The AEgis Technologies Group and RTI International are developing microsensors for High Energy Laser (HEL)
diagnostic applications. The conformal sensor array will measure the irradiance profile of an incident laser beam, and
concomitant rise in surface temperature of the target. The open mesh architecture allows 90% of the beam to impact the
surface. A critical part of this program is developing a protective coating that ensures sensor survivability at high
irradiance levels for operational lifetimes on the order of 10 seconds. The protective coating must transmit a measurable
amount of light to the irradiance sensor. We have conducted experiments to evaluate candidate heat shield materials.
In the first round of experiments, a 10kW CO2 laser was used to irradiate pure materials, including metals and carbon
foils. Although many of the metal foils were perforated by the laser, no significant amount of material was ablated
away. In fact, most of the test samples gained mass, presumably due to oxidation. Analysis of high speed video shows
that once the metal melted, surface tension caused the molten metal to coalesce into droplets around the rim of the hole.
The second and third rounds of testing, conducted with a 3kW, 1.07μm fiber laser, included samples of highly reflective
metals and ceramics, standard plasma-sprayed coatings, and multilayer stacks. We have also measured the performance
of temperature sensors and irradiance sensors fabricated from nanoparticle solutions deposited by advanced printing
technology and have completed a preliminary investigation of high temperature adhesives.
Recently we became interested in applying previous work with liquid fueled laser powered minithrusters for
spacecraft orientation to the conceptual design of a multi-newton thruster based on the same principles. Solid-fuel
configurations (such as the fuel tapes used in the Photonic Associates microthruster) are not amenable to the
range of mass delivery rates (g/s to g/s) necessary for such an engine.
We will discuss problems for this design which have been solved, including identifying a practical method of
delivering liquid fuel to the laser focus, avoiding splashing of liquid fuels under pulsed laser illumination, and
avoiding optics clouding due to ablation backstreaming on optical surfaces from the laser-fuel interaction region.
We have already shown that Isp = 680 seconds can be achieved by a viscous liquid fuel based on glycidyl azide
polymer and an IR-dye laser absorber.
The final problem is mass: we will discuss a notional engine design which fits within a 10-kg "dry mass"
budget. This engine, 80kg mass with fuel, is designed to fit within a 180-kg spacecraft, and use 3kW of prime
power to deliver a Δv of 17.5 km/s to the spacecraft in sixteen months. Its specific impulse will be adjustable
over the range 200sp<3,600 seconds and maximum thrust will be 6N, based on performance which has been
demonstrated in the laboratory. Such an engine can put small satellites through demanding maneuvers in short
times, while generating the optimum specific impulse for each mission segment. We see no reason why
Isp = 10,000 seconds cannot be achieved with liquid fuels.
In this paper, we review the history of laser space propulsion from its earliest theoretical conceptions to modern practical applicatons. Applications begin with the "Lightcraft" flights of Myrabo and include practical thrusters for satellites now completing development as well as proposals for space debris removal and direct launch of payloads into orbit. We consider laser space propulsion in the most general sense, in which laser radiation is used to propel a vehicle in space. In this sense, the topic includes early proposals for pure photon propulsion, laser ablation propulsion, as well as propulsion using lasers to detonate a gas, expel a liquid, heat and expel a gas, or even to propagate power to a remote conventional electric thruster.
In this paper, we review the history of laser space propulsion from its earliest theoretical conceptions to modern practical applicatons. Applications begin with the "Lightcraft" flights of Myrabo and include practical thrusters for satellites now completing development as well as proposals for space debris removal and direct launch of payloads into orbit. We consider laser space propulsion in the most general sense, in which laser radiation is used to propel a vehicle in space. In this sense, the topic includes early proposals for pure photon propulsion, laser ablation propulsion, as well as propulsion using lasers to detonate a gas, expel a liquid, heat and expel a gas, or even to propagate power to a remote conventional electric thruster. We also discuss the most recent advances in LSP. For the first time, it is possible to consider space propulsion engines which exhibit thrust of one to several newtons while simultaneously delivering 3,000 seconds, or greater, specific impulse. No other engine concept can do both in a compact format. These willl use onboard, rather than remote, lasers. We will review the concept of chemically augmented electric propulsion, which can provide overall thrust efficiency greater than unity while maintaining very low mass to power ratio, high mean time to failure and broad operating range. The main advantage of LSP is exhaust velocity which can be instantaneously varied from 2km/s to 30km/s, simply by varying laser pulsewidth and focal spot size on target. The laser element will probably be a diode-pumped, fiber master-oscillator-power-amplifier (MOPA) system. Liquid fuels are necessary for volumetric efficiency and reliable performance at the multi-kW optical power levels required for multi-N thrust.
For the first time, we have measured the momentum coupling coefficient and plasma expansion velocity (specific impulse) in the femtosecond region, over a laser intensity range from ablation threshold to thirty times threshold. These measurements extend the laser pulsewidth three orders of magnitude relative to previous reports. We studied several pure metals and three organic compounds as targets. The organic compounds were exothermic polymers specifically developed for the micro-laser plasma thruster, and two of these used "tuned absorbers" rather than carbon particles for the laser absorption function. The metals ranged from Li to W in atomic weight. We measured time of flight profiles for ions and found dramatic two-temperature distributions for some conditions. Specific
impulse reached record values for this type of measurement and ablation efficiency was near 100%.
The laser plasma thruster (LPT) is a new microthruster for small satellites. We report on development and testing of a prototype LPT. Some advantages of the LPT are: thruster voltage 4 V, mass less than 1 kg, power-to-thrust ratio 10 kW/newton and Isp up to 1000 seconds. Typical thrust level is 250 (mu) N with PVC fuel. Thrust of 1 mN is expected with energetic fuel. The pre-prototype continuous thrust experiment includes the laser mount and heat sink, lens mounts, and focusing mechanism, which are coupled to the target material transport mechanism. The target material is applied to a transparent plastic tape, and the laser is focused on a series of tracks on the tape. The tape drive hardware and laser drive electronics, are described, as well as the control and diagnostic software. Design, construction, and calibration of the thrust stand are described. During continuous operation, the exhaust plume is deflected in the direction of the moving tape. When the laser is operated in pulsed mode, the exhaust plume is perpendicular to the tape (parallel to the optical axis). This provides some thrust vector control.
We have developed a new type of miniature jet for pointing microsatellites. It is based on laser ablation produced by a multi-mode diode laser. The target is a specially prepared tape with a transparent layer through which the laser light passes and an absorbing layer which produces the thrust. We have achieved specific impulse up to 1000 seconds (greater than possible with chemistry), together with laser momentum coupling coefficients of order 6 dyne/W. The preprototype should achieve 100 dynes of thrust. We will discuss the target interaction physics, the materials science involved in creating the targets, and some of our measurements with the preprototype thruster.
The micro laser plasma thruster ((mu) LPT) is an efficient, long- life, low-thrust pulsed rocket engine which uses a high-brightness semiconductor or glass fiber laser as a source of energy. It uses a simple, low-voltage semiconductor switch to drive the laser, resulting in zero off-state electrical power. Results are presented of the first experimental demonstration of uLPT's. We measured single impulses covering 5 orders of magnitude from 40 micro dyn-s [< 1 nano newton-s] to 2 dyn-s, specific impulse up to 1,800 seconds and coupling coefficients up to 25 dyne-s/J. Several target materials were studied. Initial applications are orientation and re-entry at end of life for micro- and nanosatellites. Anticipated lifetime output of the prototype engine now under development is about 5E7 dyn-s [500 newton-s], sufficient to re-enter a 5 kg LEO satellite.
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