Tech Briefs

Variable-Specific-Impulse Magnetoplasma Rocket

This rocket is expected to enable long-term human exploration of outer space.

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 magneto- plasmadynamic (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.

An MPD plasma injector includes a cathode in contact with the plasma. Although the plasma at the location of contact is relatively cold, the cathode becomes eroded and the plasma becomes contaminated with cathode material (typically tungsten). The erosion and contamination can contribute to premature failure and to increased loss of energy through radiation from the contaminants in the plasma. An equal limitation on performance is exerted by nonionized propellant in a high-power amplifier cavity that is part of the MPD; the reason for this limitation is that neutral atoms and molecules in this region lead to charge-exchange losses, which, in turn reduce the overall efficiency of the engine and increase the unwanted heat load on the first wall (the liner) of the MPD thruster.

The design of the VASIMR avoids the aforementioned limiting features. The VASIMR contains three major magnetic cells — the forward, central, and aft cells. A plasma is injected into these cells, then heated, then expanded in a magnetic nozzle. (The magnetic configuration is of a type known as an asymmetric mirror.) The forward cell handles the main injection of propellant gas and an ionization system; the central cell serves as an amplifier to further heat the plasma to desired magnetic-nozzle-input conditions; and the aft cell acts as a hybrid two-stage magnetic nozzle that converts the thermal energy of the fluid into directed flow while protecting the nozzle walls and allowing efficient detachment of the plasma from the magnetic field. During operation of the VASIMR, a neutral gas (typically, hydrogen) is injected into the forward cell, where it is ionized. The resulting plasma is then heated in the central cell, to the desired temperature and density, by use of radio-frequency excitation and ion cyclotron resonance. Once heated, the plasma is magnetically and gas-dynamically exhausted by the aft cell to provide modulated thrust. Contamination is virtually eliminated and premature failures of components are unlikely.

The VASIMR offers numerous advantages over the prior art:

  • Its unique electrodeless design provides not only high thrust at maximum power but also highly efficient ion-cyclotron-resonance heating, and high efficiency of the VASIMR regarded as a helicon plasma source.
  • Because the VASIMR operates at relatively high voltage and low current, its mass is relatively low. This means that a one-ship human mission will not depend on a high-energy, complex rendezvous near Earth to achieve escape velocity. Instead, a rapid interplanetary transfer will be achieved with an adaptable exhaust, which will provide optimal acceleration throughout the mission.
  • The residual magnetic field of the engine and the hydrogen propellant will be effective as a shield against radiation.
  • Because of its continuous acceleration, the VASIMR will be able to produce a small amount of artificial gravitation, thereby reducing the physiological deconditioning produced by weightlessness.
  • The variability of thrust and Isp at constant power will afford a wide range of capabilities to abort.
  • Because hydrogen is the most abundant element in the universe, the supply of hydrogen could likely be regenerated in situ.
  • The VASIMR is flexible and adaptable to both fast transfers of humans and slower high-payload robotic missions; hence, there would be no need to develop separate propulsion systems for missions of each type, and costs would be held down accordingly.

Long-range benefits could be derived from the continued development of the VASIMR. The VASIMR can be expected to pave the way for fusion-driven plasma rockets. In addition, because the VASIMR is a high-Isp rocket, the VASIMR concept can be expected to lead to lower initial mass in low Earth orbit, relative to nuclear, thermal, and/or chemical rockets.

This work was done by Franklin R. Chang-Díaz of Johnson Space Center.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-23041.