Energy Conversion in Laser Propulsion
Abstract
Analysis of energy conversion in laser propulsion is reported and compared to experimental studies of a laboratory scale propulsion device that absorbs laser energy and converts that energy to propellant kinetic energy. The propellants studied were air and Delrin, a solid with the composition of formaldehyde H2CO that vaporizes cleanly upon laser irradiation. The Myrabo Laser Lightcraft (MLL) was studied. It 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. The results showed that between 30 and 50% of the incident laser energy is converted to propellant kinetic energy. This overall absorption/expansion efficiency was examined in terms of the thermodynamic predictions of conversion of propellant internal energy to propellant kinetic energy. Expansion of a propellant mass that was heated at constant volume was examined under conditions where either chemical equilibrium or frozen composition was maintained. For expansion with an effective area ratio of ^4, which is appropriate for the MLL, a maximum of 25 to 50% of the internal energy is predicted to be convertible to propellant kinetic energy, based on the minimization of the entropy gain of the blowdown process. With the small effective area ratio ^4, equilibrium expansion was only slightly more efficient than frozen expansion. Heating of propellant to highly ionized states resulted in lower efficiency energy conversion but higher exit velocity. The thermodynamic limitations are illustrated by process representations of blowdown in the Mollier plane (enthalpy vs entropy diagram for air). The analysis captures the equation of state of the partially ionized propellant under conditions of chemical equilibrium.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 01, 2001
- Accession Number
- ADA410649
Entities
People
- Carl William Larson
- F. B. Mead Jr.
Organizations
- Air Force Research Laboratory