Direct Drive Solar Powered Arcjet Thruster

Direct-drive electric propulsion systems are comprised of a thruster that operates with the power supplied directly from the power source (typically solar arrays) with no further power conditioning needed between those two components. This has the advantage of reducing or eliminating the power processing unit (PPU) that is typically needed to convert the spacecraft-provided power to the voltage and current needed for thruster operation. Since the PPU is typically the most expensive piece of an electric thruster system, both from a fabrication and qualification standpoint, its reduction or elimination potentially offers major reductions in system cost and risk. Arcjet thrusters are electric propulsion devices that typically process high power levels, with the power supplied as a high current at low voltage. MSFC has initiated an internal activity to investigate the possibility of scaling a thruster of this type to a size where the voltage required would be roughly equal to the output of a standard solar array. If successful, the thruster could potentially operate in a continuous steady-state mode with power directly supplied by solar arrays – taking advantage of direct-drive for the first time in a non-Hall thruster-based electric propulsion system. Propulsively, the arcjet is a lower specific impulse thruster (1,000-1800 s) relative to present SOA Hall thrusters (2000-3000 s) and ion thrusters (3000-7000 s). The advantages of the arcjet are that for comparable power the arcjet is a much smaller device and can provide more thrust and orders of magnitude higher thrust density. The thruster is also more capable of processing much higher power in a small thruster package relative to the electrostatic Hall and ion thrusters. In addition, arcjets are capable of operating on a wide range of propellant options, having been demonstrated on a wide range of propellants (for examples: H2, ammonia, N2, Ar, Kr, Xe), while present SOA Hall thrusters are limited to Xe propellants. The MSFC concept is based upon a gas-fed micro arc thruster (presently 3/8" diameter) having an applied magnetic field produced by permanent magnets. The issue of start-up/arc initiation is the most critical to address, since the available solar array voltage is typically too low to initiate a plasma discharge. The transient startup technique employed in this work is similar to that found in vacuum-arc thrusters (VAT), where a high-voltage across the electrode gap is produced when current through an inductor is quickly interrupted. Unlike in the pulsed VAT concept where the electrode is ablated to evolve the propellant, the MSFC steady-state thruster concept has a separate, independently-controlled gas feed to supply and control the rate of propellant flow. After startup, the thruster should operate in a continuous steady-state direct-drive mode. Up to four 100 W Aleko solar panels are available to provide power to the thruster for both the initial startup phase and steady-state operation. The concept is scalable in power level through the increase in solar panels, with the thruster power consuming capability increased through either the addition of micro arc thrusters to yield an array of devices or through the fabrication of a larger single unit. The prototype discharge initiation circuit is designed to perform two functions that are usually accomplished by two independent systems: a) to create a voltage spike that will ionize/break down the gas to initiate the discharge and b) to allow for a continuous, steady-state flow of current to sustain the plasma and accelerate propellant after discharge initiation. The inductive energy storage response causes breakdown, which is an arc that shorts (voltage drops to close to zero) across the argon propellant in the thruster gap. At the same junction, a high enough current needs to be drawn through the ignited plasma to ensure there is a large enough power density that the plume does not neutralize. The current and voltage response time from the solar panels will need to be quick, and their magnitudes large enough to maintain a certain unknown required power density to sustain the plasma. Some proof- of-concept testing has already been performed, providing a basis for the proposed effort. Arc discharge initiation with the initiation circuit was demonstrated using a laboratory power supply (giving a controllable voltage for proof-of-concept testing) with a prototypic thruster in a vacuum environment. With argon flowing through the thruster, the pulse circuit has been used to initiate a high voltage spark, breaking down the gas during operation. While this testing revealed some technical issues with respect to the circuit and the method in which it was operated, it nonetheless demonstrated that plasma generation in the thruster with a low bus voltage (under 100 VDC) was possible. The successful completion of the proposed CIF project will result in a direct-drive solar powered arcjet thruster with the following aspects: The thruster will have a pulsed discharge initiation circuit that has been designed to address previous component issues (such as exceeding voltage limits on some components) and will demonstrate reliable, repeatable pulsed discharge initiation. The concept will be operated in an end-to-end test with power supplied by solar arrays directly to the thruster. Thrust will be measured on a thrust stand to evaluate performance (specific impulse and efficiency). The electrical and thermal characteristics of the thruster system under continuous operation will be measured to evaluate the design and identify potential failure locations. Refinement of the design for packaging into an array-type configuration of multiple thrusters.