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

The imaginary simplified thruster can actually produce the amount of impulse equivalent to a 40 ms
square pulse if it is given a commanded pulse of only 25 ms. However, as shown in Figure 6-13, the
centroid of the actual pulse is still shifted, although now it is only located 15 ms later than the ideal
centroid. It is not possible to remove this centroid shift, but with sufficient knowledge of the thruster
timing characteristics, the location of the shifted centroid can be predicted.
Figure 6-13. Simplified diagram of adjusted command to produce impulse of a 40 ms square pulse.
Although the actual CGSE thrusters did not have linear rise and fall characteristics, and their timing
metrics all varied slightly from those assumed for the simplified thruster, the basic ideas illustrated in
Figure 6-12 and Figure 6-13 still applied. By scaling down commanded pulses and predicting centroid
shifts, impulse bits equivalent to 40 to 160 ms square pulses could be delivered on a period of 200 ms,
and the 5 Hz control cycle was considered to be feasible. However, this was not a very fast control cycle;
at 5 pulses per second and an estimated flight time of 15 s, each thruster could only produce 75 pulses
during a hop, which suggested that flight profiles would not be very smooth. Also, there were additional
difficulties created by the decrease in thrust magnitude with gas usage. If this decrease could be
predicted, pulse width commands could be gradually increased to keep the produced impulse bits
constant. The decrease was only roughly characterized, though, and there was a chance that by the end
of a hop, the thrust would fall so low that the controller would saturate. Thus, as flight testing and the
validation of the CGSE system began, the possibility of having to perform further verification testing was
also kept in mind.
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