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I recently read out about ULA's ACES (Advanced Cryogenic Evolved Stage) upper stage with IVF (Integrated Vehicle Fluids), which crazy as it sounds, has two six-cylinder engines developed by Roush that use the boiled-off H2 and O2 as fuel. These engines provide pressurized fluid for the thrusters and fuel tanks, and generate electrical power, displacing the batteries.

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

Supposedly, this allows the upper stage to remain powered for a much longer period of time (that normally would deplete batteries).

This idea strikes me as pretty cool but crazy sounding (up there with ULA catching engines for re-use with a helicopter, but that's another topic). A fuel cell stack seems like a better fit for these requirements and have been used in space applications for a long time. What are the benefits of using internal combustion engine over a fuel cell? I know ULA has been working on the IVF for some time now. Have advances in battery technology made IVF obsolete?

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    $\begingroup$ It doesn't, I mixed up my acronyms. Fixed. $\endgroup$ – masospaghetti Jun 10 at 23:14
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What a fantastic question! I learned a lot researching this one.

The use of a simple piston-in-cylinder engine on an ultra high performance in-space stage seems to be out of place in a technology landscape dominated by high speed turbomachines, fuel cells and solar panels.

Didn’t we move into the jet age? How could this possibly be a good solution? At the outset we, too, felt like we had wandered into a technological twilight zone. The key to understanding is that the high workload task for the system is not producing electricity or turning a shaft to drive a hydraulic pump. Tank pressurization is the dominant activity and it demands the delivery of enthalpy to the main tank ullage spaces – whatever the source. The IVF engine is superior to other lower temperature systems in that its waste heat is of high quality and is sufficient to turn cryogenic liquids into vapor. Half of the inefficiency of a heat engine is put directly to work pushing enthalpy into a working fluid for pressurization and the remainder is used to produce the small amount of thrust required to settle propellants.

A turbine could be used for such an application but it would be exquisitely small with extremely high rotating speeds to produce only 20kW with low density hydrogen as the working fluid. Provisions for heat and shaft power extraction could be made but the overall developmental complexity of cooling, lubrication, ignition, control and power take off at this very low power level seemed daunting compared to the IC engine. The use of such small turbines on ground based installations is virtually unheard of. Virtually a whole new technology would have to be developed at substantial cost and risk.

Similarly a fuel cell could be used to drive IVF with the advantage of no high speed machinery and an extensive history of spaceflight. Proton Exchange Membrane (PEM) cells have shown a tremendous amount of promise in recent years. However, 20kW is a relatively large fuel cell for flight applications and because all power is produced as electricity (as compared to perhaps 10% for the IC engine) it must be converted via motors to shaft power with their attendant switching systems and losses. This grows the fuel cell to address conversion efficiencies. Reactants are only consumed at a mixture ratio of 8 – which is generally insufficient for regenerative cooling so unless a bulky and costly radiator system is employed a larger flow of hydrogen must be brought to the fuel cell to maintain thermal stasis. From a consumables standpoint the fuel cell loses its advantage over the IC engine. The PEM cell efficiency is founded on low operating temperature which produces condensed liquid water which must be disposed of without providing any benefit for vehicle settling. In general, the use of a fuel cell system would be most advantageous for crewed vehicles where the water produced has a strong positive influence on vehicle mass. For cryogenic propulsive stages the cost differential between IC engines and fuel cells likely favors the former.

United Launch Alliance, Development Status of an Integrated Propulsion and Power System for Long Duration Cryogenic Spaceflight. Emphasis mine.

In short: fuel cells are too expensive & don't produce enough heat.

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  • $\begingroup$ Sort of funny how the answer is "it wasn't inefficient enough to be very efficient". $\endgroup$ – ikrase Jun 12 at 1:52

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