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

stream tests revealed that changes in feed line geometry could make chamber pressure unpredictable,
and the exact performance of the CGSE flight system could probably only be determined through
testing. Furthermore, the 40 N maximum thrust observed in the single-stream tests was attained with a
chamber pressure of 392 psia, which suggested that the optimal design chamber pressure for TALARIS
might be near 425 psia. Therefore, the nozzles for the flight system were fabricated to the same
specifications as the nozzle used in the single-stream tests, with the idea of revisiting the nozzle
dimensions in future if the chamber pressures observed in the flight CGSE were significantly far from
425 psia.
6.2 Interfacing the CGSE to the TALARIS Flight Computer
In the single-stream tests, data collection and thruster control were performed with a USB DAQ, as
described in section 5.2.1. For the full CGSE flight system on the TALARIS vehicle, these tasks were
performed with an onboard computer which carried out all the tasks required for a hop, including
collection of data from navigation sensors, execution of GNC algorithms, and control of the EDFs as well
as the CGSE. The computer selected for this purpose was a single-board reconfigurable input/output
(RIO) device, the NI sbRIO-9642. The RIO had both analog and digital input and output, as well as a real-
time processor that ran the GNC algorithms needed for flight [57]. It communicated with a ground
station computer which logged data and accepted commands from a human operator, similar to the
way in which the USB DAQ interfaced with a laptop. However, rather than using a USB cable, the RIO
was part of a local area network (LAN) that also included the ground station computer, with which it
could communicate by means of an Ethernet cable for ground tests or wirelessly for flights.
The RIO had integrated 24 V digital output channels which were used to control the CGSE thruster
solenoid valves. Originally, these channels were also used to provide power to the solenoid valves. Each
channel could output 0.25 A of steady-state current [57], so four channels were tied together in parallel
to deliver up to 1 A for each individual solenoid. The 24 Vdc coil for the SV128 solenoid valve had a
rating of 10 W, which meant that it was expected to consume only 0.42 A. However, there were enough
digital output channels available to use four for each valve, and providing extra margin for power was
considered desirable, especially in case the SV128 solenoid valves might later be exchanged for larger,
more powerful models.
The solenoid valves themselves did not require any special control circuitry. The SV128 was a normally-
closed valve, so it opened when power was applied to it, and when power was off it was closed.
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