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I've often seen that in launch sequence, "engine chilldown". I've learnt that chilling is done to avoid thermal cracking in plumbing lines and also to avoid cavitation.

While re-entry, do they chilldown the line, prior to burn ? If not , how are they avoiding the problems like cavitation etc.?

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Engine chilldown involves a slow flow of cryogenic fluids through the engine to precool the metal and, like you say, avoid thermal stresses. This is important to gradually bring the engine down from ambient to operating temps.

During the entire initial burn those same engine parts are seeing very high flow rates of those same cryogenic fluids, keeping temperatures very low.

At the time of reentry burn, the engine has only been off for a couple of minutes, meaning the components are all quite cool still. An additional chilldown would be redundant at this point.

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  • $\begingroup$ Upper stage engines are often relit after several hours. Is anything special done there $\endgroup$ – Steve Linton Oct 28 '18 at 16:54
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    $\begingroup$ The SIV-B, at least, had chilldown pumps and plumbing to chill down the J-2 engine. web.mit.edu/digitalapollo/Documents/Chapter5/saturnas501.pdf p.16 $\endgroup$ – Organic Marble Oct 28 '18 at 17:15
  • $\begingroup$ @Saiboogu, I think the environment is hotter and the cooling due to oxidizer flow will not sustain with time. $\endgroup$ – Amar Oct 29 '18 at 6:07
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    $\begingroup$ @Amar The engine has hot components after shutdown, but the 'environment' heat between MECO and reentry burn is pretty irrelevant because of the relative lack of matter to transfer heat to the engines. $\endgroup$ – Saiboogu Oct 29 '18 at 14:30
  • $\begingroup$ @SteveLinton I only have some guesses regarding that. They have mentioned upper stage chilldown during booster flight before, during webcasts. There seem to be several small vent lines in the interstage that attach to the upper stage engine to vent overboard - presumably waste O2 after chilldown. There is a cryogenic vent (presumably O2) that steadily bleeds off excess O2 during webcasts, so perhaps this is used to flow a small quantity of O2 after a long coast. $\endgroup$ – Saiboogu Oct 29 '18 at 14:33

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