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I know the basic premise of tank pressurization. A inert gas (usually Helium) is kept in small tanks in or around the large propellant tanks, and when propellant is burned the helium fills the empty space the burning propellants left. But do both stages need to pressurize the tanks, or only the upper stages? And what's stopping the helium from entering the combustion chamber?

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1) "do both stages need to pressurize the tanks, or only the upper stages?"

In general all stages need pressurization. I am unaware of any counterexample. If the engine is pressure-fed, you need higher pressure than the combustion chamber pressure. If the engine is pump-fed, you need sufficiently high pressure to keep the pumps from cavitating.

2) "And what's stopping the helium from entering the combustion chamber?"

Nothing, if you run out of propellant. But for boosters, acceleration holds the heavier propellants in the bottom of the tank, and the lighter helium stays at the top. The situation is more complicated for orbital systems, there are questions on the site about that (like this one: Propellant Settling for RCS Thrusters and this one: Starting a liquid fuel engine in zero gravity)

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  • $\begingroup$ Oh, so since helium is less dense than most propellants it stays towards the top of the tanks? $\endgroup$
    – Jake Blocker
    Commented Mar 15, 2017 at 2:21
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    $\begingroup$ yes, as long as there is acceleration! $\endgroup$
    – Organic Marble
    Commented Mar 15, 2017 at 2:23
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    $\begingroup$ An addendum: in a pressure fed system the tank pressure needs to be not just a little bit higher than the combustion chamber pressure, but high enough to maintain sufficient propellant flow, meaning that in most cases it must be quite a bit higher. $\endgroup$
    – Schlusstein
    Commented Mar 15, 2017 at 3:14
  • $\begingroup$ Also, @JakeBlocker, the helium used for pressurisation is gaseous, while the propellant in the tanks remain liquid, so it is much less dense $\endgroup$
    – user20636
    Commented Jul 1, 2018 at 20:28
  • $\begingroup$ There is no acceleration when the Falcon 9 second stage cuts off and then when it starts again what happens? Will helium enter the combustion chamber? $\endgroup$
    – Joe Jobs
    Commented Oct 24, 2020 at 20:24

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