It is well known that the rapid disassembly of Falcon 9 static testing started with the rupture of the composite fibres of the copv He tanks.

But what is the need to have the composite tanks constraining He if the system is pump fed? What role does these He gas achieve if the LOx flow is achieved by pumps?

If these He is not used for setting up the flow, then what other purpose are they meant for?

up vote 24 down vote accepted

The helium is used to continuously re-pressurise the propellant and LOx tanks as they are emptied. It is stored in a COPV inside the body of the LOx tanks to keep the He at a lower temperature (and higher density) to maximise loading capacity. The helium is cycled around the Merlin engine to be heated before being fed into the tanks. This has two main purposes:

  • To maintain a uniform pressure supplied to the turbopump throughout the operation. This in turn helps to ensure that the fuel flow rate is kept consistent. As mentioned by Organic Marble, underpressure at the turbopump inlet can result in cavitation and serious damage to the system.
  • To provide an outwards force on the walls of the tank to keep them structurally sound. The tanks themselves are strong enough to maintain form during manufacturing, but the higher flight loads require the use of pressurisation.

See this question for more detail and other examples of tank pressurisation. This article has a good run down of the He tank's purpose and of its failure.

  • Thanks for the answer! The linked discussion is even more elaborate. Thanks again – karthikeyan Jun 10 at 7:38
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    So @jack, the 1st stage too will require He pressurisation right? – karthikeyan Jun 10 at 7:41
  • Correct. This question shows the tanks in place. – Jack Jun 10 at 7:48
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    This is correct, but your first bullet is actually understating it a bit. If proper pressure isn't maintained at the turbopump inlet, a lot worse things than inconsistent fuel flow rate can happen. The pumps can cavitate resulting in Very Bad Things. pdfs.semanticscholar.org/8043/… – Organic Marble Jun 10 at 14:13
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    What about the alternative of using the propellant itself to pressurize the tanks - using waste engine heat to warm the LOX and LH2 (in their own loops) to provide heated gas to keep each tank pressurized? – Anthony X Jun 10 at 14:51

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