium, so that the present invention is markedly inferior in this regard. Moreover, it is necessary to provide the steam regeneration apparatus 5 merely to maintain lift, and this entails a weight penalty, while no correspondig apparatus is required in the case of hydrogen or helium. However, in the case of the present invention, buoyancy control is available, and this is an important beneficial feature. Furthermore, this balloon avoids the excessive danger of hydrogen and the excessive cost of helium. Yet further, as before, the great difficulty of providing helium in the field is completely eliminated, since in fact steam is even easier to produce locally than hydrogen.

COMPARISON WITH HEATED AIR BALLOONS Again, the really significant comparison for this balloon according to the third preferred embodiment must be with a conventional hot air balloon which uses air heated by a propane burner as lift gas. The advantages of steam over heated air will be seen to be very notable. In order to focus the discussion, the case will again be considered of a balloon according to this third preferred embodiment whose envelope has a radius of 10 m, so that its volume (including the conical lower portion lb) is about 4,500 m 3 and its stuface area is about I i13 00 ni.

LIFT COMPARISON In the conventional case of using heated air as the lift gas, as detailed previously, the maximum gross lift available even at the highest operationally acceptable lift air temperature of 120oC will be about 14, 355 newtons, so that the maximum gross mass that can be flown will be about 1,460 kg; and in fact the lift is usually substantially lower than this. On the other hand, with the use of steam as the lift gas, the gross lift is virtually doubled to about 28,170 newtons, which can fly a gross mass of about 2,870 kg. However, the advantage of the present invention over a conventional propane powered hot air balloon in terms of net lift, i.e. in terms of useful payload, is not as good as this, because allowance must be made for the following facts:

(1) In the case of the present invention, at any particular instant, a quantity of condensed liquid water is present within the envelope 1, some which has condensed in droplets upon the inner surface of the envelope I and is in the course of trickling down its inside, and some perhaps in mist form as described above. The weight of this condensed liquid water is entirely parasitic.

(2) The steam regeneration apparatus 5 of this third preferred embodiment will inevitably weigh more than does the typical set of propane burners and tanks used 'in a conventional hot air balloon.

The question therefore is as to how much extra mass will be entailed by these two considerations. It is not possible to put exact figures upon the masses 'involved without full scale implementation, which has not yet been performed.

However, with regard to point (1), it does not appear credible that more than about 150CM3 of water could be adhering to the inner surface of one square meter of the envelope at any time, especially since almost all of the inner surface of the envelope is sloping sharply. Accordingly an upper limit for the mass of parasitic tricklingdown water for this balloon is likely to be about 200kg. This amount of parasitic trickling- down water could be substantially reduced by treatment of the inner surface of the envelope I with a water repellent substance such as the active ingredient of the proprietary product "Rain-X" (this is a trademark).

Furthermore,, with regard to point (2), the present inventor does not believe that the extra weight entailed by a condensed water tank, a fuel tank, a boiler, and a fuel oil burner, as opposed to propane tanks and a propane burner, could be more than about 300kg. It might be much less. Furthermore, the longer the duration of the contemplated flight, the less does the weight penalty due to point (2) become, because (a) the calorific value of fuel oil per kilogram is greater than that of LPG; (b) the weight of tankage for storing a given mass of fuel oil is much less than the weight of the cylinders needed to contain the same mass of LPG; and (c) as explained below, substantially less heat energy per hour will be required for maintaining the lift of this balloon, as compared to the case of a conventional hot air balloon.

Accordingly it is considered that this free balloon according to the third preferred embodiment which utilizes steam as lift gas will have greatly enhanced net lift, as compared to a similar sized balloon which utilizes heated air as lift gas. Or, to put it another way, for lifting the same net weight, if steam is used as the lift gas, a substantially smaller envelope will be required than mi the case of heated air. In this case, the substantially reduced surface area of this substantially smaller envelope will further improve the fuel consumption even over the improved level that will be explained in the following.

FUEL CONSUMPTION COMPARISON The temperature inside the envelope of this balloon according to the third preferred embodiment which uses steam as the lift gas is a uniform I OOoC, and at sea level ISA the temperature of the outside air is 15 oC, so that the temperature differential between the inside and outside of the envelope is about 85oC. On the other hand, in the case of a similarly sized conventional balloon which uses heated air as the lift gas, as described above the average temperature within the envelope needs to be kept at about 120oC even to obtain merely half the gross lift of the balloon of the present invention, so that the temperature differential between the inside and outside of the envelope is about 1050C. Clearly, therefore, the rate of loss of heat in the case of the present invention will be substantially less than in the conventional prior art case where heated air is used as the lift gas. Of course in both cases the rate of heat loss must be balanced by the rate of consumption of fuel for reheating the lift gas, be it hot air or steam, and therefore it is considered that the fuel consumption per hour of operation of this balloon according to this third preferred embodiment of the present invention will be substantially less than that of a similarly sized conventional hot air balloon.

Now it is true that the boiler B will inevitably not be 100% efficient in boiling water, in other words that some heat will be wasted from the boiler B during the operation of this steam balloon. However, although the efficiency of the conventional method of heating the air within a conventional hot air balloon, which is to direct the flame from a LPG burner directly into the interior of the envelope through a hole in its bottom, appears to be quite good, actually this is not the case, since such a burner entrains a quantity of outside air from below into its exhaust gas stream (rather than using only air from within the envelope for combustion, which might be theoretically possible but actually is not practiced) and an equal compensating volume of air must accordingly be expelled from the downwardly facing mouth of the envelope. This displaced air is cooler than the target temperature of 120oC, but is not by any means cold, and in this manner a considerable amount of heat is wasted in the conventional case as well. The inefficiencies in both cases may be comparable; in fact, a boiler system is probably more efficient than a propane burner in terms of heat transfer to the lift gas.

