Reaction Control Thruster Thermal Design for a Mission to Mercury

Abstract

This paper describes two novel methods used to reduce the prefiring injector temperatures of thrusters on the sun-facing side of a spacecraft designed for a mission to Mercury. The Mercury Transfer Module is the propulsion module that takes the European Space Agency’s BepiColombo mission to the planet Mercury. It contains a bipropellant chemical propulsion system and a xenon-based electric propulsion system. It was predicted that, within a certain distance from the sun, the high solar loads and large internal waste heat from the electric propulsion would cause vapor lock to occur on the sun-facing hydrazine thrusters at the start of firing, rendering their performance unpredictable and inefficient. The paper reports on the development of a method to increase the emissivity of the nozzles via sandblasting and on the development of a special copper collar for directly removing heat from the injector. The design and analysis are reported, along with results from a dedicated thruster-level high solar flux thermal test and a full spacecraft-level thermal test using a flight-representative thruster to which the modifications as described in this paper. The two techniques are shown to enable unrestricted use of the thrusters all the way to Mercury.