When Robert Goddard launched the first liquid-fuel rocket from his Aunt Effie’s farm in Massachusetts, radio was young. Sixty-six years later, video is old hat and we are still using liquid-fuel rockets. Evidently rocket design has not kept pace with the other technological revolutions that have transformed our lives. And though liquid-fuel rockets have carried astronauts into Earth orbit and even to the moon, they may not be enough to push the exploration envelope out to Mars and beyond. Such long-duration missions might require more fuel than the rocket could lift.
Early this year engineers in both the United States and Russia announced that they were hard at work on a new bit of hardware that could help solve the problem: the nuclear rocket engine. While the idea of any new nuclear technology is unsettling to some, the details of the plans persuaded many observers that this technology might be worth a look. People have been planning manned missions to the planets for as long as they’ve been flying in space, says Elliot Kennel, a nuclear engineer at Space Exploration Associates in Ohio. With nuclear engines we may at last have a way to get there.
The idea of using nuclear technology in space is hardly new. Indeed, one type of nuclear device, called a radioisotope thermoelectric generator, is in use today on satellites and planetary probes, such as the Jupiter-bound Galileo. An RTG’s source of power is plutonium or some other radioactive material whose atoms spontaneously decay, shedding a particle or two and a little heat. That heat is converted directly to a few hundred watts of electricity--enough power to drive onboard instruments, but not nearly enough to propel the spacecraft itself.
The propulsion engines now being developed rely not on nuclear decay but on nuclear fission--in which an atom absorbs a neutron and splits more or less in two, releasing a lot more heat than a decaying atom. A nuclear rocket engine, in other words, would be a fission reactor like the ones in power plants on Earth, except that its heat would be used to generate thrust rather than electricity.
The idea of powering rockets with fission reactors also has a long history (as does the idea of powering them with atomic bombs, which some engineers still believe would be a safe propulsion mechanism in space). In the 1960s and 1970s, during the heady days of the Apollo program, the United States had an active nuclear effort under way. The science behind the prototype engines was relatively straightforward: Fuel rods made of uranium carbide were brought to a white heat through nuclear fission. Liquid hydrogen compressed and stored in a nearby tank was then boiled to a gaseous state and passed over the scalding rods. Unlike hydrogen in a traditional engine, the gas was not detonated but simply heated by the rods. The heating was so intense, however--the exhaust reached a temperature of 4500 degrees--that the gas went rushing out the tail of the engine at 30,000 feet per second, or twice the speed at which exhaust leaves the bell of a conventional engine. Significantly, it achieved this speed using as little as a tenth the hydrogen propellant.
The nuclear reactors, however, were extremely limited in the amount of hydrogen they could heat at once. This meant that while the engines were efficient, they could never replace powerful boosters like the Titan or the Saturn, which build up enough thrust to carry spacecraft off the Earth by moving massive amounts of fuel through their combustion chambers. Where nuclear technology would find a use, though, would be in the small onboard engines that could punch manned spacecraft out of Earth orbit. The tests we ran in the 1970s earned what NASA calls a technology readiness level six, says NASA nuclear engineer Stanley Borowski. Translated, that means we had a successful full-scale test on the ground and were ready to test in space.
But in 1973 NASA was told to scrap the effort. Facing budget constraints, the government decided to cancel its moon and Mars initiatives in favor of the cheaper, closer-to-home shuttle program. There were a lot of problems in the country at the time, says Kennel, and missions to the planets just weren’t popular. The shuttle seemed like a better choice.
For close to a generation, nuclear rocketry all but dropped out of the picture. Last January, however, it suddenly reappeared. At a meeting of nuclear scientists at the University of New Mexico in Albuquerque, a group of Russian engineers revealed that over the last two decades they had continued tinkering with fission rockets and had completed a prototype that burned hotter and more efficiently than any model ever built before. According to the Russians, their new engine produced exhaust temperatures of about 5100 degrees and burned successfully for a full hour before engineers shut it down.
The few hundred degrees improvement the Russians achieved was impressive but not colossal, says Kennel. What was significant was that they have a real, active capability.
While the Russians were sketchy about the precise design of their engine, the key improvement appeared to be the material used in the fuel rods. The rods are made of a uranium-carbide alloy, like the ones in the old NASA design, but they are strengthened by zirconium carbide. Carbides tend to be heat resistant, says Kennel. As the fissionable material in the core heats, the carbon in the alloy helps it achieve higher temperatures without breaking down itself.
The mere existence of the Russians’ nuclear engine stole the Albuquerque show. And it helped to stimulate talk once again of manned interplanetary flight. The Russians offered to share their nuclear innovation with NASA as part of the much-talked-about joint mission to Mars. I don’t think it’s unrealistic, says Kennel, to think we could have the rocket technology needed for a Mars trip worked out in ten to fifteen years.
As it turns out, the Russians were not the only ones working on the problem. Shortly after the New Mexico conference, the U.S. Air Force admitted that it has been quietly conducting its own nuclear engine research since 1987. The Air Force engine would be a particle-bed reactor-- meaning that its nuclear fuel would be in the form of sand-grain-size particles rather than rods. Particles increase the surface area of fissionable material that is exposed to the hydrogen propellant, thus, in theory, heating it faster and increasing the engine’s thrust.
It will probably be several years before a prototype is ready to test that theory. But the Air Force program has already raised some hackles. The fact that this project has been going on for five years and is only now being acknowledged is disturbing, says Steven Aftergood of the Federation of American Scientists. It’s been grossly overclassified and that should be unnecessary. The only reason they’d be so secretive about it is if they’re using civilian space exploration as a fig leaf for more dubious practices--spy satellites, for instance.
Another potential controversy is the possible environmental impact of loading nuclear material atop explodable rockets and launching them into space. In 1989 environmentalists tried to block the shuttle deployment of Galileo. Their fear was that if the shuttle were to explode, south Florida would be showered with the 49 pounds of plutonium in the probe’s radioisotope generator. A federal judge ruled that the risk was remote and allowed the launch to proceed. Many scientists believe the same reasoning should apply to the nuclear rocket program.
A uranium-fueled reactor is not highly radioactive until the reactor is operating, says Aftergood. If you had an explosion of a chemical rocket carrying a dormant nuclear engine, it would be messy, but it would not be a major nuclear accident. A reactor on an orbiting spy satellite, on the other hand, could present a greater risk; on a few occasions in the past, Soviet satellites carrying small reactors have burned up in the atmosphere, and one actually crashed--fortunately in a remote area. The Air Force tells us they’ve done an environmental impact study, Aftergood says, but so far they won’t share it. That doesn’t inspire much trust in what they’re doing.
Despite such misgivings, nuclear rocketry appears to be moving ahead. In the past two years, NASA has revived, albeit on a small scale, its program to develop a nuclear engine of the fuel-rod type. Another international conference on the subject is scheduled for September in Kazakhstan. It took us eight years to go to the moon, says Kennel, and that included inventing a lot of the technology we needed to get there. For Mars we’ve already got a lot of the science in place. At this point it looks like our biggest obstacles are less scientific ones than institutional and political ones.