In any case, even allowing for inefficiencies of the boiler, it is considered that this balloon according to this third preferred embodiment which utilizes steam as the lift gas will have much reduced fuel consumption, as compared to a similar sized balloon which utilizes heated air as the lift gas - and this even though the lift is greatly enhanced.

THE FOURTH PREFERRED EMBODIMENT STRUCTURE Fig. 7A is a schematic sectional view similar to Figs. 2, 3, and 6, showing a free balloon which is a fourth preferred embodiment of the present invention. In this figure, elements of the fourth preferred embodiment which correspond to elements of the first through third preferred embodiments shown in Figs. 2 through 6 and which have the same fiinctions are denoted by the same reference symbols, and the explanation of this fourth preferred embodiment will focus chiefly upon the features in which it differs from the first through third preferred embodiments described above, thus omitting repetitive matter. This fourth preferred embodiment is similar to the third preferred embodiment just described above, with the additional feature that the envelope I is covered over with slabs S of an insulation material. It is very desirable,, and in fact almost mandatory, for these insulating material slabs S to be flexible, in order to allow for folding up of the envelope I after use. Fig. 7B is a cross sectional view of a type of insulation material which could be used.

BENEFITS Such flexible insulating slabs S can be extremely effective in reducing loss of heat from the steam within the envelope 1; depending upon their thickness and the material from which they are composed, it is considered that it would be possible reduce heat loss, and accordingly fuel consumption, by a factor of 10 or more. The case will again be considered of the exemplary balloon discussed above whose 3 envelope has a radius of 10 in, a volume of about 4,500 in, and a surface area of about 1,300 ni, in comparison with a similarly sized conventional hot air balloon.

Since by using steam as lift gas rather than hot air the extra gross lift obtained is of the order of 14,000 newtons, it is clear that lifting a hypothetical additional mass of 500 kg of 'insulation upon the envelope I would use less than half of this extra gross lift. This allows for 380 grams of insulation to be mounted upon each square meter of the envelope fabnic.

In fact there is a cheap and widely available type of insulating material, commonly used for insulating behind household radiators and in house lofts and the like, which basically consists of two layers of metal foil bonded on either side of a sheet of plastic bubble wrap. A typical specification for such a material (called "Astro-Foil", which is a trademark) is as follows: structure - two layers of polyethylene bubble film, sandwiched between two layers of 99% pure aluminum foil; weight 382 gram/m2; thickness 8 mm; emissivity/reflectivity 95% to 97%; and claimed heat transmission around 0. 1 BTU / ft. hr. oF depending upon orientation, which translates to about 2 0 / m2. hr. oC. A schematic cross section of this material is shown in Fig. 7B. According to this specification, with an external temperature of 15oC, the heat loss per hour from the entire envelope of the exemplary balloon specified above would be 220 mJ or about 53 mCal, which would require the combustion of perhaps 8 kilograms of fuel oil to replace, allowig for inefficiencies in the boiler B. This is an extremely low fuel consumption for such a relatively large free balloon,, and although the insulation value quoted above might be difficult to realize in practice, it is clear that the provision of insulation to the envelope can be very effective for reducing the rate of heat loss and accordingly the fuel consumption, at the cost of course of lower net lift and making the envelope I more difficult to handle upon the ground. This kind of result can be obtained by the use of a cheap insulating

material which is currently available, but which is not specifically optimized for lightness. It will be clear that the development of a custom material for insulating the envelopes of LTA craft would be likely to provide ftirther benefits in terms of insulation efficiency, low weight, or both.

THE FIFTH PREFERRED EMBODIMENT STRUCTURE Fig. 8 is a vertical sectional view corresponding to Fig. 6 for the third preferred embodiment, showing a fifth preferred embodiment of the LTA craft of this invention. In this figure, elements of the fifth preferred embodiment which correspond to elements of the first through fourth preferred embodiments shown in Figs. 2 through 7 and which have the same functions are denoted by the same reference symbols, and the explanation of this fifth preferred embodiment will focus chiefly upon the features in which it differs from the first through fourth preferred embodiments described above, thus omitting repetitive matter.

This fifth preferred embodiment is an airship of the non-rigid type whose envelope I has a generally conventional external shape, and a distinctive feature of this airship is that the lift gas utilized is steam. Another feature of this envelope I is that it incorporates a ballonet system which incorporates a fore ballonet designated as BF and an aft ballonet designated as BA, both of which are shown in Fig. 8 as fully inflated. It should be noted in this connection that, in contrast to the case with a conventional airship which uses a conventional lift gas such as hydrogen or helium, it is not required that the material of this envelope I should be completely impervious to the lift gas (steam in this case), since moderate leakage of this steam lift gas can be made good during flight. By contrast, of course, in the case of a conventional airship using hydrogen or helium as lift gas, replenishment of the lift gas during flight is quite impracticable, and accordingly the envelope of such a conventional airship is required to be substantially impermeable, which means that the material from which it becomes constructed is very expensive.

