Tether solution for ISS – study

by Chris Bergin

Tethers could be used to stabilize, reboost, or even change the orbit of the International Space Station (ISS), according to a recent NASA study. That’s the promise of an electrodynamic tether – a simple wire that turns the entire planet Earth into a giant motor, according a presentation obtained by NASASpaceflight.com.

The document details how the early ideas for a space station used “gravity-gradient” configurations to stay naturally stable, but today’s ISS has deviated from that form to satisfy microgravity requirements. A simple tether could restore that passive stability, and make hardware failures – such as the recent loss of a control moment gyro – no cause for concern.

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Moving up in capability and complexity would be a conductive tether that would push electrical current and fight the aerodynamic drag forces on the station. Even at the station’s altitude, there’s still enough residual atmosphere to cause drag, which causes the station’s orbit to drop.

Regular flights of the Russian Progress spacecraft bring needed fuel to the station to fire its engines and restore the orbit, but without that propellant, the station could be lost much like Skylab in the late 1970s.

The electrodynamic tether would do the same job without using propellant since the tether can push against the Earth’s magnetic field, and use energy collected by solar panels to drive electrical current.

However, the most ambitious plan laid out in the document would be to move the station’s orbital inclination from 51.6° to 28.5°. A 28.5° inclination is the minimum inclination that can be reached from Kennedy Space Center (KSC), and allows the Shuttle to carry a maximum payload.

When the Russian Space Agency was invited to join the program, the planned inclination was changed to 51.6°, so that Russian rockets could reach the station from Baikonur Cosmodrome.

Now that the French are building a Soyuz launch site in French Guiana, there would be the opportunity to reduce the orbital inclination of the station. But it would be so difficult to do with chemical rockets that it would be nearly impossible, requiring a mass of propellant nearly as large as the station itself to be launched.

Electrical rockets like ion engines could do the job for less propellant, but would take enormous power supplies. The document lays out how an electrodynamic tether could do the same job for no propellant and a much more modest power supply.

The magic of the tether is that, ideally, it can generate the forces needed to change the orbital inclination without requiring net electrical power. To work this magic though, it must be able to flow large amounts of electrical energy out and then collect them back again at high efficiency.

The document projects that this can be achieved by using spinning flywheels to store the energy. It projects that a 50-km tether could move the ISS in four years from its current inclination to the ideal.

Furthermore, this tether technology would lead the way to momentum-exchange tethers that could be used to throw people and cargo to the Moon. (Click here for article and information).

At the very least, such advanced concepts could become the key to ensuring the US space program has alternative options that could both sustain mission flexibility with – or without – being tied into the Vision for Space Exploration (VSE).




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