AJ-10/137

The **AJ-10/137** is an American rocket engine developed and built by Aerojet as the Service Propulsion System (SPS) on the Apollo Spacecraft.

## Development:

Aerojet submitted 29 proposals to 12 different primes and all but one selected what was to become the AJ10-137. The engine thrust and size was defined before the Lunar Orbit Rendezvous (LOR) Mode was selected. The earlier concept had the entire command/service module landing and lifting off from the moon. Aerojet got the Apollo SPS engine development contract in April 1962, the final decision in favor of LOR came later that year, and even though the engine was too large for the job, no one wanted to start over with a lower-thrust engine. The resulting 20,000 lbT pressure-fed engine had a low 100 psi chamber pressure and a 314-sec specific impulse, resulting in a large engine for its thrust. The original AJ10-137 concept featured a nozzle extension gimbaled at the throat, but this was later changed to head-gimbaled design that reduced the overall spacecraft length by about 3.5 feet, saving weight.

The injector presented challenges because its inlet pressure was only 165 psia and a pressure drop of at least 40 psi across the injector was required for flow stability. The injector required cooling from both fuel and oxidizer to keep its nearly 2-foot diameter cool. The resulting injector had 22 ring channels, but this was later changed to a 15-ring design when baffles were added to address combustion instability. A brazing operation was initially tried to join the injector parts, but despite careful attention to fit and cleanliness, the brazed joints leaked. The assembly process was changed to incorporate electron beam welding, which worked well. With the low pressures and low pressure drop across the injector, the drilled orifice length was critical; sometimes the propellant streams would come out clean, and other times they would attach to the orifice edges and flow onto the injector face, causing a "pop". The problem was solved by counter-drilling the orifice to produce a standard exit length.

In order to achieve reliability and low installation weight, designers decided on a single thrust chamber with redundant valving, which guaranteed start and shutdown. The propellant valve was about two feet square, weighed about 100 lb, and consisted of a complex casting that required numerous high-precision machining operations. Four fuel ball valves were located on one side and four oxidizer ball valves on the other side. These eight valves were arranged in fuel-oxidizer pairs with one pneumatic actuator and one shaft rotating each pair. The four ball valve pairs were arranged in a series-parallel configuration so that on engine start any one actuator could fail and the engine would still start. Similarly, a single actuator failure would not prevent shutdown. The propellant valve actuators were originally powered by fuel pressure, which proved unreliable. Aerojet switched to a pneumatically-opened, spring closed actuator powered with high pressure nitrogen gas.

The original Teflon valve seal material had a tendency to cold flow, so a fiberglass-impregnated Teflon seal was substituted. Stainless steel was the material choice for the valve balls after beryllium was tried in a weight-reduction exercise and found too soft.

The ablative thrust chamber consisted of laid-up ablative material with an aluminum flange at each end. Built before the advent of impregnated tape, Aerojet hand-laid continuous coated glass fiber strings side-by-side on a sticky tape, wound the tape around a chamber mandrel and impregnated the lay up with ablative epoxy. The ablative material industry ultimately developed a pre-impregnated flat tape, which resulted in a lighter, thinner-walled chamber because the glass-to-resin ratio was higher; this material dissipated heat more effectively.

The original titanium nozzle extension consisted of 16 compound-curved gores welded together. Aerojet first tried hot forming the gores by heating and stretching them over dies. But titanium has an incredible memory and the gores tried to return to their original shape. Aerojet then tried a cold forming process developed by a toolmaker building boats in Maine. The cold-forming process turned out extremely good gores, but when the engine was tested in a vacuum the titanium nozzle curled over itself when it got hot. Aerojet then determined that the lightest metal that would handle the necessary 2,100°F was columbium, also known as niobium. The engineers embarked on a search for 0.040" thick columbium sheet and tried to make the nozzle extension using the same process as the titanium one. The columbium was very, very hard to machine. Eventually Aerojet replaced the machined nozzle extension attachment flanges with rolled ones and a newly-manufactured subscale columbium nozzle extension to be tested in the Air Force Arnold Engineering Development Center (AEDC) vacuum test chamber. The test involved multiple burns to simulate the SPS engine in its natural space environment. The first burn was flawless, but the second burn disintegrated the nozzle extension. It turned out the tiny amounts of hydrogen in the AEDC test chamber crystallized the nozzle and the shock of the restart destroyed it. Although free hydrogen was not a problem in space, Aerojet had to qualify the engine on earth. A suitable coating was eventually found that protected the nozzle extension from hydrogen poisoning. The final nozzle was made of columbium to the 40:1 area ratio point and titanium to the end. Although there were problems with developing columbium/titanium welding techniques, the process was perfected and the SPS engine successfully flew 19 times.

The gimbal actuator featured two electric motor pairs with one pair extending the actuator and the other retracting it. The motors ran continuously whenever the SPS engine was thrusting; four magnetic particle clutches geared to the motors selected primary and backup extension or retraction functions. All four motor/clutch gear trains were geared to a single ball screw, which converted rotary to linear motion. The gimbal actuator housing had to be pressurized because neither the motors or clutches would function in a vacuum. The Aerojet actuator broke ground by more than quadrupling the magnetic particle clutch diameters to nearly 3". The big clutches tended to overheat but Aerojet persisted and got them working properly.

AJ10-137 development involved more than 200 injector tests, 230 bomb tests and 124 different injector patterns to arrive at one compatible with the ablative thrust chamber. The engine assembly underwent development testing for 28 hrs and 3,200 starts, a high number for a small engine.

## Design:

The SPS engine, built by the Aerojet General Corporation Space Propulsion Division of Sacramento, California, was located in the SM center section lower half, with its nozzle extension protruding more than 9 feet below the SM aft bulkhead. The SPS engine, including the nozzle extension, was 159.944" high; the nozzle extension exit diameter was 98.4" and the engine weighed about 650 lb. The thrust expansion ratio was 6:1 at the nozzle extension joint and 62.5:1 at the nozzle extension exit. It had a 750-second service life and could be restarted a minimum of 36 times.

The AJ10-137 consisted of a bipropellant valve assembly, an injector, combustion chamber, and nozzle extension. The bipropellant valve assembly admitted propellants to the injector with a high degree of reliability. The injector distributed the propellants through orifices in the injector face so the fuel and oxidizer impinged, atomized and ignited due to hypergolic reaction. The combustion chamber was ablatively and film cooled; the nozzle extension was radiation cooled. The engine was mounted to the SM structure and gimbaled to permit thrust vector alignment through the center of mass prior to thrust initiation and thrust vector control during a thrusting.

## Modifications:

It was later adapted into serving as the primary propulsion system for the Othrys program tugs (AJ-10/147).

## See Also:

AJ-10/147

AJ-10/155

AJ-10/165