Innovations in Pulsed Plasma Thrusters to Enable Cubesat Science Missions
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Innovations in pulsed plasma thrusters to enable CubeSat science missions Paige Northway A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2020 Reading Committee: Robert Winglee, Chair Michael McCarthy Erika Harnett Program Authorized to Offer Degree: Earth and Space Sciences ©Copyright 2020 Paige Northway University of Washington Abstract Innovations in pulsed plasma thrusters to enable CubeSat science missions Paige Northway Chair of the Supervisory Committee: Robert Winglee Department of Earth and Space Sciences CubeSats and other small satellites in the 3-25 kg range are increasingly able to conduct meaningful science through advances in technology and miniaturization. However, much of the proposed science requires satellite mobility, which has advanced more slowly due to constraints on CubeSat launches. Pulsed Plasma Thrusters (PPTs) are a potential means of propulsion for these satellites that do not require fluid or gas tanks and feeds and are relatively compact. This makes them an ideal candidate as a low risk propulsion system for secondary payloads capable of passing safety concerns related to launch. Previously, the specific thrust, or thrust output per power input (mN/kW) of PPTs developed for space flight was low for the desired propulsion applications. This work examines the development of a PPT specifically for CubeSat propulsion. It also investigates how several science questions could be answered with these advances, with examples of missions to the asteroid belt and Europa. It is shown that the power, geometry, and propellant are influential factors that can lead to substantial gains in thruster performance. Where TeflonTM has been the propellant of choice for previously launched PPTs, the novel use of solid Sulfur propellant is demonstrated with a twofold increase in specific thrust, which is the highest of any material tested in this study. Furthermore, a switch from smooth to serrated coaxial electrodes provides an increase in the specific thrust by up to an additional factor of 2. These changes bring the current test system capabilities to 45 mN/kW with a specific impulse of 1200 sec. This provides the opportunity for CubeSat missions to execute orbital maneuvers with changes in velocity on a range from 50 to 500 m/s. A fully integrated flight model was built and tested to overcome issues arising from the transition from a benchtop system to a CubeSat formfactor and then further tested for launch and space environment compatibility. The miniaturized flight model achieves 25 mN/kW performance and is being tested on the University of Washington built HuskySat-1 3U+ CubeSat, which was launched Nov 2nd 2019 and deployed January 31st 2020. i Table of Contents Chapter 1. Introduction ............................................................................................................... 1 1.1 Defining CubeSats and SmallSats .................................................................................... 1 1.2 Increasing utility and use of small satellites motivating need for improved propulsion.. 3 1.2.1 Broad growth in launched nano- and pico- satellites. ............................................... 4 1.2.2 Opportunities from lowering costs of space access .................................................. 8 1.2.3 Increasing support in systems, launch, and orbit operations through industry interest 9 1.3 Current Propulsion and Science on CubeSats ................................................................ 10 1.3.1 CubeSats in Low-Earth Orbit.................................................................................. 10 1.3.2 Landing coverage on Mars with Mars CubeSat One .............................................. 12 1.3.3 Asteroid science with the Mobile Asteroid Surface Scout ..................................... 13 1.3.4 Lunar and deep space CubeSats on planned Artemis 1 launch .............................. 13 1.4 Propulsion systems background ..................................................................................... 14 1.4.1 Metrics for propulsion............................................................................................. 14 1.4.2 Ratings for propulsion technology maturity ........................................................... 18 1.4.3 Current state of the art in nanosat propulsion systems ........................................... 19 1.4.4 Summary of CubeSat propulsion systems .............................................................. 23 1.4.5 Summary ................................................................................................................. 24 Chapter 2. Science questions enabled by propulsion advances ................................................ 25 ii 2.1 Mars to asteroid belt ....................................................................................................... 25 2.1.1 Motivation and background .................................................................................... 26 2.1.2 Science Goals .......................................................................................................... 27 2.1.3 Mission concept and instrumentation ..................................................................... 30 2.2 Europa CubeSat assist .................................................................................................... 35 2.2.1 Science and Motivation........................................................................................... 35 2.2.2 Mission concept and propulsion considerations ..................................................... 37 2.3 Summary ........................................................................................................................ 44 Chapter 3. Pulsed Plasma Thruster Increased Specific Thrust ................................................. 45 3.1 Introduction to pulsed plasma thrusters ......................................................................... 45 3.1.1 Pulsed Plasma Thruster background ....................................................................... 45 3.1.2 Motivations ............................................................................................................. 48 3.2 Objectives ....................................................................................................................... 50 3.2.1 Propellants............................................................................................................... 51 3.2.2 Electrode Geometry ................................................................................................ 54 3.3 Methods .......................................................................................................................... 56 3.3.1 Testing setup ........................................................................................................... 56 3.3.2 Thrust stand ............................................................................................................. 56 3.3.3 Time of Flight ......................................................................................................... 59 iii 3.3.4 Mass ablation .......................................................................................................... 59 3.4 Results ............................................................................................................................ 60 3.4.1 Propellant Testing ................................................................................................... 60 3.4.2 Geometry Testing.................................................................................................... 62 3.4.3 Mass ablation and Time of Flight Results .............................................................. 65 3.5 Discussion of resulting thruster ...................................................................................... 67 3.5.1 Thruster characteristics ........................................................................................... 67 3.5.2 Thruster applications ............................................................................................... 70 3.6 Conclusions .................................................................................................................... 78 Chapter 4. Pulsed Plasma Thruster for in-space testing ........................................................... 80 4.1 Introduction .................................................................................................................... 80 4.2 Methods .......................................................................................................................... 80 4.2.1 Vacuum chamber testing......................................................................................... 80 4.2.2 Imaging of electrode wear ...................................................................................... 81 4.2.3 Thrust stand apparatus and calculations ................................................................. 81 4.2.4 Accelerated lifetime testing .................................................................................... 84 4.2.5 Sulfur propellant fabrication ................................................................................... 85 4.3 Results ............................................................................................................................ 87 4.3.1 Electrode material selection ...................................................................................