US3057149A - Rocket propellant spark ignition system - Google Patents

Description

Oct. 9, 1962 C. L. LEINWEBER ET AL ROCKET PROPELLANT SPARK IGNITION SYSTEM Filed Sept. 30, 1958 CONTROL CIRCU ITRY OF FIG. 2 OR FIG?) 2 Sheets-Sheet l FUEL OXIDIZER ATTORNEY Oct. 9, 1962 c. 1.. LEINWEBER ET AL 3,057,149

ROCKET PROPELLANT SPARK IGNITION SYSTEM 2 Sheets-Sheet 2 Filed Sept. 30, 1958 ECHG ECHG

INVENTORS CLARENCE L. LEINWEBER HOWARD E. CORBIT'T Z M Q ATTORNEY Unite States 3,057,149 ROCKET PROPELLANT SPARK IGNITION SYSTEM Clarence L. Leinweber, Fort Worth, Tex., and Howard E. Corbitt, Arcadia, Calif., assignors to Aerojet-General Corporation, Azusa, Calif, a corporation of Ohio Filed Sept. 30, 1958, Ser. No. 765,702 Claims. (Cl. 60-35.6)

This invention relates to ignition systems generally and to systems for igniting liquid propellants in particu- To insure reliable starting and continuous operation of a liquid propellant rocket motor or a gas generator, it is essential that ignition means be applied only when sufficient fuel and oxidizer are present in the rocket thrust or combustion chamber in the proper proportions. Should too large an amount of fuel be present, ignition of the mixture will result in explosion rather than burning characterized by rapid expansion of the gaseous products of combustion. Previous ignition systems have been known to utllize glow plugs of electrically heated wire. If elecresidual heat because of the transient nature of the spark,

thereby increasing the safety of operation. Known spark .ignition systems have been actuated by timers or at the will of an operator. Such operation affords application of ignition means irrespective of actual conditions within the ignition chamber and ignition could be premature or tardy, resulting in the disadvantages described above.

It is, therefore, the principal object of the present invention to provide an improved means for electric spark ignition of a liquid propellant fuel mixture.

Another object of this invention is to provide an improved means for automatically igniting rocket propellants in the liquid state.

Still another object of this invention is to provide a I novel method of and improved means for igniting a liquid propellant when a proper quantity of fuel-oxidizer mixture in the proper ratio is present in the rocket thrust chamber.

And yet another object is to provide novel means for the prevention of accumulation of liquid propellant in explosive quantities in a rocket thrust chamber.

In its principal aspect, the present invention comprises a spark ignition system for the ignition of liquid propellants whereby ignition occurs when the optimum amount of fuel-oxidizer mixture is present within the chamber in the proper ratio. An electrode is arranged within the rocket thrust chamber, or combustion chamber in the 1 case of a gas generator, to form an electric discharge or spark gap between the electrode and the chamber casing. The term gas generator as used hereinafter in the specification and the claims shall be understood to include rocket motors or any other device wherein ignition of combustible matter produces gaseous products of combustion.

Liquid fuel and oxidizer, one of which is a good electrical conductor with respect to the other, are injected into the chamber thereby forming an electrically conduc- Itive path between the electrode and the casing. An electric potential or voltage placed across the gap causes ice an electric discharge between the electrode and the casing when sufiicient propellant in the proper ratio is within the chamber, thereby providing ignition of the fuel-oxidizer mixture at the optimum time. Although the instant of discharge may be determined by an external timer, in the preferred embodiment the discharge is made to occur automatically as external circuitry triggers the spark discharge when the necessary degree of completion of the electrical path within the chamber is provided by the injected fuel-oxidizer mixture. In this manner, any accumulation of unburned fuel is prevented.

These and other objects, aspects, features and conditions of the present invention will be apparent to those skilled in the art from the following more detailed descriptions taken in conjunction with the appended drawings, where- FIGURE 1 is a cross-sectional view of a rocket motor incorporating the propellant injector and discharge electrode of the present invention;

FIGURE 2 is a circuit diagram of a typical spark gap trigger control circuit, employing as an integral part thereof a two-element sealed gap; and,

FIGURE 3 is an electrical circuit diagram of a typical spark gap trigger control circuit using a three-element sealed gap.

