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As I understand it, the Space Shuttle used cryogenic fuel in its main engines and it kept a supply onboard for the duration of its mission for a deorbit burn. Since Space Shuttle missions often lasted upwards of a week, how was this kept cold?

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The shuttle doesn't have fuel for the main engine inside at all - it's only in the external fuel tank. Once you detach the external fuel tank, the main engines have nothing to run on - and the tank is thrown off before the shuttle attains orbit (in part because that allows it to burn up in the atmosphere, rather than cluttering Earth's orbit). – Luaan Mar 23 at 9:32
It may be of interest to note that the Orbiter carried cryogenic oxygen and hydrogen for use by the fuel cells. – Digger Mar 23 at 17:28
@Luaan The shuttle couldn't even bring the tank into orbit if it wanted to. The shuttle had no means of lighting it's main engines. Once the engines shut down that was it. Since the orbital insertion burn must be completed at the orbital altitude that requires a coast phase and then firing engines again unless the performance of your rocket works out perfectly that you can do one burn all the way to orbit (most rockets don't work this way--they attain the needed velocity before reaching the desired altitude.) – Loren Pechtel Mar 26 at 2:01
up vote 24 down vote accepted

The fuel used in the shuttle's Orbital Maneuvering System engines and used for the deorbit burn was not cryogenic; it was storable hypergolic fuel.

The cryogenic hydrogen and oxygen fuel burned by the main engines was used only during the ascent, which took only about 10 minutes.

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Yes - and the cryogenic propellants were loaded into the tank just prior to launch. – Organic Marble Mar 23 at 11:31

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