The lowermost portion of the envelope I is denoted by I c, and a car 2 is fixed to the envelope I below this lowermost portion I c. The car 2 comprises a steam regeneration apparatus 5 which is connected by a condensed water drai hose 4 to the lowermost envelope portion I c, and which is ftu-ther communicated to the envelope I by a steam supply hose 6, which passes through the envelope I at a position somewhat higher than the lowermost portion I c, and which then opens within the envelope 1. And a condensed water drain valve 3, substantially identical to the valves 3 of the third and fourth preferred embodiments described above, is connected to the upper end of the condensed water drain hose 4 just within the envelope 1, and controls the communication between this hose 4 and the envelope 1, just as did the valves 3 of the third and fourth preferred embodiments, so as to allow liquid water accumulated in the lowermost portion I c of the envelope I to be drained to the steam regeneration apparatus 5, while preventing any of the steam in the envelope I from escaping via this path. Additionally the car 2 comprises an engine E of a per se conventional type, such as an internal combustion engine like a gasoline or diesel engine, which drives a propeller P so as to propel this airship through the ambient air. The car 2 further comprises per se conventional means, not particularly shown in the figure or Ru-ther described herein, for appropriately controlling supply of external air at appropriate pressure levels to the fore and aft ballonets BF and BA, and for appropriately controlling exhaust of air from these ballonets BF and BA.

The steam regeneration apparatus 5 of this fifth preferred embodiment operates in substantially the same manner as did the steam regeneration apparatus 5 of the third preferred embodiment, except that under the control of the operator it is capable of providing steam at a somewhat greater pressure than that of the ambient atmosphere.

OPERATION This airship is operated as will now be described.

Again, for the same reasons as before, for initial inflation on the ground from the collapsed condition it is contemplated that the boiler B in the steam regeneration apparatus 5 will not ordinarily be used. Instead, stearn for initial ground inflation of the envelope I will typically be provided from an external steam supply means (not particularly shown) like the steam truck T described above with regard to the operation of the first preferred embodiment. In the case of this fifth preferred embodiment which incorporates the fore and aft ballonets BF and BA, before takeoff, first these ballonets should be charged with appropriate amounts of air ('in fact, at ground level, the ballonets should be almost or completely filled), and then the main volume of the envelope I (not including the ballonets BF and BA) should be completely filled with steam, and should be pressurized to an appropriate pressure level somewhat higher than the external atmospheric pressure, so that the envelope I is somewhat distended with positive pressure and is thereby made rigid. When the envelope I is thus appropriately charged, this airship may be released to conimence flight. Optionally and desirably, some reserve water may be stored 'in the water reservoir VVR before takeoff.

As in the case of the previously described preferred balloon embodiments of the present invention, as soon as steam is supplied into the envelope 1, water droplets will start condensing upon the inner envelope surface due to the loss of heat to the outside atmosphere through the material of the envelope 1, and these water droplets will start trickling downwards to the lowest envelope portion I c. Thus water at a temperature of substantially I OOOC will start accumulating in this portion I c and, as explained above with reference to the third preferred embodiment of the present invention, this hot water will drain via the condensed water drain valve 3 and the drain hose 4 into the hot water reservoir WR of the steam regeneration apparatus 5.

In order to maintain the lift of this airship, and also 'in order to maintain the positive pressure differential which ensures its rigidity, drained hot water in the reservoir WR should be reboiled. The operator does this by operating the boiler B, which converts the desired amount of the hot water in the reservoir WR into steam which is supplied via the steam supply hose 6 back to the interior space within the envelope 1. As in the case of the third preferred embodiment, it may in some circumstances be advantageous for the operator so to control the steam regeneration apparatus 5 as to supply steam to within the envelope I at a temperature higher than the boiling point of water at the current ambient pressure, i.e. so as to supply superheated steam. in order to reconvert water droplets floating as mist within the interior of the envelope I back into steam.

As in the case of the third preferred balloon embodiment, it is possible for the operator to vary the lift of this airship. If the operator allows the reboiling rate of the boiler B to drop, so that the rate of condensation of water upon the inside of the envelope I is higher than the rate of reboiling of water by the boiler B, then the quantity of steam within the envelope I will start progressively decreasing, so that the airship will start progressively losing lift. At this time extra air should be progressively supplied into the ballonets BF and BA so as to maintain a proper pressure differential between the interior of the envelope I and the outside atmosphere in order to maintain rigidity of the envelope I as a whole. This can continue while sufficient extra water storage capacity is available in the reservoir VvrR (although water could be discharged overboard if deemed operationally advisable), and while extra room for air is available within the ballonets, BF and BA. Moreover during this process advantage can be taken to some extent of the elasticity of the material of the envelope I by allowing the pressure differential to decrease down to a certain acceptable lower limit dictated by the requirement for envelope rigidity, thus allowing the material of the envelope I to contract somewhat so that the volume of the envelope I decreases and accordingly its gross lift decreases. On the other hand, if the operator raises the reboiling rate of the boiler B above the rate of condensation of water on the inside of the envelope 1, then the quantity of steam within the envelope I will start progressively increasing, so that the airship will start progressively gaining lift. At this time air should be progressively exhausted from the ballonets BF and BA, so as to maintain the proper pressure differential for maintaining rigidity of the envelope 1. This can continue while a sufficient quantity of water is available in the reservoir WR, and until the ballonets BF and BA become completely collapsed. Moreover during this process advantage can be taken to some extent of the elasticity of the material of the envelope I by allowing the pressure differential to increase up to a certain acceptable upper limit dictated by the requirement for envelope integrity, thus causing the material of the envelope I to be somewhat stretched so that the volume of the envelope I increases and accordingly its gross lift increases.

