US2712283A - Propellent assembly for jet propelled device - Google Patents

Description

July 5, 1955 GOLDEN 2,712,283

PROPELLENT ASSEMBLY FOR JET PROPELLED DEVICE Filed July 20, 1944 Illllll HI \5 aw 4 m 2/ a h 1 El IDNEY GOLDEN,

nited States Patent 0 PROFELLENT ASSEMBLY FOR .lET PROPELLED DEVICE Sidney Golden, Cnrnheriand, Md, assignor to the United States of America as represented by the Secretary of War Application Juiy 2%), 1944, Serial No. 545,809

21 Claims. c1.102-49 This invention relates to a propelling charge for rockets more particularly to an assembly of individual propellent grains constituting the rocket driving charge.

In many rocket applications, particularly ground fired applications, the blast from the jet is a constant source of annoyance and frequently becomes a rather serious disadvantage to the use of rockets. If a rocket is to be fired from a projector tube held in the hands of ground per sonnel, it is important that the burning be completed within the tube. One way of achieving this result would be to have the length of the tube increased. This, of

course, would be undesirable from the standpoint of portability. The other alternative is to reduce the time required for burning the propellant.

Two important factors governing the burning time of a rocket propellant are the web thickness (the distance between burning surfaces) and the operating pressure within the combustion chamber. In all prior rocket constructions there has been a practical limit to the amount the web thickness of the propellent grain may be decreased without encountering difliculties in trapping the propellant. That is, in prior constructions the unburned portions of the propellant of low web thickness break free of the trap and escape from the combustion chamber. The smaller the original web thickness, the larger the percentage of unburned powder that escapes.

When the burning time is decreased, the muzzle velocity is also decreased (assuming constant acceleration) unless the weight of charge burned per unit time is increased. The increased weight of propellant burned per unit of time causes an increase in the operating pressure within the combustion chamber. In prior designs difiiculties are encountered in obtaining the required weight of propellant having small web thickness without an increase in volume of the combustion chamber. It is possible by my invention, however, to easily obtain a weight of propellant suflicient to produce the required velocities without increasing the size of the combustion chamber and without encountering difficulties in trapping.

Heretofore optimum performance has been obtained from conventional rocket motors by designing such motors to operate at an equilibrium pressure wherein the rate of discharge of the combustion products equals the rate of their formation. This equilibrium pressure is determined primarily by the burning rate of the propellent material and also by a factor referred to in the art as the external K. This factor K is equal to the ratio of the burning surface area of the propellant to the cross sectional area of the throat of the discharge orifice. Under equilibrium burning conditions a substantially rectangular pressure time curve is obtained. Such equilibrium burning has an inherent defect in that the equilibrium pressure developed is dependent largely upon the burning rate of the propellent material. This burning rate in turn varies greatly with the ambient temperature. As a consequence the equilibrium pressure will vary greatly with the ambient temperature of the prepellent material. The variation of equilibrium pressure with ambient tempera- 2,712,283 Patented July 5, 1955 Cal ture produces an inverse variation in the total burning time of the rocket motor.

Both of these variables produced by conventional rock'- et motors are of extreme importance in the ground firing of rocket projectiles and particularly in connection with projectiles fired from a shoulder type launcher. If the ambient temperature is high, the equilibrium pressure will be greatly increased and often reaches a value sufficient to blow out the Walls of the motor chamber. On the other hand, if the ambient temperature is low, the burning time is greatly increased and the projectile is still burning when it leaves the launcher, thus directs a sheet of flame back into the operators face.

Accordingly it is an object of this invention to provide an improved rocket motor for a rocket projectile.

Another object of this invention is to provide a rocket motor characterized by the fact that its pressure time curve has no region of equilibritun pressure.

A further object of this invent-ion is to provide a rocket motor wherein the burning time of the propellent charge is substantially uniform and independent of ambient temperature variations of the propellant.

A particular object of this invention is to provide a rocket motor and propellent assembly wherein an extremely short burning time can be realized.

It is a further object of this invention to provide an arrangement of propellent grains such that the propellent assembly can be loaded into the motor combustion chamber thru the nozzle of the chamber. Another important feature of this invention, particularly related to the object above stated, is that the amount of propellant utilized may be easily varied.

The specific nature of the invention as well as other objects and advantages thereof will clearly appear from a description of a preferred embodiment as shown in the accompanying drawing in which:

Fig. 1 is an elevational View of a rocket assembly, partly in cross section, showing the position of the propellent assembly in the motor chamber.

