FLY ME TO THE STARS
Lightweight propulsion devices might boost satellites and send probes beyond
the sun's realm
Inspired by the challenge of providing quick and easy space launches, rocket
engineers have started to think about propulsion systems that would make the
X-33, an innovative test vehicle now being built at Lockheed Martin's Skunk
Works in Palmdale, Calif., look conservative by comparison. The X-33 is
supposed to show how to cut by 10-fold the cost of lofting a payload into
orbit. But because the craft's novel engines and its large composite fuel
tanks are proving more difficult than expected, its initial flight has been
postponed until fall 2000.
One of the most intriguing new ideas is shortly to undergo a test firing at
the Massachusetts Institute of Technology. Postgraduate student Adam London
has built a prototype thrust chamber for a miniature rocket engine using the
same techniques employed to build computer chips. But M.I.T.'s neighbors in
Cambridge need not worry about their windows being shattered: the device is
about half the size of a postage stamp and will produce only up to 15
newtons (two or three pounds) of thrust.
The thruster, which will burn oxygen and methane for its test firing,
consists of six layers of silicon fused together. The whole structure is
just three millimeters (just over a tenth of an inch) thick; the main
challenge London faced was to prevent it from melting. Ethanol coolant will
circulate in minute channels around the tiny, flat thrust chamber. London
was planning a test shot in late 1999 or early 2000.
A hundred or so rocket microengines derived from London's test rocket (but
probably made of harder silicon carbide) could one day launch satellites:
the expected thrust level from micromachined devices is very high in
relation to their mass. London thinks that a two-stage microrocket vehicle
weighing some 80 kilograms (176 pounds) at launch might be sufficient to put
a Coke-can-size payload, perhaps bearing eavesdropping sensors, into
orbit--or send it undetected to the other side of the world in 45 minutes.
Microrocket engines might also be valuable for returning samples from the
surface of Mars and for maneuvering satellites in or between orbits.
London's project is an offshoot of a larger effort at M.I.T. to build a jet
engine the size of a shirt button that could power a miniature jet plane--a
possible payload for the rocket vehicle. Both the jet engine and the rocket
will need superhard, accurately micromachined parts for pumps and turbines
that rotate at extremely high speeds. The National Aeronautics and Space
Administration has funded the project for several years, and M.I.T.'s Alan
Epstein, who heads the effort, plans next year to test a gas turbine that
measures just a few millimeters across. He points out that because of the
high efficiency that should be possible, a micro gas turbine powering a
generator can in principle pack 30 times more energy into a small space than
any battery. Refueling would replace recharging.
At the University of Washington, geophysicist Robert M. Winglee has an even
more startling idea, which he calls mini-magnetospheric plasma propulsion.
Winglee envisages a chamber the size of a pickle jar attached to a
spacecraft and surrounded by a helical heating coil powered at a few
kilowatts. When a small amount of a gas such as hydrogen or helium is
injected into the device, it forms a dense, hot, magnetized plasma. Once in
space, the plasma would spread out rapidly from the open ends of the pickle
jar until it had a radius of more than 16 kilometers (10 miles).
According to Winglee, the magnetic field would spread along with the plasma
and interact with the solar wind, acting like a giant sail that would
transfer force to the heating coil and hence the spacecraft. Winglee
estimates that a spacecraft with his propulsion system could gain enough
velocity over weeks or months to exit the solar system within a few years.
He says he got his idea when studying coronal mass ejections on the sun,
which also inflate magnetic fields. Winglee and two colleagues are now
performing tests and building a prototype: NASA's Institute for Advanced
Concepts was impressed enough to give him $500,000 to work on the idea.
If the tests now under way bear out the promise on paper, Winglee's device
could greatly extend the range of unmanned spacecraft. Long after the X-33
is retired to the National Air and Space Museum, lightweight could be the
way to go in space, for launches and for the long haul.
--Tim Beardsley in Washington, D.C.