# Trouble calculating effective exhaust velocity

1. Sep 20, 2014

### Swartz55

So I'm trying to calculate the effective exhaust velocity of an engine which burned aluminum oxide and hydrogen gas at a barebones minimum temperature of 973.15 degrees Kelvin (700 C, the ignition temperature) The formula I have for specific impulse is TcxRgas, where Tc is combustion chamber temperature and Rgas is the specific exhaust velocity. It's a simplified version of the whole equation, and you can see where I got it here. First off, I believe my problem is in my equation for the Universal Gas Constant. The only one I've found that I know how to use is 8.3144621x(Tc/M) where M is the atomic weight. I've seen where the Universal Gas Constant is listed as 8.3144621x(J/mol K), but I don't know what the J, mol or K stand for, and have yet to find a definition.

When I use the weight in amu's, I get a number for the Isp ranging in the thousands (which I doubt), and when I use the weight in kg (1.66053982x10^-27) I get numbers to the power of 32, 40, 60 and so on. Another main issue is that I have been unable to get the same number twice. I know I'm doing something horribly wrong, but I don't know what. Here is everything that I know that I use for my calculations:

Exhaust total weight (amu): 43.98853
Exhaust average weight (amu): 14.66284
R: 8.3144621(Tc/m)
Tc: 973.15 degrees Kelvin
Ve (effective exhaust velocity): Rgas x Tc
Rgas: R/MM (total exhaust gas weight)