Time Dependent Measurements of Electron Temperature and Density in a Laser Lightcraft

Time dependent measurements of electron temperature and density in the plume of a Laser Lightcraft were taken with a triple Langmuir probe. In addition, instantaneous thrust measurements were obtained with a piezoelectric load cell. These data are compared with a computational aerodynamics model. While the cumulative coupling coefficients predicted by the computational model agree well with previous results, the predicted thrust width is four to five times shorter than the current data. The source of this discrepancy is not fully understood at this time. Deformation of the Lightcraft and the inertia of the thrust stand and model will broaden the pulse width in the experimental measurements, while uncertainty in the ignition volume effects the width of the pulse in the computational model. T n tr Q.dlLC.t JOJl. Beamed energy propulsion was first promoted by Kantrowitz . Since then, a propulsion system supported by a lasersustained plasma has been the subject of many studies. " The main advantage gained by laser propulsion is the low-weight system derived from decoupling the energy source from the vehicle, and high specific impulse resulting in low fuel consumption. The first ground and flight tests of a ground-based laser propelled vehicle were reported in 1998, in which active tracking and beam control were demonstrated to 122 m on a horizontal wire and spin-stabilized free flights in the laboratory were accomplished to altitude of 4 m. Six months later, the spinstabilized vertical free flights outdoors n ~ reached 30 m. These results were accomplished with axisymmetric vehicles which are of a special design in which a nosecone shaped forebody, an annular shroud, and a parabolic afterbody are the only major components. A vehicle of such design is hereby named as the "Laser Lightcraft" in this study. When the focused beam energy strikes the shroud of a Laser Lightcraft, free electrons form to invoke Inverse Bremsstrahlung (IB) and the optical breakdown ensues. In order to achieve maximum intensity and to avoid undue plasma resonance, repetitive short pulses are used. As such, repetitive Pulsed Detonation Waves (PDW) are generated to propel the vehicle, hence the Laser Lightcraft Copyright © 2001 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Government purposes. All other rights are reserved by the copyright owner. Staff, Senior Member AIAA American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. engine is also known as a PDW engine. The underlying physics involved in IB and PDW are complicated, and a simple system model is not capable of describing the phenomena, let alone predicting the thrust performance and the associated flow and thermal environments necessary for designing for structural integrity. A detailed computational plasma aerodynamics model is therefore needed for the design and scaling of future Laser Lightcrafts. From May to July 1999, performance data in the form of coupling coefficients were taken in a series of indoor tests for the Model #200 series vehicles. These test were performed under a cooperative effort between Rensselaer Polytechnic Institute, the Air Force Research Laboratory's Propulsion Directorate and NASA's Marshall Space Flight Center (MSFC), to enhance Laser Lightcraft propulsion research. These data were the first to serve as benchmark quality data for computational model validation for these systems. While these data validated the models ability to predict overall performance, validation of the physics requires time dependent measurement of properties and performance.