C. William Larson

Methodology and historical perspective of a hall thruster efficiency analysis

to ionization processes and losses that manifest as Joule heating, and contains no information about the vector properties of the jet. Propellant efficiency incorporates losses from dispersion in the jet composition and is unity for 100% ionization to a single ion species. The effect of neutrals on dispersion of the jet velocity distribution function in propellant efficiency is introduced in the neutral-gain utilization. The beam efficiency accounts for divergence of the jet and is ideal when the ion velocity vectors are parallel to the thrust axis. Plume divergence is defined as a momentum-weighted term, and the approximation as a charge-weighted term is characterized. The efficiency architecture is derived from first principles and is applicable to all propulsion employing electrostatic acceleration, including Hall thrusters and ion thrusters. Distinctions and similarities to several past methodologies are discussed, including past ion thruster analyses, early Russian performance studies, and contemporary architectures. To illustratethepotentialforenhancedunderstandingoflossmechanismsandionizationprocesseswithanarrayoffarfield plume diagnostics, a case study is presented of low-discharge voltage operation from a 6 kW laboratory Hall thruster.

Laser Propulsion and the Constant Momentum Mission

We show that perfect propulsion requires a constant momentum mission, as a consequence of Newton’s 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. We compare constant momentum 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. We also describe findings of more than 50 % efficiency for conversion of laser energy into jet kinetic energy by ablation of solids.

Vibrational spectrum of B2C in argon at 10 K

The vibrational spectrum of B2C has been observed for the first time in solid argon by Fourier-transform infrared spectroscopy. Absorption frequencies of the three boron isotopomers of cyclic symmetric B2C, previously deduced by ab initio theory, are in accord with measured frequencies, which enables assignment of an absorption at 1392.8 cm−1 to the ν1(a1) fundamental of B2C(1A1).

Comparison of Ablation Performance in Laser Lightcraft and Standardized Mini‐Thruster

Experiments on laser ablation of black and white Delrin® with a 10.6‐micron laser beam from a CO2 electric discharge laser are reported. Mass ablation and thrust generation (impulse) were accurately measured as a function of input laser energy in single‐shot experiments. The efficiency of conversion of laser energy to jet kinetic energy depended on the geometry of the energy absorption/conversion zone. The standard geometry, an axisymmetric mini‐thruster with 13‐degree conical half angle and 8:1 expansion ratio, produced ∼65 % conversion efficiency. The extensively‐studied 10‐cm focal diameter Lightcraft (with inverted paraboloid, plug‐nozzle geometry) produced ∼50% conversion efficiency. The upper limit to energy conversion was theoretically computed with a one‐dimensional chemical equilibrium code to be 73% for the well‐defined mini‐thruster geometry. Thus, total losses amount to ∼8% in the mini thruster and ∼23% in the Lightcraft. In these experiments a near‐exact match of coupling coefficients, ∼1%, w...

Laser-Driven Mini-Thrusters

Laser‐driven mini‐thrusters were studied using Delrin® and PVC (Delrin® is a registered trademark of DuPont) as propellants. TEA CO2 laser (λ = 10.6 μm) was used as a driving laser. Coupling coefficients were deduced from two independent techniques: force‐time curves measured with a piezoelectric sensor and ballistic pendulum. Time‐resolved ICCD images of the expanding plasma and combustion products were analyzed in order to determine the main process that generates the thrust. The measurements were also performed in a nitrogen atmosphere in order to test the combustion effects on thrust. A pinhole transmission experiment was performed for the study of the cut‐off time when the ablation/air breakdown plasma becomes opaque to the incoming laser pulse.

An Overview of the Experimental 50‐cm Laser Ramjet (X‐50LR) Program

In January 2001, the X‐50LR program was initiated to scale the Lightcraft concept ultimately to a 50‐cm focal diameter, and to launch a 50 cm, fully functional vehicle, into space in either a sounding rocket or suborbital trajectory by the end of FY 2009. The current work involves scaling from the 10‐cm aluminum Lightcraft to a fully composite 25‐cm laser ramjet vehicle (X‐25LR). An overview and status of this program will be given in terms of the various efforts that support this development. These efforts will include testing at the High Energy Laser System Test Facility (HELSTF), New Mexico; some results of the laser launch system study by Flight Unlimited; supporting research by the Air Force Office of Scientific Research (AFOSR); the different facets of attitude control in a small business program with Polaris Sensors Technology; continuing development of a launch model at The Pennsylvania State University; and, the development of a thrust measurement technique, and the use of a “mini‐thruster” for r...

Ablation of Liquids for Laser Propulsion With TEA CO2 Laser

Time‐resolved force sensing and intensified charge‐coupled device (ICCD) imaging techniques were applied to the study of the force generation mechanism for laser ablation of liquids. A Transversely Excited at Atmospheric pressure (TEA) CO2 laser operated at 10.6 μm, 300 ns pulse width, and 9 J pulse energy was used to ablate liquids contained in various aluminum and glass vessels. Net imparted impulse and coupling coefficient were derived from the force sensor data and relevant results will be presented for various container designs and liquids used. ICCD imaging was used in conjunction with the dynamic force techniques to examine dependencies on absorption depth, irradiance, surface curvature, and container geometry. ICCD imaging was also used to determine whether surface or volume absorption should be preferable for laser propulsion using liquid propellants. Finally, ballistic experiments were conducted in order to verify the dynamic force data and lend additional evidence as to the predominant methods of force generation.

Trajectory Simulations, Qualitative Analyses and Parametric Studies of a Laser-Launched Micro-Satellite Using OTIS

This paper will discuss Air Force Research Laboratory (AFRL) Propulsion Directorate’s theoretical and computational results regarding trajectory simulations, qualitative analyses and parametric studies of a 25‐cm focal diameter Laser Ramjet (X‐25LR) using Optimal Trajectories by Implicit Simulation (OTIS). OTIS has been used to produce an optimized trajectory simulation of a laser ramjet’s flight to Low Earth Orbit (LEO). The baseline case that has been simulated is a flight vehicle powered by a 1 MW, 10.6 μm, CO2, ground‐based laser (GBL) with an initial power capture of 82%. The fuel that is used during rocket flight is Delrin® doped with energetic additives to increase the coupling coefficient and thrust by a factor of five. Additionally, a nozzle extension was considered which increased performance by 40%. The flight trajectory was separated into three phases: 1) Air‐breathing ramjet flight to a specified altitude of ∼30 km and Mach number of ∼10; 2) Rocket powered flight into a trajectory with a final Mach number ∼27; and 3) Un‐powered coasting flight to the final altitude of 185 km. Additional sounding rocket trajectory flights with 10‐kilowatt class CO2 lasers have been assessed for a variety of laser powers. Also to be discussed in this paper are the parametric trade studies of the rocket phase comparing high thrust vs. low thrust and the effects of different‐size vehicles.

Absorption‐Enhanced Liquid Ablatants for Propulsion with TEA CO2 Laser

A technique based on measuring force as a function of time on laser‐ablated targets was applied to the study of water solutions of Sodium Tetrafluoroborate [NaBF4], a compound that strongly absorbs radiation in the 8–11 μm wavelength interval. A TEA CO2 laser of wavelength 10.6 μm, 300 ns pulsewidth and 8 J pulse energy, was used for ablation of water diluted NaBF4 contained in a conical aluminum nozzle. Net imparted impulse and coupling coefficient were derived from the force sensor data and are reported below.

Study to Determine the Effectiveness and Cost of A Laser‐Powered “Lightcraft” Vehicle System—Results to Guide Future Developments

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.