Hans-Albert Eckel

Review: Laser-Ablation Propulsion

LASER ablation propulsion (LAP) is a major new electric propulsion concept with a 35-year history. In LAP, an intense laser beam [pulsed or continuous wave (CW)] strikes a condensedmatter surface (solid or liquid) and produces a jet of vapor or plasma. Just as in a chemical rocket, thrust is produced by the resulting reaction force on the surface. Spacecraft and other objects can be propelled in this way. In some circumstances, there are advantages for this technique compared with other chemical and electric propulsion schemes. It is difficult to make a performance metric for LAP, because only a few of its applications are beyond the research phase and because it can be applied in widely different circumstances that would require entirely different metrics. These applications range from milliwatt-average-power satellite attitude-correction thrusters through kilowatt-average-power systems for reentering near-Earth space debris and megawatt-to-gigawatt systems for direct launch to lowEarth orbit (LEO). We assume an electric laser rather than a gas-dynamic or chemical laser driving the ablation, to emphasize the performance as an electric thruster. How is it possible for moderate laser electrical efficiency to givevery high electrical efficiency? Because laser energy can be used to drive an exothermic reaction in the target material controlled by the laser input, and electrical efficiency only measures the ratio of exhaust power to electrical power. This distinction may seem artificial, but electrical efficiency is a key parameter for space applications, in which electrical power is at a premium. The laser system involved in LAP may be remote from the propelled object (on another spacecraft or planet-based), for example, in laser-induced space-debris reentry or payload launch to low planetary orbit. In other applications (e.g., the laser–plasma microthruster that we will describe), a lightweight laser is part of the propulsion engine onboard the spacecraft.

Lightcraft experiments in Germany

Vertical flight and pendulum experiments have been carried out with a simple paraboloid type lightcraft in the air-breathing mode. Pulsed laser energy of up to 240 J/pulse was delivered from a highly reproducible e-beam sustained CO2-laser at repetition rates up to 45 Hz. The lightcraft mass was varied in the range between 22 and 55 g. An average thrust of 1.1 N has been derived from the flight data and the highest impulse coupling coefficient found in the pendulum experiments was 33.3(DOT)10-5 Ns/J. A double shock wave was detected that leaves the thruster exit and an attempt was made to model the thrust, using a modification of Sedovs similarity solution for a blast wave. Finally, the propulsion requirements for the launch of a 10 kg mass into low Earth orbit are presented.

Comparative lightcraft impulse measurements

The impulse coupling coefficients of two radically different laser propulsion thruster concepts (lightcrafts), each 10 cm in diameter, have been measured under equal conditions using two different pendulum test stands. One test stand and one lightcraft of toroidal shape were provided by the U.S. Air Force Research Laboratory. The other test stand and a bell shaped (i.e. a paraboloid) lightcraft were those of the German Aerospace Center (DLR). All experiments employed the DLR electron-beam sustained, pulsed CO2 laser with pulse energies up to 400 J. The laser was operated with two configurations: 1) a stable resonator (flat beam profile); and, 2) an unstable resonator (ring shaped beam profile). A first series of experiments was carried out in the open laboratory environment. Propellant, therefore, was either the surrounding air alone, or Delrin as an added solid propellant. The coupling coefficient was determined as a function of the laser pulse energy. In a second series, the same experiments were repeated at various reduced pressure levels with the German lightcraft suspended in a vacuum vessel. This simulates the conditions of a transitional flight from within the atmosphere to outer space. As an additional parameter the specific mass consumption of Delrin (gram/Joule) was measured for each parameter set, allowing the determination of the average exhaust velocity in vacuum.

Measurement Issues In Pulsed Laser Propulsion

Various measurement techniques have been used throughout the over 40‐year history of laser propulsion. Often, these approaches suffered from inconsistencies in definitions of the key parameters that define the physics of laser ablation impulse generation. Such parameters include, but are not limited to the pulse energy, spot area, imparted impulse, and ablated mass. The limits and characteristics of common measurement techniques in each of these areas will be explored as they relate to laser propulsion. The idea of establishing some standardization system for laser propulsion data is introduced in this paper, so that reported results may be considered and studied by the general community with more certain understanding of particular merits and limitations. In particular, it is the intention to propose a minimum set of requirements a literature study should meet. Some international standards for measurements are already published, but modifications or revisions of such standards may be necessary for applic...