Fig. 2 is a perspective view of a suitable propellant in disc form.

Fig. 3 is an enlarged cross sectional view of the propellent assembly showing the rod upon which the propellent grains are strung.

Fig. 4 is an elevational view of a rocket assembly, partly in cross section, showing a modified arrangement for holding the propellent grains in assembled relation within the combustion chamber.

Fig. 5 is a cross sectional view taken on lines VV of Fig. 4.

The above objects of this invention are attained by utilizing a propellant comprising a plurality of discs or washers, and supporting these discs or washers on a rod extending longitudinally of the motor chamber. These discs or washers of powder are simply threaded or strung on the rod and need not fit snugly, nor are they welded or pressed together in any way to form a unitary structure. When the propellent assembly is ignited, burning of each disc or washer occurs on all the surfaces thereof. That is, both the upper and lower surfaces as well as the lateral inside and outside surfaces of the washers are ignited. Since the web thickness, i. e., the

thickness of the Washers may be made small the burning time for a given mass of propellant may be made extremely short. Furthermore, no difficulties are encountered in trapping even though the web thickness of each disc is made exceedingly small.

Referring to the drawings there. is shown in Fig. 1 a hollow cylindrical rocket motor housing 1. A head 2 adapted to contain a pay 10a which may be a high explosive charge, a parachute flare, or a chemical charge is secured as by threads to the upper end of housing 1.

The base 3 of head 2 provides a support for axially mounting a rod 4 within chamber 1. The rod 4 is screwed into a threaded axial hole 3 in base 3. A plurality of propellent discs 5 are strung on this rod and kept in place by a washer 6, preferably of the same diameter as the propellent discs, and a nut 7. Fig. 3 shows the assembly of the rod, washer, nut and propellent discs in greater detail. The rear end of housing 1 is shaped to define a nozzle 8 of conventional Venturi construction.

It is to be observed that in the modification shown, space is left between the front end of the motor chamber and the end of the stack of propellent discs which are strung on the rod. Such free space is an important part of this invention, since it permits increasing the charge by adding extra discs or washers whenever desired, as for example, to increase the range of the rocket projectile. The loose mounting of the discs also facilitates the burning on the contiguous surfaces of each disc or washer. If the discs or washers were fastened on the rod snugly or were pressed together to in elfect simulate a single column r of propellant, there would be tendency to prevent the burning between the faces of adjacent discs or washers with the result that the burning time would be enormously increased. That is, if the burning was restricted to the arcuate areas of the discs only, an important object of this invention would be defeated.

In Figs. 4 and 5 there is shown an alternative way of fastening the stack of discs or washers to the rod at its lower end. In this modification a member 10 is provided which is generally of rod-like shape except for a plurality of radially extending arms lll which engage the interior wall of the motor housing 1. The disc portion of the member it is substantially equal in diameter to the hole in the individual propellent grains 5. In assembly the discs or washers 5 are slipped over the free end of the rod and are held thereon by the member 14). The rod 10 is then screwed into the threaded axial hole 3 in the base 3 of head 2. The disc assembly secured to head 2 is then inserted within chamber 1 and head 2 is threadably secured to housing 1 as shown in Fig. 1. The arms 11 of the member 10 engage the inside wall of the motor combustion chamber 1 just forward of the Venturi 8 to hold the assembly firmly in place.

In both of the above modifications the propellant is preferably ignited by a black powder charge 16, sufiicient in quantity to build up an initial chamber pressure to sustain ignition. The charge 16 is in turn ignited by any conventional means, such as a squib (not shown) fired electrically by current passing through the wires 21.

It is advantageous, as shown in Fig. l, to make the disc diameter slightly smaller than the throat diameter of the nozzle 8. When this is done the propellent discs can be threaded on to the rod 4 with the washer nut in place, and the whole assembly can then be slipped into the motor chamber through the nozzle and the upper end of the rod screwed into the front end of the motor chamber in the tapped opening provided therein.

It should be understood that when the alternative holding means, as shown in Figs. 4 and 5, is used, the propellent assembly must be loaded into the motor chamber through the upper end and the pay loa thereafter fixed to the motor housing. This modification is less desirable than the modification shown in Fig. 1 since, as stated above, an important advantage of this invention is that the propellant can be loaded in the combustion chamber through the nozzle if the diameter of the stack of discs is slightly less than the throat diameter. However, in some cases this advantage may be of secondary importance, therefore the invention should not be considered as limited to any particular diameters for the stack or for the throat.

