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MASS DRIVER UP-DATE

By Henry Kolm

From L5 News, September 1980

Mass Drivers were proposed by Professor Gerard O'Neill in 1974 as the logical means for transporting lunar raw material to L-5. As all but perhaps a few of the newest L-5 members know, mass drivers are electromagnetic launchers which accelerate payloads in recirculating buckets with superconducting magnet coils at a repetition rate of about ten per second. These buckets are levitated, guided and driven by a synthetically synchronized linear motor derived from the Massachusetts Institute of Technology (MIT) Magneplane. The magneplane is a cylindrical high-speed train which floats twelve inches above an aluminum trough. The mass driver ultimately evolved into a line of pulse coils surrounding a barrel of aluminum guide rails within which a stream of cylindrical buckets is accelerated and decelerated without physical contact.

Mass driver
Cutaway of a mass driver model: the current in the drive coils makdes a magnetic field which pushes on currents in the bucket coils, producing acceleration.
Mass driver
The mass driver on display at the Princeton Conference in 1979. Photo by Charles Divine.

Mass driver development has been pursued by a dedicated group, first at two NASA Ames summer studies in 1976 and 1977, and during the intervening academic year, while O'Neill was a visiting professor at MIT. Out of this collaboration came Mass Driver One, built by a group of MIT students on a shoestring budget in four months, in time to be demonstrated at the May 1977 Princeton-American Institute of Aeronautics and Astronautics (AIAA) Symposium on Space Manufacturing. It was also featured in the NOVA documentary "The Final Frontier," and was flown to California to be exhibited and nationally televised at the festivities surrounding the first piggyback flight of the Space Shuttle orbiter Enterprise in 1977.

It is now believed that a lunar mass driver several kilometers long, designed conservatively with present technology, should be able to deliver 600,000 tons a year to L-5, or more easily to L-2, at a cost of about $1 per pound, assuming only ten years of operation. Smaller caliber mass drivers could also be useful as reaction engines to propel large structures or asteroids by ejecting waste matter as reaction mass. Such devices are not as straightforward as lunar launchers since certain stability problems of long, flexible structures in space need to be solved.

I am often asked what, if anything, has happened recently. An update is about due, particularly since exciting new possibilities have emerged.

Mass Driver Two

In the fall of 1978, O'Neill and I shared a university-level NASA grant for the development of Mass Driver Two. It is to operate in an evacuated, four-inch caliber tube at an acceleration of 500 gee, with a superconducting bucket and an oscillating, push-pull coil system. It is close to an actual lunar driver, but more complicated due to the need for a vacuum tube between drive coils and bucket. O'Neill's crew chief at Princeton is Bill Snow, the last remaining mass driver veteran. Kevin Fine, Bill Arnold and Bill Wheaton have all departed for West Coast aerospace jobs. Eric Drexler is working on other problems at MIT. The Princeton group is building the actual driver with associated power conditioning equipment. They have supplemented their NASA grant with government surplus capacitors, and with funds from O'Neill's Space Studies Institute, supported by donations. The MIT crew chief is Peter Mongeau, a doctoral candidate in physics who is dedicating his thesis to a fundamental study of electromagnetic acceleration. Second officer is Fred Williams, a magneplane veteran during his undergraduate days and now a mechanical engineer with a great deal of experience in pulsed power systems, much creative ability, and tenacious faith in the future of electromagnetic launching. Our group now has two technicians, Whitney Hamnett and Al Djiauw. Two senior reseachers have joined the effort: Professor Rene Miller, past-president of AIAA and past head of the MIT Aero-Astro Department, and Dr. Peter Graneau, a pioneer in cryo-cable research, circuit breaker technology, and sophisicated electromagnetics in general. We have also taken aboard several other students at various stages of their education: Michael Paluszek, Mark Zeitman, Ken McKinney, Osa Fitch and Robert Sharp — pioneers whose names you will certainly hear in the future.

Our task is to build the superconducting bucket and cooling-charging station, which is to service the bucket and inject it into the mass driver. We are also to study the ultimate performance of superconducting coils under pulsed field conditions, and collaborate with the Princeton crew in putting it all together. We have supplemented our NASA grant with more substantial funding from the Department of Defense, with the mission of establishing a facility at the MIT National Magnet Laboratory for studying various other accelerating mechanisms besides that of the mass driver. We also helped set up a technical advisory panel to plan a national program in electromagnetic launcher research with a view to a number of terrestrial and space applications: high velocity artillery, cargo and personnel transport over inaccessible terrain, hypervelocity research, and ultimately the electromagnetic launching of space vehicles from Earth.

This expanded research has turned up several interesting results. One is that it may be possible to build mass driver reaction engines which are only several meters, rather than several kilometers, long and eject reaction mass in the form of small rings or washers (easily made of lunar aluminum, for example) without the use of superconducting buckets. Normal metals will carry even higher current densities than superconductors for very short periods of time. On the other hand, conventional mass drivers with recirculating superconducting buckets can be improved drastically by using superconducting instead of normal-conducting drive coils, and storing the launch energy inductively in the drive coils. This would eliminate the need for capacitors and feeder lines, thereby reducing the system mass, cost and complexity. The most exciting thing we learned is that mass drivers can be used to launch space cargo from Earth!

