I'm wondering if the Falcon 9 booster or upper stage regenerates any electrical power from the Merlin engines or their subsystems, or if the battery systems are perpetually draining while in flight (aside from any solar panels that may be attached to the upper stage).

I would think this could either be accomplished via an alternator on the turbopump (though probably hard to engineer, as it has to spin at 36k RPM or be geared down, and would add inertial mass to the shaft) or via the hydraulic drive pressure from the turbopump while an engine is running.

Does it do anything like this? It would seemingly reduce the required battery capacity for the first stage, which might be a worthwhile tradeoff as far as weight is concerned.

  • $\begingroup$ There are three phase electric motors for drilling PCB boards doing 120 kRPM and even more, so a generator for 36 kRPM should be possible. AC frequency would be 600 Hz, 400 Hz is used in aircrafts. $\endgroup$
    – Uwe
    Dec 8, 2018 at 14:59

1 Answer 1


We don't know for sure because SpaceX publishes very little information on its engines.

Some thoughts:

  • an alternator only works while the engines are running. The second stage does two short engine burns, with potentially several hours of coasting in between. So you still need batteries to cover that.
  • there are no solar panels on the second stage.
  • the first stage only operates for 10 minutes or so. Again, several engine burns, with coasting in between so you still need batteries.

AFAIK, alternators are rare on rockets. It's not impossible (jet engines have high-speed alternators), but the added complexity (battery charging system, making sure the power supply is uninterrupted and has no spikes during engine start and shutdown) may outweigh the saved weight.

  • $\begingroup$ Those are good considerations, thank you. Is there any information published on the hydraulic system? Is it just pressurized LP-1 distributed around the rocket and stored somehow (since the grid find operate while the engines aren't running)? If so, this could potentially regenerate some electrical power, without adding any complexity to the turbopump itself. $\endgroup$
    – Ehryk
    Dec 8, 2018 at 9:14
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    $\begingroup$ I can't see it being worth it, rockets don't really need that much electrical power. $\endgroup$
    – GdD
    Dec 8, 2018 at 10:52
  • $\begingroup$ @GdD Currently they may not - because they are using pneumatics, hydraulics, and pyrobolts to perform tasks instead of electrical motors. However they would need charging if motors were to be used. $\endgroup$
    – Ehryk
    Dec 9, 2018 at 6:56

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