Reference is made to FIGURE 1 which illustrates the present invention as incorporated in a conventional rocket motor 10 which basically comprises a chamber casing 11 within which a fuel-oxidizer mixture is burned to form hot gases which pass through an exhaust nozzle 12 to provide a propulsive force. When employed as a gas generator, the hot gases of combustion ejected from the exhaust nozzle 12 provide the force necessary to drive a turbine, not shown. The fuel and oxidizer are introduced into the chamber 13 by an injector 20 to form a stream of conducting propellant (oxidizer) surrounded by a stream of non-conducting propellant (fuel). A typical low conductivity fuel that may be used is kerosene, while suitable oxidizers include White Fuming Nitric Acid and Red Fuming Nitric Acid. Preferably the fluid propellant oxidizer is injected through the center conduit 22 which may, for example, be formed by extending a tube into the chamber 13, or be an orifice. The fuel, there-fore is injected into the chamber 13 from an annulus 24 formed around the conducting fluid tube 22. Alternatively, the oxidizer could be injected through the center conduit 22 and the fuel through the outer annulus 24; however, the arrangement described herein is preferred as it enables the fuel to vaporize more readily. Inserted into the chamber 13 in close proximity to the injector 20 is an electrode 14 which is introduced through a non-conducting pressure-tight seal 16 placed in one wall of the chamber casing 11. The casing 11 is arranged at electrical ground potential while the electrode 14 is connected to external circuitry through an electrical lead 15. A novel feature of the electrode 14 is the provision of a splash plate mixing device 18 to insure conversion of the fueloxidizer mixture stream into droplets dispersed throughout the chamber 13. In operation, fuel and oxidizer are introduced into the chamber 13 through the injector 20. Generally speaking, oxidizer fluids are relatively good electrical conductors relative to liquid fuels. Therefore,

when sufiicient oxidizer is introduced into the chamber 13, the change in leakage resistance between the electrode 14 and chamber structure 11 can be sensed by external circuitry. The oxidizer and fuel are injected in predetermined ratio, thus presence of sufficient oxidizer also indicates presence of the proper quantity of fuel. The change in resistance brought about by the presence of optimum fuel-oxidizer mixture quantities within the chamber 13 therefore causes external circuitry, such as shown in FIGURES 2 or 3, to trigger a spark discharge between the electrode 14 and chamber casing 11, thereby igniting the propellant mixture.

A typical circuit for automatic spark ignition of the mixture is shown in FIGURE 2. An external source of charging voltage is impressed across the first circuit input terminal and the second circuit input terminal 31. A current limiting resistance 33 is connected between the first input terminal 30 and one electrode 36 of a sealed spark gap 35, the other electrode 37 of which is electrically connected by a lead 15 to the chamber ignition electrode 14. A charging capacitor 32 is connected between the second circuit input terminal 31 and the point of connection between charging resistance 33 and the first gap electrode 36. Additionally, a high value resistance 34 is connected in electrical parallel across the sealed spark gap 35. The second circuit input terminal 31, one terminal of the charging capacitor 32 and the rocket chamber casing 11 are all connected together and placed at ground potential.

