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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.