United States Patent 3739574

A gas generator method and apparatus comprising an enclosed pressure chamber having an exit port, a solid combustible material disposed in the chamber, a catalyst which can increase the decomposition rate of the combustible material which has been previously ignited, the catalyst being supported in the chamber adjacent to a surface of the combustible material. Means is provided for sensing the pressure in the chamber when the combustible material is decomposing and means is provided for moving the catalyst relative to the combustible material to maintain approximately constant chamber pressure during decomposition.

Application Number:
Publication Date:
Filing Date:
Northrop Carolina, Inc. (Swannanoa, NC)
Primary Class:
Other Classes:
60/39.47, 60/218, 60/219, 60/234, 60/254, 422/112, 422/164, 422/211
International Classes:
F02K9/26; (IPC1-7): F02K9/04
Field of Search:
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Primary Examiner:
Croyle, Carlton R.
Assistant Examiner:
Garrett, Robert E.
Parent Case Data:

This is a continuation-in-part application of patent application, Ser. No. 797,038, now abandoned, filed Feb. 6, 1969.
I claim

1. A gas generator comprising:

2. The gas generator of claim 1 wherein:

3. The gas generator of claim 2 wherein:

4. The gas generator of claim 3 wherein:

5. The gas generator of claim 1 wherein:

6. The gas generator of claim 1 wherein:

7. The gas generator of claim 1 wherein:

8. A method of generating gas upon demand comprising:

9. The method of claim 8 further comprising:

10. The method of claim 8 further comprising:

The most common previous method for throttling solid propellant motors or gas generators utilizes a variable throat area wherein the chamber pressure is raised or lowered. Other methods that have been used include mass addition from a solid or liquid source and recirculation of a portion of the combustion products through ducts in the grain to vary grain temperatures and thus burning rate. In the first two mentioned prior methods, as the chamber free volume to burning area ratio increases, the transient time from one level to another increases. Thus, for a low thrust long burn time motor, the transient time can be several seconds. The time required to throttle utilizing the third method of recirculation of a portion of the combustion products is also relatively long since heat must be transferred to and from the propellant grain.

In view of the aforegoing throttle limitations, applications requiring thrust level changes in a relatively short time, such as milliseconds, utilize liquid propellants, or solid propellant motors which operate at one mass generation level and which dump the gas in excess of that required fo the desired thrust at any time. However, the feasibility of the solid propellant approach for throttling can be dependent on the excess propellant required because of dumping. Thus, to date, the solid propellants having restart capability have involved complex structures, while those having a throttling ability have severe weight penalties in certain applications.

The invention described herein offers a unique throttling approach. Throttling can be accomplished at a constant chamber pressure which overcomes the reaction time disadvantage described above.

The above and other objects of this invention are accomplished by a method and device which utilizes a propellant material whose decomposition rate is effected by the presence of a suitable catalyst. An example of this material would be guanidine nitrate. The propellant is disposed within a suitable pressure chamber of a desired configuration and can, for example, be an end burning propellant. A manifold can intersect one end of the casing to direct the outlet gases formed to thrusters or other devices which will utilize them. Adjacent to one end of the propellant is a movable grid or screen coated or covered with a catalyst such as chromic oxide which will catalyze the decomposition of the propellant liberating desired gases.

Means, such as a conventional pyrotechnic igniter and the like, is required to initiate decomposition of the propellant and raise it to a proper ignition temperature. Once this is done, the catalyst material on the screen or grid is maintained in close proximity to the burning surface of the propellant with the gases passing through the grid to be utilized for the desired end result. The propellant grain burns continuously after ignition as long as an operating chamber pressure is maintained and does not require the presence of the catalyst. The rate of production of gases, however, is affected by the proximity of the catalyst to the burning surface of the propellant.

In the herein method, the chamber is maintained at essentially constant pressure after the grain is ignited. The operation of the device then becomes pressure dependent. As gas demand is increased, the chamber pressure initially drops and the catalyst is moved closer to the propellant to maintain constant chamber pressure. As gas demand is decreased, the chamber pressure is initially increased and the catalyst is moved away from the propellant. Thus, an aspect of the invention is that the means for moving the catalyst is responsive to chamber pressure.

