Thermal Modelling and Thermal Control Optimisation of the mN-μHEMPT

Abstract

Airbus Friedrichshafen is working on the development of a milliNewton HEMPT (High Efficiency
Multistage Plasma Thruster): an electrostatic thruster concept suitable for small satellite
propulsion. An engineering model, the mN-μHEMPT, has been built and tested in vacuum,
generating thrust levels in the range of 1 to 5 mN. Although the working principle is understood,
there is still uncertainty in the loss process, in particular the heat transfer in the plasma-wall
interaction. An efficient heat management is crucial for the operation of the thruster, as the
performance of the magnets is severely hindered after reaching 250ºC. With this in mind, the
present thesis aims to produce the first thermal model of the mN-μHEMPT, with which a detailed
thermal analysis can be carried out. The model validation strategy, based on correlation
to testing results, makes it possible to overcome the uncertainty regarding the thermal losses.
By simulating the operation of the thruster in extreme load cases in a Low Earth Orbit, its
thermal performance is assessed, resulting in a detailed understanding of the temperature
evolution and heat propagation through the different components. This information is then
used to improve the performance by implementing design modifications. The result of the
thesis is a thermal model validated to within 1.65ºC as mean deviation, predicting a maximum
temperature of 180ºC at the magnet stack during operation. The application of a boron nitride
coating to the radiator and the decoupling of the heat losses at the magnet stack and at the
anode thanks to a second radiator, results in a maximum temperature of the magnet stack
of 85ºC. In conclusion, the thermal performance of the mN-μHEMPT is analysed for the first
time, and the design modifications proposed become a successful improvement.