On a Wing and a Jolt
Source: New Scientist
December 9, 2000
A JUDDERING magnet has inspired a scientist at the US
Department of Energy to investigate a bizarre new way of propelling a
spacecraft.
The idea for a "judder-drive" struck David Goodwin when
he noticed that powerful cryogenically cooled superconducting magnets
often jolt in one direction for a centimetre or two when you first
turn them on.
"If you have something metal in the magnetic field as it
is forming, you see the magnet physically shift," Goodwin, who works
at the Office of High Energy and Nuclear Physics in Germantown,
Maryland, told New Scientist.
Superconducting magnets are cooled to such a low
temperature that they have no electrical resistance. Goodwin's magnets
were made by taking superconducting wires of niobium-tin alloy and
twisting the strands into a cable. The cables were then coated with an
insulator and wound into a coil. "The coil's then put into a
cylindrical casing called a cryostat that's filled with liquid
helium," says Goodwin. The liquid helium cools the wire coil to -269
°C, when they become superconducting.
Goodwin says the metal objects create the judder effect
by inducing a "brief asymmetry in the magnetic field" as it is set up
when the magnet is turned on. This initial disturbance of the magnetic
field, he says, creates a repulsive force on the magnet and pushes it
away.
But the force produced in one jolt is very low, Goodwin
says, so you would need to turn the magnet on and off with ultrafast
switches, making a fast stream of jolts. "We've got switches now that
can work at high voltages at 400,000 times a second," he says. "If you
could use one of these switches to rapidly switch the magnet on and
off, you might get some propulsion out of it."
A colleague of Goodwin's at Brookhaven National
Laboratory in New York is now modelling the magnetic field of
superconducting magnets to work out how best to arrange a metallic
disc in the magnetic field to produce the biggest jolt. But Goodwin
admits the judder drive might be going nowhere fast. "It's very
speculative. We don't know if it'll work," he says.
Marc Millis, who heads NASA's breakthrough propulsion
physics project at the NASA Glenn Research Center at Lewis Field in
Cleveland, Ohio, has invited Goodwin to present his idea at a
propulsion conference in July next year.
The crucial thing, says Millis, is whether Goodwin's
magnet would produce any net motion at all--it might just sit there
and vibrate. "It's a definite possibility that any forces arising from
Goodwin's concept will only act within the components of the device
itself, resulting in no net force," he says. "There are a lot of
unresolved physics issues to address."
by Ian Sample
http://www.newscientist.com/nl/1209/wing.html