Tether propulsion
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Image: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.
Some current tether designs use crystalline plastics such as Dyneema. A possible future material would be carbon nanotubes, which have a theoretical strength of at least 60 GPa.
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Usage
Electrodynamic tethers
An electrodynamic tether is a conductive tether which generates current and, among other possibilities, acts against a planetary magnetic field. 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. Other applications of electrodynamic tethers are for power generation.
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 centrifugal 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.
Rotovators
The word rotovator is a portmanteau derived from the words rotor and elevator. A rotovator is a high speed rotating tether (sometimes called momentum exchange tethers), rotating so that the tips have a significant speed (~1-3 km per second).
A spacecraft in one orbit can rendezvous with one end of the tether, latch to it and be accelerated by its rotation. The tether and spacecraft would then later separate, when the spacecraft's velocity has been changed by the rotovator.
This is not free. The tether's momentum and angular momentum is changed, and must be recharged.
An example of use would be a rotovator in a circular orbit with the tip velocity zero relative to (or at) the ground that grabs an object from high altitude. However this obvious earth-to-orbit rotovator (skyhook) cannot be built from currently practical materials since the spin rate to give this tip speed is too high for current materials.
However, on airless bodies with reasonably low orbital speed (such as the Moon), a rotovator in low orbit can actually touch the ground and would provide cheap surface transport as well as launching materials into cislunar space. Although it might be thought that this requires constant reboost, in fact it can be shown to be energetically favourable to lift cargo off the surface of the Moon and drop it into a lower Earth orbit, since the moons surface is in a higher potential energy state; constant cycling of materials in loops can keep the system going continuously.
Thus rotovators can be charged by momentum exchange. Momentum charging uses the rotovator to move mass from a place that's higher in a gravity field to a place that is lower in a gravity field. 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 or near the Earth, and can use the momentum so gained to boost Earth goods to the Moon.
Systems of rotovators could theoretically open up inexpensive transportation throughout the solar system as well, as long as the net mass flow is towards a massive body, such as the Sun, or the Earth.
In a strong planetary magnetic field such as around the Earth or Saturn, a conducting rotovator can be configured as an electrodynamic tether. This can either be used as a Dynamo, which slows the tether, whilst generating electrical power and changing the angular momentum, or alternatively increasing its orbital speed and/or changing its angular momentum can be performed electrically from solar or nuclear power, by running current through a wire that goes the length of the tether. Ultimately, such a tether pushes against the angular momentum of the planet.
(One complication is that as the tether rotates, the direction of current must reverse to act against the magnetic field twice per cycle.)
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 achieve 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.
An important practical modification of a rotovator would be to add several latch points, to achieve different momentum transfers. Another useful concept would be to add a linear motor to the rotovator, to accelerate spacecraft or materials to higher speeds than the tip speed of the tether.
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).
Space elevator (beanstalk)
A beanstalk (a type of 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 rotovator 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 outside current technological limits (2006). Martian 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 objects hit the earth at near orbital speeds. Most anticipated cable designs would burn up before hitting the ground.
For a more extensive article on beanstalks, see space elevator.
Propusion challenges and other 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.
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. Disposal of waste heat is difficult in a vacuum, so over-heating may cause tether failures or damage.
In fiction
The mechanics of tether propulsion are critical in resolving the climax of the book The Descent of Anansi by Steven Barnes and Larry Niven .