NASA Goes Nuclear
When your batteries are dead and solar power is only a distant memory, you're going to need something else in your power pack.
- By Ben Iannotta
- Air & Space magazine, July 2003
NASA Glenn Research Center
(Page 4 of 5)
So, says Mike Patterson, who co-built the DS1 engine with fellow Glenn engineer Bob Roman, “when somebody asks, ‘Why are you still working on ion thrusters—I thought you flew on DS1?’ I find that laughable. It’s like saying, ‘You’re still working on chemical rockets? I thought Robert Goddard did that in 1927.’ We’re in the infancy.”
Patterson is working on the Next Generation Ion Propulsion System, a larger, more powerful, and more fuel-efficient version of the DS1 engine. That thruster was about 12 inches wide and operated at 2.3 kilowatts. The new engine will be about 16 inches wide, consume up to seven kilowatts, and perform with 28 percent greater efficiency. Patterson’s goal is to build on the DS1 work without leaping too far, too fast. One major reason is that NASA doesn’t have the budget it did in the glory days of its youth. “Back in the ’60s, when the guys were working on the [five-foot-wide engine], I suspect they thought we were probably going to go to Mars by 1975 or something like that,” he says. “It just didn’t happen.”
The greatest challenge facing Patterson’s team is proving that an ion engine can operate for 10 years. “That’s 88,000 hours of operation,” he says. “If you look at your standard automobile engine, your car only lasts about 2,000 hours. And you’re constantly maintaining it. These we can’t maintain.”
No one knows how long an ion engine can last. At NASA’s Jet Propulsion Laboratory in California, engineers continue to run an identical flight spare of the DS1 engine in a test chamber. As of January, it had operated for 27,000 hours and consumed over 430 pounds of xenon, says DS1 program manager Marc Rayman. NASA engineers were debating how long to keep the test going. “If you run it to failure, you may destroy evidence to say ‘This is the rate at which it erodes,’ ” Al Newhouse says.
Designers of traditional chemical-fuel rockets have the luxury of firing test engines for one and a half times the duration they will eventually operate in space. That isn’t possible for an ion engine intended to run for 10 years. So finding a cost-effective and accurate way to predict lifetime without firing thrusters for their full duty cycle is critical. “If we make a mistake we won’t see for six or eight years, then we’ll be six or eight years behind,” Patterson says. And if it happened during the actual mission, JIMO could be lost in space. The team is working on a system that uses lasers to measure concentrations of particles as they sputter off the electrodes during short tests. From that data, engineers would extrapolate the expected engine lifetime.
Patterson’s team also hopes to boost the power of its engine beyond that of the DS1 thrusters. In the smaller engine, ions were shot through a circular molybdenum grid that looked a little like the screen filter in a kitchen faucet. To make a more powerful engine, Patterson can’t simply shoot more ions through a grid of the same size and material—the molybdenum would erode too quickly. So he is experimenting with carbon graphite grids that are more resistant to so-called sputter erosion. A DS1-type engine built with graphite grids might erode seven times slower, says Patterson. Engineers would then have the choice of running the engine longer at the same power, or working it harder.
In all likelihood, a JIMO-type mission would be powered by a cluster of ion engines that would take full advantage of the 100 kilowatts of power produced by the nuclear reactor. One option would be to line them up in an array along a boom. “Is it going to be three thrusters, five thrusters, or 10 thrusters?” Jankovsky asks. At the moment no one knows, because no one knows for sure how much thrust a single ion engine will be able to produce.
But there’s little doubt that in terms of mission design, the combination of nuclear power and ion propulsion offers a sharp departure from the past. Prometheus and JIMO could open up whole new types of missions for exploring the solar system. “For example, if one were to go to Saturn with a follow-on to Cassini, you could envision the possibility of [placing] landers down on the solid parts of [Saturn’s moon] Titan,” Greeley says. Multiple atmospheric probes might plunge into Saturn or Neptune to assess the planet’s chemical makeup. All this would be possible because nuclear electric propulsion lets mission designers devote more of their budgeted mass to scientific instruments and less to fuel.