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

of the thruster valves never shut completely for the longer pulses, so the 10 individual commanded
pulses effectively ran together into one long pulse. Only one of the thrusters even came close to
meeting the requirements of the 5 Hz control cycle, and it still produced a minimum pulse width of only
about 60 ms reliably [59]. This performance was far worse than that observed for the single-stream
thruster (as illustrated in Figure 5-8), so an investigation of the possible reasons for this was conducted.
At first, it was thought that the thruster valve used in the single-stream tests might have been unusually
fast, and the results obtained with CGSE flight system simply represented a more typical distribution of
valve timing performance. However, the response times observed in the first round of pulse testing of
the CGSE flight system were far slower than the 30 to 60 ms range quoted in the SV128 specifications
[52], so it was suspected that some external influence might be slowing the valves down. To examine
this possibility in more detail, the valve response time was broken down into four timing metrics, as
illustrated in Figure 6-9.
Figure 6-9. Valve timing metrics [59].
As shown in Figure 6-9, the opening time of the valve was separated into two parts: open lag, which was
the time between the command to open the valve and the point at which the load cell began to
measure an increase in thruster force, and rise time, which was defined as the time between measuring
10% and 90% of the steady-state force. Similarly, closing time was broken into close lag and 90-10% fall
time. Measurements for these four timing metrics for each thruster valve as taken in the first round of
CGSE flight system pulse testing are presented in Table 6-3.
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