Analysis of pulsed laser-generated impulse in an advanced airbreathing thruster.

摘要

This thesis describes the study of an advanced beam-powered propulsion system, called an External Radiation-Heated (ERH) thruster. The repetitively-pulsed, airbreathing engine develops thrust by expanding high pressure, radiation-heated gas over an annular shroud surface. The blast waves are generated by laser radiation heating of air using Laser Supported Detonation (LSD) waves.; The phenomenology of LSD waves will be described in detail, as will the blast waves and resultant impulse they produce. Analytical simulation of the ERH thruster is accomplished with a one-dimensional model of blast waves propagating uniformly and radially outward from a laser-generated "line source" of high pressure, high temperature gas. Cylindrical blast wave scaling relationships developed by Sedov are employed in this model. The possibility of including other physical phenomena (e.g., viscosity, radiation, conduction or real gas effects) in the analysis will be reviewed.; The analyses for the ERH thruster model are performed for a sample vehicle point design. This vehicle, known as the "Lightcraft Technology Demonstrator" (LTD), may be constructed within the next five years to illustrate the potential of Earth-to-Orbit laser propulsion. The external flow over the LTD vehicle was analyzed to determine basic drag characteristics, inlet total pressure recovery and captured air mass flow rate--all projected as functions of flight Mach number and altitude.; The ERH thruster performance analysis indicates that the optimum LTD inlet air gap is about 3 cm around the 100 cm diameter centerbody, for transonic "refresh" air flow over the impulse surface. In this analysis, the principal indicator used to predict engine performance was the "impulse coupling coefficient (CC)"; i.e., the thrust developed per unit laser power input. Coupling coefficients up to 600-700 Newtons/Megawatt were found to be feasible, which are an order of magnitude larger than those for laser-heated rockets. For maximum developed thrust, laser pulse energies fell in the range of 25-50 kilojoules for pulse repetition frequencies of 1-10 kilohertz. The associated laser pulse durations ranged from 10 to several hundred nanoseconds. In the CC performance sensitivity studies, incident laser intensity was varied from 5 {dollar}times{dollar} 10{dollar}sp8{dollar} to 10{dollar}sp{lcub}10{rcub}{dollar} W/cm{dollar}sp2{dollar}. The promising results reported for the ERH thruster in this thesis support the case for continuing research on this advanced airbreathing engine.