It is believed that the invention will be better understood from the following detailed description and drawings, in which:

FIG. 1 is a partially sectioned pictorial representation of a gas generator of this invention;

FIG. 2 is an enlarged cross sectional view taken along lines 2--2 of FIG. 1;

FIG. 3 is a view taken along lines 3--3 of FIG. 1;

FIG. 4 is a graph of the burning rate of a propellant of the invention as a function of the proximity of the catalyst;

FIG. 5 is a schematic view of another embodiment of the invention.

The herein invention is based on the use of a propellant that is solid combustible material containing at least nitrogen and oxygen. The propellant must be capable of decomposition in the presence of a suitable catalyst to produce at least one gas selected from the class consisting of N2, NO and NO2. Thus, the propellant can include urea and its derivatives, nitrate esters such as triethylene glycol dinitrate, nitramines such as RDX, and guanidine derivatives such as nitroguanidine, guanidine nitrate and triaminoguanidinium hydrochloride. Further propellants contemplated are ammonia salts such as ammonium nitrate, ammonium perchlorate and 5-aminotetrazole, hydrazine derivatives such as formyl hydrazine, urea salts such as urea oxalate, nitrocellulose, glyoximes such as dihydroxy glyoxime and tetrazole derivatives such as 5-amino-tetrazole.

The propellants can, if possible, be used by themselves when pressed or melt cast into a solid form. Alternatively, the propellant can be combined with a suitable binder. Typical binders include tetrazoles such as vinyl tetrazole, polybutadienes such as polybutadiene/acrylic acid, polyesters, polyurethanes, polyethyl acrylates and nitrocellulose. A preferred form of binders are the fluorocarbon polymers such as flurooacrylates and Kel-F, a trifluorochloroethylene polymer made by The Minnesota Mining and Manufacturing Co. When a binder is used, it should not comprise more than 50 weight percent of the propellant grain. Preferably, less than 30 weight percent binder can be used.

The following table sets forth a list of some of the possible ingredients for the propellant of the invention and their relative weight percentages. As can be seen, it is contemplated that combustible materials can be combined in a single grain.


Weight Percent Ranges Ingredients A B C D E Guanidine Nitrate 0-100 0-100 0-100 0-100 Ammonium Nitrate 100-0 Ammonium Perchlorate 70-100 100-0 Kel-F Fluorocarbon Binder 30-0 5-Aminotetrazole 100-0 Triaminoguanidinium Hyrochloride 100-0

The above materials are either mixed well and pressed together, or in some cases where the material or mixture is stable at its melting point, can be melted and cast in a solid shape. It is to be noted that the composition of the propellant should preferably contain 5 to 90 weight percent nitrogen. Additionally, the propellant should preferably contain from 0 to 90 weight percent oxygen. The mixture of material chosen to be a propellant will give varying flame temperatures and varying exhaust products. One skilled in the art can calculate, for example, the theoretical flame temperature for any of the propellant compositions given in the above table, as well as analyzing the decomposition products resulting from their combustion.

The propellant grains can additionally contain small amounts of materials that will control the burning temperature and the burning rate. Such materials can include, for example, carbazides, hydrazides, amidoximes, ferrocene and ammonium dichromate. Additionally, nitrate esters can be added to serve as plasticizers for the grain.

Various materials can be utilized as catalysts to effect the desired sustained combustion of the propellant composition. However, it can be readily appreciated that not each catalyst will effect the same results with a given propellant. Typical catalysts that can be utilized are the oxides of chromium, manganese, iron, vanadium, lead, copper, nickel, boron, aluminum and cobalt. Particularly preferred are chromic oxide and vanadium tetroxide. The catalysts can be applied to the grid or screen by vapor deposition, spraying or any other suitable techniques to assure adherence of the oxide to the support structure.

In addition to the coating of the support structure with the catalyst, additional catalyst in the form of larger pellets can be imbedded in the screen or grid to further enhance the catalytic performance and insure presence of a sufficient amount of the material to achieve the catalytic effect. Additionally, a mixture of a metal oxide and the basic metal is contemplated to be applied to the support structure to obtain better adhesion to the structure.

