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

However, a solenoid is an inductor, which means that it resists changes in its state, and when power to a
solenoid is cut, the coil generates a large back voltage. The RIO had no protection against this, so to
prevent damage, the RIO manual suggested placing a flyback or snubber diode in parallel with each
solenoid [57]. This would allow current flow from the low side of the solenoid back around to the high
side in a path external to the RIO, so the induced voltage could dissipate without causing damage. Based
on this recommendation, the circuit shown in Figure 6-4 was constructed for each of the eight thruster
solenoid valves in the CGSE.
Figure 6-4. Original CGSE control circuit for a single thruster solenoid valve.
Figure 6-4 shows that an LED was also added in parallel with each solenoid such that it would light up to
provide visual confirmation when the valve was open. This was an important safety feature which
informed people of which direction the thrusters were firing, but it also became a useful tool for
debugging.
One solenoid valve had a slight variant of the basic circuit shown in Figure 6-4. For safety reasons, it was
considered necessary to have a way to vent gas from the CGSE remotely even if the RIO was not
responding to commands. Therefore, an additional line was run from the low power battery (which
provided power to all the avionics on the TALARIS hopper but not the EDFs) to one of the solenoid
valves, with a switch controlled by a remote. This modification, known as the hardline dump capability,
is illustrated in Figure 6-5.
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