According to https://en.wikipedia.org/wiki/Advanced_Cryogenic_Evolved_Stage

The Advanced Cryogenic Evolved Stage (ACES) .. proposed (using) a lightweight internal combustion engine to use hydrogen and oxygen propellant boil-off (normally wasted when boil-off gases are vented to space) ... The design included producing power, maintaining stage attitude[10][14] and keeping the propellant tanks autogenously pressurized. Using these fluids was designed to eliminate the need for hydrazine fuel, helium for pressurization,[7][15]: 4, 5  and nearly all batteries in the vehicle.

Other sources claimed the exhaust was used for settling tanks and attitude control as well. ICE are particularly well adapted for orbital fuel depots since they can operate from boil-off gasses.

The paper: Internal Combustion Engine Solar Independent Propulsion https://arc.aiaa.org/doi/10.2514/6.2019-3964 Proposed a 2-stroke ICE to serve as range extenders on lunar probes which are designed to operate in lunar Permanently Shadowed Regions. Their design had higher specific power and longer loiter time than the option which used Li batteries or fuel cells.

ICE engines running on rocket propellant are at risk of running “metal rich” on stoichiometric mixtures of oxygen and fuel due to high combustion temperatures. This is addressed in this answer from Methane internal combustion engines for rovers on Mars and Moon. Feasibility?

SpaceX has already developed one CH4/O2 ICE: The Raptor. We don’t usually think of liquid fueled rockets as ICEs, but they are: compression, ignition, expansion and all that. It is very difficult to run an ICE on oxygen (as opposed to air) due to the extremely high combustion temperatures when run stoichiometrically. Air-breathing hydrocarbon-burning ICEs (like auto engines) don’t need to deal with this complexity since air is 80% N2. This lowers combustion temperatures. Raptor’s solution for their preburners is enlightening: they are run either very rich or very lean to reduce combustion temperature. The exhaust from the two turbine preburners are then mixed to burn the uncombusted fuel (from one turbine) and oxygen (from the other). This solution could be applied to CH4/O2 ICE.

A CH4/O2 ICE on Mars will need to deal with the combustion temperature issue. It could be dealt with by the same strategy as the Raptor: staged combustion. A three cylinder engine could have different mixtures in each cylinder. One would be CH3-rich, the second O2 rich. Exhaust from each would feed into the third cylinder for completion of combustion.

So far, just talk. Have there been any ICE engines which have operated in space, except for rockets?

Edit: Definition of an internal combustion engine: Combustion products are the working fluid (Otto, Diesel, gas turbine, rocket) as opposed to external combustion engines where the working fluid is not (Sterling engines, steam engines)


1 Answer 1


With the clarification, then yes, "internal combustion engines" (ICE) as defined in the question have flown in space over 100 times.

Each space shuttle orbiter had three of these ICE installed. They were misnamed the Auxiliary Power Units (APUs) in analogy to aircraft units, but for the shuttle they were the only source of hydraulic power.

Referring to this schematic...

color schematic of APU

(APU Fam Manual)

...one may see that the APU utilized pump-fed hydrazine which passed through some pulsing speed control valves onto a catalyst bed. The combustion products passed through a gas turbine which was linked via a gearbox to a hydraulic pump.

The APUs were somewhat problematic over the life of the program notably almost destroying Columbia on STS-9 with two of them catching fire and then exploding during entry. An effort to replace them with battery-driven electric motors stalled out before it ever flew.

The APUs were started prelaunch because the Space Shuttle Main Engines required hydraulic power for gimbaling and to move the valves in their control system. After residual propellants were dumped through the engines post-Main Engine Cutoff, the APUs were shut down. Late in the mission one was started for a checkout of the entry flight control systems. On entry day, one APU was started prior to the deorbit burn, and the others were started after the burn before the vehicle needed aerosurface control.

The training manual APU/HYD/WSB Workbook is available online and gives much more detail.

Similar-but-not-exactly-the-same units provided hydraulic power for the Solid Rocket Booster (SRB) thrust vector control systems, and still do for the Space Launch System SRBs.

Information on the hydraulic pump being driven by this ICE is available here: Has any piston machine been used on a space mission?

  • 2
    $\begingroup$ Interesting answer. Like most people, when I hear "ICE" I think of automobile engines. $\endgroup$
    – Woody
    Commented May 16 at 22:41

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