Aerospace Technology Development

Riding the Highways of Light

A PROTOTYPE OF AN INNOVATIVE LASER-propelled transportation concept could someday result in space travel that is better, faster and cheaper for both Earth-bound transportation as well as space-bound flights. The Lightcraft—a small, disk-shaped, laser-propelled aircraft—virtually eliminates an on-board propellant. The absence of a propellant will allow the Lightcraft, whose revolutionary potential could parallel that of the Apollo program, to quickly and cheaply reach the speeds necessary for modern space travel. During the recent Advanced Propulsion Research Workshop held in Huntsville, Alabama, it was noted that a sophisticated descendant of Dr. Robert Goddard's first liquid propellant rocket—the Saturn V of Project Apollo—sent the first humans to the Moon just 43 years after Goddard's March 1926 launch.

Low cost, simplicity and responsiveness upon demand are the predominant reasons the Air Force Research Laboratory and NASA's Marshall Space Flight Center have joined together to research an innovative and promising method for reaching space. The evolution of ultra-lightweight high-temperature materials, dual-mode laser propulsion engines, powerful lasers and the opportunity to change science fiction into scientific fact are also driving forces in this effort.

Laser Lightcraft and their propulsion modes are a radical departure from current methods. If successful, this new energy beam propulsion technology will supplement rather than replace current manned and unmanned launch systems. "My goal has been to cut the cost of getting to space by a factor of 1,000 using a system that is completely green," explained Professor Leik Myrabo of Rensselaer Polytechnic Institute (RPI) in Troy, New York. Myrabo is the first under the sponsorship of the Laser Propulsion Program of the former Strategic Defense Initiative Organization to propose and develop the laser Lightcraft.

Called "Lightcraft" because it flies on a beam of light, the vehicle harnesses the energy of a laser beam and converts it into propulsive thrust. A parabolic mirror focuses the pulsed laser energy into a ring-shaped "absorption/propulsion" chamber. The atmosphere acts as a propellant as the focused laser light superheats the air to become a jet exhaust that pushes the craft up. With higher altitude, thinning air and a speed 5.5 times the speed of sound, the craft would use a small on-board supply of hydrogen heated by the remote laser beam.

Since 1972, Myrabo has been further developing a variation of an Arthur Kantrowitz idea: using lasers to launch satellites. The approach holds great promise for significantly reducing the launch costs of microsatellites that use today's chemical-fueled, combustion-powered rocket technology. Myrabo tried a few initial designs for NASA and then came up with the "toy top" design for ground-based lasers by reversing the laser optics of space-based lasers. With continued Air Force and NASA sponsorship, Myrabo has developed and test-flown a 15-centimeter (6-inch) diameter model of the toy top Lightcraft. In testing, a light shield erected by a crane is used to eliminate the chance of blinding an orbiting satellite's sensor.

Myrabo is now collaborating with Dr. Franklin Mead, of the Air Force Research Laboratory's advanced propulsion group, to conduct field tests to demonstrate how the craft can be propelled using available high-powered lasers. Further research may use a 100-kilowatt laser to boost a larger model and, eventually, a 1-gigawatt laser necessary to orbit satellites.

The giant leap could happen by the mid-21st century with another propellant concept. Requested by the Space Studies Institute in Princeton, New Jersey, Myrabo and his students are designing a similar craft using microwaves beamed from space. He and his students are also studying a microwave Lightcraft—an advanced derivative of a tiny, 25-gram craft that he is pushing around on a 10-kilowatt beam of infrared laser light in tests at White Sands Missile Range, New Mexico.

The concept is part airship, part microwave receiver and (smallest) part jet and rocket engine. It is powered from Earth by sunlight captured by an orbiting power station and the atmosphere heated by a laser as a propellant. Switching on the microwave transmitter would make the Lightcraft disappear in less than an eye blink. The microwaves would be focused by the internal reflector to heat the air on one side or the other of the craft and push it in the opposite direction.

Climbing a good altitude, beyond the speed of sound, where you use the magnetohydrodynamic drive, the craft tilts from flying edgewise to flying flat into the air stream, but for a reason. The microwaves are reflected forward to create a superhot bubble of air above the craft and form an air spike that acts as the nose cone as the Lightcraft accelerates to 25 times the speed of sound.

"This cleans up the aerodynamics of a vehicle that does not look like it should fly in that direction," Myrabo said. Even better, when the load is properly balanced, the craft sails through the air without leaving a shock wave and virtually no supersonic wake. Water is used by the craft to cool the rectennas and as a propellant in the last stages of ascent.

Myrabo points out that most of the technologies or principles have been demonstrated. "If successful, this will cut the cost of getting to space to whatever someone wants to charge for electricity from the orbiting power station," Myrabo said. "You could go halfway around the world in 45 minutes, or from the Earth to the Moon in about 5 1/2 hours." At the Moon, the Lightcraft would zoom down a series of ring-shaped electromagnets that would slow the craft, or it could accelerate another Lightcraft for the return to Earth.

For more information, contact David Harris at Marshall Space Flight Center.
Call: 256/544-0057, Fax: 256/544-2669, E-mail: David.C.Harris@msfc.nasa.gov
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