Small space debris objects of even a few centimeters can cause severe damage to satellites. Powerful lasers are often proposed for pushing small debris by laser-ablative recoil toward an orbit where atmospheric burn-up yields their remediation. We analyze whether laser-ablative momentum generation is safe and reliable concerning predictability of momentum and accumulation of heat at the target. With hydrodynamic simulations on laser ablation of aluminum as the prevalent debris material, we study laser parameter dependencies of thermomechanical coupling. The results serve as configuration for raytracing-based Monte Carlo simulations on imparted momentum and heat of randomly shaped fragments within a Gaussian laser spot. Orbit modification and heating are analyzed exemplarily under repetitive laser irradiation. Short wavelengths are advantageous, yielding momentum coupling up to ∼40 mNs / kJ, and thermal coupling can be minimized to 7% of the pulse energy using short-laser pulses. Random target orientation yields a momentum uncertainty of 86% and the thrust angle exhibits 40% scatter around 45 deg. Moreover, laser pointing errors at least redouble the uncertainty in momentum prediction. Due to heat accumulation of a few Kelvin per pulse, their number is restricted to allow for intermediate cooldown. Momentum scatter requires a sound collision analysis for conceivable trajectory modifications.
A requirement for efficient pulsed laser propulsion from ground to LEO is the achievement of a specific impulse of up to
800 s at a jet efficiency of at least 50%. With CO2 laser radiation at pulse lengths in the range of 10 microseconds and
polymers as propellant these numbers cannot be attained by classical laser ablation because the impulse formation by
laser ablation is limited by the premature absorption of the incident laser radiation in the initially produced cloud of
ablation products1,2. The power fraction of a CO2 laser pulse transmitted through a small hole in a POM sample has been
compared with the incident power. It was found that the transmitted power fraction is directly proportional to the inverse
of the pulse energy. 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 front of the absorption zone is found to move rapidly away from the target
surface with increasing speed. The absorption lasts twice as long as the laser pulse. It is not associated with a pressure
rise that would increase the mechanical impulse. The radial motion of the absorption wave turned out to be faster than
the shock wave seen in the Schlieren pictures.
Since the early 1970s ablative laser propulsion (ALP) has promised to revolutionize space travel by reducing the 30:1 propellant/payload ratio needed for near-earth orbit by up to a factor of 50, by leaving the power source on the ground. But the necessary sub-ns high average power lasers were not available. Dramatic recent progress in laser diodes for pumping solid-state lasers is changing that. Recent results from military laser weapons R&D programs, combined with progress on ceramic disk lasers, suddenly promise lasers powerful enough for automobile-size, if not space shuttle-size payloads, not only the 4 - 10 kg "microsatellites" foreseen just a few years ago. For ALP, the 1.6-μm Er:YAG laser resonantly pumped by InP diode lasers is especially promising. Prior coupling experiments have demonstrated adequate coupling coefficients and specific impulses, but were done with too long pulses and too low pulse energies. The properties of ions produced and the ablated surface were generally not measured but are necessary for understanding and modeling propulsion properties. ALP-PALS will realistically measure ALP parameters using the Prague Asterix Laser System (PALS) high power photodissociation iodine laser (λ = 1.315 μm, EL ≤1 kJ, τ ~ 400 ps, beam diameter ~29 cm, flat beam profile) whose parameters match those required for application. PALS' 1.3-μm λ is a little short (vs. 1.53-1.72 μm) but is the closest available and PALS' 2ω / 3ω capability allows wavelength dependence to be studied.
Pulsed laser propulsion may turn out as a low cost alternative for the transportation of small payloads in the mass range of 1 to 10 kg to high altitudes and low Earth orbits (LEO). Using a pulsed, electron beam-sustained multi-wavelength laser with pulse energies as high as 410 J in CO2 laser gas and possible repetition rates up to 100 Hz the launch of a “Lightcraft” has been demonstrated in the laboratory. A series of single pulse impulse measurements with a pendulum has been performed to derived the impulse coupling coefficient under various conditions. Simple plasma diagnostic and fluid dynamic investigations have been carried out as well.
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.
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.
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 Sedov's similarity solution for a blast wave. Finally, the propulsion requirements for the launch of a 10 kg mass into low Earth orbit are presented.