Applying thurst force to a face
 

Hi,

I am simulating drag over a rocket for different speeds. I need to apply a force representing the thrust of the rocket at the exhaust in order to determine correct pressure drag. I have tried searching for a way to do this, but with no luck. Does anyone know how this could be done?

Thanks!

Are you modelling the rocket as a moving body (so the force is required to get the rocket to move) or are you modelling the rocket fixed in its frame of reference and you want the rocket exhaust to be included so the entrained flow and drag effects are included?

I hope it is the fixed in its frame of reference case as that is the best way to do this simulation for most cases.

Yep, fixed in its frame of reference.

Excellent.

In that case the simplest approach for the rocket exhaust would seem to be an inlet boundary with the flow conditions of the rocket exhaust specified, presumably supersonic flow.

I will try that :) If I have thrust data for the rocket and I want it to be specified as an inlet - would the correct approach be to use normal speed and pressure as mass and momentum option, and then calculate the pressure equivalent to that of the exhaust from P=F/A, where F in this case would be the thrust force and A the area of the exhaust?

This web page seems to be rockets 101: https://www.grc.nasa.gov/www/k-12/airplane/rockth.html

In the thrust equation, if you know the force then you have m(dot), V and p(e) as unknowns. This assumes you know the geometry for the exit geometry. If you know the throat geometry then you can probably have a reasonable guess at the exit velocity from compressible flow equations. With any luck you know the fuel/oxidiser mass flow rate, then you have m(dot) and then you can solve for p(e).

Great, I will have a look at it! Thank you, Glenn!

The page appears to be from NASA, at their Glenn Research Centre. Any place with an excellent name like that has to be good.