Energy Conversion in Laser Propulsion II
Abstract
This paper reports on an analysis of overall energy conversion in laser propulsion. Experimental studies of a laboratory-scale propulsion device that absorbs laser energy and converts that energy to propellant kinetic energy were carried out. The Myrabo Laser Lightcraft (MLL), propelled by laser-heated air, was studied. The MLL incorporates an inverted parabolic reflector that focuses laser energy into a toroidal volume where it is absorbed by a unit of propellant mass that is subsequently expanded in the geometry of the plug nozzle aerospike. Thermodynamics predicted that the upper limit of the efficiency of conversion of the internal energy of laser heated air to jet kinetic energy, alpha, is approx. 0.30 for EQUILIBRIUM expansion to 1 bar pressure. The analysis captures the equation of state of partially ionized air under conditions of chemical equilibrium. This upper limit alpha is nearly independent of the specific internal energy between 1 and 100 MJ/kilograms, or temperature from 2000 to 24000 K at a density of 1.18 kilograms/cu m. The upper-limit efficiency for optimum FROZEN expansion of laser-heated air is alpha = 0.27. With heating of air at its Mach 5 stagnation density (5.9 kilograms/cu m as compared to Standard Temperature and Pressure (STP) air density of 1.18 kilograms/cu m) these efficiencies increase to about 0.55 (equilibrium) and 0.45 (frozen). Optimum blowdown from 1.18 kilograms/cu m to 1 bar occurs with expansion ratios from approx. 1.5 to 4 as internal energy decreases from 1 to 100 MJ/kilograms. Heating of Mach 5 air at stagnation density requires larger expansion ratios, approx. 8 to 32, for optimum expansion to 1 bar. Expansion of laser-ablated Delrin propellant appears to convert the absorbed laser energy more efficiently to jet kinetic energy because the effective density of the ablated gaseous Delrin is significantly greater than that of STP air.
Document Details
- Document Type
- Technical Report
- Publication Date
- Dec 26, 2001
- Accession Number
- ADA410873
Entities
People
- Carl William Larson
- Franklin B. Mead Jr.
- Wayne M. Kalliomaa
Organizations
- Air Force Research Laboratory