Resistojets (or, The poor man’s electric thruster)

This post is the first in a N (will be stealthily replaced with an actual number) part series on electric thrusters. I decided it’s a good idea if I condense the knowledge I pick up in small articles. Firstly, it helps my other classmates easily pick up what they would otherwise have to scour books for. Secondly, it will help me in the future when I actually start working on these topics for my summer project. So, let’s start with a light topic – Resistojets, or the poor man’s electric thurster.


A resistojet is the bare minimum that is required for a thruster to be called electric. In fact, if you are a person like me who just likes the idea of ions and blue glows (Imperial destroyer engines), you will be really reluctant to call a resistojet a new type of thruster at all. The heat that is obtained from chemical reactions in conventional rocket engines is instead obtained from resistors that convert electrical energy into heat. The propellant is hence heated by passing it over the surface of these resistors and is then expanded through a conventional nozzle to provide thrust. The thruster design itself can be done in various ways, some of which are given below.

screenshot-from-2016-12-22-23-43-02

If it isn’t already clear, the things inside the cavity are differently shaped heating elements which are empirically tested for performance. As a matter of fact, the more interesting part of this whole device is the de Laval nozzle, which has nothing to do with electricity.

As is obvious from the overly simple design, this is not the most efficient type of electric thruster. Frozen flow losses refer to the fraction of input energy that is added to the propellant gas flow which does not appear in the form of exit kinetic energy and are quite significant in these thrusters. This may occur due to a variety of reasons (propellant dissociation, ionisation and unnecessary excitation being some of them). Radiation losses are also a problem, generally occurring through the heated body of the thruster.

However, the worst part of this business is the direct contact of heating element with the propellant. This means the highest temperatures (and hence the highest exhaust velocities) that can be attained are limited by the material used for making the body of the thruster. Various propellants can be used each with their own positive and negative points. Some thruster characteristics are given in the following table.

screenshot-from-2016-12-23-21-18-35

Tungsten is the obvious choice for the theoretical maximum performance that can be obtained from these thrusters. For lower temperatures, molybdenum is generally used. These materials hence restrict the exhaust velocities obtainable to below 104 m/s that correspond to specific impulses of about 1000 sec. This calls for better designs that can sustain higher temperatures without requiring breakthroughs in materials science.

Onto arcjets !

References

[1] Robert G. Jahn “Physics of Electric Propulsion”

[2] V. V. Subramaniam “Recovery of frozen flow losses in arcjets” 

Do notify me about any errors I might have made. Constructive criticism is always appreciated.
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