Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Trouble calculating effective exhaust velocity

  1. Sep 20, 2014 #1
    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)
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted