In many rocket engines I've seen, hydrogen pumps seem to be 3 - 4 times more powerful than oxygen pumps while pumping about 5 times less mass. Why is that?

Part of this is probably driven by relative density of oxygen vs. hydrogen. Is there a simple way to figure out how density affects the required pump power? Would methane pumps need to be more or less powerful as compared to oxygen pumps? What about RP1 pumps?

A few examples:

  • Aerojet M-1 was supposed to have hydrogen pump at 56MW and oxygen pump at 20MW.
  • This design (on page 5) has hydrogen pump at 16.5MW (moving 94 kg/s) and oxygen pump at 4.3MW (moving 456 kg/s).
  • And as a counter-example, Raptor engine diagram shows 28MW for methane pump and 34MW for oxygen pump.
  • $\begingroup$ 2H2 + O2 > 2H2O For gaseous propellants you need the double volume flow rate for hydrogen than for oxygen. For cryogenic liquids the volume ratio may be a little different. The volumes have to be pumped against combustion chamber pressure. No propellants mass has to be lifted against gravity and acceleration by the pumps. $\endgroup$
    – Uwe
    Feb 5, 2020 at 10:40
  • 3
    $\begingroup$ P_pump = Δp * dV/dt = Δp / ρ * dm/dt It's as simple as this. The lower the density the higher power requirement and Oxygen is more than 15 times as dense as Hydrogen. $\endgroup$
    – Christoph
    Feb 5, 2020 at 11:54
  • $\begingroup$ Liquid oxygen has a density of 1.141 g/cm3, liquid hydrogen is 0.07099 g/cm3, so LOX is 16 times the density of LH2. $\endgroup$
    – Uwe
    Feb 5, 2020 at 12:06
  • $\begingroup$ @Christoph consider posting that as an answer. $\endgroup$ Feb 5, 2020 at 14:10


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