A combination of techniques including launch ballistics, force sensing, and time-resolved ICCD imaging was applied to the study of the mechanisms of liquid ablation in the irradiance regimes from 10<sup>6</sup>-10<sup>8</sup> W/cm<sup>2</sup>. A TEA CO<sub>2</sub> laser (λ = 10.6 μm), 300 ns pulse width and 9 J pulse energy, was used for ablation of liquids contained in various quartz glass containers in order to examine dependencies on surface tension, absorption depth, <i>etc</i>. Dominant mechanisms of force generation were analyzed in order to determine their characteristics, and the evolution of the liquid surface was studied in depth. Net imparted impulse and coupling coefficient were derived from the force sensor data and ballistics experiments, and relevant results will be presented for various container designs and liquids used. The key differences between surface and volume absorbing liquids was observed. Various mechanisms including plasma formation, vaporization, bulk liquid flow, <i>etc</i>. will be critically examined and their relevance to force generation and propulsion will be determined.
This paper reviews the basic concepts of laser propulsion and summarizes work done to date using a 10 kW device. The paper describes a candidate megawatt class CO<sub>2</sub> laser system which can be scaled relatively near-term to multi-megawatt power levels using demonstrated technology. Such a system would potentially be capable of launching micro-satellites into low earth orbits (LEO) at relatively low cost. Our projections indicate that payloads of about 1kg/megawatt are achievable. The long wavelength of a CO<sub>2</sub> laser will require the use of a large aperture telescope and/or large effective beam capture area for the lift vehicle. We believe that these limitations, not withstanding, rep-pulsed CO<sub>2</sub> in a blow-down configuration lasting 200-300 seconds could achieve the desired propulsion objectives. The laser would use a helium-free, nitrogen/carbon dioxide mixture to provide a very cost effective fuel.
Laser-powered lightcraft systems that deliver microsatellites to low earth orbit have been studied for the Air Force Research Laboratory. One result of this study has been discovery of the significant influence of laser wavelength on the power lost during laser beam propagation through Earth’s atmosphere and in space. Here, energy and power losses in the laser beam are extremely sensitive to wavelength for earth-to-orbit missions. And 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.
Conversion of pulses of CO<SUB>2</SUB> laser energy (18 microsecond pulses) to propellant kinetic energy was studied in a Myrabo Laser Lightcraft (MLL) operating with laser heated STP air and laser ablated delrin propellants. 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 subsequently expands in the geometry of the plug nozzle aerospike. With Delrin propellant, measurements of the coupling coefficients and the ablated mass as a function of laser pulse energy showed that the efficiency of conversion of laser energy to propellant kinetic energy was approximately 54%. With STP air, direct experimental measurement efficiency was not possible because the propellant mass associated with measured coupling coefficients was not known. 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 approximately 0.30 for EQUILIBRIUM expansion to 1 bar pressure. For FROZEN expansion (alpha) approximately 0.27. These upper limit efficiencies are nearly independent of the initial specific energy from 1 to 110 MJ/kg. With heating of air at its Mach 5 stagnation density (5.9 kg/m<SUP>3</SUP> as compared to STP air density of 1.18 kg/m<SUP>3</SUP>) these efficiencies increase to about 0.55 (equilibrium) and 0.45 (frozen). Optimum blowdown from 1.18 kg/m<SUP>3</SUP> to 1 bar occurs with expansion ratios approximately 1.5 to 4 as internal energy increases from 1 to 100 MJ/kg. Optimum expansion from the higher density state requires larger expansion ratios, 8 to 32. 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.