High Power Hydrogen Arcjet Performance Characterization

A MW-class hydrogen arcjet based on a water-cooled, wall-stabilized, constricted arc discharge configuration was subjected to extensive performance testing with the deliberate aim of advancing technology readiness level for potential space propulsion applications. The breadboard design incorporates alternating conductor/insulator wafers to form a discharge barrel enclosure with a 2.5-cm internal bore diameter and an overall length of approximately 1 meter. Swirling hydrogen flow is introduced into the barrel, and a DC arc discharge mode is established between a tungsten cathode button at the back plate and a ring-anode/spin-coil assembly at the exit where the heated flow is choked and accelerated in a graphite nozzle having a nominal throat diameter of 7 mm. During the performance tests, hydrogen flow rates were varied between 7-11 g/s with applied electrical power ranging up to 1.05 MW and specific input power ranging up to 105 MJ/kg. The minimal run duration for each test was 60 sec, which was adequate for the establishment of steady state operating conditions. Observed electric-to-thermal conversion efficiencies were in the range of 50-60 percent as determined via a simple heat balance method based on electrical power input and coolant water calorimeter measurements. These results were also found to closely match predictions based on an equilibrium sonic throat method. Moreover, a simple bi-linear fit was constructed which accurately correlated arc efficiency over the full range of applied power and hydrogen flow rates. Inferred specific impulse performance accounting for hydrogen recombination kinetics during the expansion process implied nearly frozen flow in the nozzle with inferred thrust efficiencies in the range of 44-56 percent. Successful completion of this test series represents a fundamental milestone in the progression of high power arcjet technology, and it is hoped that the results may serve as a reliable touchstone for the future development of MW-class regeneratively-cooled plasma rockets. The limited range of investigated specific input power constrains achievable performance, however, and represents a notable experimental deficiency in need of redress. It is therefore recommended that a follow-on test program be commissioned to extend the operating range to 800-1000 MJ/kg at MW-scale power levels.