As 'has already been indicated a rocket motor embodying this invention is characterized by the fact that the pressure time curve has no region of equilibrium pressure. Hence the maximum pressure developed is substantially independent of the ambient. temperature of motor in a shorter time.

the propellent charge. The total burning time of a rocket motor may be considered to be made up of the following two factors: (1) the burning time for the propellant; (2) the discharge time for the gaseous combustion products liberated by the burning of the propellant. In a rocket motor embodying this invention, the burning rate of the propellent material accelerates as usual with an increase in the ambient temperature and thus a maximum pressure is attained within the rocket When this condition occcurs however, the discharge time is substantially increased, hence a substantially uniform total action time for the rocket motor is obtained. Likewise at low ambient temperatures the burning rate is lower resulting in a longer burning time before the maximum pressure is reached, but under such conditions the discharge time is shorter to produce substantially the same total action time.

Such non-equilibrium burning condition is obtained by utilizing an arrangement of propellant shown in Fig. 1 together with proper proportioning of the interior diameter of the rocket motor chamber, the burning surface area of the propellant, the thickness of the web of the individual grains of propellent charge and the throat area for the discharge of the gaseous combustion prod ucts.

It is not possible at the present to define the proportioning relationship between the various elements in the form or" an equation. However the interior diameter of the rocket motor 1, the total burning surface area of the propellant, the web thickness of the individual wafers 5 and the throat area of nozzle are so proportioned that the mass rate of formation of the combustion products by the burning propellant is greater than the mass rate of discharge of such combustion products until a large quantity of the propellant is consumed. During this period the pressure is continually increasing. At a certain point, the pressure then begins to drop off because the mass rate of discharge of combustion products becomes greater than the mass formation of such products due to the consumption of the propellent charge.

The maximum pressure attained in the laminated charge rocket motor constructed in accordance with this invention depends therefore not so much on the burning rate but rather upon the consumption of propellant in the course of its burning. The maximum pressure consequently does not depend significantly upon the ambient temperature of the propellant provided that trapping losses (which affect the maximum pressure) are comparable over the range of temperatures considered.

As a further aid to performance the outside and inside arcuate areas of each disc or washer 5 can be coated with a suitable material 20 such as dimethyl-phenyl-urea or graphite to resist burning, thereby essentially limiting the burning area to the contiguous surfaces of the powder grain washers and producing a highly uniform burning area.

It should be understood that the discs or washers may be formed from any suitable propellant such as a doublebase powder. Although the propellent laminations are most conveniently made in the form of discs or washers, they can obviously assume numerous other shapes such as squares, hexagons or ovals, and still achieve the objects of this invention.

In interpreting the term relatively small as used in the claims with respect to the grain structure the drawing is to be taken as showing in general a satisfactory proportionality of dimensions.

1 claim:

1. A jet propelled device comprising in combination a head adapted to contain a pay load, a combustion chamber secured to said head, a constricted exit orifice formed in said chamber at the end opposite said head, a driving charge combustible to generate a propellent fluid under pressure within said chamber, said charge comprising a plurality of relatively thin plates having a single perforation, said plates being smaller than said orifice,

means supporting a stack of said plates longitudinally within said chamber, and spaced from the walls thereof, said means including a rod extending through said perforations longitudinally of said chamber, said rod being longer than said stack of propellent plates, and means of securing the forward end of said rod to said head.

2. A jet propelled device including the combination of a cylindrical combustion chamber having a nozzle at an end of said chamber for the discharge of combustion gases, said nozzle having a restricted portion of circular configuration, a propellent assembly comprising a plurality of separate relatively thin plates of propellent material, each of said plates having a configuration such that they may be inserted through the restricted portion of said nozzle and provided with a perforation, and means extending through the aligned perforations of each of said discs for retaining said discs in said chamber.

3. A powder grain having two burning faces of substantially equal area partially defining a disc-like body having substantially uniform body thickness between said burning faces, said body thickness being relatively small as compared to the dimensions of said burning faces, the surface area of said grain other than said two burning faces being provided with a layer of dimethyl-phenylurea.

4. A powder grain having two burning faces of substantially equal area partially defining a disc-like body having substantially uniform body thickness between said burning faces, said body thickness being relatively small as compared to the dimensions of said burning faces, the surface area of said grain other than said two burning faces being provided with a layer of graphite.

