Tether propulsion
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Tether-satellite-NASA.jpg
Tether propulsion uses long, strong strings (known as tethers) to change the orbits of spacecraft. It has the potential to make space travel significantly cheaper.
Most current tether designs use crystalline plastics such as Dyneema. A possible future material would be carbon nanotubes, which have theoretical strengths up to 100 gigapascals (though recent (2004) experiments suggest that 60 GPa may be more accurate).
There are four potential ways to use tethers for propulsion.
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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. There are two types of tidal forces: In one, the upper part of an object goes faster than its natural orbital speed, so centripetal force stretches the object upwards. The lower part moves slower than the orbital speed, so it pulls down. The other tidal force is that the top of a tall object weighs less than the bottom, so they are pulled by different amounts. On Earth, these are small effects, but in space, nothing opposes them.
The resulting tidal forces stabilize the satellite so that its long dimension points towards the planet it is orbiting. Simple satellites have often been stabilized this way, with tethers or mass distribution. Tidal stabilization is cheap and reliable. It uses no electronics, rockets or fuel. A small bottle of fluid must be mounted in the spacecraft to dampen vibrations with the friction of the fluid motion.
Electrodynamic tethers
An electrodynamic tether conducts current in order to act against a planetary magnetic field. It is a simplified, very low-budget magnetic sail. It can be used either to accelerate or brake an orbiting spacecraft. When the tether cuts the planet's magnetic field, it generates a current, and thereby reduces the energy of the spacecraft. When direct current is pumped through the tether, it exerts a force against the magnetic field, and the tether accelerates the spacecraft. The tether's end can be left bare, and this makes electrical contact with the ionosphere via the Phantom loop.
Electrodynamic tethers build up vibrations from variations in the magnetic and electric fields of the earth. Unless they are damped somehow, the vibrations grow large enough that the tether will fail in less than a month from mechanical stress. 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.
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 then stretch the tether and orient the satellite as described above. Jupiter rotates so rapidly that a tether can produce power and raise orbit passively and simultaneously [1] (http://spacetransportation.com/proseds/future.html).
Rotovators
Rotovators (sometimes called momentum exchange tethers) could theoretically open up inexpensive transportation throughout the solar system, as long as the net mass flow was toward the Sun. On airless planets (such as the moon), a rotovator in a polar orbit would provide cheap surface transport as well.
A rotovator is a rotating tether. A spacecraft in one orbit rendezvous with the end of the tether, latches to it and is accelerated by its rotation. This is not free. The tether's angular momentum is reduced or changed, and must be recharged. The tether and spacecraft separate later, when the spacecraft's velocity has been changed by the rotovator.
In a planetary magnetic field, a rotovator can be an electrodynamic tether, and its angular momentum can be charged electrically from solar or nuclear power, by running current through a wire that goes the length of the tether. When the tether turns over, the direction of current must reverse to act against the magnetic field. Ultimately, such a tether pushes against the angular momentum of the planet.
Rotovators can also be charged by momentum exchange. Momentum charging uses the rotovator to move mass from a place that's higher in the gravity well to a place that is lower in the gravity well. The energy from the falling weight speeds up the rotation of the rotovator. For example, it is possible to use a system of two or three rotovators to implement trade between the Moon and Earth. The rotovators 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.
Simple rotovators in circular orbits often can't be used because real materials are too weak. In particular, the obvious earth-to-orbit rotovator cannot be built from practical materials. This would be a rotovator in a circular orbit with the tip velocity zero at the ground.
One trick for using weaker materials is to put the rotovator in an elliptical orbit. It would pick up a load at periapsis (closest approach), then vary the tether length or attachment point to throw the load (from the top of the tether) at a later time into a higher orbit. This splits the speed-exchange into two parts, each contributing half of the final velocity. It reduces the necessary size, strength and weight of the tether dramatically. It might be called a "revovator" because it exchanges momentum in both revolution and rotation. Recharging such a rotovator is more complex, too.