In operation, a source of direct current impressed across the circuit input terminals 30, 31 charges the storage capacitor 32. The length of time required for the capacitor 32 to reach the breakdown voltage of the gap 35 is determined in part by the values of the limiting resistance 33 and the capacitor 32. The voltage charge of the capacitor 32 is impressed across the series circuit of the sealed gap 35 and ignition gap 38, the latter including the ignition electrode 14 and the chamber casing 11. Both these gaps normally have a sufficiently high resistance to preclude an electrical discharge therethrough by application of voltages of the magnitude of the charging voltage. The resistance of the ignition gap 38 is represented in FIGURE 1 by a dashed resistor 40. When a sufficient amount of conducting fluid is injected into the ignition gap 38 by virtue of the injector 20 of FIGURE 1, the leakage resistance 40 across the ignition gap 38 changes to low value. In effect, the voltage of the storage capacitor 32 is thereby imposed across the terminals of the sealed gap 35, sharply increasing the potential gradient therein and causing an electrical discharge. This discharge in the sealed gap 35 will thereby place the circuit voltage across the terminals 14, 11 of the ignition gap 38 and similarly cause a spark therein. The high tension spark within ignition gap 38 travels in the interface between the stream composed of conducting oxidizer and nonconducting fuel. The resulting electrical energy is transformed to heat and kinetic energy, heating and atomizing a portion of the fuel and oxidizer at their interface, that is, the region where the stream of oxidizer is in contact with the encompassing stream of fuel. Ignition is then propagated to the remainder of the propellant mixture within the chamber 13 which has been broken into droplets by impingement upon the electrode splash plate 18. When the capacitor 32 is discharged below the breakdown voltage of the sealed gap 35, it will cease to conduct, thereby halting the electrical discharge in the ignition gap 38. The capacitor 32 will then recharge and, when a voltage of sufiicient magnitude is reached to cause a discharge across the sealed gap 35, another spark discharge will subsequently take place within the ignition gap 38. The system thus described is a repetitive spark system as long as the conducting fluid bridges the ignition gap 38.

FIGURE 3 illustrates in schematic diagram a typical spark gap trigger control circuit of the non-repetitive or single shot type. The external source of charging voltage is impressed across the first circuit input terminal v and the second circuit input terminal 52. A current limiting resistance 56 is connected between the first input terminal 50 and the first electrode 60 of a three-element sealed spark gap 58. A storage capacitor 54 is connected between the first electrode 60 and ground. A blocking capacitor 64 ties the first gap electrode 60 to a second gap electrode 62 through the primary winding of a pulse transformer 70. One side of the secondary winding of the pulse transformer 70 is connected to ground. The other side is connected to a third or trigger electrode 61 within the sealed gap 58 through a current limiting resistor 68 and a unilateral current conducting device 66 which, in the present embodiment, is a semi-conductor diode. The second gap electrode 62 is electrically connected by a lead 15 to the ignition electrode 14 located within .the ignition gap 38, the other electrode or casing 11 of which is at electrical ground potential. The resistance between the ignition electrode 14 and the casing 11 is represented by the dashed resistance 72. Under normal conditions, no current flows through the blocking capacitor 64. When a mixture containing the electrically conductive oxidizer is injected into the ignition gap 38, the resistance 72 between the electrodes 14, 11 is suddenly decreased, thereby causing a transient current discharge from the capacitor 64 through the primary winding of the pulse transformer 70 and the fluid-bridged ignition gap to ground. A voltage pulse is thereby induced in the secondary winding of the transformer 70 which is applied through the limiting resistor 68 and diode 66 to the trigger electrode 61 of the sealed gap 58, increasing the potential gradient therein. As in the embodiment described with FIGURE 1, the sealed gap 58 breaks down, as the voltage of the storage capacitor 54 is applied thereto, consequentially causing an electrical discharge in the ignition gap 38, hence ignition of the propellant mixture takes place.

The system described herein need not be confined to a device utilizing a liquid propellant mixture composed of fuel and oxidizer. It will perform in an equally satisfactory manner with a monopropellant fuel, so long as the electrical conductivity of the fluid fuel is substantially better than that of air. Similarly, the control circuits illustrated are by way of example only, and numerous variations may be employed.

It is to be understood that the form of the invention herewith shown and described is to be taken as a preferred example of the same and that various changes in the shape, size and arrangement of the parts may be resorted to without departing from the spirit of the invention and the scope of the subjoined claims.

