Posted on 06/18/2003 9:10:00 AM PDT by RightWhale
Tether Technology: A New Spin on Space Propulsion
By Leonard David Senior Space Writer
posted: 07:00 am ET 18 June 2003
In the near future, revolutionary space hardware could put an exciting spin on spaceflight. NASA is putting money into Momentum-eXchange/Electrodynamic Reboost tether technology -- MXER for short -- an innovative concept that if implemented would station miles and miles of cart-wheeling cable in orbit around the Earth. Then, rotating like a giant sling, the cable would swoop down and pick up spacecraft in low orbits, then hurl them to higher orbits or even lob them onward to other planets. MXER is part space technology, part celestial square dancing - the ultimate dos-à-dos swing machine. The hope is to harness momentum while dramatically lowering the cost of launching space missions.
Last month, NASA picked over a dozen industry, government and academic groups to tackle novel propulsion ideas that could transform exploration and scientific study of the solar system. Under the auspices of the In-Space Propulsion (ISP) Program, NASA is footing the bill on five research areas: aerocapture; advanced chemical propulsion; solar electric propulsion; solar sail technologies; and space-based tether propulsion. The program is managed in the Office of Space Sciences at NASA Headquarters. "The MXER Tether System will serve as a fully-reusable transportation hub in orbit. It's like a 'space railroad'," said Robert Hoyt, President and Chief Scientist for Tethers Unlimited, Inc./ScienceOps. TUI was awarded funds to look into a MXER tether system based on deployment of a 62-mile (100-kilometer) long cable in orbit around the Earth.
A tether pick-up service for Earth-launched payloads offers cost-cutting pluses. By eliminating the need to launch an upper-stage rocket along with each satellite, Hoyt said that the MXER Tether System means satellites can be boosted into space atop smaller, less expensive rockets. Propulsion costs for space missions would drop by a factor of ten or more, he said.
Failsafe survival
TUI is based in Lynnwood, Washington and is actively working on tether concepts for NASA, the U.S. Department of Defense, and commercial customers. TUI's ScienceOps Division provides scientific computing services, including development, validation, and optimization of scientific software and algorithms. Hoyt said that over the next two years the group's strong focus is on developing a high-strength tether suitable for MXER applications. In addition, special TetherSim computer code will be upgraded to enable in-depth simulation of the rendezvous and capture problems of handling payloads with a MXER tether.
TUI is collaborating with the Air Force Research Laboratory's Materials Laboratory to further earlier progress on a patented "Hoytether". That work centers on creating a failsafe, multi-line tether structure. All the better to survive many years of exposure to the space environment and any nasty run in with debris. "It's sort-of like a one-hundred kilometer long fish-net stocking in space, only it's incredibly strong, and it can withstand many years of bombardment by orbital debris," Hoyt said.
Hard work ahead
Along with Tethers Unlimited, NASA has also awarded MXER tether technology contracts to the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, as well as Lockheed Martin in Denver, Colorado and Tennessee Technological University (TTU) in Cookeville, Tennessee. Work on MXER is rapidly moving ahead. For example, TTU faculty and students are readying a tether momentum exchange experiment - but it won't leave Earth. A scaled model is being devised to exhibit how to capture and release a payload with a rotating tether in a microgravity environment. The tether research will occur inside a KC-135 aircraft under the guidance of NASA through the Reduced Gravity Student Flight Opportunities Program.
How soon MXER could be up and running in space is another matter. "Realistically, I believe it will take about 7-10 years of hard work to get a MXER flight demonstration into orbit," Hoyt told SPACE.com. Under the new In-Space Propulsion contracts, research teams will be busy over the next couple of years using simulations and ground tests to show that the technical challenges of MXER tethers can be solved, he said. "If those efforts are successful, I expect that it would take about 5 years to get a flight program off the ground," Hoyt senses.
High risk, high payoff
MXER tethers are considered by NASA to be "high risk with high payoff," said Les Johnson, Manager, In-Space Propulsion Technology Projects at the NASA Marshall Space Flight Center in Huntsville, Alabama. Efforts at this time are geared to examining the technologies that will make or break MXER for use as an in-space transportation system, Johnson told SPACE.com.
Johnson said that MXER contract awards are focused on rendezvous and capture techniques, stability of the tether, as well as strength and survivability of the cable as it speeds through space. "We are years away from a system-level space demonstration, though not as far as many might naively think. If these key issues can be worked, as we believe they can, MXER tethers might be a viable candidate for use in space by the middle of the next decade," Johnson said.
Sketchy past
Hoyt said that the beauty of the MXER tether is in its reusability. Once the first system is up, fully functional and swinging away, that early hardware can be used to recoup the initial investment on developing the technology. No doubt, given a somewhat sketchy past, a MXER tether needs a solid shakeout in space. NASA has had a couple of high-profile setbacks with tether experiments over the years. But Hoyt counters that at least 17 tether trials have been conducted on-orbit so far. The majority of them have been highly successful, he said.
"Even the ones that didn't go perfectly taught us a great deal. The failures that did happen were more the result of human error and our imperfect understanding of complex technologies rather than any fundamental problem with the technology," Hoyt said. Furthermore, look at how many rockets the early astronauts watched blow up before they rode them into the sky, he added.
Building trust
To help build space community trust in tethers, Hoyt said that several low-cost flight experiments could be flown by late 2004 or early 2005. The first such confidence-builder is the "Multi-Application Survivable Tether", or MAST experiment. This NASA-funded work couples Tethers Unlimited with students and faculty at Stanford University. The project entails deployment of three tiny spacecraft along a lengthy tether. The MAST experiment would show off numerous key technologies, such as tether designs that can survive in space and momentum-exchange propulsion.
"We hope to build upon that experience to fly a very cool mission a couple of years later that will demonstrate electrodynamic tether orbit-raising, and then graduate to a full MXER tether flight demonstration at the end of the decade," Hoyt said. Space tethers have such a huge potential payoff for lowering the cost of space missions, Hoyt believes. "It's inevitable that people will keep trying them until they get it right."
I understand that you could recover the energy by taking higher spacecraft and lowering them, or de-orbiting them. But how often will this be done?
Maybe I'm answering my own question. The energy will be taken from dead spacecraft and boosters, with a side bennefit of cleaning up orbital space?
Might work, but sounds pretty complicated to organize.
In NASA-speak, this means it might get off the drawing-board stage in 7-10 decades, at which point the final remaining shuttle will have finally disintegrated into fine space dust after having been kept flying by being patched together with chicken wire and chewing gum to keep it going for just one more flight.
So9
No, we'll have to get Michael Faraday or someone else with the common touch to 'splain this.
But why? Now, in addition to just launching your satellite, you have to make it do a rendezvous with the tether, and you've opened yourself up to all sorts of collision-related risks.
There's an easier way to use tethers: just reel one out, then pump an electric current through it. Presto: electric propulsion.
You could raise the tether's orbit by pumping current through it. The tether would be just like the rotor in an electric motor, interacting with the Earth's magnetic field. Pump current, the tether moves.
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