Furthermore, if during flight it is found that this airship is becoming unduly light, presumably due to progressive consumption of fuel, and it is desired to reduce the lift without altering the amount of air in the ballonets BF and BA, then it is perfectly practicable to vent some of the steam lift gas within the envelope I and to replace it by pumping in a corresponding volume of atmospheric air. This action cannot be reversed during flight, because the only way to eliminate all air in the envelope I would be to deflate it completely and then to reinflate it with steam, and of course this can only be done upon the ground. However, it may be a useful procedure in certain operational circumstances.

BENEFITS The advantages and disadvantages of this fifth preferred airship embodiment of the present invention which uses steam as lift gas are as follows. Airships which utilize heated air as lift gas are not considered by the present applicant as serious aircraft, due to their floppiness, and hydrogen is not used for airships nowadays due to the danger which it poses, so only a comparison with a pressurized airship which uses helium as a lift gas will be made.

The lifting performance provided by the steam used as lift gas in this airship is only about 60% that of helium, so that this airship according to the fifth preferred embodiment of the present invention is markedly inferior in this regard to a helium airship. Moreover, it is necessary to provide the steam regeneration apparatus 5 to maintain lift,- while no corresponding apparatus is required in the case of helium.

Accordingly for the same net lift, i.e. for the same payload, the volume required for an airship according to the present invention employing steam as lift gas will be about twice that of a comparison airship which employs helium. Therefore its surface area will be about 2 213 times, i.e. about 1.6 times, that of the comparison helium airship, and it is expected that the energy required for propulsion, i.e. the fuel consumption of the engine E, will be likewise increased. However the cost of manufacture of the envelope should actually be less for this steam airship than for a helium airship of equivalent lift, although this steam airship is larger, because there is no requirement for complete impermeability to the lift gas; moderate percolation of steam through the envelope will prove of little consequence operationally, and accordingly a much cheaper material can be used for manufacturing the envelope.

In the case of the present invention buoyancy control for the airship is available in various different manners as described above, and this is an important benefit. This could be of great use in the case of embodying the present invention as a very large airship intended for transport of heavy cargo. However, the most significant advantage of this airship according to this fifth preferred embodiment of the present invention is in operational cost, because the excessive cost of helium is entirely avoided. The difficulty of providing helium in the field is also eliminated.

An extTemely beneficial consequence of the fact that the steam lift gas for this airship is extremely cheap and easily produced is that the operator, if he deems it operationally desirable, need feel no concern about ripping the envelope so as to vent the lift gas. In fact it is quite feasible for the envelope to be ripped at the end of every flight. This is completely impracticable due to cost considerations in the prior art case of helium lift gas. Alternatively, the boiler B may be deactivated at the end of the flight and the envelope may be left to collapse gradually by itself as the steam within condenses. In either of these cases storage of the airship on the ground is greatly facilitated, as compared to the case of a helium airship which must be stored in the inflated condition. However with this fifffi prefer-red embodiment airship, if so required, the steam within the envelope could be kept in vapor form until the next flight for a quite modest expenditure, even over a period of weeks.

THE SIXTH PREFERRED EMBODIMENT STRUCTURE Fig. 9 is a schematic vertical sectional view corresponding to Fig. 8 for the fifth preferred embodiment, showing a sixth preferred embodiment of the LTA craft of this invention, and Fig. 10 is an enlarged schematic vertical sectional view of a car thereof In these figures, elements of this sixth preferred embodiment which correspond to elements of the fifffi preferred embodiment shown in Fig. 8 and which have the same functions are denoted by the same reference symbols, and the explanation of this sixth preferred embodiment will focus chiefly upon the features in which it differs from the fifth preferred embodiment described above, thus omitting repetitive matter.

This sixth preferred embodiment is again an airship of the non-rigid type in which the lift gas utilized is steam. The envelope I of this airship is equipped with a fore ballonet BF and an aft ballonet BA, and is substantially the same as the envelope I of the fifffi preferred embodiment described above. However, the distinctive feature of this sixth preferred airship embodiment is that the propulsive means provided is not an internal combustion engine, but instead is a steam engine denoted as SE, which may be a reciprocating steam engine, or may be a steam turbine.

This steam engine SE is powered by high pressure steam from the same boiler B as is used for providing steam to the interior of the envelope I as lift gas. The steam which is supplied to the engine SE, after having been used to operate the engine SE so as to provide rotational energy for driving the propeller P, is exhausted via a steam exhaust hose 7 and is vented within the envelope 1. The boiler B should therefore be constructed as a dual purpose boiler which can provide steam at high pressure for supply to the engine SE to drive it and can also provide steam at low pressure for supply into the envelope I directly via the steam supply hose 6, i.e. not via the engine SE. The boiler B should be of a type which is capable of providing varying quantities and proportions of such high pressure steam and low pressure steam, according to ongoing operational requirements. Various possibilities for design of such a boiler will be evident to a person of ordinary skill in the relevant art without undue experimentation, based upon the functional descriptions in this specification.

BENEFITS The advantage of using the steam engine SE as propulsive means for this airship is that the same boiler B,' burner, etc. are used for two purposes, generating steam for lift and generating steam for propulsion. Furthermore, no condenser is required for the steam engine SE, because the envelope I itself serves for this purpose. Thus, according to this aspect of the present invention, the use of a steam engine for propelling an airship becomes a practical possibility, because of the great economization of weight achieved in this way by dual use of several components. It will be noted that, in general, the higher is the power level at which the steam engine SE is operating, the higher will be the air speed of the airship, and accordingly the higher will be the rate of loss of heat from the steam lift gas out through the material of the envelope 1. In other words, the greater is the requirement for condensation of steam which has passed through the engine SE, the greater is the steam condensing capacity of the envelope 1.

