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 3 of 4)
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.
In 2002, with NASA planners for the first time in years discussing trips beyond Earth orbit, agency managers requested the “non-advocate” peer review to assess VASIMR’s readiness. It came just as the project was experiencing its closest brush with death. By the time Chang-Díaz returned that June from his seventh space shuttle mission, STS-111, his rocket was within four months of cancellation. “Unless Franklin is successful in wooing money,” NASA’s Space Architect Gary Martin said at the time, “the project goes dark in October.” NASA was scraping for cash to cover a multibillion-dollar overrun on the space station, and the aerospace technology and space transportation accounts that had contributed money to VASIMR in the past were now tapped out. Even the astronaut office was holding back about $200,000 in discretionary funds it had once promised to the project. NASA tried unsuccessfully to interest the defense department in taking VASIMR off its hands, and scheduled the peer review, which was completed in November.
According to John Mankins, the reviewers were “tentative” but willing to grant VASIMR a few years’ lease on life. He says they wanted to know more about the fundamental physics and how the elements of the rocket would work together as a system. The panel recommended further study, and NASA shifted the project to a new directorate for space and astromaterials research and exploration, making it eligible for what Mankins describes as “moderate” funding. “It’s on the order of $2 million a year for a while,” he says, “something that can keep the steam heated, definitely.” VASIMR also will benefit from three NASA small-business research grants worth a total of $740,000, which will go for technologies like the superconducting magnets to be used in a next-generation prototype.
Propulsion concepts come and go, but VASIMR has shown staying power. “Oftentimes, I think of this project as one of the weeds you try to kill in your garden,” says Chang-Díaz. “It won’t die, and every time you try to kill it, it grows bigger and stronger.” Mankins takes a “let’s see how this develops” attitude. “If the physics turns out to really work, it is so cool,” he says. “But even if it turns out VASIMR is not the right answer, it seems to me some kind of plasma propulsion has to be an option for the long term.” Chang-Díaz, the dreamer turned astronaut turned rocket builder, knows a lot about long-term planning. And patience.
Sidebar: Heat Waves
Unlike other plasma propulsion concepts, VASIMR uses no electrodes to heat its propellant. Instead, antennas generate radio waves, which heat the plasma to a high state of excitation. In the first stage, a helicon antenna designed by Australian National University researcher Rod Boswell ionizes the gas, which is confined by magnetic coils surrounding the chamber. The plasma is “cold”—more than 100,000 degrees Fahrenheit—when it enters the second stage. There, an ion cyclotron resonance heater (ICRH) antenna, similar to the kind used in fusion research, adds more radio energy to get the plasma cooking at much hotter temperatures. When it is hot enough to provide thrust—a million degrees or so—it enters the inlet of the magnetic nozzle, which shapes the flow further as it exits.