US2815271A - Fuel containing nitromethane and nitroethane - Google Patents

Description

Dec. 3, 1957 F. ZWICKY ETAL 2,815,271

FUEL CONTAINING NITROMETHANE AND NITROETHANE Filed March 22, 1946 INVENTORS FRITZ ZW/CKY, FREDR/G/(J. EWING CARTER, ARTHURJ. STOS/Ck Ma-QF ATTORNEYS 2,8l5,27l Patented Dec. 3, 1957 2,815,271 FUEL coNrAmnso 1 OMETHANE AND Ntrnou'rnANE Application March 22, 1946, Serial N 0. 656,460

2 Claims. (Cl. 52--.5)

This invention relates to fuels and has for its object to provide a self-combustible fuel which may be safely employed in an internal combustion engine. A related object is to provide a partially or totally self-combustible fuel which can produce high thrust when used in a jet propulsion motor. This application is a continuation-inpart of our application, Serial No. 526,064, filed March 11, 1944, now Patent No. 2,433,943.

Decomposable and partially or totally self-combustible fuels are distinguished from ordinary fuels in that the self combustible type carries in it the oxygen required for its partial or total combustion after decomposition; whereas ordinary fuels require the addition of oxygen for its combustion. Self-combustible fuels have heretofore been employed to operate internal combustion engines but such fuels are usually limited in application due to their sensitivity to shock when exposed to the conditions which exist within the operating motor, particularly motors of the jet propulsion type. An example of a self-combustible fuel which has been used in a jet propulsion motor is nitromethane. Nitromethane, however, when subjected to the high temperatures and pressures which exist within the chamber of a jet propulsion motor, tends to become undesirably sensitive to shock and will sometimes detonate explosively; this is sometimes accompanied by the attending danger that the explosion may be propagated to the main storage supply through the lines.

In our said copending application, Serial No. 526,064, we have disclosed self-combustible fuels of nitromethane and nitroethane and of mixtures of nitromethane and nitroethane. When the mixture of nitromethane and nitroethane is used, it is a fact that a self-combustible fuel is provided over a very wide range of proportions of the nitromethane and nitroethane relative to each other. We have found, however, that the safety of the mixture, that is, its resistance to detonation from shock is enhanced when the ratio of the nitromethane to the nitroethane in the mixture is between rather definite limits. In fact, when the mixture ratio is within such limits the mixture is thereby rendered unusually insensitive to shock thereby making it safe to handle and store under conditions which have ordinarily been regarded as adverse. We accomplish this by our present invention by blending the nitromethane with the nitroethane so that the nitroethane is between 25% and 70% by volume of the nitromethane in the mixture. The effectiveness of this fuel is very ma terially improved by the addition of a suitable catalyst. We have found that the addition of chrome acetonylacetonate as a catalyst is especially effective in assisting the smooth decomposition of the compounded fuel in the motor. Such a fuel will not detonate easily under the conditions of temperatures and pressures which exist in a jet motor while it is operating and if it should detonate at some point in the system due to severe shock the explosion will not propagate to the rest of the system.

The expressions nitromethaue and nitroethane as used in these specifications and claims refer to the mononitromethane and the mononitroethane, respectively, in accordance with the common usage of these'words.

Our invention will be better understood from the following detailed description and the enclosed schematic drawing which shows a jet propulsion system suitable for employing the fuels of this invention. The drawing shows a jet motor 10 comprising a firing chamber 11 closed at one end and provided with an exhaust nozzle 12 at the other end' The blended fuel and catalyst are placed in tank 13 and are supplied to the firing chamber by conduits 14 through a nozzle 15. A suitable ignition device such as a spark plug 16 is employed to fire the charge initially. Oxygen from a storage tank 17 is connected to the firing chamber through constant pressure valve 13 and conduit 19. The flow into the chamber through a nozzle 20 is controlled by an electromagnetic valve 27. Fuel in tank 13 is placed under pressure by some suitable means such as an inert gas pressure tank 21 containing nitrogen or other suitable gas which supplies constant pressure to the fuel tank through constant pressure valve 23. An electromagnetic throttle valve 22 controls the admission of fuel into the firing chamber.

