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Tether propulsion

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Tether propulsion uses long, strong strings to change the orbits of spacecraft. It has the potential to make space travel as inexpensive as train travel.

There are several operating methods.

Table of contents

Tidal stabilization

An attitude control tether has a small mass on one end, and a satellite on the other. Tidal forces stretch the tether between the two masses, stabilizing the satellite so that its long dimension is always oriented towards the planet it is orbiting. Several of the earliest satellites were stabilized this way, or used mass distribution to get tidal stabilization. This is a simple form of stabilization that uses no electronics, rockets or fuel.

Electrodynamic Tethers

An electrodynamic tether conducts current in order to act against a planetary magnetic field. It's a simplified, very low-budget magnetic sail.

When the conductive tether is trailed in a planetary or solar magnetosphere (magnetic field), the tether cuts the field, generates a current, and thereby slows the spacecraft into a lower orbit. By pumping direct current through a tether, the spacecraft can be moved into a higher orbit. The tether's end can be left bare, and this is sufficient to make contact with the ionosphere and allow a current to flow.

An important patented application of an unpowered electrodynamic tether is to deorbit decommissioned satellites without the weight and complexity of a retrorocket. The tether deployment can be as simple as a spring. Tidal forces stretch the tether and orient the satellite as described above.

Electrodynamic tethers build up vibrations from variations in magnetic and electric fields. The vibrations grow large enough so the tether will fail in less than a month. One plan to control these is to vary the tether current to oppose the vibrations. In simulations, this keeps the tether together.

The sensors to sense tether vibrations can either be an inertial navigation system on one end of the tether, or satellite navigation systems mounted on the tether, transmitting their positions to a receiver on the end.

Rotavators

A rotavator is a rotating tether. A spacecraft in one orbit rendezvous with the end of the tether, latching onto it and being accelerated by its rotation. They separate later, when the spacecraft's velocity has been changed by the rotovator. The momentum of the tether is changed to accelerate the spacecraft.

In a planetary magnetic field, a rotavator can be an electrodynamic tether, and be charged electrically from solar or nuclear power.

Rotavators can also be charged by momentum exchange. In general, rotavators can be charged by moving mass to them from a place that's higher in the gravity well to a place that is lower in the gravity well. For example, it is possible to use a system of two or three rotavators to implement trade between the Moon and Earth. The rotavators are charged by lunar mass (dirt, if imports are not available) dumped on Earth, and use the momentum so gained to boost Earth goods to the Moon.

One trick for using real=life, weaker materials is to put the rotavator in an elliptical orbit. It would pick up a load at one perigee, then vary the tether length to throw the load (from the top of the tether) at a later perigee. This splits the speed-exchange into two parts, each contributing half of the final velocity. It reduces the size, strength and weight of the tether dramatically.

Rotavators could theoretically open up inexpensive transportation throughout the solar system, as long as the net mass flow was toward the Sun. On airless planets, a rotavator in a polar orbit would provide cheap surface transport as well.

An important modification of a rotavator would be to add several latch points, to get different momentum transfers. Another important modification would be to add a linear motor to the rotavator, to accelerate spacecraft. This would permit travel times to the outer planets that were measured in months, rather than years. This is a very valuable option, given that such performance otherwise requires extremely exotic spacecraft propulsion systems.

Beanstalks

A beanstalk is a rotavator powered by the spin of a planet. For example, on Earth, a beanstalk would go from the equator to geosynchronous orbit.

The advantage of a beanstalk over a rotavator is that it does not need to be charged.

The disadvantage is that it is usually immensely longer, and for many planets a beanstalk cannot be constructed from known materials. An Earth beanstalk would be near the limit of current known material strength. Beanstalks also have much larger amounts of potential energy, and if they should fail they would cause more damage than a rotavator.

For a more extensive article on beanstalks, see space elevator.

Problems

Simple tethers are quickly cut by micrometeoroids. The lifetime of a simple, one-strand tether in space is on the order of 5 hours for a length of 10 km. Several systems have been proposed to correct this. The U.S. Naval Research Lab has successfully flown a long term tether that used very fluffy yarn. This is reported to remain uncut several years after deployment. Another proposal is to use a tape or cloth. Dr. Robert Hoyt[?] patented an engineered circular net, such that a cut strand's strains would be redistributed automatically around the severed strand. This is called a Hoytether. Hoytethers have theoretical lifetimes of tens of years.

Beanstalks and rotavators are currently limited by the strengths of available materials. Although ultra-high strength plastic fibers (Kevlar and Spectra[?]) permit roatvators to pluck masses from the surface of the Moon and Mars, a rotavator from these materials cannot lift from the surface of the Earth. A rotavator of these materials could, however, pluck a Mach-12 aircraft into orbit from Earth, greatly lowering the cost and complexity of an orbital shuttle.

Tethers have many modes of vibration, and these can build to cause stresses so high that the tether breaks. Oscillations can be sensed by radio beacons on the tether, or inertial and tension sensors on the end-points. Mechanical tether-handling equipment is often surprisingly heavy, with complex controls to damp vibrations. Over a few tens of days, electrodynamic tethers in Earth orbit can quickly build vibrations in many modes, as their orbit interacts with irregularities in magnetic and gravitational fields. Electrodynamic tethers can be stabilized by reducing their current when it would feed the oscillations, and increasing it when it opposes oscillations.

Several conductive tethers have failed from unexpected current surges. It may be that the Earth's magnetic field is not as homogeneous as some engineers have believed. Unexpected electrostatic discharges have cut tethers, damaged electronics, and welded tether handling machinery.

See spacecraft propulsion, magnetic sail.



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