Development of a Cold Gas Propulsion System for the ... - SSL - MIT

Figure 4-4. Results of the example run of the CGSE MATLAB model.
Figure 4-4(a) shows that when fully fueled to maximum pressure of 4500 psia, the CGSE tanks hold 4.14
kg of nitrogen. By the end of the modeled hop, this has been depleted to 0.21 kg at a pressure of
approximately 900 psia. Figure 4-4(b) shows that this is still higher than the chamber pressure, which for
this run was set at 500 psia, so there was some additional margin of propellant available for the
unmodeled attitude control, and it was concluded that this example flight profile would be feasible. The
temperature of the nitrogen in the CGSE declines throughout the hop as illustrated in Figure 4-4(c), with
the chamber temperature always somewhat lower than the tank temperature due to the Joule-
Thomson effect. According to the model, the final temperature of the nitrogen is about 129 K in the tank
and 126 K in the thruster chamber. The critical point of nitrogen is at 492 psi and 126.2 K [43], so the
model indicates that at the very end of the hop, the nitrogen could begin to liquefy in the thrusters.
However, the model had many known simplifications and was designed to model the worst-case
scenario, and conditions only crossed the critical point in the last few timesteps of the model, so this
example profile was considered provisionally acceptable. Still, the possible condensation of nitrogen at
the end of a hop was a concern kept in mind until testing confirmed that it would not be a problem.
For the purposes of CGSE design, the most important outputs of the MATLAB model were plots such as
the one in Figure 4-4(d) showing the required