This is the Nozzle Extension, or bottom part, of the Apollo Service Module Propulsion System (SPS). The top part is the SPS Ring, Strut, and Thrust Chamber Assembly, simply called the SPS Thrust Chamber. The nozzle extension was cooled by radiation (dissipating its heat into space).
The SPS engine steered the Apollo Service Module (SM) towards the Moon, as well as placing it in lunar orbit, escaping from orbit upon the completion of lunar missions, and returning the SM back to Earth. The SPS was non-throttable, gimballed (steerable), and ablatively-cooled. Using storable propellants, it produced a nominal thrust of 21,900 pounds. The propellants were hypergolic (self-igniting) consisting of hydrazine dimethylhydrazine (UDMH) as the fuel and nitrogen tetroxide as the oxidizer. The control of the SPS was normally automatic, from commands from the SM's guidance and navigation subsystem, but there were provisions for manual overide.
The SPS engine was developed by the Aerojet-General Corporation of Sacramento, Calif., from 1963, though based upon earlier ablative-cooled engines. There were a minimum of techical problems in its development and after rigorous testing, including firings in simulated high altitude chambers, the SPS was first deployed in the Apollo program on 9 November 1967 on the first flight of the Saturn V unmanned Apollo 4 mission. The SPS was subsequently used successfully in all the later Apollo unmanned and manned missions, as well as in the Apollo-Soyuz Test Project (ASTP) in 1975.
Transferred from NASA/Johnson Space Center