The plasma rocket, says U.S. astronaut Franklin Chang-DÃaz, is the propulsion technology of the future.
- By Beth Dickey
- Air & Space magazine, March 2004
Advanced Space Propulsion Laboratory/NASA Johnson Space Center
(Page 2 of 4)
The output in question is thrust. If VASIMR had produced just a little, the critics say, they might think differently. Chang-Díaz claims that since it would be impractical to put the rocket in a test stand, its thrust is difficult to measure. Nevertheless, his team recently did measure a force of six or seven milli-newtons (about the same thrust that conventional ion drives produce) on a small target placed in the exhaust stream of his prototype engine, presently at the Johnson Space Center in Houston. That ought to be enough for now, he says, to prove that the concept works.
Chang-Díaz had hoped to mount a demonstration on the space station this year to show how VASIMR could be used to boost the station. But a panel of outside peer reviewers concluded that the system wasn’t ready for a flight test. So for now, it’s back to the lab, where VASIMR has been for nearly 20 years.
After finishing high school in Costa Rica in 1967, Chang-Díaz worked in a bank to help pay his way to the United States. In Hartford, Connecticut, he repeated his senior year to become fluent in the English language, and did well enough in math and science to earn a scholarship to the University of Connecticut. By 1977, the year he received a doctorate in applied plasma physics and fusion technology from the Massachusetts Institute of Technology, he had his U.S. citizenship. Before winning a spot in the astronaut corps three years later, Chang-Díaz worked at the Charles Stark Draper Laboratory in Cambridge, Massachusetts, designing and integrating control systems for fusion reactors.
At Draper he was part of a large team of scientists trying to harness fusion as a peacetime energy source. The quest aimed for nothing less than creating a small sun in the laboratory: confining plasma in a magnetic “bottle” for a long enough period to achieve fusion. As it turned out, the bottle that inspired VASIMR could not be capped tightly enough to make a good nuclear reactor. But Chang-Díaz realized that the technology of fusion also was the technology of plasma propulsion and set out to bring the two together. If the fusion chamber was leaky, why not use the leaking plasma for thrust, the way an untied balloon zips around the room if you let it go?
After joining NASA’s shuttle corps as a mission specialist, the astronaut continued his propulsion research through assistants at MIT. He squeezed in occasional lab visits between training commitments for his shuttle flights. But science in absentia finally lost its appeal. In 1993 Chang-Díaz moved VASIMR to the Johnson Space Center, whose director provided funds for a new Advanced Space Propulsion Laboratory, with the astronaut as director. Ten years later, Chang-Díaz and a team of scientists and engineers continue to work on a prototype called VX-10.
The contraption brings to mind Jules Verne or H.G. Wells. It consists of a linked series of clanking, puffing metal cylinders outfitted with hoses, tubes, valves, and dials. Through little round windows in the cylinders, the experimenters watch the colorful glow of ionized gas conforming to a nozzle shape as it streams past a series of liquid-nitrogen-cooled copper-coil magnets (the real thing will use superconducting magnets). Chang-Díaz likes to joke that when the VX-10 runs, the lights flicker.
Rockets produce thrust by shooting hot gas through a nozzle at very high speed. The hotter the material is, the faster it exits, and the better the rocket performs. A faster exhaust also cuts propellant consumption. Conventional rockets have limited room for improvement in this area, because increasing the exhaust velocity increases the danger of engine meltdown. VASIMR solves the problem by eliminating parts—such as electrodes for heating the gas—that can melt. Instead, the gas is heated by radio energy, much the way microwaves bring water to a boil.
In the VX-10 test chamber, the forward cell ionizes—electrically charges—the gas so that it stays confined in a magnetic field. The center cell then bombards the plasma with radio waves, heating it to one million degrees, and the aft cell converts the superheated plasma’s energy to rocket exhaust (see “Heat Waves,” opposite).