Re: HELP NASA CEA CODE

[Jeff Berton]

Vaibhav wrote...
> I am using Nasa CEA code for calculating Isp for different Ae/At
> ratios with chamber pressure of 1000 psi. Problem is Isp should have a
> peak at optimum expansion condition i.e. when Pexit= Patm but with
> increasing Ae/At ratios Isp's are increasing although the Pexit<Patm.
> Can anybody who has used the code help me ?

If you overexpand a convergent-divergent nozzle beyond ideal conditions
(i.e., when Pexit < Patm), the exit velocity will indeed be greater than the
ideal exit velocity. However, you need to account for the "base pressure"
thrust loss, Aexit * (Patm - Pexit). This drag term grows with
overexpansion and it prevents overexpanded thrust from exceeding the ideal
thrust.

Note: There are limits to how much viscous attachment effects allow a
nozzle to be overexpanded. Eventually, the flow will separate from the
divergent wall.

Regards,
Jeff Berton