A propulsion system architecture is proposed in this dissertation. The intent of the work was to explore the feasibility of this proposal and to carry out fundamental research to provide engineering data that can be used to build a prototype of the system, or at least identify key areas that need further investigation before prototyping. The intent of the proposed architecture was to fill a hypothetical need based on an intended ‘use-case’. The proposed architecture is illustrated in Figure 1.3.
The proposed propulsion system is a heavy metal subliming electrostatic propulsion system. This architecture is novel and no such system has been found in the literature. The proposed system is novel because of the propellant generation paradigm and a new type of thruster
Figure 1.3: Schematic of the proposed propulsion system.
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manufacturing technology. The system operates by subliming a corrosive material that etches a heavy metal. The corrosive material is stored in a subliming chamber and the heavy metal is stored in an etching chamber. Both chambers’ temperatures are regulated by a thermal control system and separated by a valve. These components together make up the propellant generator. The purpose of this arrangement is to be able to store propellant in a maximally dense form which, as has been shown theoretically, helps to optimize the achievable delta-V that the system can provide.
The corrosive material that will be explored in this dissertation is XeF2 and the heavy metal is W. These materials react to form a stream of Xe and WF6 gases and this flow of gas is used as propellant. The process would also work using XeF4 and W as reactants and could theoretically achieve slightly better performance. This approach allows for the highest propellant storage density that has ever been reported. The theoretical maximum propellant storage density is 5.44 g/cm3 for XeF2 and W and 5.70 g/cm3 for XeF4 and W. XeF4 was not explored in this research because it is not readily available commercially.
The propellant generator is separated from an electrostatic thruster by a valve. The electrostatic thruster is composed of a gas flow regulator, power and control electronics, and a thruster body. The flow regulator controls the mass flow rate of propellant entering the thruster body which is a critical operational parameter for any propulsion system. The electronics coordinate valves, flow control, and DC and RF power delivery to the thruster. The thruster body is the component that ionizes propellant and accelerates it to produce thrust. The thruster is based on a classical thruster design, the electrostatic RF ion thruster, but is manufactured with an entirely new technique, the LTCC process and materials system. An advantage of using LTCC is that the electrodes can be embedded in a tough and chemically resistant ceramic material which enhances grid lifetime. Additionally, the manufacturing technique allows for a very efficient packing of
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functional elements of the thruster to improve how compact it can be produced. The ceramic material also allows for the thruster to be capable of very high voltages and temperatures, limited only by the drive electronics or other ancillary equipment. This research focused on studying the behavior of the three novel elements of this propulsion system, the sublimation chamber, etching chamber, and thruster body. The heaters, valves, flow control, and electronics are well understood engineering components and are not academically interesting in this context.
The intended use-case for the proposed propulsion system provided context so that performance targets were not arbitrary. The hypothetical use-case was an interplanetary 3 U CubeSat mission that needs a delta-V in excess of 1000 m/s. The design was further constrained by placing a 10 W power limit for propellant beam power and a propellant storage volume of only 0.1 L. This mission profile was selected in 2014 based on the assertion that would have been exceedingly difficult with the technology of the day. The only technology that could compete was an iodine propellant based propulsion which was in development at the time [71, 9]. That mission has been delayed due to propulsion system development challenges stemming from propellant corrosion. It was originally intended to be capable of a 200 m/s delta-V maneuver to lower its orbit from a 600 km circular orbit to a 300 km circular orbit.
The method of defining a tradeoff space, as described by Equation 1.2.16 and Equation 1.2.17, was investigated for the above use-case. Again, this method involved determining the intended power, propellant, and propellant storage volume. This method assumed that the application was a power limited and volume constrained spacecraft such as a CubeSat. The propellant selection that was made determined the average propellant storage density as well as the average propellant molar mass. From these parameters, the thrust and specific impulse as a function of effective accelerating voltage were calculated. A plot showing this relationship is
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shown in Figure 1.4. These calculations determined that an accelerating voltage of 318 V would result in a delta-V of 1000 m/s and net a thrust of 106 àN. This delta-V would take ~112 days to achieve on the use-case CubeSat. This represents the performance target of this work.
Figure 1.4: Thrust versus delta-V tradeoff space for the intended use-case CubeSat fitted with the proposed propulsion system.
The sublimation of XeF2 was studied as a function of its surface area and temperature and this work is presented in Chapter 2. The etching behavior of XeF2 on W was studied, and this work is presented in Chapter 3. The design, fabrication, and testing of the LTCC electrostatic thruster was performed and is presented in Chapter 4. The conclusions of this work and a discussion of future work and potential lines of research stemming from this dissertation are discussed in Chapter 5.
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Thrust (àN)
Specific Impulse (s)
Accelerating Voltage (V)
Specific Impulse versus Thrust - Tradeoff Space
Specific Impulse Thrust