It is believed that the invention can be further understood from the drawings, and attention is thus directed to FIG. 1. An outer case 11 of suitable metal construction contains a propellant 13 which, for example, can be pressed or melt cast guanidine nitrate. The case 11 is of cylindrical construction having a head end closure 15 affixed at a flange portion 17 thereto. A metal oxide covered curved screen 19 is shown adjacent to a correspondingly curved end surface 21 of the propellant material. The screen 19 is affixed to a rod 23. The rod is enclosed in an elongated cylindrical housing 25 extending outwardly from the case 11. The movement of rod 23 and attached catalyst covered screen 19 is controlled through gas pressure from a source 27 as will be further explained.

A fitting 29 extends inwardly into the case about the rod 23 and serves to limit its relative movement away from the surface 21 of the propellant. The details and operation of the fitting will be discussed with relation to FIG. 2. The exhaust gases produced by the decomposition of the propellant surface 21 will exit the case 11 through a manifold 31 and can, for example, be directed to a plurality of thrusters 33. Additionally, a relief valve 35 can be provided in the manifold to dump excess gas. A conventional solid propellant igniter 37 can be also provided in the head closure 15 and serves to start the initial combustion of the propellant surface 21 which is subsequently sustained by the catalyst screen 19. Though only one igniter is shown, a plurality can be used for multiple restarts.

As shown, a layer of insulation 39 covers the head closure 15 and tapers inwardly toward the casing wall of the gas generator. The insulation is preferably thicker at the head end since this portion experiences heat longer than the opposite end of the case. The main purpose of the insulation is to protect the external hardware from heating. Typical insulation material utilized in solid propellant technology can be suitably utilized. This material, for example, could be a silica or asbestos filled phenolic or rubber.

The exposed surface 21 of the propellant grain is shown in the Figure to be slightly curved. This allows sufficient space to be occupied by the fitting 29, yet maximizes the quantity of propellant contained in the housing 11. The fitting 29 serves actually to control the rearward movement of the screen 19, as will be explained. The chamber pressure during the burning of the propellant grain exerts a force corresponding to the pressure times the diameter of shaft 23 to move the grid 19 away from the grain face 21. A constant opposing force is supplied by the pressure bottle 27 which contains inert gas such as nitrogen. This pressure is maintained on the opposite end 41 of the shaft. A pressure regulator valve 43 serves to control the amount of gas pressure exerted. If the chamber pressure decreases, offsetting the balance, the screen moves toward the grain which increases the burning rate. Alternatively, if the chamber pressure increases, the screen separates from the grain.

The mechanism 29 shown in detail in FIG. 1 is incorporated to limit the backup distance of the screen to prevent long travel distances. As can be seen, the mechanism of fitting 29 comprises an outer shell 45, having double O-ring seals 47 to prevent the nitrogen gas from seeping through to the propellant chamber, or alternatively preventing the gases in the chamber from seeping toward the nitrogen source. Disposed in a recess area 57 in housing portion 45 is a ramp piece 51 which circumferentially surrounds the rod 23.

A pair of metal balls 53 are enclosed by ramp piece 51 and held in position by a spring 55. The ramp piece 51 is connected to a backing plate 56 by spring 57. The backing plate in turn is held within the recessed area 49 by a locking ring 59. Thus, it can be seen that as the rod 23 moves forward toward the propellant grain due to pressure exerted thereon by the inert gas, no restraint is placed upon its movement. However, when the chamber pressure builds up above a desired point, the rod is limited in its rearward movement due to the effect of the balls 53 gripping the rod as they move within the ramp piece. The limit of rearward movement is determined by the distance between the ramp piece and the backing plate.

Once the ramp piece strikes the backing plate, it can be seen that no further rearward movement of the rod 23 is possible. Generally it is desirable to limit the backup distance to 0.10 inch to prevent long travel distances. Alternatively, a typical screen velocity in a forward direction toward the grain could be about 5 inches per second. This is normally sufficient to maintain a pressure to within ±10 percent with a mass flow ratio of 8 to 1.

