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
HE BUILT HIS FIRST ROCKET WHEN HE WAS SEVEN. It was a big cardboard box, fueled with the imagination of a Costa Rican boy who in October 1957 went to great heights for a closer look at Sputnik: He climbed a mango tree. From his perch, the satellite that launched the space race was a twinkling star racing past all the others in the heavens. Franklin Chang-Díaz knew then that his future was in physics. “I wanted to be like Wernher von Braun and Robert Goddard,” he says. “I wanted to design rockets.” That he spoke no English and lived in a country with no space program were inconveniences, not obstacles, to the young student, who eventually worked his way to the United States, immersed himself in the language, became a citizen, and made his dreams come true as an American astronaut.
The boy who climbed the mango tree is now 53. He has been to space seven times, and, probably more than most astronauts, he ponders the fact that in nearly half a century, not much has changed about the way humans get there. They still rely on rockets that use chemical combustion and travel no faster than the missiles that carried the first astronauts into Earth orbit. The chemical combustion rocket “has given us the capability to go to the moon and establish a permanent presence in space, but it cannot go farther than it already has,” he says. “If we are going to do any serious exploration of the solar system, we have to develop a new type of propulsion that increases performance by orders of magnitude.”
Today, one of the most controversial proposals for a new type of propulsion is a product of Chang-Díaz’s imagination. His Variable Specific Impulse Magnetoplasma Rocket is one exotic cardboard box. Rather than burning liquid or solid rocket fuel, VASIMR would use radio waves to heat a stream of ionized gas, or plasma, which would then be expelled at terrific speeds. VASIMR’s exhaust nozzle is an electromagnetic field that can change shape to change the rocket’s speed. The concept borrows heavily from technology developed through years of research on controlled fusion.
VASIMR’s promise is that it could get to Mars in four months, half the time conventional rockets would need. Closer to home, the rocket could recycle waste hydrogen from the International Space Station to keep the lab in orbit without fuel deliveries from Earth. Chang-Díaz calls it a quantum leap in space transportation; “I am very much convinced that this is the way we’re going to go to Mars,” he says matter-of-factly.
With his passion and charm, the astronaut has convinced a fair number of experts that he’s on the right track. Many scientists, though, question whether the rocket will ever get off the ground. Partly out of professional courtesy, and partly for fear of jeopardizing their own NASA funding, those contacted for this article would not attach their names to specific criticism. But the complaints have a common theme: VASIMR continues to win more than its fair share of scarce research money, even though it has yet to produce what critics consider measurable results. One plasma scientist who is intimately familiar with the project goes even further: “VASIMR does not work, even on paper.”
Of course, no one is heading to Mars any time soon, so NASA can afford to gamble on ideas that may not ultimately pay off. The agency is putting nearly all its money for advanced space transportation into nuclear-powered ion-drive engines, under the heading of Project Prometheus (see “NASA Goes Nuclear,” June/July 2003). Only a pittance—meaning tens of millions of dollars in NASA’s $15 billion annual budget—goes to fund far-off, conceptual studies of propulsion methods, which range from solar sails to space elevators to VASIMR.
Chang-Díaz has scraped by for a decade with roughly the same amount of money every year—$1 million, give or take a few hundred thousand dollars. He has gotten some funding from NASA, including the astronaut office, and scrounged the rest wherever he could—the Department of Defense, other government agencies, research foundations, academic collaborators. He has established scientific liaisons with Department of Energy fusion researchers and with international institutions like the Australian National University, the Alfven Laboratory in Sweden, and the Center for High Technology of Costa Rica. John Mankins, who directs a NASA headquarters office called THREADS—Technology for Human/Robotic Exploration and Development of Space—says that perseverance as much as anything explains why the project is still alive: “VASIMR has persisted because Franklin has been a champion of it.”
The astronaut’s critics agree. They say Chang-Díaz is pursuing what amounts to a government-subsidized hobby. Says one: “It’s a big waste of taxpayer money to have all those beautiful toys and no scientific output to speak of.”
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).
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.