A study of the lifetime of miniaturized ablative pulsed plasma thrusters

Abstract

Miniaturized satellites are one of the fastest growing sectors in the space industry, allowing cheap access to space. These satellites are currently limited by their lack of orbit control and their lifetime is therefore determined by the natural, drag-induced, de-orbiting. These issues can be solved using a dedicated propulsion system. Cold-gas micro-thrusters, successfully flown on miniaturized satellites, are characterised by a very low specific impulse (typically less than 100 s), which considerably limits their capabilities. Moreover, they require pressurized tanks, pressure regulators and valves that may significantly increase the dry mass of the system. Pulsed Plasma Thrusters (PPTs) represent an excellent alternative to the chemical thrusters thanks to their high scalability in terms of geometry, power input and performance.

Within this research programme, the factors that affect the lifetime of solid state propellant PPTs for miniaturized satellites (e.g. Cubesat and nanosatellites) applications have been studied and addressed. Moreover, efforts have been made to optimise a PPT as a sub-system to be integrated into a satellite spacecraft. This included the characterization of the electromagnetic noise that PPTs generate and that may cause failures on the other Cubesat subsystems and a study aimed at the optimization of the PPT ignition process to produce a lighter and more reliable conditioning electronics.

To complete all these tasks, a miniaturized PPT (called PPTCUP, which stands for PPT for Cubesat Propulsion) has been developed and tested in collaboration with Mars Space Ltd and Clyde Space Ltd. Results from the PPTCUP test campaign have proved the reliability of the thruster, being able to perform a number of shots almost two times bigger than the nominal number required to demonstrate the discharge chamber lifetime. Moreover, it has been found that the unit can work correctly in the range of the Cubesat operating temperatures, withstands the mechanical vibrations during launch and has main natural frequencies compliant with the Cubesat requirements. The results of the EMC characterization test show that the electromagnetic noise generated during the main PPT discharge is mostly compliant with the requirements or small enough to be impossible to distinguish from the facility background noise.
The dedicated study to identify the best spark plug system working with a relative low breakdown voltage has shown that the semiconductor- coated spark plug insulator are able to trigger the PPT main discharge at a lower voltage (i.e. about 1700 V against the 7500 V needed for the no-coated propellant rods) without affecting the overall thruster performances.

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