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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
Organic Marble
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uhoh
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    "cryogenic" doesn't necessarily mean liquid. I'd make sense to cool the gaseous He to the same temperature as the LOX. – Hobbes Sep 26 '16 at 17:14
  • @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... – uhoh Sep 26 '16 at 17:18
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    I think Helium Loading and Topping are perfectly normal terms to use while filling the vehicle tanks for launch. What would you subsitute? – Organic Marble Sep 26 '16 at 19:15
  • Launch facilities use LHe pumps and space vehicles use LHe tanks in order to generate high-pressure GHe for pumping and purging liquid propellants. The LHe tank is located inside the LOX tank to simplify insulation. – amI Sep 26 '16 at 22:16
  • @OrganicMarble if I could edit the comment I'd change "you'd" to "I'd". I was around cryo systems for long strings of superconducting magnets and those guys usually reserved the words like loading, transfer, and topping off for liquids, namely LHe and LN2. There was always some initial gas passed to cool the lines before the liquid would pass easily. Anyway I'll grant you rockets are not (primarily) magnets. I'd like to know if any substantial fraction of the LHe transferred to the F9 in the last hour or so is used for cryopumping or cold-trapping. It could of course have other uses as well. – uhoh Sep 26 '16 at 23:31
  • @amI this sounds interesting! "Purging" sounds straightforward - pressure from GHe derived from LHe displaces whatever is being purged. For "pumping", do you mean the GHe is used as a mechanical power source in some way to drive liquid pumps, or do you mean vacuum pumping as in cold trapping and cryopumping? Or both? – uhoh Sep 26 '16 at 23:33
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    An LHe tank would be unlikely to be pressurized to 380 bar. – Hobbes Sep 27 '16 at 11:14
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    @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? – uhoh Sep 27 '16 at 16:16

1 Answers1

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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?

  • 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. https://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. – uhoh Apr 16 '18 at 03:21
  • 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! – uhoh Apr 16 '18 at 03:21
  • 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. –  Apr 16 '18 at 06:00
  • 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.

     –  Apr 16 '18 at 06:01
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    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". https://space.stackexchange.com/q/14456/12102 – uhoh Apr 16 '18 at 06:48
  • Since you brought up cryopumps in the first place, I thought you know they were vacuum pumps. I have added the explicit connection in my answer. –  Apr 16 '18 at 07:09
  • Thanks for the edit. The real value of answers in Stack Exchange lies in their relevance to future readers, so it is best to keep a generic reader in mind rather than to customize an answer based on what you think the OP knows. – uhoh Apr 16 '18 at 07:15