Simulation and design of a hydrogen arcjet thruster seeded with cesium

Abstract

In conventional arcjets the largest loss in efficiency is the energy required to ionize the propellant. Adding a small amount of a readily ionizable material should provide adequate electrical conductivity, without the main propellant being ionized, thereby reducing this frozen loss. To study the effects of seeding a hydrogen arcjet with cesium, a two-dimensional numerical model was developed. The ions and atoms are treated as one fluid and the electrons as a second. The flow was modelled by Navier-Stokes equations modified to account for thermal and chemical non-equilibrium. The equations were numerically integrated by MacCormacks predictor-corrector scheme and a TVD scheme. The electric potential was solved by successive over-relaxation. Results show that the seeded arcjet can be run with the cesium fully ionized but the hydrogen having minimal dissociation or ionization. When the flow passes through the high current density region of the anode arc attachment, the hydrogen was found to not ionize as previously feared even when run at comparatively high currents. The cesium is fairly uniformly spread throughout the constrictor, as is the current density, so there is no formation of a narrow arc as in conventional thrusters.

(cont.) Consequently the constrictor must be an insulator or else the arc will not extend down the constrictor as desired. Modelling predicts that seeding can cut the frozen losses from fifty percent to less than one percent. Following this an experiment to test a prototype was designed. Performance mapping was done using a lower order model which was shown to be consistent with the full two dimensional code. Varying the parameters mapped out the performance range and provided a design for a prototype. A thermal model led to a structural design including regenerative cooling. Supply and extraction of the cesium was examined and the pumping requirements determined. Finally a mission analysis showed that a seeded arcjet is the best choice for LEO-GEO orbit raising with transfer times between 30 and 160 days.