US3000312A

US3000312A - Igniter

Info

Publication number: US3000312A
Application number: US306030A
Authority: US
Inventors: Ernest R Roberts
Original assignee: Aerojet General Corp
Current assignee: Aerojet Rocketdyne Inc
Priority date: 1952-08-23
Filing date: 1952-08-23
Publication date: 1961-09-19
Anticipated expiration: 1978-09-19

Description

P 1961 E. R. ROBERTS 3,000,312

IGNITER Filed Aug. 23, 1952 2 Sheets-Sheet 1 ERNEST R. ROBERTS A TTORNEY P 9, 1961 E. R. ROBERTS. 3,000,312

IGNITER I Filed Aug. 25, 1952 2 Sheets-Sheet 2 IN VEN TOR. ERNEST R. ROBERTS United States Patent 3,000,312 IGNITER Ernest R. Roberts, Pasadena, Calif., assignor, by mesne assignments, to Aerojet-General Corporation, Cincinnati, Ohio, a corporation of Ohio Filed Aug. 23, 1952. Ser. No. 306,030 7 Claims. (Cl. 102-702) This invention relates to jet propulsion and in particular to an improved ignition system for use in rocket motors.

The object of this invention is to provide an igniter which avoids the source of ignition shock which is commonly encountered in igniters used heretofore.

A further object of the invention is to provide an igniter that does not require nozzle closure members.

Another object of the invention is to provide an igniter that avoids undesirably long ignition delays when the propellent charge to be ignited is at a low temperature.

Heretofore the majority of igniters employed for initiating combustion of jet propulsion charges, such as solid propellants, have usually made use of a nozzle closure device to insure suflicient pressure combined with the temperature developed by the igniter to assist in igniting the principal charge. The ordinary black powder type igniter, develops after ignition what is commonly known as an ignition shock. This shock is in part due to the force developed by the igniter and has the undesirable feature that it causes malfunction of sensitive instrumentation often employed in connection with rocket motors.

The second source of ignition shock is caused, as stated above, by nozzle closures used in conjunction with the black powder ignition system. The ignition shock occurs when the closure bursts and result in severe shock load- It is evident that in order to remove these two principal sources of ignition shock it is desirable to use a diflerent type of igniter than the conventional type; and it may also be desirable to avoid the use of conventional nozzle closure members.

In accordance with my invention I provide an igniter that gives reliable and reproducible results at ambient temperatures in a wide range from about 75 to +170 F., and which also produces a gradual rate of thrust increase accompanied by a relatively low peak of ignition thrust, and also by a relatively short time delay of ignition.

I have discovered that the foregoing desired characteristics can be obtained by use of a screenlike cage or basket containing combustible grains or pellets. This construction has an advantage in that the mesh spaces of the basketlike container may be sealed by a suitable substance such as a thin screen of lacquer or soft plastic, forming a case sufliciently strong to contain the combustible as well as such pressure as is desired within the case before the case bursts upon combustion.

This construction has the advantage that the sides and end of the igniter cage are unobstructed thus permitting free escape of flame and combustion products in many directions, and thus produce little or no thrust and resulting ignition shock.

A particularly desirable type of combustible to use in the igniter is a type described in Gongwer application Ser. No. 175,397, filed July 22, 1950, and assigned to the same assignee as the present application. In said Gongwer application there is described a stoichiometric mixture of aluminum and potassium perchlorate suitable for the generation of large amounts of heat. Such a mixture liberates more than three times the energy of an equivalent weight of black powder when burned and produces a flame temperature exceeding 7000 F.

3,000,312 Patented Sept. 19, 1961 Since it produces an extremely high flame temperature there is sufficient heat to vaporize all of the combustion products and develop the required pressure. As condensation of the combustion products occurs rather quickly, an advantage is that the latent heats are liberated at high temperature. Such an aluminum-potassium perchlorate mixture will produce said grain ignition Without any nozzle closure at ambient temperatures as low as F.

In some instances it is desirable to compound the ignition charge by adding small amounts of black powder to assist starting.

It has been discovered that the thickness of the wall of the igniter case should be controlled so that little or no pressure is required to burst the case; thereby avoiding explosive burning of the igniter charge before rupture of the case and materially reducing the shock that would normally occur.

The case should be capable of retaining the igniter material until the combustion is essentially complete and the case should be constructed so as to always allow the escape of the combustion products over .a wide spherical angle thus preventing a directed blast in any one direction. This eliminates or reduces the component of force producing thrust and shock.

