Wolfgang O. Schall

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.

Laser Propulsion Activities in Germany

Activities related to laser supported propulsion concentrate on investigations of the fundamental phenomena arising from the interaction of high‐power CO2 laser pulses with a simple bell engine. Breakdown dynamics, plasma, and lightcraft accelerations are carefully measured using optical and laser diagnostics. In a first‐order approach the expanding plasma can be described by the point explosion model with counterpressure. Wire guided vertical flight experiments in the laboratory have been undertaken and analyzed. Comparative impulse measurements in ambient air and ablated material are presented for a series of experiments performed either at atmospheric pressure or at reduced pressures down to vacuum. Impulse coupling coefficients, average exhaust velocities,specific impulse, and jet efficiencies are derived from the experimental data. The repetitively‐pulsed CO2 laser device used in all experiments shows a potential of achieving 50 kW average power. Finally, long‐term perspectives of laser propulsion wi...

Power extraction investigations for a 10-kW-class supersonic COIL

In a stable resonator configuration, the output power and the power density distribution of a chemical oxygen-iodine laser (COIL) were measured for various outcoupling geometries: Diaphragms with slit apertures of various size were introduced intracavity in front of the outcoupling mirror. The slit was placed at different positions along the flow axis. Furthermore, outcoupling mirrors of various reflectivity were used. For all experimental conditions the measured power density distribution at the outcoupling mirror reveals strong symmetry effects: The beam patterns are always symmetric to the resonator axis. The beam shape and the beam size are defined by the hardware aperture that is nearest to the resonator axis. As a function of the slit width, the laser output power saturates well before the aperture of the resonator is fully opened. Fifty percent of the maximum output power were achieved at a width of 6 mm only. Good agreement is found between the measured data and theoretical calculations, when taking into account the specific flow conditions. The data highlight the significance of deactivation processes and the strong iodine repumping mechanism within the cavity. For standard operating conditions a Rigrod type analysis reveals a small signal gain that is nearly constant throughout the cavity exceeding 1.3% cm-1. The outcoupling reflectivity for the maximum power output was found at a value of about 94%. These experimental data agree well with a simplified analytic model for gain saturation and power extraction as derived by G. D. Hager et al.

Supersonic COIL operation at DLR, Germany

The German Aerospace Research Establishment (DLR) is routinely operating its supersonic chemical oxygen-iodine laser (COIL). Meanwhile over 200 single tests have been performed at run times extending to 1 minute. Power levels of 5 KW have been exceeded. Parametric studies were performed resulting in chemical generator efficiencies of about 43% for the baseline operation with high power output. The BHP molarity was found as one of the most important parameters for a stable and reproducible operation. Yield measurements revealed lower numbers than expected from theoretical calculations. The paper gives an overview of the COIL device and discusses the experimental and calculated results of the investigations.

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.

CO2 Laser Absorption in Ablation Plasmas

The impulse formation by laser ablation is limited by the premature absorption of the incident laser radiation in the initially produced cloud of ablation products. The power fraction of a CO2 laser pulse transmitted through a small hole in a POM sample for pulse energies of 35 to 150 J focused on a spot of 2 cm2 has been compared with the incident power. The plasma formation in vacuum and in air of 3500 Pa and the spread of the shock wave with velocities of 1.6 to 2.4 km/s in the low pressure air was observed by Schlieren photography. A sharp edged dark zone with a maximum extension of 10 to 12 mm away from the target surface develops within 5 μs independently of the pressure and is assumed to be a plasma. In order to find out, if this is also the zone where the majority of the incident laser radiation is absorbed, a CO2 probe laser beam was directed through the expansion cloud parallel to and at various distances from the sample surface. The time behavior of the absorption signal of the probe beam has b...

Investigations on the efficiency of a rotating disk type oxygen generator.

For a rotating disk type generator the dependencies of utilization and yield on the generator operating mode are experimentally investigated. The fundamental effects of rotational speed, reduced gas volume, gas flow and liquid phase composition on the efficiency of singlet delta oxygen generation are discussed together with theoretically predicted generator performace from literature.

A Review of Laser Ablation Propulsion

Laser Ablation Propulsion is a broad field with a wide range of applications. We review the 30‐year history of laser ablation propulsion from the transition from earlier pure photon propulsion concepts of Oberth and Sanger through Kantrowitz’s original laser ablation propulsion idea to the development of air‐breathing “Lightcraft” and advanced spacecraft propulsion engines. The polymers POM and GAP have played an important role in experiments and liquid ablation fuels show great promise. Some applications use a laser system which is distant from the propelled object, for example, on another spacecraft, the Earth or a planet. Others use a laser that is part of the spacecraft propulsion system on the spacecraft. Propulsion is produced when an intense laser beam strikes a condensed matter surface and produces a vapor or plasma jet. The advantages of this idea are that exhaust velocity of the propulsion engine covers a broader range than is available from chemistry, that it can be varied to meet the instantaneous demands of the particular mission, and that practical realizations give lower mass and greater simplicity for a payload delivery system. We review the underlying theory, buttressed by extensive experimental data. The primary problem in laser space propulsion theory has been the absence of a way to predict thrust and specific impulse over the transition from the vapor to the plasma regimes. We briefly discuss a method for combining two new vapor regime treatments with plasma regime theory, giving a smooth transition from one regime to the other. We conclude with a section on future directions.Laser Ablation Propulsion is a broad field with a wide range of applications. We review the 30‐year history of laser ablation propulsion from the transition from earlier pure photon propulsion concepts of Oberth and Sanger through Kantrowitz’s original laser ablation propulsion idea to the development of air‐breathing “Lightcraft” and advanced spacecraft propulsion engines. The polymers POM and GAP have played an important role in experiments and liquid ablation fuels show great promise. Some applications use a laser system which is distant from the propelled object, for example, on another spacecraft, the Earth or a planet. Others use a laser that is part of the spacecraft propulsion system on the spacecraft. Propulsion is produced when an intense laser beam strikes a condensed matter surface and produces a vapor or plasma jet. The advantages of this idea are that exhaust velocity of the propulsion engine covers a broader range than is available from chemistry, that it can be varied to meet the instantan...