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

6.3.3 Thrust Output Characterization
To make measurements of static thrust production, thrusters were fired for at least 1 s, with at least 2 s
of space between successive firings. This allowed time for transient effects to damp out. The following
thruster combinations were fired:
• Each individual thruster
• Each pair of vertical thrusters
• Each station pair
• Each set of three vertical thrusters
• All four vertical thrusters together
• All four vertical thrusters plus each individual horizontal thruster
• All four vertical thrusters plus a pair of horizontal thrusters firing to provide translational force
(e.g. thrusters 2 and 8)
• All four vertical thrusters plus a pair of horizontal thrusters firing to provide a torque to roll the
vehicle about its X axis (e.g. thrusters 2 and 6)
This set of combinations (31 in total) was chosen for several reasons. First, the single thruster firings
allowed for the easiest comparison to the results of the single-stream tests. Second, the wide range of
combinations tested allowed for some characterization of the effects on an individual thruster’s
performance depending on how many other thrusters fired with it. This was useful in planning future
changes to the CGSE; for instance, it gave some sense of how performance might change if horizontal
thrusters were added in the Y direction. Finally, the combinations most likely to be used in flight were
tested. During a hop, the four vertical thrusters would essentially be firing constantly (though not at
100% duty cycle); the horizontal thrusters would only fire some of the time, and then most likely in pairs
to translate or rotate the vehicle, although under certain conditions a single horizontal thruster might be
fired to make a small attitude correction. All of these situations were included in the static testing plan.
As testing of the various planned combinations began, it became clear that one effect that had not been
fully considered was the state of gas in the CGSE when a given thruster was fired. The starting pressure
of the flight tanks did not seem to matter, as long as it was high enough for the regulator to maintain a
given chamber pressure; if thruster 1 was the first thruster fired in a test, it produced approximately the
same thrust level whether the tanks started at 4500 psia or 1500 psia. However, as a given test
progressed and gas was depleted from the flight tanks, thruster output decreased even if no other
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