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I apologize in advance if this question has already been asked and answered elsewhere, but why do RP-1/LOX rocket systems use helium for propellant tank pressurization? I understand the fact that O2 and N2 have boiling points in close proximity precludes the use of N2 for pressurizing the oxidizer tank, but couldn't the O2 be pressurized autogenously and the fuel tank with N2?

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  • $\begingroup$ Found this and this which may shed some light on the question, also this stackexchange question. Bottom line: seems there is a concern of nitrogen dissolving into the LOX. $\endgroup$ – Anthony X Feb 21 '17 at 1:19
  • $\begingroup$ Thanks, Anthony. I suspect that the boiling points of the two gases are just too close, meaning that the partial pressure of N2 relative to O2 in an N2/O2 gas mixture at cryogenic temperatures is too low for N2 to be an efficient pressurant. And I can only assume that for the purpose of mechanical simplicity, rather than use autogenous pressurization for the oxidizer, and a separate N2 gas pressurization system for the fuel, it is more straightforward to pressurize both oxidizer and fuel with the same low-density gas. Please anyone - correct me if my reasoning is wrong or incomplete. $\endgroup$ – Joel Neatrour Feb 21 '17 at 3:15
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    $\begingroup$ Possible duplicate of Why do pressure-fed systems have to be pressurized with helium or nitrogen? $\endgroup$ – Russell Borogove Feb 21 '17 at 3:51
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    $\begingroup$ Note to those voting to close as a duplicate: the suggested duplicate specifically states it's not asking about "large rockets launched from the ground" but maneuvering/attitude control systems only. $\endgroup$ – Organic Marble Feb 21 '17 at 14:14
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Bottom line: like so many things in rocketry, it's a design trade, usually at the system level.

The book Pressurization Systems for Liquid Rockets goes into great detail on this. I highly suggest you read Chapter 2.1.2 "Selection of System Type" which pretty much covers your exact question. There is even a case description on page 33 where the author goes through the trades for selecting helium versus autogenous pressurization on the S-IVB.

Here's one of the charts from this book describing some of the trade space where weight/compatibility/complexity are considered.

enter image description here

And here is the S-IVB discussion (I am posting it as an image, because whatever format Google Books uses does not allow me to copy the text). Note that the trades were made on weight and reliability.

enter image description here

Edit: The linked document is available at NTRS which is nice because the Google Books format is annoying.

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  • $\begingroup$ It would be nice to add a conclusion on why helium won for a complete answer. $\endgroup$ – Antzi Feb 21 '17 at 5:10
  • $\begingroup$ Good suggestion, incorporated. $\endgroup$ – Organic Marble Feb 21 '17 at 14:07
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    $\begingroup$ OM - great sources! Seems it's just as I suspected and as you pointed out in your preamble: design trade-offs. Where there is a prolonged period of coast, as there was with the Saturn V and as there likely would be with any GTO (or beyond) payload, autogenous pressurization is simply not feasible due to inevitable pressure decay. In fact, the prolonged coast (or, for that matter, parking orbit) parameter alone rules out autogenous pressurization. $\endgroup$ – Joel Neatrour Feb 22 '17 at 0:15

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