Prediction of Damage to Structure resulting from Recirculation of Particles from a Magnetoplasma Spacecraft Engine
A magnetoplasma spacecraft engine, such as the Variable Area Specific Inpulse Magnetoplasma Rocket (VASIMR?), uses magnetic fields and a magnetic nozzle to constrict and accelerate plasma to produce thrust. Most of the ejected plasma particles are expected to detach from the magnetic field lines and escape to provide thrust but some particles may not and could impact the spacecraft structure resulting in surface erosion and electrical charging. The plasma plume for a magnetoplasma engine was modeled computationally and scaled to determine what percentage of particles remained in the magnetic field and the kinetic energy of all impacting particles. Factors such as average particle velocity at the engine exit, magnetic field strength, and plume density distribution (i.e. width) were varied in a full factorial experiment to ascertain the effects of each factor and the important inter-relationships. The results are presented for a generic magnetoplasma engine and for the specific VASIMR? case.
Detachment was found to be occurring with 99.42% of particles escaping under the worst conditions and only 0.0172% of particles impacting structure. It was determined that three things led to an increase in the number of impacting particles on spacecraft structure: a stronger magnetic field, a lower exit velocity of particles into the plume, and a wider plume. In addition, there was an ?erosion zone? where an increasing particle exit velocity led to more erosion until the number of impacting particles was negligible and erosion dropped significantly.
For the specific case under nominal conditions, the erosion rate was 1.386 nm/month of engine operating time on aluminum and 0.611 nm/month on silicon. The electrical charging on spacecraft surfaces was found to be -27.85 V DC, which can be mitigated with current plasma contactor technology or some variant. Therefore, magnetoplasma spacecraft engines can be shown to cause minimal erosion and electrical charging and should be capable of operating safely with current technology by varying the three parameters previously mentioned.