Moreover, the typically quiet operation of a steam engine eliminates the requirement for a heavy silencer and renders this airship environmentally fiiendly in operation. Furthermore, since the torque of a reciprocating steam engine is at its highest at low rpm, it is possible and indeed optimal for the steam engine SE to operate at high torque and low rotational speed, and to rotate the propeller P slowly, without the use of any gearbox. This means that the propeller P can be of a very large and slow spinning type. Large slow spinning propellers are well know to excel in thrust efficiency, especially at the low airspeeds typical of airships. Also low rotational speeds for the engine SE mean that its service interval can be extended, and further reduces the generation of noise.

THE SEVENTH PREFERRED EMBODIMENT STRUCTURE Fig. I I is a vertical sectional view corresponding to Fig. 8 for the fifth preferred embodiment, showing a seventh preferred embodiment of the LTA craft of this invention. In this figure, elements of the seventh preferred embodiment which correspond to elements of the fifth preferred embodiment shown in Fig. 8 and which have the same functions are denoted by the same reference symbols, and the explanation of this seventh preferred embodiment will focus chiefly upon the features in which it differs from the fifffi preferred embodiment described above, thus omitting repetitive matter.

This seventh preferred embodiment again is an airship of the non-rigid type whose envelope I has a generally conventional external shape, and a distinctive feature of this airship is that the lift gas utilized is steam. However the feature of this seventh preferred embodiment which distinguishes it from the fifth preferred embodiment described above is that its envelope I incorporates no ballonet. Furthermore, the envelope I is made to be elastic, preferably in a manner explained subsequently in this specification.

As with the fifth preferred embodiment, a car 2 is fixed to the envelope 1. The details of the car 2 and of the steam regeneration apparatus 5 which it incorporates are substantially the same as in the fift preferred embodiment described above, and accordingly their description will be curtailed.

OPERATION This seventh preferred embodiment airship is operated as will now be described.

Again, for the same reasons as before, for initial inflation on the ground from the collapsed condition it is contemplated that the boiler B in the steam regeneration apparatus 5 will not ordinarily be used. Instead, steam for initial ground inflation of the envelope I will typically be provided from an external steam supply means (not particularly shown) like the steam truck T described above with regard to the operation of the first preferred embodiment. In the case of this seventh preferred embodiment, the envelope I should be completely filled with steam before takeoff, and should be pressurized to a desired pressure level somewhat higher than the external atmospheric pressure, so that it is somewhat distended with positive pressure against the force of its elasticity, and is thereby made rigid. When the envelope I is appropriately charged, this airship may be released to commence flight. Optionally and desirably, some reserve water may be stored in the water reservoir WR before takeoff.

As in the case of the other preferred embodiments described above, as soon as steam is supplied into the envelope 1, water droplets will start condensing upon the inner envelope surface due to the loss of heat to the outside atmosphere through the material of the envelope 1, and these water droplets will start trickling downwards to the lowest envelope portion I c. Thus water at a temperature of substantially 100 - C will start accumulating in this portion I c and, as explained above, this hot water will drain via the condensed water drain valve 3 and the drain hose 4 into the hot water reservoir WR of the steam regeneration apparatus 5.

Drained hot water in the reservoir WR must be reboiled in order to maintain the lift of this airship, and also in order to maintain the positive pressure differential which ensures its rigidity between its upper pressure limit (defined by the requirement not to overstretch the envelope 1) and its lower pressure limit (defined by the requirement to maintain a certain degree of envelope rigidity). The operator does this by operating the boiler B, which converts the appropriate amount of the hot water in the reservoir WR into steam which is supplied via the steam supply hose 6 back to the interior space within the envelope 1. As in the case of the third through the sixth preferred embodiments, it may in some circumstances be advantageous for the operator so to control the steam regeneration apparatus 5 as to supply steam to within the envelope I at a temperature higher than the boiling point of water at the current ambient pressure, i.e. so as to supply superheated steam, in order to reconvert water droplets floating as mist within the interior of the envelope I into steam.

Thus, in this seventh preferred embodiment, the envelope I has been greatly simplified in that no ballonet has been incorporated, and this also simplifies the operation of the airship in that no pressure fan for ballonet pressurization needs to be operated or monitored, and these are notable advantages. However, as described above, it is required that the operator should increase or decrease the rate at which the steam regeneration apparatus reboils the water in the water reservoir VVR in accordance with the trend of the pressure differential, which can vary due to change of the external atmospheric pressure caused by change of altitude of the airship or by the weather, or due to insolation, so as on the one hand to avoid damage being caused to the envelope I by overpressure, while on the other hand avoiding the envelope I sagging or becoming floppy due to under-pressure. A high elasticity for the envelope I will materially aid in this pressure control by allowing more latitude to the lift gas (steam) to expand and contract without much varying the pressure differential; and, of course, the relaxation of the prior art requirement for the envelope to be absolutely impermeable to the lift gas makes it easier to select a material for the envelope I which can be sufficiently elastic without being unduly expensive.

