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

4 Modeling and Flow Control Components
Once the architecture of the TALARIS CGSE propulsion system had been defined and quantitative
performance requirements had been set, detailed design could proceed. Since cold gas propulsion
systems are a mature technology, the types of components to be used in the CGSE were known from the
start. The design process thus involved sizing these components, particularly the thruster solenoid
valves and regulator, as well as the thruster nozzles. In order to accomplish this, a computer model was
written in MATLAB. This model was designed with several simplifying assumptions, which are discussed
in the following sections. Therefore, the model was not expected to produce highly accurate predictions
of the real CGSE’s performance. Rather, it was intended to serve as an aid to component selection in the
early stages of design, and the results produced by the model were sufficient for this purpose.
4.1 MATLAB Model
The MATLAB model written to assist the CGSE design process was centered on the solution of
differential equations tracking the thermodynamic state of the gas in the system. Various hopper flight
profiles could be simulated; knowledge of the thrust needed to execute a certain profile was combined
with information about the state of the gas to determine the gas flow rate necessary at each point in the
flight. From this, the sizes of the thruster solenoid valves and regulator necessary for the system to
produce sufficient thrust to execute the intended flight profile could be determined. Digital copies of the
model code are held by the TALARIS project and the author, but the derivation, structure, and content
of the model are discussed in detail here.
The structure of the model was based on a previous MATLAB model that had been written in the fall
2008 16.898 design class. However, nearly all of the working equations within the model were modified
or exchanged for more applicable equations in light of the evolutions of the CGSE design. The method of
entering flight profiles was changed to reflect the thruster configuration illustrated in Figure 3-2. A tank-
sizing routine that had been included in the 16.898 model was removed, since tanks had already been
selected for the CGSE by the time the work described in this thesis began. Most importantly, the 16.898
model used the ideal gas assumption throughout for simplicity. However, the high initial tank pressure
as well as the low temperatures that the gas would reach suggested that real gas effects would be non-
negligible for the majority of the conditions that the CGSE would experience. Thus, the MATLAB model
was modified to use real gas equations. This increased the complexity of the model, but it was believed
that it would provide more accurate results for the design process described in this thesis.
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