Laser fusion propels a dream of travel to the stars
Livermore, Calif. — Roderick Hyde has starships in his eyes. At Lawrence Livermore Laboratory here, he has done what may be the most advanced work to date on a spacecraft propulsion system based on nuclear fusion, the energy source that powers the stars.
Such an engine would bring the planets in the solar system to within a few weeks travel time. It would even make voyaging to nearby stars conceivable.
While fusion engines for space travel are purely theoretical at this point, progress in efforts to harness nuclear fusion for terrestrial power applications in the last decade has brought such a possibility light years closer to reality.
''I'll live to see it,'' Dr. Hyde asserts simply.
A big, almost burly man with a scraggly beard, Hyde's casual clothes and low-key manner attest his rural Oregon upbringing. But when the physicist begins talking about spacecraft engines, his gaze becomes piercing and his speech quickens.
The story begins in 1972, when the young scientist was finishing up his bachelor of science degree at Massachusetts Institute of Technology in little more than two years. There he met Lowell Wood, one of Livermore's leaders in the new field of laser fusion. They discovered a shared passion for spacecraft engine design. So Wood offered Hyde a summer job.
''It was a great change from the agricultural jobs I had been getting back home,'' Hyde recalls.
During this summer Hyde put together a remarkably detailed design for a laser-fusion-powered spacecraft. Since getting his degree and coming to Livermore to work, he has periodically updated the design as research in this area has progressed, aided by powerful computers used in nuclear weapons research.
Laser fusion is one of two basic approaches being pursued to harness the energy source of the stars - and of the hydrogen bomb - to provide power.
This year the US is spending $145 million on research in the area. The details are tightly classified, but the basic concept involves making pellets less than a millimeter in diameter out of the basic ingredients of a hydrogen bomb. Five to 10 of these are detonated per second by intense pulses of light from giant lasers. To do this, the pellet must be compressed to 1,000 times the density of liquid and heated to 100 million degrees Celsius.
A spaceship powered by a laser-fusion drive would look little like the vessels created by the imaginations of Hollywood special-effects people.
In outline, it would be a slender cylinder. At the front is an enclosed area for crew and cargo. The engine itself is a cylindrical lattice work. Along its axis 70 to 100 lasers are aligned. These are placed on a series of rotating cylinders, like the barrels on an old-fashioned Gatling gun. After each laser has released its fiery bolt of light, the cylinder rotates and another laser chamber moves into position. A series of mirrors split the laser light beams and direct them back to the thrust chamber where the pellets are burned. The light itself would be a deeper violet than human eyes can see, so would be invisible.
Most of the thrust chamber, the fusion engine's version of the rocket nozzle, is also immaterial. That's because it consists mainly of a magnetic field. The only thing physical is a circular magnet coil. Fashioned out of superconducting material, this generates a magnetic field strong enough to divert the blast of charged particles from the nuclear explosions to the rear of the craft.
In this design, 700 tons of fuel is enough to take you any place in the solar system. Forty percent of the energy released by the microscopic bombs is converted to thrust.
Still, getting rid of waste heat is a major problem. In the vacuum of space there is no air to carry heat away, so it must disposed of by bulky radiators. Hyde's design keeps the amount of waste heat down to only a few percent of the energy being released. This makes it possible to keep the weight of the nuclear engines to 700 tons as well.
The scientist calculates that such a spaceship should be able to sustain a steady acceleration of half a gravity. It would also be capable of achieving speeds of 0.1 to 0.2 times that of light. At this rate, a quick trip to see the red sands of Mars would take one to two weeks. A voyage to the gas giant Jupiter would require 5 weeks or so. To inspect the outermost planet, Pluto, would consume half a year. And to fly past the nearest star would take 20 years.
Of course, there are a number of technical hurdles that must be crossed before Hyde's starship design becomes feasible. The major one is development of the powerful lasers required.
''Here I hope we can piggyback on terrestrial efforts,'' Hyde explains. Recently lasers have been increasing 10-fold in power every three years. One type of recently developed laser, called a krypton-fluoride excimer, would be suitable. However, it has not yet received as much attention as he would like.
A laser-fusion drive would only work in outer space.
''There will still be the problem of getting off Earth. Actually, I'm more worried about that than the engine design,'' Hyde says.
Such a drive would have some environmental problems. Operated in near-Earth orbit, it would create some radioactive contamination. This would begin causing significant environmental problems when there are enough such craft for several hundreds flights per year. But this could be alleviated by improved design of the pellets or, at worst, by operating from the Moon.
''Theoretically, laser fusion is the best propulsion system for interplanetary travel,'' Dr. Hyde asserts. So now it's only a matter of waiting for practice to catch up to the theory.