As in the case of the fifth and sixth preferred embodiments described above, it is possible for the operator to vary the lift of this airship, but in a somewhat different manner, as follows. If the operator allows the reboiling rate of the boiler B to drop, so that the rate of condensation of water upon the interior surface of the envelope I is higher than the rate of reboiling of water by the boiler B, then the pressure of the steam within the envelope I will start dropping and the envelope I will start shrinking due to its elasticity, so that the airship will start losing lift. At this time the pressure differential between the interior of the envelope I and the external atmosphere will naturally be dropping, and it is incumbent upon the operator to ensure that it does not drop so far as to compromise the required rigidity of the envelope 1. On the other hand, if the operator raises the reboiling rate of the boiler B above the rate of condensation of water upon the interior surface of the envelope 1, then the pressure of the steam within the envelope I will start rising and the envelope I will start being expanded against the force of its elasticity, so that the airship will start gaining lift. At this time the pressure differential between the interior of the envelope I and the external atmosphere will be rising and the stretching (strain) of the envelope I will be increasing, and it is incumbent upon the operator to ensure that the envelope I is not stretched so far as to damage it; in other words, he must ensure that the elastic limit of the material of the envelope I is not surpassed at any point of its surface along any direction. Again, a high elasticity for the envelope I will materially aid 'in this lift control by allowmig more latitude for change o f volume relative to variation of the pressure differential.

As with the airship according to the fifth preferred embodiment of the present invention described above, if during flight it is found that this seventh preferred embodiment airship is becoming unduly light due to progressive consumption of fuel, it is quite practicable in some circumstances to vent some of the steam lift gas within the envelope I and to replace it by pumping in a corresponding volume of atmospheric air.

The advantages and disadvantages of this airship which uses steam as lift gas are similar to those of the fifth preferred embodiment, with the additional benefit that no ballonet needs to be provided and no ballonet management needs to be performed. This concept has a particular synergy with the concept of using steam as the lift gas, since only when steam is used as the lift gas is it possible in practice to provide additional lift gas to the airship envelope during flight, or to temporarily withdraw lift gas from the envelope during flight with the possibility of replacing it.

THE RELATION BETWEEN PRESSURE IN AN AIRSHIP ENVELOPE AND THE LINEAR TENSIONS OF THE ENVELOPE MATERIAL The relation between the pressure differential P between the pressure of the lift gas within an airship envelope and the outside air, and the linear tensions of the envelope material, will now be explored with reference to Fig. 12A; however, advanced geometrical considerations such as would involve differential geometry will be eschewed. Fig. 12A schematically shows a small substantially rectangular element E of envelope material of dimensions dx in the x-direction and dy in the ydirection, and thus of area dxdy. In this region of the envelope, it is supposed that the radius of curvature in a section taken along the x- direction is Rx and the radius of curvature in a section taken along the y-direction is Ry, and that the linear tension of the envelope material along the x-direction is Tx newtons/meter, while the linear tension of the envelope material along the y-direction is Ty newtons/meter. Since the values of dx and dy are small relative to the radiuses of curvature Rx and Ry, and since sines of small angles are substantially equal to the angles themselves, accordingly the component in the inward direction of the envelope of the force upon the element E due to the tensions across the two dx sides is equal to:

(Ty.dx)(dy/Ry) (1) and similarly the component in the inward direction of the envelope of the force upon the element E due to the tensions across the two dy sides is equal to:

(Tx.dy)(dx/Rx) (2) and so the total force upon the element E in the inward direction of the envelope due to the tensions in all four of its sides is equal to:

dx.dy.(Tx/Rx+Ty/Ry) (3) and this must equal the force upon the element E in the outward direction of the envelope due to the pressure differential P which it restrains, which is P.dx.dy; and therefore:

P = Tx/Rx + Ty/Ry (4) Fig. 12B schematically shows an airship envelope of the general shape utilized the seventh preferred embodiment of the presentinvention. It will be clear that, in the case of this envelope, the radius of curvature along the longitudinal direction L of the envelope and the radius of curvature along the circumferential direction C of the envelope both attain maxima approximately at the points of the envelope material which are arranged circumferentially around the envelope at the position longitudinally therealong at which the element E shown in this figure is located. This locus M - which is a circumferential circle extending circumferentially around the fattest portion of the envelope - will hereinafter be termed the locus of maximum curvatures. Since the pressure P within the envelope is of course the same at all points within it, and since both the radius of curvature along the longitudinal direction L of the envelope and along its circumferential direction C are at their maxima on this locus M of maximum curvatures, it will be clear from Equation (4) that both the linear tension of the envelope material along its longitudinal direction and its linear tension along its circumferential direction attain maxima along this locus M. In the case of other envelope geometries, however, the longitudinal linear tension and the circumferential linear tension might attain their maxima along different loci, which would not invalidate the following arguments.

A NON-ISOTROPICALLY ELASTIC ENVELOPE MATERIAL Fig. 12C shows in schematic enlarged plan view a novel type of envelope material that may be used for constructing the envelope I of the seventh preferred embodiment airship described above.

The first notable characteristic of this envelope material is that the strands from which it is woven are made from an artifical fiber material the elasticity of which is pronounced. A possible choice for this material is Nylon 6, which is widely used for fishing line because of its elasticity. It is known that Nylon 6 can be stretched through at least Mand will still return to its original length, even if it has remained stretched for some time. Alternatively various other artificial fibers of high elasticity could be considered; for example, a possible candidate is Lycra (this is a trademark). Nylon 6.6 also is a possibility.

