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I do see the need to provide pressurizing gas with non cryogenic fuels (like RP-1) but if you require to have it at very low temperatures anyway to keep it liquid and the pressure in the tank drops, all you'd have to do is make the cryogenic fuel a bit warmer to increase pressure again?

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  • $\begingroup$ When the engines are ignited the pressure gas flow should increase very fast to achieve the tank pressure needed by the turbo pumps. To make the cryogenic fuel a bit warmer is too slow. Heating a tank of this size is very slow. $\endgroup$
    – Uwe
    Commented May 21 at 12:45
  • $\begingroup$ I'd imagine an electric heater at the top of the tank that would just heat up the gaseous part and surface layer of the liquid fuel. No need afaics to warm the entire volume. $\endgroup$ Commented May 22 at 16:58
  • $\begingroup$ Forget the electric heater, it would need too much power. Using battery power for heating of a cryogenic fuel does not make sense. The batteries needed for heating would be much too heavy for a rocket. It is true there is no need to warm the entire volume. The cryogenic liquid should be well below its boiling temperature, the turbo pumps could be destroyed by a mixture of liquid and gas bubbles. $\endgroup$
    – Uwe
    Commented May 22 at 20:59

2 Answers 2

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Rocket fuels are pumped into the rocket engines.

Pumping a liquid near or at its vapor pressure is a very good recipe for cavitation.

Pumps don't like cavitation.

They lose a great deal of efficiency in a hard to estimate manner.

They also tend to lose material from their blades, teeth or whatever the particular pump uses to interact with the liquid. The particles end up in the liquid pumped and could damage the downstream gear (e.g. engines).

This is bad enough in pumps for garden irrigation or car engine cooling and does not get any better in a high-performance pump pumping an exotic liquid in an extreme environment.

In short, use inert gas.

This also allows you to have pretty much known pressure in the tank, generally independent from the inevitable, and unknown to some extent, heat ingress. This simplifies the engineering tasks around.

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  • $\begingroup$ I suggest to insert the word cold: In short, use cold inert gas. $\endgroup$
    – Uwe
    Commented May 22 at 21:06
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It is true that warming cryogenic propellants will increase the gas pressure in the headspace. But this will take some time to achieve if left to sunlight or even if left to electrical heating. In addition as soon as the liquid enters the engine the headspace will increase in volume and the pressure will decrease rapidly unless extra gas is introduced at a high rate. It would be difficult to achieve this controllably using electrical heating.

An inert pressurized gas can be applied via a pressure reducer at constant pressure whenever required and will allow the gas pressure in the headspace to remain constant as it expands whilst the engines start up before autogenous pressurization is available. As soon as the engines are running autogenous gas can be used to pressurize the headspace.

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