FIG. 3 is a pictorial representation of the catalytic screen. The screen 19 can be comprised of stainless steel wire having a mesh of 12. Alternatively, it can be more of a grid where the wires 61 are a greater diameter and/or of a flat ribbon. Obviously any metal suitable to sustain the relatively high temperatures without being affected by the exhaust gases is suitable for the grid 19. Such metals include chromium, high temperature nickel and chromium based steel alloys. The grid can, if desired, be flat rather than curved as shown. The oxide catalyst can be flame sprayed to cover the grid 19. Additionally, as previously indicated, individual pellets 63 can be disposed in the grid to further aid in catalyzing the decomposition of the propellant grain.

Many applications such as attitude control systems have mass flow requirements which change so rapidly that time is not available to vary the mass flow of a conventional solid propellant motor by changing pressure from one level to another or by extinguishing combustion and restarting. The time required to pressurize or de-pressurize the chamber free volume is longer than the response time requirements for the application.

The above problem is solved by the instant solid propellant motor or gas generator in which pressure is held constant and the burning rate of the propellant is varied by catalytic means as rapidly as mass flow requirements change. Burning rate of the propellant can be varied at least over a 10 to 1 range at constant pressure by changing the proximity of a catalytic screen to the propellant burning surface a fraction of an inch. Hence, the mass flow from the propellant grain can be changed as rapidly as the time required to move the catalytic screen this distance. The response time would be approximately 0.020 seconds with a screen velocity of 5 inches/seconds.

In FIG. 4, a typical burning rate curve is shown as a function of the proximity of the catalytic screen to the propellant burning surface, at constant chamber pressure. A gas generator can be so designed that the burning rate at Point A, with the catalyst in contact with the propellant, can satisfy the maximum mass flow requirements for an application. In operation, the propellant grain burns continuously but at a rate such that the pressure in the chamber remains constant. The catalytic screen is actuated toward or from the propellant surface as chamber pressure falls below or exceeds the nominal pressure value due to output flow rate changes caused by opening and closing of thruster valves and the like.

Two type control systems, proportional and bi-stable, can be employed. A proportional system will continuously send a signal to move the catalytic screen to maintain a separation distance between catalyst and propellant surface that corresponds to the instantaneous burning rate and mass flow rate requirement in order to maintain pressure constant. The bi-stable type system moves the catalyst to contact the propellant surface, Point A, when pressure falls below a limiting value. The catalyst is maintained in contact with the propellant surface until pressure rises above a limiting value, at which time the catalyst is moved a fixed distance from the propellant surface, Point B, to lower burning rate to the lowest value obtainable at the nominal operating pressure. During periods of maximum demand the catalyst will be at Point A a large proportion of the time. During periods of low demand the catalyst will be positioned at Point B a large proportion of the time. For intermediate demands, the proportion of the time at Point A and Point B will vary according to the level of the requirement. If the mass flow rate at Point B exceeds the demand during certain periods of operation, the gas in excess of that required to maintain constant pressure must be dumped from the system via a pressure relief valve 35, thruster valves, or a small orifice or orifices which flow continuously.

The catalyst screen can be actuated by hydraulic, penumatic or electrical means. Chamber gas can be employed for actuation in the same manner as with conventional gas-actuated valves.

Generally the system of this invention is designed to provide the desired thrust with all thrusters open. The chamber pressure is then controlled for a lesser operating number. For example, in a bi-stable system, with all eight thrusters open, the catalyst would be on the grain about 90 percent of the time. With one of the eight thrusters open, the catalyst would be on the grain only about 40 to 50 percent of the time. It is not desirable to have a system where the catalyst must be on the grain 100 percent of the time with all thrusters open because there should be a contingency for temperature effects and the like.

The aforegoing describes a two-position control system, as previously discussed. Alternatively, a proportional system can be utilized which will continuously send a signal to move the catalytic screen to maintain a separation distance between the catalyst and propellant surface that would correspond to the instantaneous burning rate and mass flow rate requirement in order to maintain constant pressure. In the proportional system, somewhat lower screen velocity is required than in the two-position one described, because the screen continually moves to maintain constant chamber pressure.