The second notable characteristic of this envelope material is that the thickness and the number per meter of the warp strands thereof are suitably chosen relative to the thickness and the nurnber per meter of the weft strands, in order to provide a different cross sectional area of warp material per transverse linear measure than of weft material. In other words,, this. envelope material is not isotropic. It is contemplated at the present time by the applicant that the cross sectional area of weft per meter circurriferentially around the envelope may be about three times as much as the cross sectional area of warp per meter longitudinally along the envelope, but the value of this factor must depend upon the results of experiments, taking into account the particular geometry of the envelope I which it is desired to construct.

The purpose of this construction is to ensure that the envelope I can be distended by positive internal gas pressure to as great an extent as possible without damaging it, i.e. with such distension remaining reversible. A possible route for the design process will now be outlined, supposing for the sake of discussion that the material from which the envelope material is to be formed is Nylon 6.

As schematically shown 'in Fig. 12B, the envelope I is typically made by sewing together a plurality of longitudinally extending gores G, with the warp strands of the material extendig along the envelope, i.e. in the longitudinal direction L thereof, while the weft strands of the material extend around the envelope, i.e. in the circumferential direction C thereof. (However, the nose portion N of the envelope I is typically reinforced or made from a specially strong fabric, because it is required to withstand particular stresses both during travel through the atmosphere and when the airship is tethered to a mast; therefore, the nose envelope portion N must be understood as being excluded from this discussion.) Further, it is a practical requirement for weaving of an envelope fabric that the warp strands should all be uniform and of the same thickness as one another (i.e. should come off one uniform spool), and similarly it is a practical requirement that the weft strands should all be uniform and of the same thickness as one another.

There is a minimum pressure differential Pri,, that is required to be maintained between the interior of the envelope I and the external atmosphere, for maintaining the shape of the envelope I with sufficient rigidity; this minimum acceptable pressure differential P,6,, will vary according to the shape and dimensions of the particular envelope I under consideration. Assume that the envelope I is filled with lift gas at this minimum acceptable pressure differential Pni.,,. The thickness of the weft strands and their number per meter along the longitudinal direction of the envelope should be so chosen that at this time those of them which extend along the locus M of maximum curvatures are stretched (strained) by about 2%. Of course, the other weft strands which extend around other circumferential lines around the envelope at different longitudinal positions therealong will be strained rather less, since the radiuses of curvature of these circumferential lines are smaller. Similarly, the thickness of the warp strands and their number per meter around the circumferential direction of the envelope should be so chosen that, when the envelope is thus filled with gas at the minimum acceptable pressure differential P.,i,,, their portions at the locus M of maximum curvatures are stretched (strained) by about 2%. Of course, the other portions of these warp strands at other longitudinal positions along the envelope will be strained rather less, since the radiuses of curvature of the warp strands at these positions are smaller.

Thus, in summary, the definition of the design procedure which is advocated is so to proportion the amount of warp per transverse linear meter (thickness of each warp fiber multiplied by the number of warp fibers per transverse linear meter) and the amount of weft per transverse linear meter to one another that the maximum over the entire envelope of the warp fiber strain is substantially equal to the maximum over the entire envelope of the weft fiber strain. Depending upon the geometry of the exact envelope in question, these maxima may not be attained along thesame loci, as has been suggested may be the case with the exemplary envelope shown, but the above definition will still hold.

Suppose now that the pressure differential P between the interior of the envelope and the external air is gradually increased from the minimum acceptable pressure differential Pmffi. It will be easily understood that, according to this construction for the envelope material, the maximum warp fiber strain over the entire envelope and the maximum weft fiber strain over the entire envelope will both continue to be attained upon the same locus or locuses as before, and will continue to be substantially equal as they both rise. Suppose that the maximum acceptable strami for full restitution for the material of the warp and weft fibers, hereinafter termed Pm,,x, is 6%. When this strain P. is attained upon the locus M, the circumference of the envelope at this longitudinal position thereof will have increased by 4% from its original value when the pressure differential was Pmj", and accordingly the cross sectional area of the envelope at this longitudinal position thereof will have increased by about 8% from its original value. Similarly the length of the portion of the envelope generally at this longitudinal position thereof will have increased by 4% from its original value when the pressure differential was P.,i,,, since its strain is now 6%, and accordingly at this maximum acceptable pressure differential Pniaxthe volume of this portion of the envelope will have increased by about 12% from its original value when the pressure differential was Pmj".

However,, this does not mean that the volume of the entire envelope will have increased by 12%, because only the fattest portion of the envelope, generally at the longitudinal position of the element E in Fig. 12B, will have been strained to its maximum permissible amount in this manner. Other portions of the envelope towards the nose portion N or the tail portion T will have been strained less, because the radiuses of curvature at these portions are smaller. However, these other portions of the envelope contribute less towards its volume than does its fattest portion, because they are thinner. The exact proportion by which the entire volume of any particular envelope increases as the pressure differential rises from P. to Pma,. can only be determined by numerical computer calculation or by experiment, but is estimated to be about 10% for the parameters defined above.