5. A powder grain for propelling a rocket comprising a substantially right cylindrical annular body of powder, the axial dimension of said annular body being relatively small as compared to the diameter, each of the arcuate surfaces of said annular body being provided with a layer of dimethyl-phenyl-urea material.

6. A powder grain for propelling a rocket comprising a substantially right cylindrical annular body of powder. the axial dimension of said annular body being relatively small as compared to the diameter, each of the arcuate surfaces of said annular body being provided with a layer of graphite material.

7. In a rocket projectile, a housing defining a motor chamber, a propellent charge within said chamber comprising a stack of powder grains, each grain being in the form of a right cylindrical annulus having an axial dimension relatively small as compared to the diameter of said cylinder, each of the arcuate surfaces of said annulus being provided with a layer of incombustible material.

8. A powder grain having two burning faces of substantially equal area partially defining a disc-like body having substantially uniform body thickness between said burning faces, said body thickness being relatively small as compared to the dimensions of said burning faces, the surface area of said grain other than the said two burning faces being provided with means whereby said area is rendered substantially incombustible.

9. The powder grain of claim 8 in which the means for rendering incombustible the surface area of said grain other than the said two burning faces is a layer of incombustible material positioned on said surface area.

10. A powder grain for propelling a rocket comprising a substantially right cylindrical annular body of powder. the axial dimension of said annular body being relatively small as compared to the diameter thereof and each of the arcuate surfaces of said annular body being provided with a layer of incombustible material.

11. The powder grain of claim 8 having additionally means comprising an axial perforation therein whereby said grain may be positioned on a rod.

12. The powder grain of claim 10 having additionally means comprising an axial perforation therein whereby said grain may be positioned on a rod.

13. In combination, a rocket motor including a cylindrical combustion chamber with an outlet orifice, a rod extending axially through said chamber and means supporting a propellent charge loosely on said rod, said charge comprising a plurality of perforated powder grains, each of said grains comprising a right cylindrical annular body of powder, the axial dimension of said annular body being relatively small as compared to the diameter thereof, each of the arcuate surfaces of said annular body being provided with a layer of incombustible material, each said annular body having formed therein an axial perforation whereby each said grain may be positioned on a rod.

14. In combination, a rocket motor including a cylindrical combustion chamber with an outlet orifice, a rod extending axially through said chamber and means supporting a propellent charge loosely on said rod, said charge comprising a plurality of perforated powder grains, each of said grains having two burning faces of substantially equal area partially defining a disc like body having substantially uniform body thickness between said burning faces, said body thickness being relatively small as compared to the dimension of said burning faces, the surface area of each said grain other than said two burning faces being provided with means whereby said area is rendered substantially incombustible, and means comprising an axial perforation in each said grain whereby each said grain may be positioned on a rod.

15. A propellent charge for a reaction motor comprising a plurality of substantially identical perforated grains of powder retained loosely on a rod, each of said powder grains having two burning faces of substantially equal area partially defining a disc-like body having substantially uniform body thickness between said burning faces, said body thickness being relatively small as compared to the dimensions of said burning faces, the surface area of each of said grains other than the said two burning faces being provided with means whereby said area is rendered substantially incombustible.

16. A propellent charge for a reaction motor comprising a plurality of perforated discs of powder loosely retained on a rod, each of the arcuate areas of said discs being provided with means to prevent the combustion thereof.

17. A propellent charge comprising a plurality of perforated plate like laminae of powder loosely retained on a rod, each of the peripheral areas of said laminae being provided with means to prevent the combustion thereof.

18. A propellent charge comprising a plurality of perforated discs of powder loosely retained on a rod, each of the arcuate areas of said discs being provided with a layer of non-combustible material.

19. In a rocket propellent, a plate like lamina of powder having a central axial perforation, and combustion inhibiting means covering the axially-extending surfaces of said lamina.

20. The propellant in claim 19 wherein said combustion inhibiting means is a layer of dimethyl-phenyl-urea.

21. The propellant in claim 19 wherein said combustion inhibiting means is a layer of graphite.

References Cited in the file of this patent UNITED STATES PATENTS 826,293 Unge July 17, 1906 1,274,258 Gerdom July 30', 1918 1,812,010 McBride June 30, 1931 2,008,889 Wagner July 23, 1935 FOREIGN PATENTS 125,447 Great Britain Apr. 24, 1919 516,865 Great Britain Jan. 12, 1940 207,540 Switzerland Feb. 16, 1940

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