<title>Fission-activated laser as primary power for CW laser propulsion</title>

Abstract

Recent advances in the development ofreactor-pumped lasers (RPL's) have stimulated renewed interest in the concept of laser-powered propulsion. This paper surveys a number oflaser propulsion concepts and identifies the ooe that is most promising from the standpoint ofpracticality. It is proposed that a ground-based FALCON (Fission-Activated Laser CONcept) RPL can provide primary power for this launch vehicle design. The laser-vehicle system could launch small payloads into low-earth orbit (LEO) with high repetition rates and at low costs per kilogram. For the favored design, thruster efficiencies are currently estimated to be about 50% with 80% being seen as a potentially realizable goal after further design refinements Laser launch system simulations indicate that, with a buy-in laser power of 10 MW, it will be possible to obtain specific impulses in the range of600 to 800 seconds and payload-to-power ratios of 1 to 3 kg/MW. RPL PROPULSION FOR LEO MISSIONSThe concept ofRPL-propulsion to LEO is illustrated in Figure 1. A ground-based laser (GBL) system is used to beam radiant power to an ascending launch vehicle. The laser beam is focused onto the launch vehicle's heat exchanger (HX) with the use ofa steerable beam director having a deformable mirror. The mirror has the capability ofpre-distorting the beam so as to compensate for wavefront distortions caused by atmospheric turbulence. Real-time control signals for the mirror are provided by an adaptive optics unit which uses a low-power laser beam to sense the degree ofturbulence and distortiorL With these corrections, the laser beam incident upon the vehicle's HX is nearly diffraction-limited.A number oflaser-powered propulsion system designs have been proposed during the last decades. Many ofthese are variations on the basic continuous-wave (CW) direct-heating thruster design, which will be described later.14 A more recent advance in 1aserwered vehicle design has been developed and promoted by J. T. Kare at Lawrence Livermore National g9-Iz The Kare design was used to obtain the performance parameters displayed in Figure 1.The design by Kare uses a CW laser source, a metallic HX and liquid-hydrogen propellant This so-called CW/HX launch vehicle can provide specific impulses of600 to 800 seconds, which is well above that Obtained with conventional chemical thrusters. The overall thruster efficiency is estimated to be about 50%, but it may be possible to achieve 80% efficiency after additional design refinements. The ratio ofdeliverable payload mass to laser power is about 1 to 3 kg/MW.913The GBL system design depicted in Figure 1 assumes a CW4aser power output of 10 MW, which is the estimated buy-in power for such systems. '3yl having masses in the range of 10 kg and greater are of importance, and have acquired the descriptive name "microsat". With a 10-MW laser it would be possible to cost-effectively deliver very large total payloads to LEO as sequences of small payloads having masses of about 10 kg and greater. System simulation results indicate that the Kare...