The heroic engine that brought Apollo back from the moon, well except for Apollo 13. Assembled and tested in Q2, 1969.
With only one combustion chamber and expansion nozzle in space, engineers designed a simple SPS 20,500-pound thrust engine to perform flawlessly each time. Instead of mechanical pumps with moving parts, helium forced fuel and oxidizer into the combustion chamber. With no throttle, the engine either burned wide open or was off. To simplify the ignition system, the SPS engine’s fuel and oxidizer ignited on contact with each other (hypergolic fuels requiring no igntion system).
The SPS engine was the Aerojet AJ10-137, which used Aerozine 50 as fuel and nitrogen tetroxide (N2O4) as oxidizer to produce 20,500 lbf (91 kN) of thrust.
An artifact in the Future Ventures’ 🚀 Space Collection.
From the top: 
Assembled Q2, 1969:
Part numbers: 
And the other side 
Vibration test on 5/9/69 (two months before Apollo 11):
For a sense of scale, the valve assembly is on top of the combustion chamber, and the expansion bell below is huge, for ideal operation in a vacuum 
The propellants were pressure-fed to the engine by 39 cubic feet of gaseous helium at 3,600 PSI, carried in two 40-inch diameter spherical tanks. I have them too, 
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