arcjet

Arcjet is a form of electric propulsion used in spacecraft that generates thrust by heating a propellant with an electrical arc. It belongs to electrothermal propulsion and differs from resistojets, which heat propellant by resistance, and from electrostatic thrusters, which accelerate ions with electric fields. In an arcjet, a powerful electrical arc is struck between electrodes, heating a supplied propellant (commonly hydrogen, ammonia, hydrazine, or inert gases) to a high temperature. The hot gas expands through a nozzle to produce thrust. The design must balance arc stability, propellant flow, and electrode life, since the arc erodes the electrodes over time.

Operation involves directing propellant into the arc region, where it is vaporized and heated before exiting through a converging-diverging nozzle. The thrust level is higher than typical ion engines at comparable power, but the specific impulse is lower because much of the energy is dumped as heat in the gas rather than being stored in high exhaust velocity. Efficiency is curtailed by heat losses to electrodes, nozzle, and power electronics as well as by erosion and recombination losses in the heated gas.

Arcjets have been demonstrated at various power scales, from fractions of a kilowatt in laboratory units to tens of kilowatts in testbeds. Specific impulse values are typically in the several hundred to about a thousand seconds range, with thrust ranging from millinewtons to a few newtons depending on power and propellant. They were researched in the 1960s–1980s for orbital maneuvering and main propulsion on certain missions and as a technology option when high thrust was required with moderate Isp. In practice, arcjets have been superseded for many missions by ion and Hall-effect thrusters due to higher Isp and longer lifetimes, though arcjet concepts continue in research and in some niche demonstrations. Today, arcjet research continues, exploring advanced electrodes, propellants, and cooling methods.