add vacuum ode analysis:

I ran a quick and dirty ODE.  I found the net mdot over time and threw this into and ODE45 solver.  Note the following assumptions:

1) gas constant of water (until I can get my hands on CEA stuff)
2) initially a perfect vacuum inside chamber
3) rocket instantaneously achieves steady state
4) Vacuum pump Vdot is constant over full range of pressure
5) ientropic expansion from engine chamber to vacuum chamber
6) no heat exchangers yet  --> adiabatic