# Has any piston machine been used on a space mission?

Inspired by question about engine lubrication in space.

Piston engines are not used for propelling spacecraft, but there are some possible uses for especially Stirling engines and compressors:

But I didn't find a reference of any such being actually used in space, even as a demonstration. It appears that even air conditioning heat pumps used in space applications are not using the piston compressors that are common on Earth.

Has any reciprocating piston machine (either engine or compressor) been operated in space?

Apparently JWT uses piston pumps in its Cryocooler system:

The only moving parts in the cryocooler are the two 2-cylinder horizontally opposed piston pumps in the CCA, and by having horizontally-opposed pistons that are finely balanced and tuned and move in virtually perfect opposition, vibration is mostly cancelled-out and thus minimized.

• +1, great find. Apparently a lot of cryocoolers work that way space.stackexchange.com/a/35620/6944 I had forgotten Jan 24 at 18:42
• Actually the praise should go to the "Real Engineering" channel on Youtube. This is something I remembered from one of their excellent videos (and then I looked up for an easily linkable source, so kudos to Nasa for their wonderful mission pages). Jan 24 at 18:51
• +1 yes I'd linked to that video in comments elsewhere I think, it's quite informative! I noticed another reference to a Real Engineering video here as well.
– uhoh
Jan 24 at 21:46
• Two great answers! I chose to accept this one because it's currently in operation :)
– jpa
Jan 25 at 18:06

Not sure that this is exactly what you seek, but the main pumps in the Space Shuttle hydraulic systems were "reciprocating piston variable displacement pumps". This type of pump was not unique to the shuttle but is used in other aircraft hydraulic systems (and perhaps hydraulic systems in other machines, I do not know).

The pumping mechanism consists of a rotating barrel and a tilting "head" with the pistons attached to it. The "head" can vary its angle to control the amount of fluid displaced by the pistons, based on the demand on the system. In the shuttle the pumps were driven by the hydrazine-powered Auxiliary Power Units.

Drawing from Space Shuttle Systems Handbook Volume 2 sheet 12.3

Addressing the "been operated in space" part of the question: The orbiter hydraulic systems were used to power other systems mainly important for ascent and entry such as main engine thrust vector control, aerosurfaces, landing gear, etc. The pumps ran until completion of the Main Propulsion System dump well after Main Engine Cutoff during ascent. One system was started prior to the deorbit burn on entry. So these piston pumps definitely operated in space.

• I didn't consider piston pumps, but they are essentially the same mechanism so it definitely counts!
– jpa
Jan 24 at 18:04
• The tilting head and pistons mechanism is widely used down here as well. Your car's A/C compressor look more or less the same. Jan 24 at 18:28
• @fraxinus thanks! I tried to leave that possibility open in my answer. I'm pretty ignorant of non-aerospace hydraulic systems. Jan 24 at 18:40