Technology Watch: Rocket Science

How do you launch something into orbit without using any fuel? A prototype spacecraft relies on a ground-based laser to push it aloft.

By Fenella Saunders|Wednesday, April 01, 1998
Whenever rockets leave Earth, the satellites and other cargo they carry account for just a fraction of their weight. Most—up to 90 percent—of a rocket’s weight comes from its fuel. This presents a problem for rocket designers: the more a rocket weighs, the more fuel you need to boost it into space. But how do you reduce a rocket’s weight without reducing the fuel needed to launch it? Leik Myrabo, an aerospace engineer at Rensselaer Polytechnic Institute in Troy, New York, says the solution is to take the power source off the rocket and leave it on the ground, reducing launch costs to at least a hundredth of what they are now. Working at the Air Force Research Laboratory in California with aerospace engineer Franklin Mead, Myrabo has built a number of six-inch-wide prototypes that in early tests have reached a height of more than 70 feet without so much as a drop of onboard propellant.

The model looks like a big acorn, but its odd design allows it to use an unconventional means of propulsion: a high-power laser. Before launch the aluminum prototype, called Lightcraft because of its power source, balances on a six-inch-long pole. To launch the device, Myrabo and Mead first fire a jet of compressed air at it, making it spin. The spinning stabilizes the craft. Then a laser pulse shines up from below and hits the polished underside of the craft, which acts as a mirror to focus the beam into the air beneath the craft. The laser power is so concentrated that it superheats .6 cubic inch of air a fraction of an inch below the craft to 54,000 degrees Fahrenheit. Such a cloud of gas is called a plasma, and it expands explosively to power the craft. Between beam pulses, which fire 20 times a second, air rushes back beneath the craft to fuel the next laser-induced explosion.

Myrabo keeps the Lightcraft’s power source on the ground because a laser powerful enough to create a plasma would be too heavy to mount onto the craft. To stay positioned over its power source, the Lightcraft has to move upward in a very straight line, which is why Myrabo and Mead spin the craft before liftoff, creating gyroscopic forces that steady it. Myrabo says that within the next five years he and Mead plan to make a version that will be up to five feet in diameter, which he hopes will reach a height of 124 miles, putting it into low orbit. Myrabo says he even has a 66-foot-wide model on the drawing board that could carry 12 passengers. That model would have onboard jets as stabilizers.

What about bad weather or birds coming between the Lightcraft and the laser? As with conventional rockets, Lightcraft launches would have to be canceled during storms. But you would launch from the top of a mountain or a desert to avoid many atmospheric problems, says Myrabo. And if birds flew through the beam, it would be a very momentary interruption, so I don’t see any particular problem. And unlike conventional rockets with their burden of onboard fuel tanks, if something does go wrong with a launch of the Lightcraft, the laser can be shut off and the craft can parachute safely back to the ground for a later attempt.

Right now the acorn-shaped nose of the Lightcraft is empty, but Myrabo hopes to fill the scaled-up, five-foot-wide model with electronics and turn the entire craft into a microsatellite once it reaches orbit. With some special optics, you’ll be able to use it as a telescope or for communications, he says. However, above about 18 miles, there’s not enough air to form the plasma, so the craft would have to switch to a reserve of liquid hydrogen fuel, which the laser would ignite to boost it into space.

Myrabo and Mead readily admit they have a lot of work to do before going from their current height of some 70 feet to 124 miles. One thing they’ll need is a far more powerful laser, about the size of a small house, which they now have unassembled in a warehouse. We’re trying to follow Goddard’s track, says Myrabo. About 70 years ago, Robert Goddard, the American rocket pioneer, spent more than a year trying to get a liquid-fuel rocket to go from 90 to 2,000 feet. If we match his rate, we should be able to put microsatellites into orbit in four and a half years.
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