Pulsed plasma thrusters for small satellites

摘要

Since the Russian launch of the Zond-2 satellite in 1964 there have been over fifty years of research dedicated to the understanding of the first electric propulsion device to be flown in space, the Pulsed Plasma Thruster. The Pulsed Plasma Thruster originates from the evolution of the vacuum arc switch, but due to its microsecond operation time, the internal dynamics and nature of operation have remained unclear. The Pulsed Plasma Thruster is generally cheap to manufacture and to operate, which keeps it a popular device to research within institutes worldwide and has contributed to its longevity. As a satellite propulsion device it has unique capabilities that other propulsion systems cannot provide. The thruster operates by accelerating plasma formed in the accelerating electrodes (or nozzle) in short discrete packets of thrust or impulse. The pulsed nature of the thruster means that between pulses energy can be stored in capacitors, ready for the next discharge. The storage of energy over time means the power draw is variable and is only dependant on the frequency that the system is pulsed at. This property of the thruster makes the Pulsed Plasma Thruster extremely versatile, allowing the thruster to perform both velocity correction and control manoeuvres and attitude control manoeuvres. The Pulsed Plasma Thruster is mechanically scalable but the performance of the thruster has been shown to depend linearly on the energy storage ability of the thruster’s capacitor. The work presented here covers two areas. Firstly is the critical analysis of the physical mechanisms that occur within a Pulsed Plasma Thruster through a review of literature, experimentation and the development of a high current plasma flow model. The second area is the design, development, manufacture and evaluation of the Pulsed Plasma Thruster for use on a nanosatellite platform known as the CubeSat. Several novel observations and contributions were made during the critical analysis of the physical mechanisms of the Pulsed Plasma Thruster. The most significant was realising how the erosion of the metal electrodes affected the overall discharge process. It is postulated that the expulsion of material from emission sites (or cathode spots), the ionisation of that material and the resulting freed electrons, create a pinched plasma column between the electrodes. It is postulated that the interaction of the electrode sheath region and the intersecting plasma column cause the current flow to become limited. This was then shown to affect the efficiency with which the stored energy of the capacitor was converted to energy to accelerate the plasma. Understanding this issue is key in improving future designs of the Pulsed Plasma Thruster. The observations and conclusions made during this work were put into practice to create an eight μPPT propulsion module for a 3U CubeSat. Initial results show that a μPPT with a specific impulse of 321s, an impulse bit of 0.56μNs and a mass bit of 0.17μg has been developed. The thruster was developed for two technology demonstration CubeSats. STRaND-1 is a joint collaboration between Surrey Space Centre and Surrey Satellite Technology Limited and UKUBE-1 is a joint collaboration between Surrey Space Centre and the UK Space Agency. Both CubeSats are scheduled for launch late 2011, early 2012. The propulsion module for the STRaND-1 CubeSat will be the first to provide full axis control and the first to provide electric propulsion on this class of satellite, showing the advantages of the Pulsed Plasma Thruster for Small Satellites.