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I understand why an engine like the AJ10 needs pressurisation to feed it fuel, but why does a stage like the S-IVB/ J-2 need it if it has turbomachinery?

Andykins
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    It's LH2 and LOX isn't it? How would you keep those liquid if they were not pressurized? – Wyck Dec 01 '22 at 18:50
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    The fine answers to this question explain why propellant tanks need to be pressurized to avoid the pumps cavitating https://space.stackexchange.com/q/46967/6944 – Organic Marble Dec 01 '22 at 19:43

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Both engines run on cryogenic propellants. Unless they are very close to their tripple point, they will be at equilibrium with significant vapor pressure. However, tanks were actively pressurized as well.

For the Saturn IB version of the S-IVB, this was the only necessary propellant tank pressurization system, as the S-IVB fired only once, placing its playload into orbit. The engine was not reignited and stage's mission was essentially over.

On the Saturn V, however, the S-IVB stage initially burned for a few minutes to place the Apollo spacecraft into Earth orbit. A little over two hours later, the S-IVB's J-2 engine would reignite to perform its trans-lunar injection (TLI) burn, sending the Apollo spacecraft (and usually the S-IVB itself) toward the moon.

During its parking orbit, the S-IVB's fuel tank was allowed to vent; the LH2 tank was routed through a "propulsive vent" which provided a small amount of thrust to keep the propellants settled at the aft end of their respective tanks. However, this meant that the propellant tanks had to be repressurized prior to J-2 reignition for its TLI burn. This repressurization system was unique to the Saturn V version of the S-IVB.

http://heroicrelics.org/info/s-ivb/s-ivb-v-propellant-pressurization.html

Woody
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  • "Both engines run on cryogenic propellants" No, there are two turbopumps for the two propellants, the fuel and the oxidizer. This is true for rockets with one as well as two or more engines. To be close to the triple point may be bad, there should be no solid propellants in the tank and the temperature should be above the triple point – Uwe Dec 01 '22 at 21:21