Another trick to lower stresses is that rather than picking up a cargo from the ground, at zero velocity, a rotovator can pick up a moving vehicle and sling it into orbit. For example, a rotovator could pick up a Mach-12 aircraft from the upper atmosphere of the Earth, and move it into orbit without using rockets. It could likewise catch such an aircraft, and lower it into atmospheric flight. This would save tons of fuel per flight, and permits both a simpler vehicle and more cargo. If it rotated so slowly that the lower tip always pointed at Earth, it would be re-classified as a skyhook.
An important practical modification of a rotovator would be to add several latch points, to achieve different momentum transfers. Another important modification would be to add a linear motor to the rotovator, to accelerate spacecraft. This would permit travel times to the outer planets measured in months, rather than years. This is a very valuable option, given that such performance would otherwise require extremely exotic spacecraft propulsion systems.
Skyhooks
A tidal stabilized tether is called a "skyhook" since it appears to be "hooked onto the sky".
They are also called "hypersonic tethers" because the tip nearest the earth travels about Mach-12 in typical designs. Longer tethers would travel more slowly. At the limit of zero ground speed, it would be re-classified as a beanstalk.
An aircraft or sub-orbital vehicle transports cargo to one end of the skyhook.
Skyhook designs typically require climbers to transport the cargo to the other end (like a beanstalk).
Beanstalks, or Space Elevators
A beanstalk (more formally a space elevator) is a rotovator powered by the spin of a planet. For example, on Earth, a beanstalk would go from the equator to geosynchronous orbit.
A beanstalk does not need to be charged as a rotavator does, because it gets the required energy directly from its planet's angular momentum. The disadvantage is that it is much longer, and for many planets a beanstalk cannot be constructed from known materials. A beanstalk on Earth would require material strengths just within the current technological limits (2004). Mars and Lunar beanstalks could be built with modern-day materials however.
Beanstalks also have much larger amounts of potential energy than a rotavator, and if heavy parts should fail they might cause multiple impact events as heavy objects hit the earth at orbital speeds. Most anticipated cable designs would burn up before hitting the ground.
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 five hours for a length of ten km. Several systems have been proposed to improve this. The US Naval Research Laboratory 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. In low Earth orbit, a tether could be wiggled to dodge known pieces of space junk.
Beanstalks and rotovators are currently limited by the strengths of available materials. Although ultra-high strength plastic fibers (Kevlar and Spectra) permit rotovators to pluck masses from the surface of the Moon and Mars, a rotovator from these materials cannot lift from the surface of the Earth. In theory, high flying, supersonic (or hypersonic) aircraft could deliver a payload to a rotovator that dipped into Earth's upper atmosphere briefly at predictable locations throughout the tropic (and temperate) zone of Earth.
Tethers have many modes of vibration, and these can build to cause stresses so high that the tether breaks. Oscillations (also called vibrations or mechanical transients) 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. The one ton climber proposed by Dr. Walter Edwards may detect and suppress most vibrations by changing speed and direction. The climber can also repair or augment a tether by spinning more strands.
Cargo capture for rotovators is nontrivial, and failure to capture is generally catastrophic. Several systems have been proposed, such as shooting nets at the cargo, but all add weight, complexity, and another failure mode.
Currently, the strongest materials in tension are plastics that require a coating for protection from UV radiation and (depending on the orbit) erosion by atomic oxygen.
Over a few tens of days, electrodynamic tethers in Earth orbit can 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. Unexpected electrostatic discharges have cut tethers, damaged electronics, and welded tether handling machinery. It may be that the Earth's magnetic field is not as homogeneous as some engineers have believed.
Disposal of waste heat is difficult in a vacuum, so over-heating may cause tether failures or damage.
External Links
- NASA IAC report on orbital systems (http://www.niac.usra.edu/files/library/meetings/fellows/nov99/355Bogar.pdf)
- Hoytethers (http://www.tethers.com/Hoytether.html)
- ProSEDS, a tether-based propulsion experiment (http://spacetransportation.com/proseds/)
- Tethers Unlimited Incorporated (http://www.tethers.com/)
- spacetethers.com
- NASA Tether Overview (http://www1.msfc.nasa.gov/NEWSROOM/background/facts/momentum.pdf)