The invention will be better understood with references to the following description and accompanying drawings in which:

FIGURE 1 is a cross-section view of an assembled igniter mounted in a motor boss.

FIGURE 2 is an end view of the igniter of FIGURE 1.

FIGURE 3 is a view of the igniter safety diaphragm.

FIGURE 4 is an isometric view of the igniter basket.

Referring to the drawings: The igniter comprises a rocket motor adapter 11 provided with legs 12 that support an igniter basket ring clamp 13. A wire mesh basket 14 having a wire mesh lip 15, as shown in FIG. 4, is held against the annular section 13 by means of a washer 16 secured to the ring 13 by maens of a plurality of screws 17.

The lower end of the adapter 11 adjoining the portion to which the legs 12 are fastened is reduced in diameter for a suitable distance and this reduced portionis provided with threads 18 by which the adapter is secured to a threaded boss 19 attached to the rocket motor. In the igniter shown in FIGURE 1, the end of the adapter opposite to the end from which the legs 12 are secured, is provided with a bore 20 which extends throughout the length of the adapter. Bore 20 is provided with a counterbore 21 which starts at the opposite end from the end to which the legs are secured. Counter bore 21 continues for a substantial distance then terminates abruptly forming a shoulder 22. Counter bore 21 is provided with threads 23 which engage a flanged annular nut 24 preferably having a bore extending through it that corresponds to bore 20 of adapter 11.

A diaphragm 25 shown in FIGS. 1 and 3 made of a substance capable of withstanding the normal pressures developed in the normal operation of rocket motor is inserted in counterbore against the shoulder 22 and clamped in position by threaded flanged nut 24 when tightened on threads 23.

The adapter 11 is provided with suitable electrical connectors 26 which may be in the form of insulated plugs that extend into bore 20 of the adapter. The projecting electrodes 27 of the plug are insulated from the metallic portions of the adapter by means of a liner 28 that fits within the adapter and is continuous against the diaphragm 25. Liner 28 is provided with cylindrical projections 29 that extend a substantial distance into threaded bores 30 in adapter 11 to which the threaded portion of electrode plugs 26 are secured.

The space in bore 20 between the insulating liner 28, in contact with the diaphragm 25, up to within a short distance of the point at which electrode 27 enters the bore 20 is filled with a suitable heat insulating material 31. This may be for example a chromate paste. The insulating material 31 is retained in place by a cover member 32.

The igniter cage 14 is packed with a number of pellets 33 of the igniting material, which in this case is the stoichiometric mixture of aluminum with potassium perchlorate.

The screen wire of the igniter cage should be of such a material and thickness that it will not burn out or melt until the combustion is essentially complete. A preferred form of igniter case comprises an 8-mesh, deep drawn, cylindrical screen cage using approximately .028 in. wire. The case may be made of galvanized iron, soft steel or stainless steel as desired. Where it is desired to obtain a slight burst pressure the screen is dip coated with a vinylite-type lacquer in order to fill the meshes with a thin, tough film.

1 Within the igniter cage 14 there is also located a squib 34. This squib is preferably annular and may be of the black powder type provided with a resistance element 35 to ignite the powder charge whenever the circuit element is heated by completing the circuit from a source of electrical energy (not shown) through the electrode 27 and connecting Wires 36. In some instances it may be desirable to use a resistor 37 in the line to reduce the current flow through resistance element 35 to a desirable value in cases where the voltage applied at the electrical terminals is unduly high.

The pelletized material employed in the cage may also, if desired, be a mixture of potassium perchlorate, aluminum and a small amount of black powder. Other suitable combustible materials may also be used.

The preferred igniter pellets are made as follows: The aluminum and potassium perchlorate are formed into solid elliptical shaped pellets by subjecting the mixture to high pressures. These pellets have a burning rate of between .2" and .5 per second and develop a pressure of between 15 and 100 p.s.i.g. The dimensions of the pellet are determined by the burning rate desired at 100 p.s.i.g. This is done by using the formula H=2RT, in which H is the thickness of the pellet, R is the burning rate, and T is the total burning time. Lens-shaped pellets having a thickness of between .07 to .10 inch and a diameter of inch are preferred over flat discs since the discs would tend to become stacked in the case and materially reduce the surface of the pellets exposed to the flame. When stacking of this type occurs combustion of the pellets is materially delayed.

