Newswise — NASA supports Tennessee Tech University mechanical engineering professor Steven Canfield's efforts to move beyond the technology's conceptual stage, developing and testing models on Earth and bringing it one step closer to reality.
"Space tethers have made headlines since NASA began investigating a way to harness the earth's magnetic field for in-space propulsion," said Canfield, who has worked as a faculty researcher each summer since 1999 for the In-Space Propulsion Technology group at NASA's Marshall Space Flight Center.
Currently, a tether project called a "space elevator" is receiving a lot of national publicity, offering a vision of satellites or space vehicles shimmying up a 62,000 mile cable into space. However, the technological challenges that exist for such a tether make its implementation a far-fetched dream.
On the other hand, NASA's In-Space Propulsion group has contracted with groups, including Canfield's Tennessee Tech team, to investigate a rotating tether concept that can be implemented using technology that should be available within the next five years. The concept has the potential to provide a much-needed space transportation vehicle.
As one of only a dozen industry, government and academic groups recently funded by NASA to tackle novel ways to propel payloads into orbit, Canfield leads Tennessee Tech's work with a tether project called momentum-exchange electrodynamic reboost tether technology — MXER for short.
Think of a tether as a really long space string or cable equipped with a catching mechanism, a space glove of sorts, to capture payloads. In the MXER tether approach, the rotating tether transfers some of its momentum to a satellite through a brief but controlled capture and release process. After this energy transfer, the tether uses the Earth's magnetic field to propel itself into an orbit ready to launch another satellite, all without the use of onboard propellants.
Pulling a pair of earplugs on a string from his desk, Canfield simplified the complex concepts into an understandable illustration. Making the string cartwheel, rolling end over end along a line parallel to his desktop, or "Earth's lower orbit" in the illustration, he demonstrated how MXER works.
"The tether moves like a wheel in orbit around the Earth, storing kinetic energy as it travels, much the same way ice skaters play 'crack the whip,'" said Canfield, rotating the string. "The payload, which could be a satellite or spacecraft, would be launched into low orbit to arrive for a rendezvous with the tether.
"The tether would snare the payload at the precise moment they both arrive at the same spot, then toss it into another orbit," he continued. "The energy is then transferred from the tether to the payload."
MXER technology could dramatically reduce the cost of raising the orbits of spacecraft, including those destined for deep-space missions. The process could eliminate or reduce the need for a booster rocket, which is not reusable and carries high fuel costs. And because a MXER tether can reboost its own orbit with stored energy and no additional propellant, it could perpetually capture and toss payloads.
Canfield's work with MXER has made its way up the "ladder" NASA uses to evaluate a technology's readiness for space use. Projects are tracked from stage one, an idea, to stage nine, daily operation.
"To advance technology, you have to work your way up a series of tasks," said Canfield. "With MXER, we're on our way to level four and possibly five, where we'll test elements on the ground in a simulated environment and try to replicate the results."
NASA considers MXER technology "high-risk, high-payoff." Several key challenges must be resolved before MXER technology is feasible. Current materials available to make tethers have a hard time lasting in the space environment. Stability is also another key issue, along with reliable rendezvous and capture mechanisms.
For more than a year, Tennessee Tech and Lockheed Martin have teamed up on the capture device development and process vital to MXER's future. The capture demands precise design and execution. For instance, unlike the deliberate crawl and delicate docking seen when the space station and a shuttle connect in orbit, this payload capture ideally would take place in less than a second.
"Testing space devices on the ground is hard," said Canfield, "but tethers actually create their own 'artificial gravity' when they rotate, so we don't have as much trouble simulating how they would act in space. Think of the force you feel on a centrifugal ride at the fair; you are spinning around and feeling that force created by the motion pushing against you. It's the same concept."
The more difficult simulation involves replicating how a payload acts in low orbit.
"We've built a machine that will accurately throw a payload, but now we have to introduce variables to more accurately simulate what could and would happen in space," he said. We have to create a capture window to account for potential errors."
A group of Canfield's students earned a spot in NASA's microgravity experiment facility aboard a KC-135 aircraft in April 2003, which allowed the team to early evaluation of their simulations.
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