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After putting the rocket on the launch pad the propellants are filled.

But before that how to maintain the lower temperatures of the tanks so that when filling the propellants don't boil off?

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Short answer, they let some boil to keep the rest cool and also 'waste' some to cool the plumbing and tanks before loading.

The bulk cryogenics are made by compression, not cooling and from that point on normally the liquid is kept liquid by evaporation during shipping and storage.

This can become a complex engineering problem for large rockets to handle the boiled off (but still very cold) gases and keep them from reacting/exploding or just icing up other parts of the launch pad/rocket or people.

Edit: in the case of supercooled/densified propellant you need to use a cooling system on the pad and circulate the fluid. This cooling system will use a gas with a boiling point lower than your target temperature, possibly Helium.

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  • $\begingroup$ better - no need to mark it as an edit, just improve the answer. Helium can be used to precool the tanks, but a second fluid need not be involved, the propellant can be cooled by lowering the pressure and evaporating some off before filling. $\endgroup$ – JCRM Jul 21 '18 at 21:21
  • $\begingroup$ Helium is considerably more expensive than cryofuels so using it for precooling the infrastructure to reduce cryofuels boil-off would kinda miss the point. $\endgroup$ – SF. Jul 21 '18 at 21:47
  • $\begingroup$ @SF - Helium in this case is used in closed loop refrigeration systems that your other fluids get pumped through, see the linked patent. Do not have a source but suspect initial tank/plumbing cooling is still done by evaporation of the final fluid, though liquid nitrogen may be used if you have surplus from O2 production and a method to flush it out afterwards. $\endgroup$ – GremlinWranger Jul 21 '18 at 23:34
  • $\begingroup$ @GremlinWranger, Are there any rockets which uses LN2 for chilling the tanks and plumbing lines? If yes, then are they carrying the LN2 with the whole rocket or they use it externally? $\endgroup$ – Amar Nov 21 '18 at 14:41

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