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In a staged combustion cycle rocket engine, the fuel and oxidizer are fed into turbo pumps which are in turn powered by a pre-burner. My question is how do they ensure that the fuel and oxidizer are always fed in the proper amounts into the engine?

As in, there must be a maximum flow rate that the pumps can push fuel and oxidizer into the engine. There must also be a maximum flow rate that the tanks can push fuel and oxidizer into the pumps. But wouldn't that flow rate decrease as the rocket expends the fuel/oxidizer in the tanks? So is there ever any time in which the flow rate of the pump exceeds the flow rate of the incoming fuel/oxidizer from the tanks? In that case, wouldn't gas bubbles form inside the pump? I'm assuming that would be very bad for the engine. How do they account for this problem?

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  • $\begingroup$ "My question is how do they ensure that the fuel and oxidizer are always fed in the proper amounts into the engine?" – Careful. Very careful. $\endgroup$
    – Jörg W Mittag
    Commented Jan 21, 2022 at 18:47
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    $\begingroup$ The tanks don't push propellants into the pumps. They provide a minimum pressure to keep the pumps from cavitating. I believe what you are asking about is, in fact, cavitation. Cavitation has been discussed a lot on this site; try searching for that and you might find your answer. $\endgroup$
    – Organic Marble
    Commented Jan 21, 2022 at 18:49
  • $\begingroup$ @OrganicMarble It must be an important distinction, or you wouldn't bring it up, but I don't understand it. Providing a minimum pressure to keep the pumps from cavitating seems the same to me as pushing fuel into the pumps: If you don't push the fuel hard enough, the pumps cavitate. What am I missing? $\endgroup$
    – Wayne Conrad
    Commented Jan 21, 2022 at 18:55
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    $\begingroup$ @WayneConrad pressure drop and flow are inextricably linked; I suppose one can choose to think about it however one likes. The questioner seems to think that the tanks "push" propellant into the turbopumps at flowrates equal to those flowrates exiting the pumps. That doesn't work for my mental picture because of the small delta p between the tank ullage and pump inlet. In my mind the pumps are sucking in the propellant and the tank provides a small pressure to prevent cavitation. I fear this is heading the way of "is air sucked or blown out of the the airlock" discussion though, so I'm out. $\endgroup$
    – Organic Marble
    Commented Jan 21, 2022 at 19:02
  • $\begingroup$ @OrganicMarble You may be out, but you've won me over to your sensible point of view. Thanks for taking the time to explain it. $\endgroup$
    – Wayne Conrad
    Commented Jan 21, 2022 at 19:06

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Cavitation occurs when fluid pressure is below the fluid's vapor pressure. In other words, the liquid boils.

In a rocket, the propellant tank outlet is pressurized by ullage and acceleration. It is easy to prevent pressure drop between tank outlet and turbine inlet with correct pipe sizing for the flow. This prevents feed starvation cavitation.

The challenge is preventing cavitation inside the turbine, against the low pressure side of the blades at the turbine "eye". That's why they pay the engineers big bucks and give them fancy computers.

But wouldn't that flow rate decrease as the rocket expends the fuel/oxidizer in the tanks?

As the propellant is burned, the rocket gets lighter and acceleration increases. This helps counter the loss of hydrostatic pressure as the tank empties. In addition, remaining high vapor pressure propellants (like methane) can boil to replace the decreased partial pressure of propellant as the tank empties. This reduces the temperature of the remaining liquid propellant, lowering the vapor pressure and reducing cavitation. Think of frost on the tank just before your BBQ runs out of propane.

enter image description here Cavitation damage to pump housing

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    $\begingroup$ You might want to mention that most (all) booster tanks have pressurization systems that use helium, nitrogen, or autogenous propellants, not just self-boiling. Generally these systems provide regulated pressure. $\endgroup$
    – Organic Marble
    Commented Jan 21, 2022 at 20:32
  • $\begingroup$ Ah interesting. "It is easy to prevent pressure drop from tank to turbine inlet with correct pipe sizing for the flow." Can you expand on this a little bit? I'm guessing it has to do with the Bernoulli principle somehow, but I'm not versed enough in engineering to know the specifics. "The challenge is preventing cavitation inside the turbine, against the low pressure side of the blades at the turbine "eye". That's why they pay the engineers big bucks and give them computers to use." This is exactly what I'm looking for. Where can I go to read up more on this? $\endgroup$
    – JN18
    Commented Jan 21, 2022 at 21:14
  • $\begingroup$ @JN18 there's a lot of arguing in the comments but there is also good info about turbopump cavitation in the answers here space.stackexchange.com/q/18486/6944 $\endgroup$
    – Organic Marble
    Commented Jan 21, 2022 at 21:21

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