What are some of the measures engineers would take to ensure that the LOX and other similar fuels are kept below their boiling point during flight?
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2$\begingroup$ It is not possible to keep LOX and LH2 below their boiling points by insulation only. Installing a machine within the rocket to liquefy boiled of gases would be too heavy. Sub cooled LOX and LH2 may stay below their boiling point for a short time after launch. $\endgroup$– UweCommented Jan 23, 2018 at 19:10
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2 Answers
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As small amounts of LOX boil off heat is removed from the remaining volume. Boiloff actively cools the fluid and helps keep the remainder liquid. Rockets typically have vents to aid in managing the boiloff rate and fluid feed lines to replace lost fluid while waiting on the pad.
During flight the consumption rate outpaces any boiloff losses and it is not an issue.
Cryogenic propellants aren't used for long term functions in flight, so it is not an issue after launch either. Storable propellants like solid fuel or hydrazine are used when long term storage is necessary.
Superchilled rockets like the Falcon 9 cannot manage propellant temperatures as easily. As the propellant warms it expands before boiling off, so they cannot hold indefinitely while topping off fluids like a near-boiling-point rocket. Instead they must time propellant loading to end shortly before launch, and a delayed launch would typically result in a scrub while the warmed propellant is drained and replaced with cold propellant.
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4$\begingroup$ Sub cooling LOX and LH2 may be done within the tanks of the rocket by bubbling cold helium gas from the bottom of the tanks. Topping off should be done shortly before launch. $\endgroup$– UweCommented Jan 23, 2018 at 19:23
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$\begingroup$ @Uwe I've heard that technique before, but haven't heard of it being used in practice. Are there any vehicles doing that? $\endgroup$– SaibooguCommented Jan 23, 2018 at 19:30
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2$\begingroup$ LH2 may be an unlikely or unsuitable choice for a storable propellant, but it was used on Apollo (service module) with LOX to power fuel cells for electric power. Note that Apollo missions were up to 12 days duration. $\endgroup$ Commented Jan 24, 2018 at 2:16
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$\begingroup$ @AnthonyX I found this document about Apollo scrub procedures, which mentions CSM cryogenics being a consideration in scrub turnarounds, sometimes needing topping up. I haven't found anything else about storage duration or methods yet. Still looking. $\endgroup$– SaibooguCommented Jan 24, 2018 at 4:25
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Adding to Saibogu's answer, ULA is currently developing technology called Integrated Vehicle Fluids (IVF) that will use boiloff gas normally vented during second stage coast phases. The waste gas will be used generate electricity using an internal combustion engine and also to power reaction control thrusters and pressurize tanks.