What is claimed is:

1. In a gas generator, an ignition system comprising, a chamber, an ignition electrode arranged within said chamber, said chamber and electrode together forming a spark gap, an energy storage capacitor, a second spark gap, said capacitor being in series electrical connection with said first and second spark gaps, means injecting an electrically conductive fluid stream concentrically surrounded by an electrically non-conductive fluid stream within said chamber, and means mounted on said ignition electrode within said chamber for converting said fluid streams to a plurality of droplets, whereby said capacitor discharges electrical energy into said first and second spark gaps causing spark discharges therein.

2. In a gas generator, an ignition system comprising, a chamber, an ignition electrode arranged within said chamber and electrically insulated therefrom, said chamber and electrode together forming a first electric discharge device, an electric energy storage device, a second electric discharge device, said energy storage device being in electrical series circuit connection with said first and second discharge device, and means for injecting within said chamber in a predetermined ratio fuel and oxidizer in concentric streams, at least one component of which is electrically conductive, said fuel and oxidizer streams being intermixed within said chamber.

3. A system for igniting a liquid rocket propellant comprising, an electrically conductive structure containing a combustion chamber, an ignition electrode arranged within said combustion chamber and electrically insulated from said structure, a first resistor, a two-electrode spark gap of sealed construction having a first electrode which connects to said first resistor and a second electrode which connects to said ignition electrode, a second resistor in electrical parallel connection across the electrodes of said spark gap, a capacitor connected between said first electrode and said conductive structure, means connecting a two-terminal source of direct current between said first resistor and said structure, and means injecting an electrically conductive liquid propellant within said chamber.

4. A fuel ignition system comprising, a combustion chamber casing fabricated of electrically conductive material and at ground potential, an ignition electrode arranged within said chamber to form an electric discharge gap therewith, a unidirectional source of electrical current one terminal of which is at ground potential, a first current limiting device, a spark gap of sealed construction, a second current limiting device connected in electrical parallel with said sealed spark gap, means serially connecting in the order named, said current source, said first limiting device, said sealed gap, and said ignition electrode, an energy storage device arranged in electrical connection between ground and the junction between said first current limiting device and said sealed gap, and means for injecting an electrically conductive propellant in the fluid state into said chamber, whereby said propellant is ignited by an electric discharge between said ignition electrode and said combustion chamber casing when the proper quantity of propellant is within said chamber.

5. A system for igniting liquid propellant comprising, an electrically conductive structure containing a combustion chamber, an ignition electrode arranged within said combustion chamber and electrically insulated from said conductive structure, a first resistor, a spark gap of sealed construction containing first, second and third electrodes, said first resistor being connected to said first electrode, a storage capacitor connected between said first electrode and ground, a blocking capacitor connecting to said first electrode a pulse transformer having primary and secondary windings, the primary winding of said pulse transformer connecting between said blocking capacitor and said second electrode, a unilateral current conducting device, a second resistor, the secondary winding of said transformer connecting between ground and said second resistor, said unilateral current conducting device connecting said third electrode and said second resistor, means connecting said second gap electrode to said ignition electrode, means connecting said conductive structure to ground, and means injecting within said chamber a propellant which is electrically conductive.

References Cited in the file of this patent UNITED STATES PATENTS 1,960,685 Thompson et al May 29, 1934 2,090,039 Goddard Aug. 17, 1937 2,194,081 Block Mar. 19, 1940 2,493,073 Kinneen Jan. 3, 1950 2,549,806 Hall Apr. 24, 1951 2,572,209 Smits Oct. 23, 1951 2,643,284 Putnam June 23, 1953 2,655,617 Laird Oct. 13, 1953 2,789,632 Smits Apr. 23, 1957 2,826,721 Smits Mar. 11, 1958 FOREIGN PATENTS 655,121 Great Britain July 11, 1951 747,593 Great Britain Aug. 11, 1956 OTHER REFERENCES Ignition Systems Must Keep Up With Engine Performance, by K. R. Stehling, Aviation Age, September 1957, vol. 28, No. 3, pp. 566l.

A Mechanical Igniter for Liquid Rocket Engines, by Stehling and Freeland, Jet Propulsion, August 1957, pp. 896 and 897.

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