Development of Cubesat Thrusters
Date
2022
Authors
Tsifakis, Dimitrios
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The popularity of the cubesat form factor has increased dramatically in the last few years, resulting in unprecedented access to space by smaller or medium sized organisations, universities and even smaller countries that could not afford such access in the past. However, the vast majority of the cubesats currently in orbit do not have a propulsion system. Such a system is invaluable as it can extend the satellite's time in space, assist with space debris avoidance manoeuvres, coordinate constellation orbits and facilitate a timely and controlled re-entry at the end of the mission. Various satellite propulsion methods have been developed and tested successfully in space over the past 50~years but it is not always possible to scale down a thruster to match the strict requirements of cost, power, size and so on, set by the cubesat platform.
The objective of this thesis is to explore a number of options that, directly or indirectly, can help with the development of cubesat thrusters.
The presented work includes a cost efficient method of measuring the performance of a cubesat thruster in the lab, using a load cell on a hanging pendulum thrust balance. The method shows good performance in comparison with the most expensive displacement measurement method, normally achieved by a laser interferometer.
The thesis continues with the enhancement of the Pocket Rocket, an existing cubesat electrothermal RF thruster. The changes introduced are centred around a change from a capacitively coupled plasma system to an inductive one. These changes result in a more compact and efficient RF matching system which improves the overall efficiency of the thruster system, which is an important improvement for power-constrained cubesats. The new Inductive Pocket Rocket is placed in the Wombat space simulation chamber and a set of direct thrust measurements are made, showing the increase in thrust of up to 50%, then the plasma is ignited.
Continuing on that theme, the thesis presents a very efficient RF source produced in collaboration with the Stanford Power Lab. It is based on the Class-E amplifier topology, which operates the power semiconductor at the two extremes - fully conducting to fully cut off, spending little time in the linear, lossy region of the transfer curve. This RF source, matched with a suitable DC power supply, exhibits electrical efficiency of over 91% resulting not only in better use of the limited power resources on-board the cubesat, but also less waste heat.
The thesis continues with exploring an alternative propellant, naphthalene. Naphthalene sublimates at relatively low temperatures, producing enough vapour pressure to sustain the operation of a cold gas thruster. Cold gas thrusters generally have lower performance compared to other types of thruster systems for satellites but have the engineering advantage of simplicity. To that, naphthalene adds the advantage of having a propellant that is stored in solid state, resulting in negligible propellant storage pressure and higher propellant storage density, compared to the more common gas propellants such as krypton and xenon. Naphthalene also has excellent compatibility with the materials commonly used in the construction of satellites, eliminating the materials compatibility problems (corrosion), that increase the engineering challenge of other, more reactive solid propellants such as iodine. A functional proof of concept thruster is built and its performance is tested in the space simulation chamber, showing the reliable production of up to 0.6 mN of thrust.
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