Variable-Specific-Impulse Magnetoplasma Rocket: "Variable-Specific-Impulse Magnetoplasma Rocket

This rocket is expected to enable long-term human exploration of outer space.
Lyndon B. Johnson Space Center, Houston, Texas

Johnson Space Center has been leading the development of a high-power, electrothermal plasma rocket - the variable-specific-impulse magnetoplasma rocket (VASIMR) - that is capable of exhaust modulation at constant power. An electrodeless design enables the rocket to operate at power densities much greater than those of more conventional magnetoplasma or ion engines. An aspect of the engine design that affords a capability to achieve both high and variable specific impulse (Isp) places the VASIMR far ahead of anything available today. Inasmuch as this rocket can utilize hydrogen as its propellant, it can be operated at relatively low cost.

The design of the VASIMR is so original that a prototype is being developed in collaboration with the Department of Energy and with the Oak Ridge National Laboratory and its Center for Manufacturing Technology. The VASIMR is expected to be commercially useful for boosting communication satellites and other Earth-orbiting spacecraft to higher orbits, retrieving and servicing spacecraft in high orbits around the Earth, and boosting high-payload robotic spacecraft on very fast missions to other planets. Similarly, the VASIMR should make it possible for robotic spacecraft to travel quickly to the outer reaches of the Solar system and begin probing interstellar space. By far, the greatest potential of the VASIMR is expected to lie in its ability to significantly reduce the trip times for human missions to Mars and beyond. This reduction in times is expected to enable long-term exploration of outer space by humans - something that conventional rocket designs now preclude.

Because the VASIMR uses plasma to produce thrust, it is related to several previously developed thrusters; namely, the ion engine, the stationary plasma thruster (SPT) (also known as the Hall thruster), and the magnetoplasmadynamic (MPD) thruster [also known as the Lorentz-force accelerator (LFA)]. However, the VASIMR differs considerably from these other thrusters in that it lacks electrodes (a lack that enables the VASIMR to operate at much greater power densities) and has an inherent capability to achieve high and variable Isp. Both the ion engine and the SPT are electrostatic in nature and can only accelerate ions present in plasmas by means of either (1) externally applied electric fields (i.e., applied by an external grid as on an ion engine) or (2) axial charge nonuniformity as in the SPT. These ion-acceleration features, in turn, result in accelerated exhaust beams that must be neutralized by electron sources strategically located at the outlets before the exhaust streams leave the engines.

In the LFA, acceleration is not electrostatic but electromagnetic. A radial electric current flowing from a central cathode interacts with a self-generated azimuthal magnetic field to produce acceleration. Although LFAs can operate at power levels higher than those of either the ion engine or the SPT and do not require charge neutralization, their performances are still limited by the erosion of their electrodes.


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