I am doing the math required to find the thrust of a rocket engine (more specifically backwards, I have the thrust required and I am designing the engine).

In my looking over the equations I have found on the NASA website:


$$F={\dot {m}}*Ve+(Pe-Po)*Ae$$

In this equation: ${\dot {m}}$ = Mass flow rate $Ve$ = Exit velocity $Pe$ = Exit pressure $Po$ = Chamber pressure $Ae$ = Exit area

It seems to me that if the ambient pressure and exit pressure are equal, that the equation would just be:

$F={\dot {m}}*Ve$

Would this be the correct because $(Pe-Po)*Ae $ is then zero?

  • $\begingroup$ Welcome on Physics SE :) Are you asking whether $a-b=0$ for $a=b$ or is your question really whether the equation you found is correct? For the latter question, it might be of advantage to edit your post to (i) include a reference and (ii) explain the variables you are using $\endgroup$
    – Sanya
    Dec 22, 2016 at 21:29
  • $\begingroup$ related: physics.stackexchange.com/q/158750 $\endgroup$
    – Phoenix87
    Dec 22, 2016 at 23:10

1 Answer 1


Yes, that is correct, with $V_e$ the velocity of the gases at the nozzle exit.


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