Mars, and Step on It
When it’s not the journey but the destination that counts.
- By Michael Klesius
- Air & Space magazine, September 2009
(Page 3 of 4)
Rocket scientists rate the efficiency of rocket engines in “specific impulse,” a measure like miles per gallon, but with a time component: the number of seconds that a pound of rocket fuel will create a pound of thrust. For chemical engines, the best figure barely tops 450 seconds, paltry compared to ideas on the drawing board. A chemically propelled ship would exhaust much of its fuel at the beginning of a trip to Mars, like a drag racer that floors it, then coasts the rest of the way. To slow enough to enter orbit around Mars, it would depend on friction with the atmosphere as a brake. Unlike Earth’s atmosphere, thick and fluffy as a down comforter, with some room for error, the Martian atmosphere is a bed sheet.
“I’d love to go to Mars, but not on that ship,” says Emrich. “You’re going down to just a few thousand feet above the surface. It would be a very scary ride.” Come in too steep, he says, and you plow into the ground. Come in too shallow, and you skip off the atmosphere to become the next Voyager. “Very little room for error,” he says. “You get one crack at it.”
His solution: a nuclear thermal rocket. It would produce thrust the way chemical rockets do: by heating a propellant—in this case, hydrogen—and ejecting the expanded gas through a nozzle. Instead of heating hydrogen through combustion, however, the nuclear rocket vaporizes it through the controlled fission, or splitting of atomic nuclei, of uranium. Because nuclear fuel has a greater energy density, it lasts a lot longer than chemicals, so you can keep the engine running and continue to accelerate for half the trip. Then, with the speedometer clicking off about 15 miles per second—twice the speed reached by returning Apollo astronauts—you’d swing the ship around to point the other way and use the engine’s thrust to decelerate for the rest of the trip. Even when factoring in the weight of the reactor, a nuclear engine would cut the transit time in half.
In a program called NERVA (Nuclear Engine for Rocket Vehicle Applications), NASA built a nuclear thermal rocket in the 1960s. It delivered a specific impulse of 850 seconds—twice the efficiency of the best chemical rockets—and could have been tweaked to deliver up to 1,000 seconds. As NASA prepared to follow the moon missions with human voyages to the planets, the nuclear thermal rocket was a serious candidate to replace chemical engines in the Saturn V launch vehicle’s upper stages. Instead, despite more than 20 successful test firings in the Nevada desert, NERVA died in the mid-1970s.
Nuclear thermal rockets are limited by the heat tolerance of the uranium fuel and the engine’s structure, so engineers have experimented with new fuel elements and heat-resistant materials. At Marshall, Emrich has constructed a simulator that can test a nuclear rocket’s components by subjecting them to some of the conditions that fission would produce—the temperatures and pressures, though not the radioactivity. Because work on NASA’s Ares rocket, which will boost astronauts to the moon, has taken over the propulsion lab, Emrich is moving his simulator to another facility.
Not far from NASA’s Johnson Space Center in Houston, Franklin Chang Díaz, a former NASA astronaut and veteran of seven space shuttle flights, is developing an alternative to the nuclear thermal rocket. VASIMR, the Variable Specific Impulse Magnetoplasma Rocket, combines features of the high-thrust/low-specific-impulse chemical rocket, and the low- thrust/high-specific-impulse nuclear rocket. VASIMR is a plasma rocket. Instead of a combustion chamber, it uses three staged, magnetic cells that first ionize hydrogen and turn it into a super hot plasma, then further energize it with electromagnetic waves to maximize thrust. Chang Díaz promises his rocket could attain a speed of 31 miles a second, and would reduce a one-way trip to Mars from three months to one. His team has made slow progress on the concept since the late 1980s. Last fall, his VX-200 rocket prototype’s first stage, powered by argon, reached a milestone: a successful, full-power firing in his Webster, Texas lab. Having spent about $25 million from several government sources so far, and with equipment, lab space, and personnel from NASA, Chang Díaz is coming closer to a flight test. NASA is considering testing the rocket on the International Space Station, perhaps as soon as 2011 or 2012, where it may contribute to maintaining the huge laboratory’s orbit.
After VASIMR, the next step up in velocity is a nuclear fusion rocket. Scientists haven’t yet re-created sustained, controlled fusion, the chemical process that powers stars and promises enormous benefits as a power source on Earth, but that hasn’t stopped them from getting a lot of money from governments to try. The International Thermonuclear Experimental Reactor, being built in southern France, is a joint project of the European Union, Japan, China, India, South Korea, Russia, and the United States. The reactor will cost at least $15 billion, is not expected to begin operation until 2018, and is the size of an office building, but scientists hope that once they achieve fusion on the ground, reactors can be downsized for space travel. Fusion gives off more energy and less radiation than fission, and could propel a ship at high speed. In one scenario, its exhaust would be contained by a string of superconducting magnets shaped like huge washers, each perhaps 15 feet in diameter. The string of magnets would reach back from the reactor for the length of several football fields.
“The problem is not so much the amount of energy; you have gobs and gobs of energy,” says Emrich. “The problem is power, which is how fast you get the energy out of the system. A hydrogen bomb releases a huge amount of energy instantly but melts everything in sight.”