Catching a
Falling Star

By John Woodford

Gallimore

n the poet John Donne challenged his readers to do something miraculous, he bid them to "go and catch a falling star ...." If we interpret it as catching up with a falling star, that's no longer a far fetched achievement, as it was four centuries ago. Alec Gallimore and his team of comet-catchers in U-M's electric propulsion lab are designing engines with the speed and durability to overtake many an astral body in outer space.

Gallimore's story is one of speed in outer space, of the development of rocket engines that go faster and farther than any has gone before.

Let's begin with the basics: Chemical propulsion (CP) rocketry was the way to go in the dawn of rocket science, Gallimore says in his office in the Francois-Xavier Bagnoud Building, where he is an associate professor in the Department of Aerospace Engineering. "Propelling a rocket involves heating a gas. In chemical propulsion, the heat is provided by an oxidizer, like oxygen, interacting with a fuel, like hydrogen. The ejection of gases from this reaction provides thrust to the rocket. The energy provided in chemical propulsion is limited because it takes a relatively large mass of fuel to achieve the needed levels. That limits the exhaust velocity. In the space shuttle, for example, the natural limit imposed by CP is less than three miles a second or about 10,000 miles per hour."

ELECTRIC PROPULSION is a category. Just as a bicycle is a category that can include mountain, touring and racing types, electric propulsion is a category that includes electrothermal (a type like lightning bolts or "arc-jets"), electrostatic (ion engines) and electromagnetic systems. Ion engines or thrusters have a potential top speed above 150,000 mph in space. Electromagnetic thrusters produce thrust through electric and magnetic body forces interacting with charged particles, or plasma. Plasma, sometimes called the fourth state of matter, is an electrically charged gas made up of charged particles, in this case from a field of positively charged particles, or ions, and negatively charged electrons.

Instead of accepting that natural limit of chemical combustion, scientists theorized that an electric propulsion system could put far more energy into the propellant via electricity. The pioneers in rocketry were aware of CP's limitations. Both the Russian rocket scientist Konstantin Tsiolkovsky (1857-1935) and his American counterpart Robert H. Goddard (1882-1945) discussed the potential of electrical propulsion even as they worked on earlier phases of rocketry Gallimore says. They realized that EP, as we call it, would permit us to move the gas at much higher speeds," Gallimore explains. "In one form of electric propulsion, electrothermal, we use an arc that has about the energy density of a lightning bolt in heat. So the speed limit jumps to about 45,000 miles per hour." Ion propulsion introduces even more energy and can go to 100,000 mph now, with faster speeds on the horizon.

"With electric propulsion, electricity heats the gas and the ions formed in that process supply the thrust—that is the basic concept," Gallimore says. "Chemical propulsion produces higher thrust. Put in automotive terms, you'd say its zero-to-60 miles per hour is much greater than electric propulsion's. It takes EP much longer to get to high speed. EP is used once a craft is in space. It can't provide the thrust to accelerate off the ground and into space.

Gallimore uses the analogy of the tortoise and the hare to compare the systems. "Chemical propulsion burns all of its fuel in several hours at most, then coasts at that speed in space. It's the hare. Electric propulsion takes weeks or months to build to its top speed. In the future, it may even take years before top speed is reached. But it will go several times faster. With the same amount of fuel it can speed spacecraft by a factor of 10 over conventional chemical-propulsion rocket. For long-term travel the tortoise will overtake the hare.

"EP is better for interplanetary travel. It will take much less fuel to power an equivalent space craft, or let you propel a spacecraft that is much lighter than its chemical fueled counterpart, or let you go several times faster. We say EP is mission-enabling and mission-enhancing, meaning it will let you do things you can't do now and/or let you do what you're doing now much more quickly or cheaply."

Galaxy XI, launched in December 1999, uses a xenon ion propulsion system developed by Hughes Space and Communications (which was acquired in 2000 by Boeing). Illustration from Boeing

EP took off in the late 1950s, and the first advanced use of ion thrusters took place with the SERT spacecraft in the mid-1960s. More than 100 different craft are using EP now. Most are communications satellites linked to pagers, direct TV and that sort of technology. EP enables the controller to keep the satellite positioned in the right spot to provide effective service.

"The cost savings are significant," he points out, "when you consider that powering a spacecraft costs $100,000 per kilo of fuel. Cutting fuel costs in half or more or sending up a double or triple payload at the same fuel cost appeals to businesses operating in space."

EP isn't used yet for manned spacecraft but is likely to be, because to send humans or even huge robotic payloads to Mars would be too expensive with a chemical system. The trip would require a series of huge liftoff rockets and take at least two years, according to Gallimore. "And since the rocket coasts, you have to get everything lined up with Earth's and Mars's orbits to ensure the shortest travel distance. With EP, you could get there quicker. and you would have more flexibility as to the planets' orbital alignments as far as choosing your departure date. That's why EP is the propulsion system many people are thinking about with human exploration of Mars."

NASA's whimsical DeepSpace I mission poster, developed in cooperation with SpectrumAstro Inc., JPL's primary industrial partner.

Artist's conception of Deep Space I's encounter with asteroid 9969 Braille.

Other uses of similar high-energy ion beam technologies include making extra-hard alloys for tools, special metal tape and steel, high-strength glasses and the thin metal vapor barriers used to seal potato chips and other foods in extra-tight bags. The technology lets you accelerate particles at high speeds, so to make the bags, you direct an ion beam into a vapor of aluminum and then press it into the polymer that provides the air-tight seal.

A craft powered by nuclear electric propulsion (NEP) could be lighter and smaller than a solar EP craft, the kind in use now, but there are obvious difficulties in testing NEP technology. Researchers are experimenting with NEP with everything but the radioactive materials. "To go beyond, say, Jupiter," Gallimore says, "we will need power other than solar—which both chemical and electrical systems use—because there isn't enough sunlight to power a solar craft when you get past Jupiter."