Propellant tank pressurization is a critical aspect of liquid propellant rocket design. Many designs use high pressure helium, heated and recirculated, however the propellant gases themselves can be used - at least in the case of LH2 as explained in this answer. In the case of Falcon 9 this is mentioned here and here as I've learned in the discussion and links associated with this question.

Another question Why do pressure-fed systems have to be pressurized with helium or nitrogen? addresses the question of why the choice of pressurization gas must be helium or nitrogen, and self-pressurized by their own boil-off gas. This is a different question. I am asking about the function or purpose of the pressurization, and the relative importance of two following possibilities.

I had thought the pressure was necessary only to feed the propellants into the pumps and other plumbing of the engine fast enough, but then I saw this line in the CSMonitor article: SpaceX launch explosion traced to helium system. Now what?:

Helium is injected into fuel tanks to keep them structurally sound as the launcher burns fuel during flight. This system apparently leaked during the static test.

Thinking about it, overpressure would certainly help maintain rigidity of the tank. Anyone who's seen the "crush the can" experiment can't forget it.

Question: Is the pressurization of propellant tanks actually necessary for structural integrity? And while maintaining pressure above ambient may be necessary to prevent buckling, is further positive pressure used in the mechanical design of a rocket to substantially stiffen the structure?

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above: Image of the "Crush the Can" experiment, Ronald Lane Reese, Johns Hopkins University (1999).

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    $\begingroup$ Possible duplicate of Why do pressure-fed systems have to be pressurized with helium or nitrogen? $\endgroup$ – David Hammen Sep 24 '16 at 17:36
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    $\begingroup$ Not posting as answer, because not helium, but I found a reference in the shuttle flight rules saying that loss of ullage pressure in the H2 tank could cause a structural failure: Underspeeds resulting from two GH2 flow control valves failed closed or a plugged GH2 pressurization leg for specific engine configurations, or three GH2 flow control valves failed closed in all cases, will potentially result in loss of crew and vehicle due to either early engine shutdowns (due to LH2 NPSP) or ET structural failure. Reference www.jsc.nasa.gov/news/columbia/fr_generic.pdf, rationale for A5-155 $\endgroup$ – Organic Marble Sep 25 '16 at 2:02
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    $\begingroup$ We called the ET autogenously pressurized because it used the same propellants it contained as pressurants (heated up in the engines). $\endgroup$ – Organic Marble Sep 25 '16 at 2:11
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    $\begingroup$ Here's one that failed due to loss of pressure (of course this was the famous balloon tank) Missile 5C (February 20) was a complete failure when the fuel disconnect valve did not close properly at booster separation. Fuel tank pressure was lost, leading to reversal of the intermediate bulkhead and missile self-destruction at T+172 seconds. from en.wikipedia.org/wiki/SM-65C_Atlas $\endgroup$ – Organic Marble Sep 25 '16 at 2:13
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    $\begingroup$ For the Shuttle O2 tank, the pressurization was far less important for cavitation, for the reasons you list (LOX is, IIRC, 6 lb/gallon) and plus it sat up on top of the LH2 tank giving a high rho x g x h number but LH2 is only 1 lb/gallon and NPSP (cavitation) became an issue before structure for that tank. (its tank outlet actually curved up instead of down) $\endgroup$ – Organic Marble Sep 25 '16 at 2:42

Pressure stabilization is used in some rockets, and to varying degrees.

  • the Atlas and Centaur use 'full-scale' pressure stabilization. The tank walls were so thin, an unpressurized stage would collapse under its own weight (huge PDF). The stage had to be pressurized (or kept in a support jig) at all times.
  • the Falcon 9 uses flight pressure stabilization. The tank walls are thick enough that a stage can bear its own weight, and does not need pressurization during manufacturing or transport. It does need pressurization in flight, to withstand the flight loads.
  • Saturn V used no pressure stabilization. The stage structure is strong enough to withstand flight loads on its own.
  • $\begingroup$ Aha! A rocket scientist to the rescue! Thanks for putting these all together in one place - and citing and documenting examples of each. This is the kind of answer that is useful to keep in mind and refer back to later on. I would have never guessed that the 'full-scale' situation would be even considered, much less be true for such established rocket designs! $\endgroup$ – uhoh Sep 26 '16 at 12:43
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    $\begingroup$ I changed the link. That's an old SpaceX brochure. $\endgroup$ – Hobbes Sep 26 '16 at 12:46
  • $\begingroup$ note to everyone - follow the links within the Atlas link, or tl;dr to the punch line: youtube.com/watch?v=o7A6GBqre1k - presumably the loss of a (in this case) spy satellite during a prelaunch test due to rocket buckling due to loss of pressurization... $\endgroup$ – uhoh Sep 26 '16 at 12:57
  • $\begingroup$ There's a nice graph showing ullage pressure predicts and the structural and engine ICD limit lines for the shuttle tanks...but I cannot find it on the internet :( $\endgroup$ – Organic Marble Sep 26 '16 at 16:01
  • $\begingroup$ @OrganicMarble there is certainly room for more Q&Q related to propellant tank pressure, static and dynamic issues thereof! $\endgroup$ – uhoh Sep 26 '16 at 18:18

Lets take a rubber boat as an example. The tubes of the boat must be filled with air to let the boat float and carry some load. But to give the boat stability in the waves, a certain amount of pressure is needed. To much pressure inside the tubes of the boat will destroy them. The pressure has to be kept between certain limits.

The fuel tank of a rocket has to be as light as possible, a certain pressure inside will make the tank structure stiff against bending forces. Again the pressure has to be kept within the limits, too low and the tanks may be destroyed when bended (because they are made from metall, not from rubber), too high and the pressure itself destroys the tank. The pressure limits must be observed when the rocket stands at the launch with empty tanks, during the fuel loading, while the rocket is waiting for ignition and LOX and LH2 boils off, when the fuel is pumped into the combustion chamber and also when the air pressure outside decreases from sea level to vacuum. It is not the absolute pressure inside the tank that has to be maintained, it is the relative pressure to the outside.

A tank which is stable when the pressure inside is equal to the pressure outside may be build too, but it will be heavier and thus the velocity at engine shutdown will be lower. The tubes of the rubber boat demonstrate what additional stiffness is possible when inside pressure is above the lower limit.

  • $\begingroup$ Uwe I did not ask if inflation can make things stiffer. I asked specifically if the pressurization of propellant tanks (of liquid propellant rockets) is necessary for structural integrity. Can you try to answer that question specifically, and support your answer with a link or two. For example, has a rocket which required pressurization for structural integrity ever buckled under flight when pressurization wasn't sufficient? Or maybe there is some technical documentation of a particular rocket design when this failure mode was discussed? Check the comments above - thanks! $\endgroup$ – uhoh Sep 26 '16 at 12:33
  • $\begingroup$ The tanks should be as light as possible, if internal pressure allows us to build a lighter tank, we should use it. If the tank was constructed to use internal pressure for stability, it is dangerous to use it with a pressure significantly below the limits. $\endgroup$ – Uwe Sep 26 '16 at 13:17

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