Riding the Highways of Light
1999: It looks like another fine product of Area 51, and it really is shaped like a
flying disc, and would even fly like one. If it works, the family tree will trace back not
to Roswell, New Mexico, but Troy, New York.
"It came out of a trans-atmospheric vehicle design course at Rensselaer Polytechnic Institute," which is in Troy, explained Prof. Leik Myrabo.
The microwave Lightcraft being studied by Myrabo and his students is shaped that way because that's how the physics works. It's 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, N.M.
Myrabo discussed his work last week during the Advanced Propulsion Research Workshop held in Huntsville.
"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," he explained of his passion for the past three decades. Since 1972, he has been building on an idea developed by Arthur Kantrowitz to use lasers to launch satellites. Myrabo introduced a variation using the atmosphere as the propellant heated by a laser. At higher altitude and at 5.5 times the speed of sound, as the air thins, the craft would use a small supply of on board hydrogen, still heated by the remote laser beam.
Myrabo's initial design for NASA was a 5-meter (16.5 ft) diameter, four-person craft - "ma and pa in the front, two kids and a dog in the back" - in a shape that he dubbed Acorn. The front is shaped to reflect the coherent laser light into a narrow region between the body and a shroud. The focused light superheats the air to become a jet exhaust that pushes the craft up. In this case, the laser power station would be based in orbit.
He next worked on the Toy Top design which reversed the optics for lasers based on the ground. The Strategic Defense Initiative Organization was interested in this approach for rapid launches of satellites weighing around 100 kg (220 lbs).
Under continued Air Force and NASA sponsorship, Myrabo has developed and test flown a 15 cm (6 inch) diameter model of the Toy Top Lightcraft.
"We just passed Goddard's second flight of 92 feet (28 meters)," Myrabo said. That limit, though, is set at 120 ft (37 m) by a light shield erected by a crane to stop the light beam and eliminate the chance of blinding a satellite sensor.
The next step is to develop a 150 kilowatt laser that would boost a larger model to 30 km (18 mi) altitude. Eventually, a 1 gigawatt laser would be needed to orbit satellites as Kantrowitz, Myrabo, and others have long envisioned.
It's possible, though, that it all might be supplanted by mid-21st century by the microwave Lightcraft. When word of his work got around to the Space Studies Institute in Princeton, Myrabo was asked if he could design a similar craft that used microwaves beamed from space since microwave transmitters were a more mature technology than lasers.
Myrabo (right) watches as students at Rensselaer run a computer simulation for the air spike wind tunnel test on the Lightcraft model held by the student at left. (RPI photo)
The concept that evolved is a part airship, microwave receiver, and (the smallest part) jet and rocket engine, and as green as any space concept. The 12-person, 20-meter (66 ft) craft would be powered from the Earth's surface to the Moon by sunlight captured by an orbiting power station (1 km diameter, 20 GW power), converted to microwaves, and beamed to rectennas (rectifying antennas) that turn it back into electricity on the Lightcraft. That's where the saucer shape comes from.
The airship part is a pressurized helium balloon-type structure made of advanced silicon carbide film (transparent to microwaves) to make the craft partly buoyant and to provide for a large parabolic reflector for the energy beamed from space. The craft would be encircled by two superconducting magnet rings and a series of ion engines, and topped with solar cells.
At launch, the Lightcraft would use electricity from its solar cells (powered by an infrared space-based laser at night) to ionize the air and move the craft through electrostatic discharges. The craft could move at 80 to 160 km/h (50-100 mph).
That's just low gear. 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.
"This is used to climb out to a good altitude and beyond the speed of sound where you use the magnetohydrodynamic drive," Myrabo continued. Now the craft tilts from flying edgewise to flying flat into the air stream. That seems wrong but for another trick. 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.
At least initially, during the prototype phase, it won't be for everyone, just NASA and military test pilots. The hyper-energetic performance will require that the crew ride in liquid-filled escape pods to protect them from g-forces greater than even fighter pilots occasionally endure. In some Air Force Space Command schemes, the crew would breath an oxygenated fluid to protect their lungs.
It all sounds a bit too much like science fiction, but Myrabo points out that most of the technologies or principles have been demonstrated. Faculty and students at Rensselaer have demonstrated the MHD slipstream accelerator and the air spike concept in a high-speed wind tunnel, and will test new models of other parts of the propulsion system later this year.
"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 could accelerate another Lightcraft for the return to Earth.
"This would require a fully mature infrastructure to support these vehicles," Myrabo said. But it could bring about an era of airline-like space travel on "highways of Light."
More information about the Lightcraft program, including additional images and Quicktime movies, are available at the Lightcraft web site at RPI.