Development and Testing of Injectors for Space Shuttle Orbital Maneuvering Engine Non-Toxic Upgrade

Vladimir Bazarov

Moscow Aviation Institute (State Technical University)

Robert J. Santoro

The Pennsylvania State University

The Paper contains some results of the research of different conceptions of combustible mixture formation and injectors that can provide high combustion efficiency and stability in Space Shuttle liquid -rocket engines (LRE) of Orbital Maneuvering System (OMS) when using non-toxic propellants. In this work liquid oxygen (LOX) and hydrocarbons (ethanol, propane) were tested. This study was performed with the financial aid of NASA, according to NASA Research Announcement #9-NRA-BE-96-1 technical conditions, as a first stage of the development of next generation space rocket engine conception.

The description and comparative analysis of OMS engine of Space Shuttle and Buran spacecraft were presented. Space Shuttle OMS engine with hypergolic and toxic nitrogen tetraoxide N₂H₄ and mono-methyl hydrazine (MMH) as propellants contains two combustion chambers (CC) with the thrust of 27.2 KN each and pressurized by compressed helium feed system. CC pressure is only 0.88 MPA and specific impulse not more 320 s. Buran spacecraft OMS engine, which is now widely used as booster engine of "Block DM" spacecraft, final stages of Proton and Zenit launchers, is also characterized by high reliability. It contains two CC with the thrust of 90 KN each, but differently from its American analogue it has staged combustion cycle with ox-rich gas generator, uses non-toxic LOX and kerosene as propellants, has 7.85 MPA of CC pressure which provided specific impulse I_sp = 362 s. None of them fits totally NASA requirements, applied to the space rocket engine of the 21-st century.

Three conceptions of combustible mixture formation were considered in the research:

-        By impinged jets or films of propellants in multi-injector assembly, typical for US rocket propulsion and already used in Space Shuttle OMS engine, presented by "Aerojet Co".

-        By pintle injector with axial hollow flow of LOX and radial fuel sprays, presented by TRW.

-        By atomization and mixing of coaxial swirled flows of propellants in coaxial bi-propellant swirl injector elements, typical for Russian rocket propulsion.

Experimental studies consisted from cold flow and fire tests. Cold flow tests, performed partially in The Moscow Aviation Institute (MAI), included definition of injector's hydraulic and dispersion characteristics, thickness of liquid films, specific mass flow rates and O/F local ratios via radius and angle of spray. Fire tests were performed at the cryogenic fire test facility of The Pennsylvania State University Propulsion Engineering Research Center. Sectioned model CC with square 2"32" interior size was used, supplied by quartz windows near the combustion zone to check burning droplets, changeable water cooled nozzle and injection head. There either one model pintle injector (1/60 from original) or model impinged jet injectors with five full scale sized jets, or one of versions of full scale swirl bi-propellant injectors were placed.

Tests of impinged jet injector elements, which worked successfully with hypergolic propellants, showed that with liquid oxygen that requires pre-chilling of the engine systems and CC, they can't be used without complication of the feed system by means of purge of injector cavities by neutral gas. The attempt to fire such an injector without a purge led to the explosion of propellant due to the penetration of liquid oxidizer in the fuel cavity during pre-chilling. The same problems appeared in swirl bi-propellant injectors with central supply of liquid oxygen.

Fire tests with two types of pintle injectors with mass flow rates of 149 and 457.5 g/s showed reliable ignition (with the help of autonomous hydrogen burner), smooth going to the regime, stable operation and smooth check out. Combustion efficiency h_t for low mass flow rate injector was 0.87 - 0.995. More narrow margins (0.9 - 0.99) were typical for high mass flow rate injector. Value of h_t significantly depends on O/F ratio K_m. Temperature of fire face with pintle injectors was the highest from all other tested injectors.

Firings with bi-propellant swirl injectors (low mass flow rate - for 90-element injector assembly, high mass flow rate - for 60-element injector assembly) were also characterized by smooth operation. Combustion efficiency h_t with combustion chamber pressure P_c = 21 bar was in the range of 0.93 - 0.98, but with low P_c combustion efficiency significantly decreased (h_t = 0.675 - 0.685). Special investigation of traces of burning drops in the combustion zone showed that it was due to the boiling of LOX in the exterior vortex chamber of swirl injector. Gaseous flow of oxygen separated liquid propellants from each other and prevents their mixing. Several means to prevent propellants separation were tried. Two of them were successful.  The first was the usage of the reverse injector scheme (central oxidizer and external fuel supply). It provided higher combustion efficiency than pintle injector (h_t = 1.02). As exact definition of heat losses in the model, thick walls CC is problematic, further evaluation will be relative to best value obtained with pintle injector. With the usage of direct scheme injectors (oxidizer outside, fuel - inside) the most effective mean to improve combustion efficiency was in increased recess of the fuel nozzle in oxidizer one up to 1". Exit part of the injector LOX nozzle, covered by cooling liquid film, served as a small pre-chamber for central spray of the rotating fuel, atomized by gaseous oxygen. Formed there combustible mixture partially burned inside the injector's nozzle. Combustion efficiency of CC with such an injector was 6.5% higher than best values obtained with pintle injector. The temperature of the fire face was higher that for classical swirl injector with exterior mixing but still lower than for pintle injector.

So, each type of tested injector conceptions can be used for non-toxic upgrade of Space Shuttle OMS engine. Best results with the respect of reliability, simplicity of usage and combustion efficiency showed bi-propellant swirl-swirl injector with internal propellants mixing.