NASA spins out gridless plasma rocket design

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VASIMIR offers boost to specific impulse compared with Hall Effect and ion drives

A plasma rocket that does not need the high-voltage grids or electrodes normally associated with electric propulsion systems such as Hall Effect thrusters and ion drives is to be built over the next two years.

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Fitted to a spacecraft, Houston-based Ad Astra Rocket’s Variable Specific Impulse Magnetoplasma Rocket (VASIMIR) would use hundreds of kilowatts of power from solar panels for payload orbit boosting.

“We can achieve a 20,000s specific impulse [Isp],” says Ad Astra research director Jared Squire. This compares with ion drives that have Isps – a measure of effciency – of around 4,500s. Higher Isps are difficult for Hall thrusters and ion drives because the acceleration electrodes and grids become degraded by ion collisions. VASIMIR was developed to overcome this problem.

In VASIMIR, radio-frequency antennas direct the energy for the rocket’s two stages. The first stage is the helicon, which turns the propellant gas into a plasma, and the second is the ion cyclotron (RF booster) wave, which accelerates it. During this process the plasma reaches a temperature of several million Kelvin. The helicon provides for efficient gas ionisation and the ion cyclotron wave, a transverse oscillation of the plasma’s ions, propagates them almost perpendicular to the magnetic field. The magnetic field for the magnetic nozzle that directs the plasma is provided by superconducting magnets. However, VASIMIR’s magnets are magneto-static and do not consume power.

The rocket prototype would initially use hydrogen as propellant, but researchers are looking at heavier elements.

The traditional fuel for ion drives, xenon, is not a candidate. The company is licensing the technology from NASA, where it was originally developed.

ROB COPPINGER / LONDON