LASER PROPULSION: Craft heads for free flight and higher altitudes

Barring inclement weather, laser propulsion researchers were heading for the White Sands Missile Range (White Sands, NM) last month in their continuing quest to launch a small space vehicle into orbit in the next five years. The device currently under test, which in one particular design looks like a cross between an ice-cream cone and a flying disc, is called a "lightcraft" and it glides through the air on the beam of a ground-based, 10-kW, pulsed, carbon-dioxide laser (see Laser Focus World, March 1998, p. 91).

The cone portion of the lightcraft is actually an elongated parabolic mirror. The mirror reflects and focuses incident laser energy into the lower ring of the flying disk (an annular shroud), and the resulting 54,000°F temperature turns the air molecules into an explosive plasma that detonates out of the bottom of the lightcraft 28 times/second. Fresh air rushes back into the lightcraft through the "ice-cream" portion (an air-breathing engine inlet) to refresh the cavity between detonations (see photo).

Currently, the researchers envisage using a 100-kW-class carbon dioxide laser at the high-energy laser system test facility (HELSTF) in White Sands to propel a 230-lb, 4-ft lightcraft about six miles up to the edge of the atmosphere and a speed of about Mach 5. When operating in space, the air inlet would close, and the vehicle would switch over to an orbital rocket mode, supplying its own airflow as a sort of plasma fuel for satellite-supplied laser detonation, with the flying disk and cone acting as a conventional rocket thrust chamber.

Leik Myrabo, an associate professor of engineering physics at Rensselaer Polytechnic Institute (Troy, NY), started developing lightcraft in the late 1960s. He received government assistance, initially from the Ballistic Missile Defense Organization (formerly known as the Strategic Defense Initiative or "Star Wars") in 1986 and afterwards from the Air Force Research Laboratory (AFRL; Edwards Air Force Base, CA) and the NASA Marshall Space Flight Center (Huntsville, AL). The first lightcraft flight took place four years ago, and last year Myrabo, while on sabbatical from his university responsibilities, started Lightcraft Technologies, Inc. (LTI; Bennington, VT) to commercialize the technology.

Record set

Government researchers led by Franklin Mead, project manager for the AFRL Propulsion Directorate at Edwards, were setting out for White Sands last month with an altitude goal of 1000 feet. Their goal represented an order-of-magnitude improvement over the 128-foot record set by a 4-in., 1-oz, laser-propelled vehicle. The record was set toward the end of 140 previous lightcraft flights, which were intentionally limited in altitude by a black plywood backstop positioned over the test to prevent safety incidents due to laser illumination of overhead aircraft or satellites.

In addition to removing the backstop, the researchers planned to maintain a tighter beam over a longer distance by routing the beam through a hand-off mirror system on the ground, before sending it skyward to buoy the lightcraft. The Air Force Space Command will track the free flight experiments for the researchers, who will also have to time their bursts of propulsive laser light to stay within cleared airspace.

Concurrent with the development of the beamed-energy propulsion technology, the researchers are also developing miniature propulsion and guidance components in preparation for an envisaged time when lightcraft graduate into nano- and microsatellites. Beyond that, some of the researchers look to passenger-carrying space vehicles, perhaps in this century, powered by 100-GW lasers.

While developing such laser sources may take some doing, the fact that they will be ground based eliminates size and weight restrictions that might limit reliability or power. In an article published last spring, Myrabo was quoted likening the magnitude of the tasks ahead to the accomplishments of Robert Goddard with liquid-fueled rockets in the early twentieth century. "This is where we are now," said Myrabo, while pointing to a photograph of Goddard standing beside the first successful liquid-fueled rocket.