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Laser Propulsion and the Four P's

by John G. Cramer

Alternate View Column AV-21
Keywords: laser, propulsion, LEO, space, launch
Published in the August-1987 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 1/9/87 and is copyrighted © 1987, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.


    We live at the bottom of a very deep gravity well: the lowest part of the well is the surface of the Earth and its top is interplanetary space. Each kilogram of mass on the Earth's surface has an energy debt of 63 million joules that must be paid to get out of the Earth's well. That in itself isn't so bad; at standard commercial power rates 63 megajoules of electrical energy costs less than $1. But this isn't the true cost. Our gravity well has no bucket, no rope, no crank. The lifting can only be done very inefficiently with expensive, complicated, and not completely reliable chemical rockets. Using that technology it costs between $4,000 and $10,000 per kilogram to ferry payload mass to low earth orbit with the Shuttle.

    But there may be another way. Laser powered launching to orbit is an emerging space technology that may eventually provide a techno-fix for the large expense of getting payloads into orbit, a way around the high cost of Shuttle payloads. Extremely powerful lasers are now on the hi-tech horizon, and their development promises to make this new technology feasible. In July of 1986 a group of experts gathered at Lawrence Livermore National Laboratory to consider the key issues of laser propulsion. They focused on three basic questions: (1) What laser launching will be possible with the big lasers now under development? (2) What groundwork of research and development is needed to prepare for launch testing when the big lasers are ready? And (3) What characteristics should be pushed for in these big lasers so that they will be more useful for such launching? This Alternate View column follows some of their discussion.

    The idea of laser-powered propulsion is not new. It was first proposed by Arthur Kantrowitz in 1972, but recently it has been given a new twist. The twist, which will be discussed further below, is to eliminate engine hardware altogether from the launch vehicle and to obtain thrust instead from the laser-sustained detonation (LSD) of a thin flat layer of inert fuel that pushes explosively against the vehicle's planar rear surface. This new concept emerged from discussions at the Livermore workshop and seems to offer the long sought inexpensive highway into space, the cheap elevator for lifting payloads from our gravity well.

    Until recently there seemed little prospect of developing lasers large enough to launch useful payloads from the ground. However, the DOD's Strategic Defense Initiative Office (SDIO, a.k.a. "star wars") is presently spending quite large amounts of money on the development of gigawatt-level free electron lasers for possible use in the military "defensive shield" that we read about in the newspapers. These powerful lasers, while they may never meet the SDIO goals, appear likely to provide a nearly ideal power source for laser launching. It is somehow ironic that an unplanned spinoff of the big federal investment in SDI may be a cheaper way to put bulk loads and later people into space.

    How does laser power propulsion work? Let's start by contrasting it with conventional chemical-fuel propulsion. In conventional rockets the fuel serves two separate and distinct functions: (1) it burns, converting chemical energy to the kinetic energy of the high speed exhaust particles, and (2) the burned fuel supplies the reaction mass of exhaust particles ejected at the exit nozzle. The dual function of the fuel makes any chemical rocket dangerous and expensive because all that stored chemical energy in the fuel is a significant explosion hazard. Rocket design requires careful attention to this problem. The trick in the laser powered launching scheme is to separate the two functions of the fuel: let a ground-based laser supply the propulsion energy, far more energy per fuel mass than chemical burning could supply, while a safe chemically inert "fuel block" supplies the reaction mass. Kantrowitz (now on the Dartmouth University faculty) and Dr. Jordin Kare, the Livermore workshop coordinator, describe this latest laser powered launch scheme as the "4-P" launch technology. It leaves everything on the ground except "Payload, Propellant, and Photons ... Period!"

    How might such a "4-P" vehicle operate? Imagine the launch vehicle as a pyramid about 200 cm high (about the size a pyramid camping tent) made of solid material with about the density of water. The top 91 cm of the pyramid (mass about 1 metric ton) is the payload. The bottom 109 cm (mass about 9.7 tons) is the expendable "fuel block" to be consumed during the launch. This vehicle would probably receive its initial velocity from a Jules-Verne-style launch cannon which accelerates at 10 g's and fires vertically. After the vehicle leaves the cannon it is tracked by the intense beam of the free electron laser, which begins detonating fuel on the backside of the vehicle to propel it into orbit. The laser, probably delivering average power of 100-1000 megawatts, provides the propulsion energy while the fuel block provides the reaction mass.

    The laser must be pulsed in a carefully programmed way. A relatively low energy "metering pulse" from the laser vaporizes a thin layer of fuel from the flat rear surface of the fuel block. The gas thereby produced drifts away from the rear surface of the vehicle. Then, when the dispersing gas reaches the proper density, the laser hits again with a far more powerful pulse, converting the vaporized fuel to a very hot plasma of dissociated electrons and ionized atoms. This plasma absorbs energy rapidly and detonates explosively. The shock wave from the LSD wave provides the push. This is the pulsed LSD propulsion scheme mentioned above. It's a revolutionary concept in rocketry. There are no on-board engines, no plumbing, no pumps, no valves, no potentially explosive fuel, no nozzles, no coolants, no stage-separation explosives, no solid-fuel boosters, no O-rings, ...

