A study of lightcraft propulsion systems in general has led to the conception and analysis of such a system for launching a small (having a mass no more than a few kilograms) satellite into a low orbit around the Earth. This study built on theoretical and experimental investigations of the feasibility of lightcraft, performed by a number of researchers during the past two decades. The word “lightcraft” signifies an aircraft or spacecraft that derives its propulsive energy from a laser beam aimed toward it from an external platform that, for the purpose of the present study, would be a ground station.

The conceptual lightcraft would operate in two different propulsion modes during successive phases of flight: Immediately after launch and during flight through the lower atmosphere, it would operate in an air-breathing (detonation- wave) mode, in which the laser beam would be used to heat ingested air to make the air expand in a rear-facing nozzle and thereby generate thrust. Once it exceeded a speed of about Mach 5 and an altitude of about 30 km, the lightcraft would operate in a rocket mode, in which the laser beam would be used to generate thrust through heating of a propellant material stored on board.

This Conceptual Lightcraft would be a single-stage launch vehicle thatwould derive its propulsive energy from a laser beam incident from therear. It would operate in an air-breathing mode in the altitude range up toabout 30 km, then in a rocket mode at higher altitude.

Viewed from the side, the conceptual lightcraft would resemble an acorn (see figure). The forebody would have an approximately conical shape designed to effect compression of incident air during operation in the air-breathing mode. An annular cowl surrounding the wide end of the forebody would constitute the outer wall of a ring-shaped air inlet and laser-energy-absorption/ propulsion chamber. The aft body would have a paraboloidal mirror outer surface that would serve as the primary optic for reception of the laser beam and as an external plug nozzle expansion surface. The aft-body mirror would focus the laser beam into the ring-shaped chamber. In the air-breathing mode, the laser beam would heat the compressed air, and the resulting rearward expansion of the air would generate thrust. In the rocket mode, the air inlet would be closed.

The analysis performed in the present study included consideration, not only of the lightcraft proper, but also of the ground-based laser system and other supporting systems. Major attention was given to the incident laser power needed for propulsion. In taking account of the degrading effects of distance and propagation through the atmosphere upon the laser beam, it was found that energy and power losses are extremely sensitive to the laser wavelength and that this significantly affects the amount of mass that can be placed into orbit for a given maximum amount of radiated power from a ground based laser. In a comprehensive analysis of costs associated with development and operation of the conceptual lightcraft propulsion system, it was estimated the cost of launching into orbit would lie between $20 and $200 per kilogram of payload — of the order of a thousandth to a hundredth of the standard space-launch industry cost per kilogram of payload launched by use of conventional chemical rocket propulsion systems.

This work was done by Franklin B. Mead, Jr. of the Air Force Research Laboratory and Eric W. Davis of the Institute for Advanced Studies at Austin. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Mechanics/Machinery category. AFRL-0079

This Brief includes a Technical Support Package (TSP).
Lightcraft Propulsion for Launching a Small Satellite

(reference AFRL-0079) is currently available for download from the TSP library.

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This article first appeared in the February, 2010 issue of Defense Tech Briefs Magazine.

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