To chill down an engine, cryogenic propellant is pushed through the engine. This is fine for the first stage because the chill down propellant can just flow out the nozzle and on to the ground/the atmosphere. The upper stage, however, is closed up. How do you chill down the upper stage engine if there is nowhere for the propellant to go after it exits the nozzle?

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    $\begingroup$ What do you mean by "there is nowhere for the propellant to go"? It burns and then the exhaust flies off to empty space and pushes the rocket in opposite direction. $\endgroup$
    – Naktibalda
    Commented Sep 22, 2021 at 11:40
  • $\begingroup$ Which part of the upper stage is "closed up". Do you mean this ? $\endgroup$
    – AJN
    Commented Sep 22, 2021 at 12:45
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    $\begingroup$ @Naktibalda and AJN : of course he means the space between second and first stage before the separation. If the second stage engine should be chilled down before ignition, this has to be done before the stage separation and igniton of the second stage. $\endgroup$
    – Uwe
    Commented Sep 22, 2021 at 14:41
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    $\begingroup$ AFAIK, the space between the stages (and space between the upper stage and the fairing enclosing the upper stage) is not air tight. @Uwe $\endgroup$
    – AJN
    Commented Sep 22, 2021 at 16:44
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    $\begingroup$ Not sure where the VTC for this question is coming from. The question is valid, is not trivially answerable via googling, and is stated sufficiently clearly. $\endgroup$ Commented Sep 23, 2021 at 7:04

2 Answers 2


Different vehicles use(d) different schemes.

The S-IV stage on the Saturn I vehicle had overboard vents for the LH2, and fabric blowout panels for the LOX. One of the LH2 vents is indicated, the blowout panels are the half-ovals at the lower end of the stage.

enter image description here

The Saturn V second stage used a recirculating chilldown system that didn't require overboard venting

A chilldown of the S-II stage engines begins prior to liftoff with the chilldown of the thrust chambers. Propellants are circulated though the pump and feed lines during first stage operation until a few seconds before first plane separation.

The Saturn V third stage also used a recirculating system.

The engine propellant pumps and gas generators must be chilled prior to start. This is accomplished during S-IC boost phase. LH2 is circulated, Figure 22-17, by means of stage mounted pumps through the engine LH2 feed lines, engine LH2 pumps, and gas generator LH2 bleed valves and then returned to the container. LOX is circulated, Figure 22-18, by means of thermal convection through the engine LOX feed lines, engine LOX pumps and gas generator LOX bleed valves and returned to the LOX container.



There is plenty of space to go for the propellant. For example, the nozzle of a vacuum engine itself is a giant empty cone. Then there's the interstage.

On rockets which perform hot staging, the interstage must necessarily be open.

And if all else fails, you can always just punch a hole into the interstage. I mean, obviously if someone builds a rocket, they will think of this. It's not like they perform the first orbital launch and only realize 3 minutes into flight that they never figured out how to chill the second stage engines.

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    $\begingroup$ This answer is ok, but " I mean, obviously if someone builds a rocket, they will think of this." could apply to about half the questions on this site. $\endgroup$ Commented Sep 23, 2021 at 0:07
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    $\begingroup$ "There is plenty of space to go for the propellant." is pure nonsense. The second stage purges at 30-80km altitude, depending of which rocket family. At that altitude, 100 grams of propellant will fill the rocket nozzle and interstage volume to ambient pressure, if there is no gas outlet or recirculation! This answer contains false information, and is rude and condescending in its tone. $\endgroup$ Commented Sep 23, 2021 at 7:08

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