    In a normal rocket engine the explosion of burned fuel is roughly spherical and continuous, so that carefully cooled engine walls and nozzles must convert the omnidirectional pressure of the exploding fuel into directed thrust. But the LSD explosion comes in a pulse and has the geometry of a plane, not a sphere. The exploding gas does not have to be redirected. Half of the gas molecules will push against the surface of the fuel block, providing thrust, and the other half of the molecules will dissipate in the opposite direction. Only at the edges of the plane LSD wave is there deviation from the plane geometry of the explosion, and this has negligible effects. The LSD wave propulsion scheme is a chamber-less nozzle-less engine-less engine.

    It is worth noting that the SDI people have some very severe problems to overcome is making their energetic lasers destroy missiles. It is difficult to make free electron lasers operate at the near-infrared wavelengths that SDI needs. But laser powered propulsion works quite well at more accessible far-infrared wavelengths. Another problem for SDI involves the LSD waves themselves: if the target missile has a simple vaporizable coating the laser beam's energy may be dissipated in heating the vaporized coating rather than destroying the missile. But for laser powered launching the shielding effect of the plasma is a benefit because it insures that the laser energy is absorbed by the vaporized fuel rather than the payload or the solid fuel block.

    What are the problems with the scheme? The biggest one, of course, is getting that big laser to use. Free electron laser technology will be discussed a bit later. A second problem is finding a suitable fuel block material that will operate efficiently and that will not be eroded too rapidly. Ice, plastic, and lithium metal have been discussed, but more research is needed. The structural and hydrodynamic effects of the detonation waves must be understood. And of course the aerodynamics of the vehicle will have to be carefully considered. It will require attitude-control hardware, vanes or jets perhaps, to keep it oriented properly in the beam. The laser tracking to orbit will also require careful design. When the vehicle had reached some altitude and velocity, it must be turned so that the back surface of the fuel block is at an angle to the incoming laser beam. This allows the proper tangential velocity to be added to achieve a stable orbit. Notice that the thrust in the tilted configuration remains at right angles to the block surface and is quite independent of the laser beam angle. If this technology is to become widespread, the environmental effects of noise pollution from the launch site, the atmospheric effects of the laser beam (e.g., nitrous oxide generation), interference with air traffic, etc., must also be carefully studied. But at present the biggest problem is to find a funding agency that will pay for this research. Unfortunately NASA is reportedly not interested.

    Let's now turn to the technology of the free electron laser (FEL). It's a spinoff, just 10 years old, of accelerator technology developed for nuclear physics basic research in the past few decades. Basically an FEL is a large electron linear accelerator that accelerates perhaps 10 amperes of electrons to nearly the speed of light, giving them energies around 100 million electron volts. At such energies an electron weighs about 200 times more than it would at rest due to relativistic mass increase. The acceleration of the electrons may either be continuous, using resonant electrical cavities powered by high frequency electrical power, or it may be pulsed with a set of microwave "transformers" that use the electron beam as the effective secondary winding. The latter acceleration scheme appears to be the more useful for LSD wave propulsion, which requires a pulsed laser beam in any case.

    After acceleration the electron beam passes into a "wiggler", an intense and rapidly alternating set of static magnetic fields. The wiggler efficiently converts the energy contained in the electron beam to coherent photons and a few hundred megawatts of coherent light energy emerges from the machine. The FEL's conversion of electrical energy to light energy is remarkably efficient. Overall efficiencies of 20% or more have already been achieved with low power FEL devices. A low power FEL was first demonstrated at Stanford University a decade ago, but until the SDIO made FEL development a national priority, it progressed rather slowly. But now with SDI funds as the driving force, two national laboratories and several aerospace companies are in a development horse race. A major facility to test high power FEL devices is being constructed at White Sands Proving Grounds in New Mexico. It's curious to consider that that site, where captured V-2 rockets were tested after WWII, may eventually become our first laser spaceport.

    This brings us to bottom line: the cost of putting a kilogram of mass in low earth orbit with the laser power launch scheme. It's estimated that it will cost a billion dollars or so to produce a FEL installation powerful enough for laser launching. If that cost must be amortized as part of the launch cost, that cost at 10% of maximum capacity is about $180 per kilogram. At 100% utilization the cost drops to $30 per kilogram. If one uses a laser "built for other purposes" such as SDI and does not have to amortize the capital cost, the minimum cost drops to $18 per kilogram. Compared to the $4000 to $10,000 per kilogram cost of Shuttle freight, laser launching looks very attractive indeed.


Free Electron Lasers:
"Scientists Focus on FEL Technology to Counter ICBM's" Aviation Week and Space Technology, pp.40-45, August 18, 1986.

Laser Propulsion:
"Propulsion to Orbit by Ground Based Lasers", A. Kantrowitz, Aeronautics and Astronautics 9, #3, pp.34-35, May, 1972.
"Proceedings of the SDIO/DARPA Workshop on Laser Propulsion", J. T. Kare, Ed., CONF-860788, Lawrence Livermore National Laboratory, November, 1986.

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