Arcjets

An arcjet is a direct absorption electro-thermal thruster wherein the propellant is heated by an electric arc. It essentially consists of a negative (cathode) and a positive (anode) annular electrode, held close together by an insulating material and surrounded by a chamber, with an open connection to a nozzle, see figure. The narrow passageway between the cathode and anode is called the constrictor.

 

 


Figure: Schematic of arcjet thruster

To operate the thruster, it is filled with gas and voltage is applied. When the applied voltage is high enough (in excess of the breakdown voltage) and the power supply allows for sufficient current flowing (in excess of the holdover current), an arc occurs. The gas in between the electrodes is ionised and the conducting path closes the circuit and electrons will flow from the cathode to the nozzle (anode). As the electrons leave the cathode, the electric fields that exist between the cathode and anode accelerate them. Through collisions with the propellant gas, the propellant is heated to a high temperature. In the arc, the temperature can be in excess of 10,000-20,000 K. The injection of the propellant gas near the base of the cathode usually is with an azimuthal swirl. One can picture the gas flow by imagining that a section of gas flows around and along the cathode in a helix, the centre of which is the middle of the cathode/thruster. This flow surrounds both the cathode and then the electric arc in the constrictor. The swirl prevents the arc from kinking and touching the walls, thereby keeping the constrictor from melting. Only a small portion of the gas flow actually experiences direct electric arc heating; the other portion surrounding the central core is cooler, mixes with the very hot gases from the arc, and helps to protect the walls from excessive temperatures. The electrodes typically are made from tungsten alloy allowing for electrode temperatures in the range of 2000-2300 K. With regard to the propellants there are no important limitations.

Highest performance is achieved by using hydrogen and lithium, due to their low molecular mass. Depending on temperature and propellant an exhaust velocity of up to 10-15 km/s (hydrogen propellant) and 4-5 km/s (hydrazine propellant) seems feasible.

Arcjets can use electrical power from solar cells or batteries, and any of a variety propellants.Thrust efficiency of arcjets is typically less than about 50% [G. P. Sutton, Rocket Propulsion Elements] meaning that less than half of the electrical energy goes into the kinetic energy of the jet. The residual non-kinetic energy (heat and ionised particles) of the exhaust jet is the largest loss. 10-20% of the electrical power input is usually dissipated as radiated heat to space from the hot nozzle anode or transferred by conduction to other parts of the thruster.

Operational life of arcjets currently is limited by electrode erosion to about 1000-1500 hours. 

Some typical developments are described in Research and Development of Arcjets. Typical data of power augmented hydrazine thrusters are given in "Power augmented hydrazine thruster characteristics".
Arcjets have amongst others been applied on GE Astro 7000 (currently Lockheed Martin) satellite bus, as well as for the orbit-fine-control of the amateur radio satellite AMSAT P3-D.

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[1]The space between and shape of the electrodes, the nature of the surface treatment of the electrodes, and the gas and the gas pressure determine the breakdown voltage of the thruster.

[2]With temperature is meant here the effective temperature and not the electron temperature. The latter is usually much higher. For example, to ionise hydrogen an electron should have an energy of at least 13.6 eV. This resembles an electron temperature of close to 150,000 K.

 

Naam auteur: SSE
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