In all liquid rockets, you've got to inject the fuel into the combustion chamber at a higher pressure than the combustion chamber itself. This is why turbopumps may be used--to raise the pressure behind the injector to higher than the chamber--otherwise the contents of the combustion chamber would flow backwards.

In broad terms, about what velocity do the propellants come out of the injector plate? And does it contribute to the total thrust of the engine in any significant way?

Tim Dodd (update: not Tim Dodd) asked a very similar question not too long ago. However, the answers are almost completely unsourced. Based on the Stack Exchange blog, I think this slightly differently worded question is still worth asking, just in the case it gets some sourced answers.

  • $\begingroup$ Only a small part of the propellants are used to drive the turbo pumps, the main part is burned within the chamber. So the thrust provided by injection may be only a very small part of the total thrust. $\endgroup$
    – Uwe
    Commented May 20, 2020 at 19:20
  • $\begingroup$ See the related question. $\endgroup$
    – Uwe
    Commented May 20, 2020 at 19:28

1 Answer 1


Partial answer

In broad terms, about what velocity do the propellants come out of the injector plate?

For the instrumented 50 klbf LOX/LH2 engine used in this study they varied the LH2 injection velocity from ~300 to ~700 ft/s. LO2 injection velocity was about 1/6th of that.

enter image description here

For the other part of the question, it seems what you are asking is, what would the thrust produced by an engine be if it didn't ignite. Some systems (Shuttle, Saturn V upper stages) dump residual propellants through the engines, which might give some clue, although the flow rates are tiny compared to that seen during the burn.


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