If this airship is flying in the International Standard Atmosphere, this value of 10% possible safe volume increase for the envelope will be sufficient, for example, to cope with a rise from sea level altitude to about I kilometer, bearing in mind the following two helpful effects that have not been considered above: (1) the expansion of the lift gas as the airship rises does work upon the external atmosphere and hence removes heat from the lift gas (in other words, the expansion is not completely isothermal but has an adiabatic component due to the non-zero speed of ascent) which causes an extra quantity of the steam lift gas to condense into water; and (2) the increased envelope tension compresses the lift gas to a greater extent. When combined with the fact that, as the airship starts to rise, the operator of this airship may simultaneously reduce the rate of production of steam by the boiler B, so that the volume of steam within the envelope I diminishes progressively as the airship rises, , and vice versa, it will be understood that this airship is sufficiently able to cope with variations of ambient pressure such as may be caused by change of altitude,, variation of insolation,, and weather, without the requirement for any ballonet system.

Claims (20)

1. DISCLAIMER

   Although the present invention has been shown and described with reference to various embodiments thereof, the form and the details of any particular embodiment could be varied without departing from the scope of the invention. The concepts of the various embodiments could also be combined 'in various ways. For example, the concept of affixing slabs of flexible insulation material to the envelope of this LTA crA as was done in the fourth preferred embodiment disclosed above, can be applied to any embodiment of the present invention, and particularly can be applied to the fifth through seventh preferred airship embodiments disclosed above. Similarly, the concept of not providing any ballonets to the envelope of the airship, as was done in the seventh preferred embodiment disclosed above, can be applied to any airship embodiment of the present invention, and particularly can be applied to the sixth preferred embodiment disclosed above, in which a steam engine is utilized for propulsion. Other variations are also possible. Accordingly it is desired that the scope of the present invention should be determined, not by any of the perhaps purely fortuitous details of particular embodiments, but solely by the claims.

   CLAIMS 1. An LTA craft comprising: an envelope; and means for passing liquid water lying at a low point of the inside of said envelope out from said envelope while intercepting gas from escaping from said envelope.
2. 2. An LTA craft as claimed in Claim 1, said LTA craft being a free balloon, and Rirther comprising means for discharging said passed out liquid water into the atmosphere.
3. 3. An LTA craft as claimed in Claim L fin-ther comprising: means for boiling said passed out liquid water into steam; and means for supplying said steam into said envelope.
4. 4. An LTA craft as claimed in Claim 3, wherein the rate of operation of said boiling means is variable.
5. 5. An LTA craft as claimed in Claim 42 ftuther comprising means for temporarily storing at least a portion of said passed out liquid water.
6. 6. An LTA craft according to any one of Claims 3 through 5, said LTA craft being a powered airship, wherein said means for supplying said steam into said envelope comprises a steam engine developing motive power to propel said LTA craft, which is supplied with pressurized steam 0 from said boiling means, and whose exhaust steam is supplied into said envelope.
7. 7. An LTA craft according to Claim 6, wherein said means for supplying said steam into said envelope ftwther comprises a bypass conduit which supplies steam from said boiling means directly into said envelope, bypassing said steam engine.
8. 8. An LTA craft according to Claim 7, wherein said means for supplying said steam into said envelope ftirther comprises means for varying the amounts and proportions of steam supply from said boiling means into said bypass conduit and to said steam engine.
9. 9. An LTA craft according to Claim 1, wherein said water passing and gas intercepting means comprises a trap valve located proximate to said low point, and comprising a valve seat and a valve float which: rests against and intercepts said valve seat when less than a predetermined amount of liquid water is present around said float, thus preventing the passage of liquid water through said valve seat; and is lifted by buoyancy force away from and opens said valve seat when more than said predetermined amount of liquid water is present around said float, thus allowing the passage of liquid water through said valve seat.
10. 10. An LTA craft comprising: an envelope filled with a lift gas which mainly consists of steam; and means for passing liquid water condensed from said lift gas and lying at a low point of the inside of said envelope out from said envelope while intercepting said lift gas from escaping from said envelope.
11. 11. An LTA craft as claimed in Claim 10, said LTA craft being a free balloon, and further comprising means for discharging said passed out liquid water into the atmosphere.
12. 12. An LTA craft as claimed in Claim 10, , further comprising: means for boiling said passed out liquid water into steam; and means for supplying said steam into said envelope.
13. 13. An LTA craft as claimed in Claim 121, wherein the rate of operation of said boiling means is variable.
14. 14. An LTA craft as claimed in Claim 13, ffirther comprising means for temporarily storing at least a portion of said passed out liquid water.
15. 15. An LTA craft according to any one of Claims 12 through 14, said LTA craft being a powered airship, wherein said means for supplying said steam into said envelope comprises a steam engine developing motive power to propel said LTA craft, which is supplied with pressurized steam from said boiling means, and whose exhaust steam is supplied into said envelope.
16. 16. An LTA craft according to Claim 15, wherein said means for supplying said steam into said envelope further comprises a bypass conduit which supplies steam from said boiling means directly into said envelope, bypassing said steam engine.
17. 17. An LTA craft according to Claim 16, wherein said means for supplying said steam into said envelope further comprises means for varying the amounts and proportions of steam supply from said boiling means into said bypass conduit and to said steam engine.
18. 18. An LTA craft according to Claim 10, wherein said water passing and gas intercepting means comprises a trap valve located proximate to said low point, and comprising a valve seat and a valve float which: rests against and intercepts said valve seat when less than a predetermined amount of liquid water is present around said float, thus preventing the passage of liquid water through said valve seat; and is lifted by buoyancy force away from and opens said valve seat when more than said predetermined amount of liquid water is present around said float, thus allowing the passage of liquid water through said valve seat.
19. 19. An LTA craft as claimed in any previous claim, further comprising means for insulating said envelope.
20. 20. An LTA craft substantially as described and as shown in the accompanying drawings.