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. One Wee Hop For A Laser 'craft' Might Also Be A Giant Leap

This almost imperceptible hop on June 7, 2004, was the first laser-fired 'rocket' launch in a vacuum. Photo credit: UAH's Laser Propulsion Group.
by Phil Gentry
Huntsville AL (SPX) Oct 18, 2004
In rocket travel, half a millimeter hardly qualifies as a measurable distance. But the half-millimeter hop of a tiny plastic "craft" in a UAH lab this summer might turn out to be a giant leap in the history of rockets.

The June 7 hop in a lab in UAH's Optics Building was the first successful demonstration of laser-powered rocket propulsion in a vacuum, according to Dr. Andrew Pakhomov, an associate professor of physics at UAH and a leader in the fledgling field of beamed energy propulsion.

"It was awesome," said Tim Cohen, the undergraduate student research assistant responsible for setting up the test shot in Pakhomov's lab. "The whole time we were doing it we were worried about it not working, so when we saw that little movement ... that was really wonderful."

Cohen is scheduled to make the first public presentation on this event at 8:50 a.m. on Wednesday, Oct. 13, at the Third International Symposium on Beamed Energy Propulsion at Rensselaer Polytechnic Institute in Troy, New York.

As part of a NASA-funded research project, UAH's Laser Propulsion Group is studying what has now become a new type of rocket engine. They use lasers firing pulses that last only tenths of nanoseconds - picoseconds or tenths of billionths of a second - at target materials, typically a piece of metal.

When the laser hits, the target absorbs some of the laser energy. Electrons fly away from energized atoms, turning them into ions which explode off the face of the metal.

Instant rocket.

This laser "ablation" technology is also efficient, with each pound of material generating five to ten times as much thrust as a pound of chemical rocket fuel and oxidizer.

That first half-millimeter hop sounds a lot more impressive when Cohen and Pakhomov say that the 35 millijoule laser pulse that caused it lasted only 100 picoseconds (10-10 seconds), generated about one Newton of thrust for a microsecond and burned off only about half of a microgram of the laser craft's silver coating.

Repeat that small laser pulse often enough to add up to one second of thrust and you generate almost enough force to accelerate one kilogram of material from zero to one meter per second (just over two mph) in one second.

And you burn off only half a gram of metallic "fuel."

(Silver isn't even the best metal to use for ablative fuel. It was chosen as a matter of convenience for the people building the laser craft. Aluminum and carbon are both significantly more efficient at converting laser energy into thrust.)

In addition to reducing the amount of fuel needed to boost rockets, since the laser itself can sit on the ground (or on a satellite for boosting objects already in orbit), a spacecraft doesn't have to cart around heavy engines.

"And you don't have to bring big fuel tanks because your energy source is on the ground," said Cohen. "We estimate that we might reduce the cost of getting a payload into space down from the present $10,000 per kilogram to as little as $100 per kilogram."

Earlier experiments in laser propulsion used powerful laser pulses to heat air under a metal shroud to the point that it exploded like lightning, forming a plasma and a shock wave that pushed against the shroud. The system works, but it isn't very efficient (it needs really powerful lasers) and it requires that there be air inside the shroud.

Pakhomov and others realized early that firing the laser at the shroud or some other metallic target and peeling off, or ablating, the target one layer of ions at a time could be more efficient than firing it at the air - at least in theory.

The first laser rocket systems might use powerful lasers mounted on the ground to give spacecraft a boost during launch, when a spacecraft has to fight through the thickest layer of the atmosphere and overcome inertia. A similar system aboard the space shuttle might help to boost satellites or probes out of low Earth orbit.

A third potential use might employ ablative plates and small lasers mounted aboard a satellite or interplanetary probe as a weight-efficient replacement for the caustic chemical rocket steering and pointing systems that are used today.

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