Stefan Scharring

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.

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.

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.

Flight experiments on energy scaling for in-space laser propulsion

As a preparatory study on space‐borne laser propulsion, flight experiments with a parabolic thruster were carried out on an air cushion table. The thruster was mounted like a sail on a puck, allowing for laser‐driven motion in three degrees of freedom (3 DOF) in artificial weightlessness. Momentum coupling is derived from point explosion theory for various parabolic thruster geometries with respect to energy scaling issues. The experimental data are compared with theoretical predictions and with results from vertical free flights. Experimental results for the air‐breakdown threshold and POM ablation inside the thruster are compared with fluence data from beam propagation modeling.

Remotely Controlled Steering Gear For A Laser‐Driven Rocket With A Parabolic Thruster

A new steering concept for laser‐driven parabolic thrusters is presented. As an alternative to the well‐tried spin‐stabilization, our concept provides rocket stabilization and trajectory control and is suitable for the injection of a laser‐driven launcher into a planetary orbit: Angular and lateral momentum components are systematically applied to the rocket by specific variation of the ignition configuration inside the parabolic thruster. The effect of the tilting angle of the steering gear from the axis of symmetry on momentum coupling is examined, as well as the influences of pulse energy and ignition geometry. Based on the experimental results, an off‐axis detonation is modeled with respect to the fluence distribution at the ignition area and the resulting force components. A demonstrator model of a laser‐driven rocket with a parabolic thruster has been constructed, including a remotely controlled steering gear and a separate payload fraction. Test flights employing a high energy CO2 laser have been p...

Flight Analysis of a Parabolic Lightcraft—Ground‐based Launch

An experimental environment has been developed for free flight experiments with a parabolic lightcraft in a laboratory scale. An electron beam sustained CO2 laser is employed as source for energy beaming with 10.6 μm wavelength, ∼10 μs pulse duration, pulse energies up to 200 J and repetition rates up to 40 Hz. The free flight range of 1.7 meters enables to monitor several subsequent pulses in one flight as well as, in the case of Delrin (POM) as a propellant, a single pulse with a large momentum transfer. The impulse coupling is derived from flight trajectories and analyzed with respect to the temporal course of the flight. The influence of beam‐related parameters like pulse energy and repetition rate are discussed regarding the flight performance. Finally, an insight is given into actual work on the transformation of the testbed for flights in vacuum.

Spaceborne Lightcraft Applications – an Experimental Approach

An experimental approach is proposed for the near‐term demonstration of space‐borne laser propulsion. A feasibility study at the ZARM Drop Tower Bremen is planned. The facility provides microgravity conditions within a drop capsule for ∼9 seconds. An excimer laser is used for energy beaming operating at a wavelength of 248 nm with max. 500 mJ pulse energy and a repetition rate of 250 Hz. Within the drop capsule, free flights of a lightcraft are intended to be conducted in air as well as under vacuum conditions. Different propellants are reviewed regarding their features for propulsion with a UV laser. The scalability of previous ground‐based flight experiments is discussed with respect to microgravity conditions and moderate pulse energies. Space logistic and sample return missions are discussed as possible applications.