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.

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As he enters the nearby Electric Propulsion Research Building, Jankovsky points to a huge circular engine in a corner. It measures five feet across and looks like the housing of a large industrial fan. Engineers tested this 200-kilowatt ion engine in a vacuum tank here in 1967, back when it was assumed nuclear reactors would be generating millions of watts of electricity for future missions to Mars.

The engines being developed in this building are far less ambitious, but still an advance over the ion engines that have flown in space so far. At one end is a cathode tube that spits out electrons. They collide with a neutral gas, in this case xenon, knocking off more electrons and creating positively charged xenon ions. Other fuels could be used—krypton gives off a greenish glow, neon glows red. Xenon is popular because its electrons orbit farther from their nuclei, and that makes them easier to bump.

In 1998, a NASA technology demonstration mission called Deep Space 1 used a xenon engine; solar panels instead of a nuclear reactor supplied electricity. Although ion propulsion had already flown on U.S. commercial satellites and dozens of Russian military satellites, its use on DS1 was the first time it was included on a spacecraft dispatched beyond Earth orbit—in this case, to a comet and asteroid. The ion drive worked like a champ. The test proved that the engine could be throttled up or down, that its exhaust would not corrupt scientific readings, and that the ions wouldn’t short out electronics or block radio signals.

But DS1’s engine was not a powerful one, even by ion thruster standards. And although it ran for 678 days and was still going when NASA ended the mission in 2001, that wasn’t long enough to demonstrate the years-long operation required for the JIMO mission.

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.

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