High speed analysis of free flights with a parabolic thruster

A laser‐based rangefinder with high temporal resolution, synchronized with a laser burst, is employed for fast on‐site analysis of pulsed free flights. Additional high speed recordings from two different angles of view allow for full 3D‐reconstruction of the trajectory and calibration of the rangefinder data. This reveals the whole dynamics of the flyer including the lateral and angular impulse coupling components as well as information on the detonation process. The employment of an ignition pin enhances the reproducibility of the momentum coupling due to a more reliable plasma ignition during the flight. The impact of initial lateral offset is studied and shows beam‐riding properties of the parabolic craft within a small range. Back‐driving forces are derived and compared with the theoretical model. The flight stability is evaluated with respect to the minimization and compensation of the lateral and angular momentum in a hovering experiment. Stable laser acceleration ranges up to 3 m altitude. Ballisti...

Beam-Riding of a Parabolic Laser Lightcraft

The impulse coupling characteristics of a parabolic laser-driven thruster (‘lightcraft’) are investigated in free flight experiments using a pulsed CO2 high energy laser. The analysis of 3D high speed recordings reveals lateral force components as well as angular momentum re-orientating the lightcraft towards the laser beam in the case of slight misalignment. Beam-riding properties are examined with respect to the initial lateral offset at the launch position. The results are compared with model data derived from raytracing analysis of the intensity distribution on the surface of an ignition pin which is located on the lightcraft’s symmetry axis. Based on model data, beam-riding abilities are characterized with respect to initial offset and inclination by means of Julia sets. The parameter space of tolerable misalignment is explored with respect to laser burst parameters and compared with experimental data.

Experimental Determination of the Impulse Coupling Coefficient - Standardization Issues

In research on beamed energy propulsion, the momentum coupling coefficient cm is a central figure of merit to characterize a propulsion system. The determination of cm is based on the measurement of imparted impulse and laser pulse energy. Nevertheless, the knowledge of laser pulse length, laser spot area and ablated mass is of great importance for the comparability of experimental results in laser ablative propulsion. The use of a great variety of measurement techniques for these parameters throughout the scientific community implies the risk of misunderstandings and might impede the comparability of results. In this paper, we present critical issues concerning the measurement of the aforementioned key parameters with respect to possible standardization issues. As an example, a simple laser propulsion experiment will be presented and compared with an experimental model from a different research group.

Ablation Performance Experiments With Metal Seeded Polymers

The specific impulse of plain polymers has been found too low for application in pulsed laser propulsion for single stage to orbit flights. Therefore, ablation tests with polymers, seeded with Al and Mg powder in various concentrations to reduce the absorption depth, have been conducted with CO2 pulses up to 280 J and ∼12 μs pulse length to measure the coupling coefficient and specific impulse in air and in vacuum. A large and increasing loss of pulse energy, presumably deposited in a decoupled absorption wave, has been found for increasing laser pulse energy. This loss prevents the achievement of better results compared to unseeded material.

High-power CO-overtone laser

A modified electron beam controlled pulsed CO2 laser is used as a multi spectral multi purpose test bed in order to generate high power fundamental and first overtone laser transitions in CO. The revisited concept includes an all solid state power supply which provides a highly reproducible operation at pulse repetition frequencies of up to 100 Hz. The active gas mixture is recirculated in a closed loop and kept at near room temperature using conventional water cooling. Discrimination of the CO fundamental band is obtained by using specially coated dielectric mirrors and introducing additional intracavity diaphragms. Unprecedented laser pulse energies of 25 J are reported in the overtone transitions covering a spectral range between 2 micrometers and 3.5 micrometers . Further scaling of pulse energies is expected in the near future using larger diameter resonator mirrors.

Laser-based removal of irregularly shaped space debris

Abstract. While the feasibility of laser space debris removal by high energy lasers has been shown in concept studies and laboratory proofs of principle, we address the question of the effectiveness and responsibility associated with this technique. The large variety of debris shapes poses a challenge for predicting amount and direction of the impulse imparted to the target. We present a numerical code that considers variation of fluence throughout the target surface with respect to the resulting local momentum coupling. Simple targets as well as an example for realistic space debris are investigated with respect to momentum generation. The predictability of the imparted momentum is analyzed in a Monte Carlo study. It was found that slight variations of the initial debris position and orientation may yield large differences of the modified trajectories. We identify highly cooperative targets, e.g., spheres, as well as targets that are strongly sensitive to orientation, e.g., plates, and exhibit a poor performance in laser debris removal. Despite limited predictability for the motion of a particular debris object, the laser-based approach appears to be suitable for space debris removal, albeit not with a deterministic but rather with a probabilistic treatment of the resulting trajectory modifications.