The aluminum-potassium perchlorate mixture does not appear to undergo deterioration or lose its ignition characteristics when subjected to temperatures of 165 F. for a period of three weeks or longer or when subjected to conditions where the relative humidity is 100% at 140 F. for ninety-six hours.

The operation of the device is as follows: The igniter adapter is threaded into the rocket chamber boss 19 usually positioned so as to direct the flame from the igniter against the propellent charge. The electrical circuit is closed through the wires 36 and resistor 37 and element 35 causing the powder in squib 34 to burn. The burning powder in squib 34 causes ignition of the surrounding pellets of aluminum and potassium perchlorate which burn developing a high temperature. The flames and heat from these pellets can escape uniformly through the openings in the wire mesh basket 14, since there are openings in all directions. The igniter in this manner cannot generate any shock due to the gases or combustion products escaping in any one direction and does not require any prolonged period of time for the flames to escape the cage and reach the propellent charge.

With this type of igniter charge it is generally unneces- '4 sary to employ' a closure member in the throat of the nozzle leading from the motor chamber; therefore, there is no ignition shock developed such as usually would oc cur, when a closure member is ruptured after a relatively high pressure has built up in the closed combustion chamber sufficiently high to burst the closure member.

If desired the wire mesh basket in which the pellets and the squib are inclosed may be clipped or otherwise coated with a thin film of a low tensile strength plastic to protect the pellets and the squib from moisture and to permit a slight pressure to develop within the igniter to assist the ignition of the more difiicultly ignitable pel lets of aluminum and potassium perchlorate placed in the wire mesh basket.

I claim:

1. An igniter comprising a metallic body member having an opening therethrough, a wire mesh cage, supporting means attaching said cage over the opening, in such a manner that said cage is free to discharge gases in many directions, safety diaphragm means secured to said body member at a position of the opening spaced from the cage, primer means within said cage for initiating combustion therein, a plurality of self-combustible pellets within said cage in proximity to the primer means said pellets being larger in size than the interstices of said cage, whereby they are retained within said cage until they are substantially consumed by combustion, and means for igniting the primer means.

2. An igniter comprising a metallic body member having an opening, a wire mesh cage, supporting means attaching said wire cage to said body member and over the opening, in such a manner that the cage is free to discharge gases in many directions, a plurality of self-combustible pellets within the cage, said pellets comprising an intimate mixture of aluminum and potassium perchlorate in substantially stoichiometric proportions, and means in proximity to the pellets for igniting the pellets.

3. An igniter according to claim 2 in which the open spaces of the wire mesh cage are closed by a combustible resinous film.

4. An igniter comprising a metallic body member having an opening, a wire mesh cage, means attaching said cage to said body member at an end of the opening, in such a manner that the cage is free to discharge gases in many directions, a plurality of self-combustible pellets within the cage, said pellets comprising an intimate mixture of aluminum and potassium perchlorate in substantially stoichiometric proportions and means including black powder in proximity to the pellets for igniting the pellets.

5. An igniter for solid propellent rocket motors comprising a metallic body member having an axial opening, a wire mesh cage, means attaching said cage to said body member at an end of the opening, in such a manner that the cage is free to discharge gases in all directions, a plurality of self-combustible pellets within the cage, said pellets comprising an intimate mixture of aluminum and potassium perchlorate in substantially stoichiometric proportions and black powder.

6. A igniter comprising a metallic body member, a wire mesh cage, supporting means attaching said cage to said body member in such a manner that said cage can discharge gases in many directions, primer means within said cage for initiating combustion therein, a plurality of self-combustible pellets within said cage in proximity to the primer means said pellets being larger in size than the interstices of said cage, whereby they are retained within said cage until they are substantially consumed by combustion, and means for initiating the primer means.

7. An igniter comprising a body member, a cage structure having a plurality of openings therethrough, supporting means attaching said cage structure to said body member in such a manner that said cage structure can discharge gases in many directions, primer means within said cage structure for initiating combustion therein, a plurality of self-combustible pellets within said cage structure in proximity to said primer means, said pellets being larger in size than the openings in said cage structure, whereby they are retained within said cage struc- 5 ture until they are substantially consumed by combustion, and means for initiating the said primer means.

References Cited in the file of this patent UNITED STATES PATENTS 2,434,652 Hickman Jan. 20, 1948 2,693,757 Brandt Nov. 9, 1954 FOREIGN PATENTS 280,216 Swizerland Apr. 16, 1952