Definitions

specific impulse

Variable Specific Impulse Magnetoplasma Rocket

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electro-thermal thruster for spacecraft propulsion. It uses radio waves to ionize a propellant and magnetic fields to accelerate the resulting plasma to generate thrust.

The method of heating plasma used in VASIMR was originally developed as a result of research into nuclear fusion. VASIMR is intended to bridge the gap between high-thrust, low-specific impulse propulsion systems and low-thrust, high-specific impulse systems. VASIMR is capable of functioning in either mode. The Costa Rican scientist and astronaut Franklin Chang-Diaz created the VASIMR concept and has been working on its development since 1979.

Performance

Current VASIMR designs should be capable of producing specific impulses ranging from 3,000 to 30,000 seconds (jet velocities 30 to 300 km/s). The low end of this range is comparable to some ion thruster designs. By adjusting the manner of plasma production and plasma heating, a VASIMR can control the specific impulse and thrust. VASIMR is also capable of processing much higher power levels (megawatts) than existing ion thruster electric propulsion designs. Therefore it can provide orders of magnitude higher thrust, provided a suitable power source.

Applications

VASIMR is not suitable to launch payloads from the surface of the Earth due to its low thrust to weight ratio and its need of a vacuum to operate. Instead, it will function as an upper stage for cargo, reducing the fuel requirements for in-space transportation. The engine is expected to perform the following functions at a fraction of the cost of chemical technologies:

  • drag compensation for space stations.
  • lunar cargo transport.
  • in-space refueling.
  • in space resource recovery.
  • ultra high speed transportation for deep space missions.

Other applications for VASIMR such as rapid transportation of people to Mars requires a very high power, low mass energy source, such as nuclear.

Current development

The principal developer of the VASIMR has been the Ad Astra Rocket Company. Currently, efforts have been focused on improving the overall efficiency of the engine by scaling up power levels. According to company's data, current VASIMR efficiency is at 67%. Published data on VX50 engine, capable of processing 50kW total radio frequency power, shows efficiency to be 59% calculated as: 90% NA ion generation efficiency * 65% NB ion speed boosting efficiency. Model VX100 is expected to have overall efficiency of 72% by improving the NB ion speed boosting efficiency to 80%. There are, however, additional (smaller) inefficiency losses related to DC electric current conversion to radio frequency power and superconducting magnets' energy consumption. By comparison current state of the art, proven ion engines designs like NASA's HiPEP operate at 80% total thruster/PPU energy efficiency. Published test data on VASIMR engine model VX50 show it to be capable of 0.5N thrust. The Ad Astra Rocket Company plans to test a ground prototype rocket in early 2008, the VX-200 rated at 200kW total radio frequency power, to demonstrate the required efficiency, thrust and specific impulse.

Negotiations have taken place with NASA to arrange the placement and testing of a flight version of the VASIMR, the VF-200, on the ISS. Its launch is expected to be in 2011-2012.

References

Additional resources

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