Research Project 2010-2012
I have always been fascinated by electrical propulsion and whilst an undergraduate student at Olin College I spent some time on research and self-study explorations in this space. Between 2010 and 2012 I formed a student research project to better understand the mechanics of arcjet thrusters, a specific thruster variant of electrical propulsion systems, with the additional goal to build a working prototype of such a propulsion system.
An arcjet thruster is an electrothermal poprulsion device that heats a propellant stream by passing a high-current electrical arc through it, before the propellant is expanded through a downstream nozzle. Gas flows through the region between the nozzle body, which is treated as an anode, and a central cathode, where it is ionised by the anode-cathode arc before exhausting through a converging/diverging (deLaval) nozzle to provide thrust.
Though an imperfect geoemetry, the DeLaval configuration is a cost efficient design since the conical geometry nearly approximates a more contoured bell geometry, which would more perfectly direct exhaust axially, and the net conversion efficiency of random thermal energy to directed kinetic energy is in fact similar for both nozzle geometries. This geometry coupled with a large nozzle surface area for heat radiation, where the joints are located upstream of the heated region, leads to optimum thermal efficiency and a minimization of detrimental hot spots at high heat flux regions.
In the design and development of such a thruster system, we elected to pursue a modular design with a front mounted anode-nozzle, which could be removed for interchange, repair, and cleaning. The smallest realistic nozzle throat that were able to machine was 4mm, and using a length/diameter ratio of 1.0 which is proven to provide optimal expansion, the result was an area ratio of 1:225 with a conical nozzle at a 20 degree half angle and a 50 degree half angle converging section for a good low pressure region. Note: this magnitude of area expansion ratio is necessary in a vaccum to provide positive pressure thrust and allow the flow to reach a more ideal expansion, however we were unable to source a vacuum chamber for our testing purposes, and so realistically an expansion ration between to 1:3 and 1:6 would have been more appropriate.
The papers linked below provide details of the results of this project, as well as further details on design, development, and materials research. Thanks to Robert McMullen, who helped manufacture the arcjet thruster and test stand.