C. W. Larson

Energy conversion in laser propulsion III

Conversion of pulses of CO2 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/m3 as compared to STP air density of 1.18 kg/m3) these efficiencies increase to about 0.55 (equilibrium) and 0.45 (frozen). Optimum blowdown from 1.18 kg/m3 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.

Laser-Powered, Vertical Flight Experiments at the High Energy Laser System Test Facility

Abstract : In 1996, the Air Force Research Laboratorys Propulsion Division at Edwards AFB initiated a program that had as its main objective to launch a laser-propelled vehicle into a suborbital trajectory within a period of 5 years in order to demonstrate the concept and its attractive features. The concept is a nanosatellite in which the laser propulsion engine and satellite hardware are intimately shared. The Lightcraft Technology Demonstration Program was planned in three phases. Phase I, Lightcraft Concept Demonstration, was to demonstrate the feasibility of the basic concept. This phase ended in December 1998. Phase II, Lightcraft Vertical Launches to Extreme Altitudes, was initiated in January 1999, and is a five-year effort designed to extend Lightcraft flights in sounding rocket trajectories to 30 km with a 100 kW CO2 laser. Phase III, Lightcraft Dual Mode Vehicle, is a two year effort designed to launch the first laser-propelled vehicle, fully functional, into space. This phase will require the construction of a megawatt class CO2 laser with appropriate optics to meet the power beam propagation requirements.

Launching of Micro‐satellites Using Ground‐based High‐power Pulsed Lasers

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 CO2 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 CO2 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 CO2 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.

Launching of microsatellites using ground-based high-power pulsed lasers (Plenary Paper)

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 CO2 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 CO2 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 CO2 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.

Benefit of Constant Momentum Propulsion for Large Delta V Missions -- Applications in Laser Propulsion

Abstract : The authors show that perfect propulsion requires a constant momentum mission as a consequence of Newtons second law. Perfect propulsion occurs when the velocity of the propelled mass in the inertial frame of reference matches the velocity of the propellant jet in the rocket frame of reference. They compare constant momentum propulsion to constant specific impulse propulsion, which, for a given specification of the mission Delta V, has an optimum specific impulse that maximizes the propelled mass per unit jet kinetic energy investment. They also describe findings of more than 50% efficiency for conversion of laser energy into jet kinetic energy by ablation of solids.