An inert gas storage tank 21 is also connected to the firing chamber directly through a line 24 provided with a constant pressure valve 25. 'The flow of gas into the firing chamber through nozzle 28 is controlled by an electromagnetic throttle valve 26.

The fuel employed in the apparatus, and which is the subject of this invention, is compounded by blending nitromethane with nitroethane. We have found that a self-combustible relatively non-detonable fuel may be obtained when a mixture is compounded containing nitromethane and nitroethane in which the nitroethane is from 25% to 70% by volume of the nitromethane. If the nitroethane is less than 25% by volume of the nitromethane-nitroethane mixture the mixture will be undesirably sensitive to shock and may be detonated under some conditions. However, such a mixture forms a low order of explosion and does not seem to propagate the explosion to other parts of the system through the connecting lines. The propagation of the explosion does not begin to take place unless by volume of the nitromethane is employed in the nitromethane-nitroethane mixture. Likewise if the proportion of nitroethane exceeds 70% of the nitroethane-nitromethane mixture the fuel becomes deficient in oxygen and is no longer self-combustible to a satisfactory degree. This fact is sometimes evidenced by the formation of carbon during the operation of the motor.

A preferred fuel mixture suitable for operation in jet motors is compounded by mixing equimolecular proportions of nitromethane and nitroethane. However, the factors which will determine the actual percentages to be employed are the specific impulse required and the chamber temperature at which the motor shall operate. There fore, any desired combination of nitroethane and nitro; methane within the limits stated above may be employed.

We have discovered that the decomposition of our nitroethane-nitromethane mixture is materially assisted by the presence of a suitable amount of catalyst introduced into the firing chamber such as chrome acetonylacetonate. The catalyst is mixed with the fuel prior to its transfer to the storage supply tank 13 of the motor assembly. The catalyst is added to the nitroparaffin mix{ ture in amounts varying from a trace to 5% by weight of the mixture. It has been observed, however, that sat. isfactory results will be obtained when about 3% of the chrome acetonylacetonate is employed in the fuel and successful runs have been made with lower amounts. The use of amounts greater than 3% of the catalytic material does not appear to improve the rate or manner of decomposition of the nitroparafiin substantially and apparently the main effect derived from increasing the amount to over 3% is only to add to the cost of the propellant.

To facilitate starting the jet motor when cold and to permit the fuel to produce the highest thrust on decomposition as soon as possible, a stream of oxygen is introduced into the combustion chamber simultaneously with the fuel during the initial firing period. The supply of oxygen may be discontinued after a short period which has been found to vary between 5 and seconds. After the motor has once assumed smooth operation it is unnecessary to supply additional oxygen.

The manner of operating the motor with our novel fuel is as follows: The combustion chamber of the motor 11 is flushed with an inert gas supplied from storage tank 21 to eliminate any unburnt materials and oxygen which may be present in the chamber. Fuel which contains the catalyst and stored in tank 13 is then introduced into the firing chamber as a mist or fine spray simultaneously with a stream of oxygen from container 17. The fuel in chamber 11 is ignited by spark plug 16 and the combustion takes place in the presence of excess oxygen for a period of 5 to 15 seconds, starting from the time the initial charge is ignited. This period serves as a warm ing up period to insure a sufficient temperature within the firing chamber to produce the smooth decomposition of the nitroparaflin fuel in the motor. After the warming up period the oxygen supply is discontinued and the nitroparaflins are allowed to decompose without further addition of oxidizer. The percentage of oxygen used varies generally between 5% and 15% by weight based on the weight of the fuel. It is preferable to employ a slightly greater percentage of oxygen than indicated above when the mixtures contain the higher percentages of nitroethane. For example, the wei ht of oxygen employed in a 5050 mixture of nitromethane-nitroethane was found to be preferably 15% of the weight of the fuel used; whereas a mixture of 65% nitroethane and 35% nitromethane by volume operated more efficiently when 17 /2 of oxygen by weight of the fuel used was employed in the initial starting period.

