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

and if the cold gas system was found to be lacking in performance after initial demonstration, the
question of investing more resources into development of a hydrogen peroxide propulsion system might
be revisited. (As discussed in section 2.1, this is the path that the NASA Marshall team followed in
moving from the CGTA to the WGTA. However, it was anticipated that because the TALARIS hopper had
air-breathing EDFs for weight relief, the cold gas propulsion system on TALARIS would be less taxed than
the propulsion system of the Marshall CGTA, and therefore flight times for TALARIS would not be limited
quite so severely.) A possible framework for the process of upgrading from cold gas to monopropellant
hydrogen peroxide is presented in section 7.3 as part of the discussion of future work.
2.4 Selection of Nitrogen Propellant
One final decision was made before proceeding with the detailed design of the TALARIS cold gas
spacecraft emulator (CGSE) propulsion system: which particular gas to use as a propellant. This affects
component selection in terms of sizing and allowable materials. Many different gases have been used
for cold gas propulsion, including nitrogen, helium, carbon dioxide, air, and Freon, among others [24,27].
Selection among the numerous possible gas options was based on several factors. Chief among them
were non-reactivity, to minimize materials compatibility issues, and readiness of availability. Eventually,
the field was narrowed to four gases that were considered in greater detail, as described below.
Carbon dioxide (CO2)
Carbon dioxide was considered mainly because it was used effectively for small satellite cold gas
propulsion for SPHERES (Synchronized Position Hold Engage Reorient Experimental Satellites), another
MIT project with which many TALARIS team members were familiar [36]. In SPHERES, the carbon dioxide
is stored in mixed liquid and gas phase. As the gaseous CO2 is used as propellant, the liquid CO2
evaporates to replenish it, keeping the pressure inside the propellant tank constant until all the liquid
evaporates [37]. This helps keep thruster performance constant, and storage in the high-density liquid
phase means that a relatively large mass of propellant can be carried. However, the drawbacks to liquid
hydrogen peroxide discussed in section 2.3.2 apply to liquid CO2 as well, and these were considered
serious enough that carbon dioxide was discarded as an option. (Also, modeling of hops as described in
section 4.1 revealed that TALARIS uses propellant at a much more rapid rate than SPHERES, raising
concerns as to whether carbon dioxide could evaporate rapidly enough to continuously supply gaseous
propellant to the TALARIS thrusters.)
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