Star Power

The plasma rocket, says U.S. astronaut Franklin Chang-Díaz, is the propulsion technology of the future.

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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).

VASIMR produces much higher exhaust velocities than conventional ion-drive engines, and more mass can be expelled. The rocket, therefore, can produce much greater thrust. And because the shape of the magnetic nozzle can change, the thrust is variable, one of VASIMR’s big advantages. Chang-Díaz, a Corvette enthusiast, designed the rocket to cross the gravitational hills and valleys of space the way an automobile shifts gears to cross a mountain range. For the cruise to Mars, low thrust—expelling less mass at high exhaust velocity—saves fuel. Higher thrust would be used for entering or exiting a planet’s gravity “well.”

First, though, the VASIMR team has to solve several knotty problems. The plasma tends to cling to the magnetic field that confines it, and for the rocket to go anywhere, it has to detach before exiting the nozzle. Detachment happens in nature: The sun spits out plasma in the form of solar flares. But in laboratories, it has been virtually impossible to demonstrate. VASIMR’s critics are convinced the detachment problem is a showstopper. Chang-Díaz is equally confident that “in time, this will be shown to be a non-issue.” Sensitive to the criticism, however, he has engaged U.S. and Swedish collaborators in experiments to demonstrate detachment. But he cautions that obtaining conclusive results may require investing in a larger and more expensive test chamber.

The space shuttle main engine, the best chemical rocket in use today, has a specific impulse—a measure of fuel efficiency—of 465 seconds. Recently, the VX-10 achieved a specific impulse of 11,000 seconds, using deuterium, or heavy hydrogen, as fuel.

Some of VASIMR’s critics say heavier elements would give even better results. The peer reviewers suggested lithium, which has been used in low-thrust plasma propulsion experiments at NASA’s Jet Propulsion Laboratory in Pasadena, California. Chang-Díaz is reluctant, because lithium is highly toxic and could pose a contamination hazard in space. The team has experimented with argon, helium, and xenon, among other propellants, and plans to try ammonia. Chang-Díaz originally chose hydrogen and deuterium for several reasons. Both can be stored at cryogenic temperatures, meaning they can be used as propellant as well as coolant for the magnets that confine the plasma. They also are among the most abundant elements in the universe, so space travelers would find ample supplies anywhere they go.

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