The chamber pressure which should be maintained during the operation of the motor lies approximately between 350 and 700 p. s. i., and it is preferable to maintain the chamber pressures about 400 p. s. i., since it has been observed that when the pressure drops below the minimum value of 350 p. s. i. there is a tendency for the decomposition to proceed only partially with the attendant formation of intermediate products such as formaldehyde and other noxious gases. The maintenance of desired chamber pressure is obtained in a manner well understood in the art, that is, by proper selection of the proportion between the exhaust nozzle and the combustion chamber with relation to the rate at which the fuel is injected. Formation of such toxic gases is not only undesirable but their presence may impair the efficiency of the apparatus.

The fuels according to our invention possess the advantage that they are self-combustible propellants and may be practically and efficiently employed in a jet motor without exposing the operator or apparatus to undue hazards from explosion and to a still greater hazard of the propagation of an explosion to the main storage supply through the fuel lines.

Another advantage which has been observed is that some blends of our fuels are capable of producing specific impulses which are substantially as high as those obtained when nitromethane alone is employed. It will be recognized, of course, that different motor dimensions and injection rates will produce different impulses. The following examples are illustrative: A 25% nitroethane to nitromethane by volume mixture under ideal conditions generates a specific impulse of approximately 200 lbs. sec./lb., and a chamber temperature of approximately 2500 Kelvin, when injected at a definite rate into a particular combustion chamber equipped with a suitable exhaust nozzle. A mixture containing 70% nitroethane, 30% nitromethane by volume when injected at the same rate into the same motor as above develops a specific impulse of lbs. see/lb. at a chamber temperature of 1700 Kelvin. When nitromethane alone is employed under the same conditions the specific impulse developed is 215 lbs. sec./lb. with a corresponding chamber temperature of 2460 Kelvin. These values in actual operation are subject to modifications due to the variations in design of the apparatus and to the experimental losses encountered in the operation of these apparatuses.

If it is desired to increase the specific impulse or the chamber temperature when employing the same motor as above, and employing the same fuels, it is possible to do so by conducting the combustion in an atmosphere containing various percentages of oxygen by weight of fuel burned. When the fuel is burnt in'the presence of added oxygen the specific impulse may be raised materially. For example when 10% of oxygen by weight of the fuel used was employed (the fuel being a mixture of 70% nitroethane and 30% nitromethane by volume) the impulse was increased from 175 lbs. sec./lb. to 208 lbs. see/lb. with a corresponding increase in chamber temperature up to 2000 Kelvin.

It is apparent that a wide range of specific impulses as well as chamber temperatures are made available by employing the mixtures described in our invention and this desirable feature accompanied by the relative freedom of detonation and the propagation of detonation opens a new field in the operation of jet motors which have heretofore been confined to installations in which safety was not regarded as an important factor. A characteristic of the above fuels is that after the motor has once started and after the chamber temperatures have been raised sufiiciently the motor will continue to operate with a practically invisible flame escaping from the jet orifice and the discharge from the motor will be substantially free from smoke and noxious fumes.

We claim:

1. A self-combustible, shock-proof, stable fuel comprising a mixture of nitromethane with about 25% to about 70% of the volume of the mixture of nitroethane, and a substantial proportion not exceeding about 5% based on the weight of the mixture of chrome acetonyl acetonate.

2. A self-combustible, shock-proof, stable fuel comprising a mixture of nitromethane with about 25% to about 70% of the volume of the mixture of nitroethane, and a substantial proportion not exceeding about 3% based on the weight of the mixture of chrome acetonyl acetonate.

References Cited in the file of this patent UNITED STATES PATENTS 2,086,775 Lyons et al. July 13, 1937 2,317,557 Senkus Apr. 27, 1943 2,325,064 Lawrence July 27, 1943 2,389,837 Bogin Nov. 27, 1945 2,397,657 Goddard Apr. 2, 1946 2,419,043 Urbanski Apr. 15, 1947

Claims (1)

1. A SELF-COMBUSTIBLE, SHOCK-PROOF, STABLE FUEL COMPRISING A MIXTURE OF NITROMETHANE WITH ABOUT 25% TO ABOUT 70% OF THE VOLUME OF THE MIXTURE OF NITROETHANE, AND A SUBSTANTIAL PROPORTION NOT EXCEEDING ABOUT 5% BASED ON THE WEIGHT OF THE MIXTURE OF CHROME ACETONYL ACETONATE.

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