THEMIS Propulsion System Performance Enhancement Utilizing a Repressurization Subsystem

THEMIS (Time History of Events and Macro-scale Interaction during Substorms) is a NASA MIDEX (Medium-class Explorers) program investigating magnetospheric substorm instability utilizing five spin stabilized probes in elliptical Earth orbits. The science mission is achieved when the one day, two day and four day orbits are aligned to achieve three dimensional magnetic mapping. The spacecraft designated Probe 1 after stage separation must reach a distance of 20 Earth Radius (ER). Early on in the spacecraft design phase, performance analyses showed that the baseline blowdown monopropellant propulsion system needed to be augmented to achieve the delta-V to reach 20 ER. After several trade studies, the solution was to provide a pyro-valve activated repressurization system that would achieve the increased delta-V while optimizing weight, propellant load, and probe propulsion performance within the existing power and real estate limitations. This paper considers the decision factors that were evaluated in determining what and how to enhance the blowdown propulsion system to achieve the necessary delta-V.