Development of a Tridyne Microthruster for CubeSat Applications

Abstract

A warm gas thruster using a catalyzed tridyne propellant (85% N2, 10% H2 and 5% O2 by volume) was developed at the University of Washington in collaboration with Aerojet Rocketdyne for the purposes of advancing micro-propulsion technologies. Chemical equilibrium analyses were conducted to estimate performance and establish a design parameter space. Trade analyses were performed to establish the theoretical design parameters of a 1.5U (10x10x15 cm^3) Tridyne propulsion unit and the ability to compete with the current state-of-the-art in low-complexity micropropulsion applications. A 1.5U Tridyne micropropulsion system is estimated to achieve total impulses up to 1400 N-s dependent on propellant storage pressure. A high-level computer aided design was completed with use of commercial off-the-shelf, flight-heritage components and a custom engineered, additively-manufactured propellant tank by Aerojet Rocketdyne. A bench-top prototype was designed, constructed and tested to demonstrate the technology and performance capabilities of the flight-weight design while providing greater diagnostic access, measuring mass flow rate, pressure drop across the catalyst bed, and temperatures at the beginning, middle and end of the catalyst bed. The nominal mass flow rate is 1.0 g/s with an anticipated thrust of 1 N. A laboratory environment was set-up to test the bench-top thruster, including an electro-mechanically controlled mass flow system accessing gas cylinders of both Tridyne and nitrogen and an inline accumulator bottle with comparable volume to the flight-weight propellant tank to simulate a passively pressure regulated flow regime. The mass flow delivery and data collection and storage is controlled by a custom programmed LabVIEW GUI. The bench-top thruster was tested for two different Tridyne reaction configurations: ignition via flow over a heated pebble bed, and ignition via flow over a catalyst bed composed of the iridium-based S-405 catalyst. A complete reaction reaching the adiabatic flame temperature was observed in both thruster configurations, with the heated pebble bed only reaching a steady state reaction at the thruster exit for flow rates under 0.8 g/s. The catalyst bed configuration thruster reached a complete Tridyne reaction after 0.6" or less of S-405 catalyst flow length. The catalyst bed configuration thruster has a characteristic velocity efficiency in the range of 0.78 - 0.94 depending on the mass flow rate and preheat temperature, with a much higher efficiency in range of 0.92 - 1.0 anticipated for the flight-weight configuration due to a shorter distance between the reaction front and nozzle. Chemical reaction startup rise times in the range of 5-10 seconds and a maximum estimated propellant flow time of 720 seconds indicate a Tridyne thruster technology is best suited for missions requiring a minimal number of startup cycles. Phase II will perform a full-scale integration of flight-geometry hardware and extensive testing under both laboratory (as in this study) and simulated space conditions.