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Turbopump are driven either by a preburner or by an electrical engine (OK this technology is quite new and currently in 2019 only used on the Rutherford engine).

If I understand correctly, using electric turbopump have some pro and cons, namely thrust to weight ratio, throttle precision, startup time, startup complexity, dropped consumable (either battery or propellant), pipes complexity.

My question is: is it possible to use hybrid drive to drive the trubopump? This way we can take advantages of both precision and startup simplicity due to electric drive and thrust to weight ratio and no need to drop battery due to "classic" preburner.

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Probably not in a way analogous to a hybrid-car's system, where either system could drive the car, as @hobbes discusses.

However, perhaps in a lesser capacity an electric augmentation may make sense. Especially if battery was already needed for other reasons: having and electric motor/generator attached to a turbo pump axle - driven off or charging this battery - could allow some of the benefits you speak of. Namely extra power to assist startup, a way to help throttle and finer control, etc. The down side that this would add dry mass, which is a massive (pun intended) drawback still remains however to a lesser extent. It would also cause additional cost and complexity. Significant benefit would be needed to overcome these disadvantages, and it seems unlikely that this would be the case for the current rockets. However that doesn't mean it would never be a good trade-off.

For example: a highly reused, low-total-dV first stage, where dry mass and cost are less significant and slight increases in simplifying operations are more significant this may be a useful approach. This is deep into hypothetical ground though.

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Such a system would be the worst of both worlds: you would have the weight and complexity of two systems, with very little benefit compared to just using one system.

  • precision: no. As soon as you switch from the electric motor to the rocket preburner, you lose that advantage.
  • startup simplicity: no. Startup may be simple, but now you have to switch from electric drive to the rocket preburner while the turbopump is running. This complicates things.

You need throttle precision not so much during startup and initial flight, but in low-throttle situations (e.g. when you're landing a rocket stage).

A petrol-electric drivetrain for a car makes sense, because the two systems supplement each other and mitigate each other's weaknesses, so the entire system is more efficient than a petrol-only drivetrain despite the added weight.

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  • $\begingroup$ Hybrid does not mean either one or the other, both may drive the trubopump simultaneously (as in hybrid cars). The electric drive may be here to add precision and to drive the tubopump alone when low thrust is demanded. $\endgroup$ – Manu H Aug 11 '19 at 19:20
  • $\begingroup$ The same points apply when you use both drive methods simultaneously. Toyota's HSD is a brilliant bit of engineering that allows you to run both motors without having to sync them up exactly. You can't do that here, so you'd have to control them very precisely to avoid using one motor to push the other (if the rocket preburner runs too fast, your electric motor turns into a generator). $\endgroup$ – Hobbes Aug 11 '19 at 19:27
  • $\begingroup$ Also, a tiny hiccup due to unpredictable turbine power when transitioning from pure-electric to hybrid or back is no big deal in a car but likely disastrous in a rocket engine (through combustion instability, pump overspeed/cavitation, you name it). $\endgroup$ – TooTea Aug 13 '19 at 7:49
  • $\begingroup$ I think the idea has some merit when using the preburner turbopump as the main, primary source of power working start-to-finish during the launch. Electric motors are simple and robust enough that embedding a relatively weak one into the structure of the turbopump shouldn't be too difficult - and then it could "supercharge" the original pump providing some extra torque. Then it could be used alone for low-thrust maneuvers; mitigating some of the reignition problems, allowing precision control, but not nearly as much torque (and throughput) as the preburner. $\endgroup$ – SF. Aug 13 '19 at 13:40

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