The Era Of Earth-Based Mass Drivers

Electromagnetically launched space vehicles are an old dream. Arthur C. Clarke and Robert Heinlein have used them for decades, and a Princeton professor named Northrup proposed them in the Twenties. The Germans attempted electromagnetic launching unsuccessfully during World War Two, before they embarked on the development of rockets. Actually the most successful catapult launch was achieved by chemical means in the Sixties when a passive missile was almost accelerated to orbital velocity from the Barbados Islands by welding together two large naval guns. It would be nice to be able to launch pure payload, unaccompanied by over 100 times its mass in expensive rocket engines and fuel. Nevertheless, space technologists never took direct Earth-launching seriously. After all, consider the ablation problems we face when entering the atmosphere from the top, where it is very dilute. Imagine the energy and ablation loss when a vehicle enters at full speed from the Earth's surface, where the atmosphere is very dense. Even if a vehicle could be launched at escape velocity of 11 km/s, or even at a lower orbital velocity, it would certainly burn up before traversing the atmosphere, right?

Wrong! At least one dreamer refused to accept this extrapolation: Fred Williams has talked about Earth-launching ever since the days of the Magneplane Project. The question is: just how large would an Earth-launched vehicle have to be to survive its passage through the atmosphere? The first time this question was considered seriously in a quantitative way, to the best of my knowledge, was at the 1977 NASA Ames summer study. The theory of ablation in a dense atmosphere had received recent attention in connection with the outer planet probe program, and two members of the Ames team applied the resulting software to the problem of the Earth launcher: Chul Park and Stuart Bowen. They found, much to everybody's surprise, that an Earth-launched vehicle would not have to be prohibitively large to survive: a vehicle the size and shape of a telephone pole could be launched out of the Solar System with a loss of only about 3% of its mass, and 20% of its energy to the atmosphere. There are two reasons for this result. First, the atmospheric transit is short and vertical rather than long and tangential (as required for astronauts to survive the deceleration); and second, the high atmospheric density leads to highly opaque ablation products which reduce radiation heating from the hot air to the projectile's surface.

A reference design telephone pole launcher would have the specifications shown below.

Vehicle: Telephone Pole Shaped, Mass of 1,000 kg
Launch Velocity: 12.3 km/s
Velocity at Top of Atmosphere: 11 km/s (escape velocity)
Kinetic Energy at Launch: 76 x 109 joule
Ablation Loss, Carbon Shield: 3% of mass
Energy Loss: 20%
Acceleration: 1,000 gee
Launcher Length: 7.8 km
Launch Duration: 1.26 second
Average Force: 9.8 x 106 newton = 2.2 x 106 pound
Average Power: 60 x 106 kilowatts
Charging Time From 1,000 MW Power Plant: 1.5 minute

This launcher is about as long as the deepest well hole ever drilled, and therefore represents the longest launcher which can be installed vertically by present technology. If it were made longer to decrease the power requirement or increase the payload size it would have to be installed up a mountainside at an inclination of perhaps 30 to 45 degress. This would increase mass and energy losses due to the lengthened path through the atmosphere.

The cost of the launcher itself in terms of installed copper, steel and concrete would be only 24 million dollars. But a device to store 76 gigajoules by conventional technology (generators and capacitors) would cost 11 billion dollars. This estimate may not be very meaningful, because it is based on cost estimates for quantities of capacitors which have never been manufactured before, but even at half the price, the investment would be formidable. The energy cost of the launch would only be about 65 cents per pound, but amortization of capital would add 10 to 20 dollars per pound, even if the launcher were used continuously, day and night, every 12 minutes.

Actually, it is more useful to think in terms of power compression rather than energy storage. The reference launcher could be operated by storing energy from one single large (1,000 megawatt) power plant for 1.5 minutes, and releasing it in 1.5 seconds, a 60-fold compression. Perhaps sixty power plants could be tapped simultaneously during off-peak hours by using superconducting transmission lines. On the other hand, if the power requirement were reduced by a factor of 60, there would be no need for energy storage at all. This could be done either by making the launcher 60 times longer (468km), or by making the vehicle 60 times smaller (17kg). Neither alternative is reasonable. A compromise might be to apply a factor of the square root of 60 to each: a 60km long launcher with a 129kg vehicle. Unfortunately this launcher would be too long even for installation up a mountainside, and the payload ratio of such a small vehicle would be very poor.

There does, however, appear to be a solution to the energy storage problem. If the entire drive coil system of a mass driver is made superconducting, as well as the bucket coils, enough energy can be stored inductively by charging the system with current. It is then merely necessary to quench the current in each individual drive coil as the bucket passes. This loses some of the energy efficiency of a push-pull capacitor system, but the loss is more than offset by eliminating capacitor feeder line losses. A preliminary calculation indicates that a "quench gun" of this type of 12-inch caliber, only 1 km long, would store enough energy to launch a 20kg vehicle to 10.5km/s at an energy conversion efficiency of 80%, at an average acceleration of 5,600 gee.

There are technical problems to be solved, of course, but not any of a fundamental nature. The benefit-to-risk ratio of the enterprise certainly justifies an immediate, serious study. The possibility of launching cargo into space at a cost approaching about one dollar per pound by using off-peak electric power has mind-boggling consequences. To name only the most obvious: we could dispose of nuclear waste by launching it out of the Solar System; we could begin constructing solar power satellites; and we could establish fueling stations in low Earth orbit where Shuttle travellers would take on fuel and reaction mass for the trip beyond: to geosynchronous orbit, to the Moon, and to L-5.

 

Henry Kolm Henry Kolm, Professor of Physics at M.I.T.'s National Magnet Laboratory, is one of the foremost workers on the mass driver project.

L5


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