# Quick question about exhaust velocity

1. Nov 8, 2015

### Jason Weiner

I'm working on some propulsion homework for rockets. Basically combustion happens, and the gas velocity increases.
sometimes we use:
exhaust velocity = sqrt ( 2*Cp*exit temp)
other times we use
exhaust velocity = sqrt (2*Cp*(initial temp - final temp))

so the difference is, sometimes exit temp is used, others the 'change' in temp is used. I can't tell when or why the difference!

2. Nov 8, 2015

### Staff: Mentor

How are those three temperatures defined?

3. Nov 8, 2015

### Jason Weiner

I'm not sure what you mean by 'defined'
in both cases, the initial (combustion temp) was given, and exit was found and needed for other calculations.
So for two different questions, of the same 'basic' principals (rocket combustion)
to find exit velocity:
for one problem, we used exit temp
for another problem, we were supposed to use delta temp (change between chamber and exit)
even though they both seem to be combustion flowing out a nozzle!