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April 6, 1999: Atoms locked in snow, a teaspoon from the heart of the sun, and the stuff that drives a starship will be on the agenda of an advanced space propulsion conference that opens today in Huntsville.
The tenth annual Advanced Propulsion Research Workshop will be held at the University of Alabama Tuesday through Thursday. It's sponsored by NASA, Marshall Space Flight Center, the Jet Propulsion Laboratory, and the American Institute of Aeronautics and Astronautics.
Sessions in the conference include solar and microwave sails, beamed energy, tethers, advanced chemical rockets, nuclear propulsion, fusion research, and antimatter rockets.
Today, travel to other planets follows the New England farmer's cautionary note: "You can't get there from here." In other words, you have to go someplace else. That first stop is low Earth orbit, followed by a boost that puts a probe on a long arc to its destination. Sometimes, the probe goes in the "wrong" direction, like the Cassini mission to Saturn which first is making "gravity slingshot" passes by Venus and the Earth as the price of using a conventional rocket to leave the ground.
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Improving conventional rockets is one of the near-term steps sponsored by NASA/Marshall. In a sense, rockets are batteries comprising chemicals that have been prepared and stored so they hold a tremendous amount of potential energy. Pack more energy into a smaller volume, and you can send heavier probes deeper into space, and faster. The challenge is reaching beyond the sophisticated fires that we have now. The Space Shuttle Main Engines, for example, burn oxygen and hydrogen stored in liquid form. Its Solid Rocket Boosters burn aluminum and oxygen locked in a rubbery compound.
"One of the areas we're studying, at Glenn Research Center, is solid hydrogen and atoms," explained John Cole, a manager in the Advanced Space Transportation Project office at NASA/Marshall. "You can get as much as 10 times as much energy out as you would from conventional combustion." Cole will review Marshall-sponsored research during this morning's opening session.
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Many atoms prefer to join with their own kind to form molecules. When they join, the surrender a tremendous amount of energy. If they are cold enough, they won't join, so NASA is experimenting with methods to separate carbon or boron molecules into atoms, then trap them in hydrogen ice pellets. The ice, in turn, would float in liquid helium.
Cole said this slushlike mix is unstable, and requires several years of work before test engines could be built. But if successful, it would yield an engine with a specific impulse - a measure of efficiency - of 750 seconds. The Shuttle Main Engine produces 455 seconds.
Another way to enhance chemical rockets is with "strained ring hydrocarbons," Cole explained. Benzene burns quite well because it's built around rings of six carbon atoms. Taking three carbon atoms out produces bicyclopropylidene that has even greater potential energy.
"It's amazingly stable," Cole noted, "so it's not shock sensitive." Its value is not in higher energy - a bicyclopropylidene is only 5 percent more efficient than the refined kerosene used as a first-stage fuel in many rockets - but its greater density - more than double.
"This is very dense, so it doesn't take much tank mass to carry the propellants," Cole continued. It also reduces the size of the tank, so there's less surface area to push through the dense lower atmosphere at liftoff.
The atmosphere itself would be pressed into service with a radical concept being tested by Prof. Leik Myrabo of Pennsylvania State University in a project jointly sponsored by NASA/Marshall and the Air Force Research Laboratory at Edwards Air Force Base, Calif. The Lightcraft would rise on a series of hot air pulses produced by a high-energy laser.
The laser beam would be focused just aft of the Lightcraft and superheat the air, which expands, pushing the craft upward. Slots in the craft's skirt push more air into the now-empty region and another pulse of light pushes the craft a little farther. A 15-cm (6 in.) diameter model has been tested in brief flights at the White Sands Missile Range at White Sands, N.M.
An advanced version of the Lightcraft would be a large helium-filled balloon that focuses microwaves beamed from the ground or space. The balloon would be ringed by ion engines that would electrify the air to push the craft upwards.
"It's a technical stretch in a lot of areas," Cole admits.
Advanced technologies for nuclear propulsion also are under study.
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"If NASA decides to send people to Mars with a nuclear rocket, we want to make sure that the rocket is as safe as possible, and perhaps to improve the performance," Cole said.
The simplest nuclear rocket would pump liquid hydrogen over a reactor core, and expel the superhot gas out a rocket nozzle. Cole said the University of Florida is studying high-temperature, high-strength alloys that would withstand the extreme temperature difference.
NASA/Glenn is studying an intriguing variation that would give a nuclear rocket the equivalent of a military jet's afterburner. In this scheme, liquid oxygen would be pumped into the exhaust nozzle. This would help cool the hydrogen enough that it could burn, combining with the oxygen to form water vapor.
A more powerful rocket would use nuclear fusion, the same power source at the heart of the sun. Controlled fusion - combining the nuclei of two lightweight atoms and reaping energy from the process - has eluded the best efforts of scientists and engineers since an early attempt in the late 1930s.
(In the 1930s, Arthur Kantrowitz, then with the National Advisory Council for Aeronautics - NASA's predecessor - made the known fusion attempt at Langley Field Station, now Langley Research Center. Kantrowitz and several colleagues rigged a pressure vessel normally used with a wind tunnel and tried to produce controlled fusion. They were far below the necessary pressures and temperatures needed, but it does mark the first attempt. It was described several years ago in American Heritage Invention & Technology.)
"It is not NASA's intent to solve fusion power production problems," Cole said. "We're trying to understand enough of the physics to understand typical plasmas that might be used in propulsion. That way, we'll be ready to take advantage of fusion whenever the problem is solved."
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Cole said that solving this problem is essential to large missions - including manned - to the outer planets."Mars is as far as we can go because the trip time is too long," Cole said, "unless we get specific impulses of 50,000 to 100,000 seconds." That's more than 200 times the efficiency of the Shuttle Main Engine, although the thrust would be lower and the fusion engine would fire for weeks at a time.
"Fusion could get us to the outer planets - and perhaps so could antimatter," Cole continued.
Antimatter is the stuff that powers fictional starships, but NASA is not planning to reach warp speeds with it. Again, the antimatter would be like a battery storing energy on Earth for later use in space. When expended, the antimatter would combine with matter to superheat a gas that would be expelled out a nozzle to power the spaceship.
And if you thought antimatter propulsion wasn't far-enough out on the research frontier, Mark Millis of NASA/Glenn will review the Breakthrough Physics program that is investigating some subtle and genuinely odd aspects of quantum physics that might be exploited in the next millennium for propulsion.
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MSFC Advanced Space Transportation Programs Office.
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