I am wondering if the "cryogenic helium" system discussed in articles about the 1-Sept-2016 SpaceX Falcon 9 anomaly (see this answer for a good summary to date) is not actually related to the high pressure gaseous helium in the COPVs which were moved inside of the LOX tanks, but instead this is actually for liquid helium used in more standard cryogenic applications, to either maintain the sub-cooled LOX well below its boiling point, or for fast pumping speed cold trapping or cryopumping.

Looking at a typical Falcon 9 countdown timeline from here as linked in this answer, I've pasted a selected, discontinuous subset of items below. You should go to the original link for the complete thing.

This filling near the end seems like a bad idea for an extremely high pressure, heat generating pressurization of the COPVs which will end up swimming in sub-cooled LOX. Instead, this looks like filling and later topping off of cold traps.

Is this Liquid helium for cold trapping and cryopumping? Are there mechanical compressors or is this just dewar-supplied LHe?

T-0:33:30   Stage 1 Liquid Oxygen Loading

T-0:29:30   Stage 1 Helium Load

T-0:25:00   All three Cryo Helium Pumps active

T-0:22:00   Stage 2 Fuel Loading Complete

T-0:19:30   Stage 2 Liquid Oxygen Loading

T-0:17:20   Stage 1 LOX Flowrate Adjustment for Stage 2 Fast Fill

T-0:13:15   Stage 2 Helium Loading

T-0:13:00   Stage 2 LOX Flow Adjustment for Helium Cryo Load

T-0:09:15   Stage 1 Helium Topping

T-0:06:45   Stage 2 Helium Transition to Pipeline

T-0:01:25   Helium Loading Termination 
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    $\begingroup$ "cryogenic" doesn't necessarily mean liquid. I'd make sense to cool the gaseous He to the same temperature as the LOX. $\endgroup$
    – Hobbes
    Commented Sep 26, 2016 at 17:14
  • $\begingroup$ @Hobbes yep I know, But "Helium Loading" and "Helium Topping" are things you'd normally say about a liquid, not a gas - thus the question... $\endgroup$
    – uhoh
    Commented Sep 26, 2016 at 17:18
  • 1
    $\begingroup$ I think Helium Loading and Topping are perfectly normal terms to use while filling the vehicle tanks for launch. What would you subsitute? $\endgroup$ Commented Sep 26, 2016 at 19:15
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    $\begingroup$ An LHe tank would be unlikely to be pressurized to 380 bar. $\endgroup$
    – Hobbes
    Commented Sep 27, 2016 at 11:14
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    $\begingroup$ @Hobbes could there be in fact more than one kind of helium tank? Could there be both high pressure GHe and some LHe present as well - for different uses? $\endgroup$
    – uhoh
    Commented Sep 27, 2016 at 16:16

1 Answer 1


You should consider that helium's Joule–Thomson coefficient is negative above 45 K. This means that it actually cools when compressed.

This is, in fact, the reason why solid oxygen formed around a F9 COPV helium tank.

I get the impression you believe "cryogenic" means a substance which is typically a gas at room temperatures is in a liquid state. But the true definition of cryogenic is simply "very cold". So a tank of gaseous helium at LOX temperatures would be described as cryogenic but the helium would be a gas.

Finally, I don't see a cryopump or cold traps are likely to be used on a rocket. It may be in a payload, but not the rocket itself. Both cryopumps and cold traps only work at very low pressures like 10e-3 torr or lower and why would a rocket need that?

  • $\begingroup$ I'm not sure what you mean here. Standard cryopumps used around the world are based on compression, liquification, expansion and boiling of helium, this is a fact. If you are trying to explain why this would not work, then your answer makes no sense. en.wikipedia.org/wiki/Cryopump#Operation Also, I've recently explained here what "cryogenic" really means. You are right it does not refer to a phase, change, but you are wrong that it simply means "cold". In any event, your answer does not address the question. $\endgroup$
    – uhoh
    Commented Apr 16, 2018 at 3:21
  • $\begingroup$ More about helium compressor based cryopumps can be found here and here and here for example. If I've misunderstood your answer in some way, can you clarify or edit? Thanks! $\endgroup$
    – uhoh
    Commented Apr 16, 2018 at 3:21
  • $\begingroup$ And I'm not sure of your point. My statement is I don't see where a F9 would use a vacuum pump, which is what a cryopump essentially is. $\endgroup$
    – user8269
    Commented Apr 16, 2018 at 6:00
  • $\begingroup$ Regarding definitions, MW gives this: 1 a : of or relating to the production of very low temperatures b : being or relating to very low temperatures Are you claiming a rocket like the F9 produces low temperatures? I think in this context, a cryogenic fuel means a cold fuel. $\endgroup$
    – user8269
    Commented Apr 16, 2018 at 6:01
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    $\begingroup$ Currently your answer doesn't mention vacuum at all. If your "statement is I don't see where a F9 would use a vacuum pump," then it might be good to include that explicitly. As far as "cryogenic" goes, popular dictionaries tend not to reflect the way the word is used in practice. A home deep freezer at -30C wouldn't be called cryogenic. While some people call subcooled RP-1 "cryogenic" I think that's only because they are lumping it in together with the LOX. Otherwise it would just be called "cold kerosene". space.stackexchange.com/q/14456/12102 $\endgroup$
    – uhoh
    Commented Apr 16, 2018 at 6:48

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