Ascent Propulsion System

After visiting the USSRC and deciding to visit all of the manned Apollo command modules in North America, the Michigan Space & Science Center was one of the first museums I visited. This was before I'd done much reading as an adult on the space program; when I took this picture, I'm not sure I knew what it was.

However, I now know that this is a lunar module Ascent Propulsions System (APS) engine thrust chamber, used on the ascent stage of the lunar module. None of the various valves, propellant lines, or associated parts are installed.

Since the APS required absolute reliability (if this engine didn't work, the astronauts would be stranded on the moon), it was a relatively simple engine: It was pressure-fed, which means that there were no turbopumps to fail. It used hypergolic propellants, so the fuel and oxidizer ignited on contact, without the need for an ignition system. The thrust chamber was ablatively, rather than regeneratively, cooled, which removed the associated complexity.

Bell Aerospace was the original engine contractor, but they were experiencing difficulty with combustion instability. Rocketdyne was eventually enlisted to develop an injector which did not suffer from combustion instability. Rocketdyne came into the picture rather late, and it wasn't until August 1968 (after LM-1 flew) that the Bell engine with Rocketdyne injector was fully qualified. This means that this engine was tested in space for the first time, manned, on Apollo 9's lunar module! (Additional information about the Bell/Rocketdyne partnership in this endeavor can be found in Rocketdyne: Powering Humans into Space.)

Additional information on the APS is sprinkled throughout the Apollo Lunar Module News Reference; the "APS Functional Description" section begins at page MP-15 (p. 3 of the part 8 [direct link to 3.8 meg PDF] PDF file).

Today, this APS thrust chamber (along with a second thrust chamber in the same state) is in storage at the Air Zoo.