starship-design: Ions In The Sky

["L. Parker" <lparker@cacaphony.net>]

Ions In The Sky



by Marcia Goodrich
Houghton - Jan 14, 2001

It may sound like Isaac Asimov made it up, but on at least one level,
there's nothing fancy about ion space propulsion. Many satellites fly in a
steady, 24-hour orbit, so they essentially hang in one spot above the Earth.
After awhile in space, however, they can get wobbly and then drift uselessly
out of their intended orbit along with their gazillion dollars worth of
electronics. To prevent this expensive debacle, ion propulsion engines
mounted on the satellites send out streams of ions almost daily to stabilize
the spacecraft, a process known as station keeping.

"There's nothing special about ions," says Assistant Professor Brad King
(ME-EM). "You could throw toasters off the back and it would do the same
thing."

It would certainly be cheaper to use toasters, if that were the only issue.
Ion-propulsion engines are typically fueled with xenon, one of the so-called
noble gases on the periodic table. "I think God has a sense of humor," King
said. "The gas with the best performance also happens to be the most
expensive that you can buy."

The nice thing about ions, however, is that, unlike toasters, they throw
themselves. "We use ions because they are positively charged, and the
spacecraft is given a positive charge, so they repel each other," King said.
"The ions come screaming out the back at 20 km per second." That's pushing
45,000 mph.

Pound for pound, that's a lot of thrust, which is why the aerospace industry
is becoming so fond of the ion-propulsion engines known as Hall-effect
thrusters. Satellites have been around a lot longer than Hall thrusters, and
much of King's work has focused on integrating these engines into existing
satellite designs.

There's plenty of capital available to do that. Getting mass into orbit is
astonishingly expensive, and fuel for station keeping can eat up to 60
percent of a satellite's weight. "If we can cut the amount of fuel in half
for a given satellite, we can save $100 million by using a smaller launch
vehicle to put it in orbit," King said. For that kind of money, companies
will gladly undertake a high-end retrofit on their traditional satellites.

Now, however, King is turning his attention to bigger and better
ion-propulsion engines.

The typical 1-kilowatt ion engine developed in the 1990s generates thrust
equal to the weight of a couple paperclips, which is quite adequate for
station keeping. (Ion thrusters are also good for long trips; one has been
powering NASA's Deep Space 1 probe on its drive to the outer planets.) To
expand their capabilities, however, much more powerful engines are needed.

"Now we're working on the next generation of thrusters," King said. In the
next five to 10 years, researchers expect to design thrusters that work at
over 30 kilowatts. "Those would be powerful enough to significantly raise
the orbit of a satellite, rather than just keep it in place," he said,
something that until now only chemical propellants have had the power to do.
"A 30-kilowatt thruster would cut chemical rockets out of the loop for most
satellite maneuvers," King said, drastically reducing the cost of launching
a satellite.

As efficient as they are, Hall thrusters have an unfortunate side effect.
Xenon ions traveling at 20 km per second can wreak havoc on a molecular
scale, which brings us to another aspect of King's research.

Once they're in orbit, satellites get most of their power from a solar
array, which converts sunlight to electricity. As the ions fly out, they
disperse and can slam into the arrays, blowing away molecules from the lens
that collects the sunlight. Over time, the effect is not unlike
sandblasting. "That ruins the efficiency of the solar array," King said.

To solve the problem, King and graduate student Alex Kieckhafer are
investigating "beam divergence": designing the Hall thruster and satellite
to keep the ions from spraying into the spacecraft.

Working on ion propulsion systems requires an unusual grade of equipment.

Trying to make outer space in the U.P. can be difficult," King notes as he
ushers a visitor into his lab. The first thing you need is a place to hold
your vacuum, in this case, a cylindrical stainless steel tank four meters
long and two meters wide with walls three-quarters of an inch thick. "I got
it at an aerospace surplus place in California," King said. "They didn't
know what it was, so they sold it cheap." Its two massive doors were built
closer to home, at Calumet Machine.

To make outer space in the basement of the ME-EM building, a powerful pump
sucks most of the air out of the tank. Then liquid helium circulates through
a special carbon pad on its inside; virtually all of the remaining air
freezes onto the pad. The interior is then nearly as air-free as outer
space, making it a good place to test ion propulsion engines. The pumps must
also be able to remove the ejected xenon propellant, which they can do at a
rate of 60,000 liters per second.

Clearly, Hall-effect thrusters are not the easiest type of engine to study.
King doesn't seem to mind.

"They're fun," he says. "Kind of like Buck Rogers, but real. And you have an
audience -- you don't have to sit in the basement, write papers and stick
them in file cabinets."

For his next project, he envisions another Star Trekkian device that would
hold a group of information-gathering satellites in formation. If each
carried a small telescopic lens, they could function as a many-faceted
insect eye, providing better images less expensively than a single, huge,
Hubble-style lens. But to work, they'd have to maintain a very tight flight
pattern.

What would that take?

King smiles. "I'm trying to build a realistic tractor beam."
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