Reaction Engines recently announced a milestone completion in their SABRE development program:

we have tested the engine’s advanced hydrogen pre-burner at energy delivery levels in excess of two megawatts and proved output temperature uniformity under pressurised conditions

2MW sounds like a lot, and is enough to power a large village, but is around a tenth of the power of an SSME. The heat from this is used to drive both the air compressor and the hydrogen pump in something vaguely similar to an expander cycle type set up.

Which engines had a comparable level of pre-burner (or gas generator) power?

  • $\begingroup$ I wonder how much thermal energy drives the generator turbine of a power plant that size/ $\endgroup$ Oct 1, 2021 at 0:06
  • 3
    $\begingroup$ A 2MW engine isn't "big" at all. For example many diesel-electric train engines have higher power output than 2MW. $\endgroup$
    – alephzero
    Oct 1, 2021 at 1:00

3 Answers 3


a tenth of the power of an SSME.

17.3MW is just the oxygen pump output, the hydrogen pump is by neccesity larger at an additional 53MW.

For some more comparable gas-generator turbopump power outputs:

  • The French Vulcain engine, with an oxygen turbopump at 3MW (hydrogen pump at 12MW)
  • The retired American J-2 of Saturn V, oxygen pump at 1.6MW (hydrogen at 5.8MW).

Those are still some pretty big engines, with comparable thrust to the projections of the SABRE engine.

  • 1
    $\begingroup$ Hmmm, this provides the thermal difference that powers both the air compressor and the hydrogen pump, but most recent thrust specifciation that I'm aware of for the test unit was 200kN - which I guess puts the engine in the YF-77 area of performance. $\endgroup$
    – user44085
    Oct 1, 2021 at 9:37
  • $\begingroup$ @user44085 Then your thrust estimate is more up to date, and considerable lower than they initially claimed. $\endgroup$ Oct 1, 2021 at 9:39
  • $\begingroup$ yeah, it is just a technology demonstration to test whether the cycle works and performs as expected -- but nobody was going to give them the big bucks to test a full-sized unit first. $\endgroup$
    – user44085
    Oct 1, 2021 at 9:49

The SpaceX Raptor's preburners run at about 75 MW so about 37 times as much.

The SpaceX Merlin 1D turbopump (not strictly a preburner, as it dumps the exhaust) runs at about 7.5MW

The Shuttle's RS-25 preburners did some 17.3MW

On the other end of the scale, the Rocketlab Electron's pumps run at 38KW (*2) per engine. But those are purely electrical engines, thus much more efficient in power use, but much less efficient in power source.

All this is a bit of a case of comparing apples to kumquats to pink flamingoes though. The engine topology, fuel type, pump type and performance all vary wildly between these, so comparison via one metric is a bit of fluffy exercise.

  • $\begingroup$ none of those are comparable -- either too big or too small $\endgroup$
    – user44085
    Sep 30, 2021 at 12:12

While is quite small compared to other engines, it is, per patent US20150101333A1, used differently - it isn't the sole source of power for the compressor and hydrogen pumps. It's purpose is to provide static thrust, and to top up the heat collected from the air cooling (and/or regenerative cooling)

  • $\begingroup$ Did mean "compared to other engines" or "combined to other engines"? $\endgroup$
    – Fred
    Apr 4 at 13:10
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