Some current tether designs use crystalline plastics such as ultra high molecular weight polyethylene, aramid or carbon fiber. A possible future material would be carbon nanotubes, which have a theoretical tensile strength of at least 60 GPa.
History of tether propulsion
Some of the earliest writings on space tethers can be found in the work of Tsiolkovsky. He proposed a tower so tall that it reached into space, held there by the rotation of the Earth. However, there was no realistic way to build it.
Hans Moravec wrote extensively on synchronous and non synchronous tethers, and performed detailed simulations of tapered tethers that could pick objects off and place objects onto the Moon, Mars and other planets.
More recently Brad Edwards has done a very great deal to popularise the subject again in the scientific community, and it is now an area of active research. Many people consider it likely that this can actually be achieved.
To achieve maximum performance and low cost, tethers need to be made of materials with the combination of high tensile strength and low density. Depending on the type of tether, the design equations describe the material by one of three typical quantities.
Space elevator equations typically use a ‘characteristic length’ (Lc). Lc is also known as its 'self-support length' and is the length of untapered cable it can support in a constant 1g gravity field. Lc=σ/ρg.
Hypersonic skyhook equations use the material’s ‘specific velocity’ which is equal to the maximum tangential velocity a spinning hoop can attain without breaking. Vs=√(σ/ρ).
Finally, for rotating tethers (rotovators) the value used is the material’s ‘characteristic velocity’ which is the maximum tip velocity a rotating untapered cable can attain without breaking. Vc=√(2σ/ρ). The specific and characteristic velocities are the same except for the factor of two inside the radical.
These values are used in equations similar to the rocket equation and are analogous to specific impulse or exhaust velocity. The higher these values are, the more efficient and lighter the tether can be in relation to the payloads that they can carry. Eventually however, the mass of the tether propulsion system will be limited at the low end by other factors such as momentum storage.
One material that has great potential is M5 fiber. This is a synthetic fiber that is lighter than Kevlar or Spectra. According to Pearson, Levin, Oldson, and Wykes in their article The Lunar Space Elevator, an M5 ribbon 30 mm wide and 0.023 mm thick, would be able to support 2000 kg on the lunar surface (2005). It would also be able to hold 100 cargo vehicles, each with a mass of 580 kg, evenly spaced along the length of the elevator. Other materials that could be used are T1000G carbon fiber, Spectra 2000, or Zylon. All of these materials have breaking lengths of several hundred kilometers under 1g (10 m/s²).
To exceed the self-support length the tether material can be tapered so that the cross-sectional area varies with the total load at each point along the length of the cable. Correct tapering ensures that the tensile stress at every point in the cable is exactly the same. For very demanding applications, such as an Earth Space Elevator, the tapering can result in excessive ratios of cable weight to payload weight.
In addition the cable must be constructed to withstand micrometeorites and space junk. This can be achieved with the use of redundant cables, such as the Hoytether; redundancy can ensure that it is very unlikely that multiple redundant cables would be damaged near the same point on the cable, and hence a very large amount of total damage can occur over different parts of the cable before failure occurs.
Gravity-gradient stabilization, also called "gravity stabilization" and "tidal stabilization", is cheap and reliable. It uses no electronics, rockets or fuel.
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 ways of explaining 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. Another way to explain tidal force is that the top of a tall object weighs less than the bottom, so they are pulled by different amounts. The "extra" pull on the "bottom" of the object stretches it out. 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. A small bottle of fluid must be mounted in the spacecraft to damp pendulum vibrations with the friction of the fluid motion.
A tidal stabilized tether is called a "skyhook" since it appears to be "hooked onto the sky". This term was introduced relating to satellites and orbital mechanics by the Italian scientist Giuseppe Colombo. Skyhooks rotate precisely once per orbit and hence are always oriented the same way to the parent body.
Some are called "hypersonic skyhooks" because the tip nearest the earth travels about Mach-12 to 16 in typical designs. Longer tethers would travel more slowly. At the limit of zero ground speed, it would be re-classified as a space elevator or 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).
Robert Raymond Boyd and Dimitri David Thomas (with Lockheed Martin Corporation) patented the Skyhook idea in 2000 in a patent titled "Space elevator".
Space elevator (beanstalk)
A beanstalk (a type of space elevator) is a skyhook that is attached to planetary body. For example, on Earth, a beanstalk would go from the equator to geosynchronous orbit.
A beanstalk does not need to be powered as a rotovator does, because it gets the required angular momentum directly from the planetary. 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 (2007). Martian and Lunar beanstalks could be built with modern-day materials however. A space elevator on Phobos has also been proposed.
Beanstalks also have much larger amounts of potential energy than a rotovator, 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.
Tether cable catapult system
A tether cable catapult system is a system where two or more long conducting tethers are held rigidly in a straight line, attached to a heavy mass. Power is applied to the tethers and is picked up by a vehicle that has linear magnet motors on it, which it uses to push itself along the length of the cable. Near the end of the cable the vehicle releases a payload and slows and stops itself and the payload carries on at very high velocity. The calculated maximum speed for this system is extremely high, more than 30 times the speed of sound in the cable; and velocities of more than 30 km/s seem to be possible.
Challenges and other problems
Objects in low earth orbit are subjected to noticeable erosion from monomolecular oxygen, due to the high orbital speed with which the molecules strike as well as their high reactivity.
Micrometeorites and space junk
Simple tethers are quickly cut by micrometeoroids and space junk. The lifetime of a simple, one-strand tether in space is on the order of five hours for a length of ten kilometers. This was originally a show stopper for the use of tethers.
Several systems have since 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 P. 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.
Very large pieces of junk would still cut most tethers however, but these are currently tracked on radar and run on predictable orbits, and a tether could be wiggled to dodge known pieces of space junk or thrusters used to change the orbit well before a collision could occur.
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.
Computer models frequently show tethers can snap due to vibration.
Mechanical tether-handling equipment is often surprisingly heavy, with complex controls to damp vibrations. The one ton climber proposed by Dr. Brad Edwards for his Space Elevator may detect and suppress most vibrations by changing speed and direction. The climber can also repair or augment a tether by spinning more strands.
The vibration modes that may be a problem include skipping rope, transverse, longitudinal, and pendulum.
Tethers are nearly always tapered, and this can greatly amplify the movement at the thinnest tip in whip like ways.
Cargo capture for rotovators is nontrivial, and failure to capture can cause problems. 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.
Control and modelling issues
A tether is not a spherical object, and has significant extent. This means that, as an extended object, it is not directly modellable as a point source, and this means that the center of mass and center of gravity are not usually colocated, and the inverse square law does not apply except at large distances to the overall behaviour of a tether. This makes prediction and modelling extremely complex.
This was a simple tether launched March 9, 1994; and was successfully deployed, and met the mission objectives including having minimal swing and good deployment length. It was expected to last almost two weeks, but in fact was cut after 3.7 days and the lower end quickly experienced atmospheric entry.
NASA's space tether experiment
NASA deployed an electromagnetic tether as an experiment. Unfortunately it burned through due to excessive current flow.
Published - July 2009
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