In designing a gas generator in accord with this invention, one first determines the desired end result. Thus, one might be interested in the specific flame temperature, a specific burning rate, and/or specific exhaust gas composition to be obtained from a given propellant utilized. One skilled in the art can readily calculate the aforegoing requirements from a propellant composition together with an overall motor design. For example, one might desire a gas generating propellant that does not contain hydrogen chloride in the exhaust products and has a maximum flame temperature requirement not exceeding 2,300°F. A guanidine nitrate propellant containing ammonium nitrate does not, for example, produce exhaust gases containing hydrogen chloride. Such a propellant containing 90 % guanidine nitrate and 10% ammonium nitrate has a claculated flame temperature of 2,274°F.

Though pyrotechnic type igniters have been shown in the figures, the propellant can be ignited through the use of electrical means. For example, an electrical lead can be connected to the grid which would be resistively heated to the desired auto-ignition temperature and brought into contact with the propellant.

Turning to FIG. 5, there is seen a schematic view of an embodiment of the invention utilizing two catalyst screens 71 movable in opposite directions. The generator 72 has two end burning grains 73 disposed therein. Each grain 73 burns from the center outwardly toward each end of the cylindrical housing 75. This arrangement provides an effective increased burning surface and thus a greater amount of produced gas within a given volume of gas generator. Further, by moving the screen 71 simultaneously in opposite directions, through the use of a single actuator (not shown), axial acceleration effects are neutralized, and thus less force is required to advance the screen toward the burning grain. In addition to the above two screen arrangements, a plurality of screens could be used, if desired. Further, gas generator and propellant can be generally spherical in shape to minimize case weight.

Though the invention has been described with relation to a plurality of thrusters, it is also applicable for a single one. In this case a variable control valve would be disposed between the gas generator and the single thruster to change thrust levels upon command. The different thrust levels could be delivered instantaneously to the thruster, depending only upon the response of the valve, by maintaining a constant pressure in the gas generator.

It can be seen from the above description of the invention that the device and method herein relate to controlling the burning rate of the propellant by the proximity of the catalyst. Though the aforegoing discussion has related to moving the catalyst screen toward and away from the propellant, the same effect can be achieved by moving the propellant, if desired, relative to the screen. Obviously, all the matter is relative movement between the two.

It should be pointed out that U.S. Pat. No. 3,460,348 discloses a solid prpellant gas generator in which the rate of decomposition of the grain is controlled by contact with a catalyst, and thus would appear initially to be similar to the herein invention. The patent includes ammonium nitrate as a solid propellant contemplated and potassium dichromate as a catalyst. The patent only contemplates the operation of the gas generator in on-off mode, since the grain therein will only burn when in contact with the catalyst. In other words, the patented process does not contemplate the constant pressure concept of the instant invention and thus cannot be successfully used where there are changing demands upon the gas level generated. In fact, the patented process is essentially opposite to the present invention in that it must inherently be operated at chamber pressure levels whereby the grain is extinguished when the catalyst is removed therefrom. In other words, the prior process cannot be operated at a constant chamber pressure while demand requirements are changing. It should be further noted that the patent only discloses inorganic salt oxidizers to be used in the propellant. There is no teaching of the use of organic materials such as guanidine nitrate.

The invention will be further understood from the following example:


A 10-inch diameter propellant grain comprised of 95 weight percent guanidine nitrate and 5 weight percent Kel-F was compression molded and fitted into a phenolic-asbestos sleeve which served as insulation for the grain. The insulated grain was placed in a pressure chamber along with a chromic oxide coated stainless steel screen fixed to a rod. The screen and rod were movable and controlled by a pneumatic bi-stable control system to adjust the proximity of the screen to the propellant burning surface similar to the device shown in FIG. 1. The chamber was fitted with a fixed vent or nozzle of 0.0125 square inch area and a second vent of 0.0256 square inch area which could be opened and closed with an on-off valve such that the total vent area could be changed from 0.0125 square inch to 0.0381 square inch on command. The propellant grain was ignited by pyrotechnic means and the catalyst screen controlled to maintain chamber pressure around 650 psi by moving the screen to the burning surface when pressure exceeded the desired value. The on-off valve was opened and closed four times during approximately 80 seconds of operation to demonstrate variable thrust with chamber pressure cycling about 650 psi.