Tether propulsion

Tether propulsion systems are proposals to use long, very strong cables (known as tethers) to change the velocity of spacecraft. The tethers may be used to initiate launch, complete launch, or alter the orbit of a spacecraft. Spaceflight using this form of spacecraft propulsion may be significantly less expensive than spaceflight using rocket engines.

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.

Later, another Russian, Yuri Artsutanov wrote in greater detail about a tensile cable to be deployed from a geosynchronous satellite in Komsomolskaya Pravda (July 31, 1960).

Jerome Pearson explored synchronous tetherscite web| url=http://www.niac.usra.edu/files/studies/final_report/1032Pearson.pdf| last=Pearson| year= 2005| title=Lunar Space Elevators for Cislunar Space Development Phase I Final Technical Report| first=Jerome| coauthors= Eugene Levin, John Oldson and Harry Wykes| format=PDF] further, and in particular analysed the lunar elevator that can go through the L1 and L2 points.

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.

Construction

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’ (L_c). L_c is also known as its 'self-support length' and is the length of untapered cable it can support in a constant 1"g" gravity field. L_c=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. V_s=√(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. V_c=√(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.

Building materials

Materials proposed include Kevlar, Ultra high molecular weight polyethylene, Carbon nanotubes, M5 fiber, diamond.

One material that has great potential is M5 fiber. This is a synthetic fiber that is lighter than Kevlar or Spectra. [Bacon 2005] 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 1"g" (10 m/s²).

hape

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.

Tether systems

Tidal stabilization

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.

Electrodynamic tethers

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 and changes the angular momentum whilst generating electrical power, or alternatively, its orbital speed and/or angular momentum can be increased electrically from solar or nuclear power by running current through a wire that goes the length of the tether. Thus the tether can be used either to accelerate or brake an orbiting spacecraft.

In both cases the tether pushes against the planet, and thus the momentum gained or lost ultimately comes from the planet.

One complication to these techniques is that if the tether rotates, the direction of current must reverse (such as is the case in alternating currents).

Momentum exchange tethers

A momentum exchange tether is a high speed rotating tether, spinning so that the tips have a significant speed (~1-3 km/s).

The maximum speed is limited by stress tolerances of the tether but this can be greatly increased if it is of thicker cross-section in the middle and tapers and is lighter, thinner at the tips.

A spacecraft could rendezvous with one end of the tether, latch to it, and be accelerated by the tether's rotation. The tether and spacecraft would then separate at a later point 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 this costs energy that must be recouped. The idea is that the recharge could be done with some form of energy (for instance solar panels generating current for electromagnetic propulsion) that is far cheaper than multi-stage-rocket fuel.

Rotovators can also be used to slow down incoming spacecraft, thus increasing the rotovator's momentum. If the average momentum gained from inward traffic equals that imparted to outward traffic, there is no net energy cost, and thus nothing to recoup.

Rotovators

The word rotovator is a portmanteau derived from the words "" and "elevator". Rotovators would be momentum exchange tethers, with a retrograde motion of the tip closest to their parent body relative to the center of the tether.

Because the tips have a significant speed (typically 1-3 km/s), it can be possible in some cases to cancel the orbital speed such that the tips are stationary at their lowest point with respect to a planetary surface or lunar body.

On bodies with reasonably low orbital speed (such as the Moon and possibly Mars), a rotovator in low orbit can actually touch the ground, thereby providing cheap surface transport as well as launching materials into cislunar space.

Cislunar transportation system

Although it might be thought that this requires constant energy input, it can in fact be shown to be energetically favourable to lift cargo off the surface of the Moon and drop it into a lower Earth orbit, and thus it can be achieved without any significant use of propellant, since the moon's surface is in a comparatively higher potential energy state. [ [http://www.tethers.com/papers/LDC00Cislunar.pdf THE CISLUNAR TETHER TRANSPORT SYSTEM ARCHITECTURE Robert P. Hoyt Tethers Unlimited, Inc.] ]

Rotovators can thus 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". 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 exports are not available) dumped on or near the Earth, and can use the momentum so gained to boost Earth goods to the Moon. Thus the momentum and energy exchange is balanced with equal flows in either direction, and the system is stable.

Similar systems of rotovators could theoretically open up inexpensive transportation throughout the solar system.

HASTOL — Earth launch assist rotovator

Unfortunately an Earth-to-orbit rotovator cannot be built from currently available materials since the thickness and tether mass to handle the loads on the rotovator would be uneconomically large. A "watered down" rotovator with two-thirds the rotational speed, however, would halve centripetal acceleration stresses.

Therefore another trick to achieve lower stresses is that rather than picking up a cargo from the ground at zero velocity, a rotovator could 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, and could likewise catch such a vehicle and lower it into atmospheric flight. It is easier for a rocket to achieve the lower tip speed, so "Single Stage To Tether" has been proposed. One such is called the Hypersonic Airplane Space Tether Orbital Launch (HASTOL). Either air breathing or rocket to tether could save a great deal of fuel per flight, and would permit for 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. [ [http://www.npr.org/templates/story/story.php?storyId=9574513 Space Tethers: Slinging Objects in Orbit?] by Nell Boyce - National Public Radio - 16 April 2007]

Skyhooks

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" [http://www.google.com/patents?vid=USPAT6491258] .

The company Tethers Unlimited Inc (founded by Dr. Robert Forward and Dr. Robert P. Hoyt) has called this approach "Tether Launch Assist". [ [http://www.tethers.com/LaunchAssist.html Tethers Unlimited Inc, "Tether Launch Assist"] ]

pace 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. [ [http://www.isr.us/Downloads/niac_pdf/chapter7.html Space Elevator - Chapter 7: Destinations] ] A space elevator on Phobos has also been proposed. [ [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030065879_2003074809.pdf Space Colonization Using Space-Elevators from Phobos Leonard M. Weinstein] ]

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. [US Patent 6290186]

Challenges and other problems

Monomolecular oxygen

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 deploymentFact|date=April 2007. 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.

Material strength

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.

Vibrations

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. [ [http://liftoff.msfc.nasa.gov/academy/tether/dynamics.html Tether dynamics] ]

Tethers are nearly always tapered, and this can greatly amplify the movement at the thinnest tip in whip like ways.

Cargo capture

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.

Life Expectancy

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.

Real Missions

EDS-2

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. [ [http://www.tetherapplications.com/seds2.htm SEDS-2] ]

MAST

The MAST tether experiment was launched aboard a Dnepr rocket in April 2007. Unfortunately, the tether did not deploy successfully.

NASA's space tether experiment

NASA deployed an electromagnetic tether as an experiment. Unfortunately it burned through due to excessive current flow. [ [http://www-istp.gsfc.nasa.gov/Education/wtether.html The Space Tether Experiment] ]

YES2

The YES2 satellite was launched September 14, 2007 from Baikonur. The YES2 satellite employed a 30 km long tether to deorbit a small re-entry capsule. [ [http://www.yes2.info YES2] ]

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.

ee also

*Lofstrom launch loop - cable-like launch system
*rope splicing - how tethers can be joined and terminated
*Spacecraft propulsion

References

External links

* [http://data.engin.umich.edu/Icarus/proseds_ed.html ProSEDS, a tether-based propulsion experiment]
* [http://cfa-www.harvard.edu/~spgroup/ Special Projects Group]
* [http://www1.msfc.nasa.gov/NEWSROOM/background/facts/momentum.pdf NASA Tether Overview]
* [http://www.tethers.com/ Tethers Unlimited Incorporated]
* [http://www.tethers.com/papers/TethersInSpace.pdf "Tethers In Space Handbook" M.L. Cosmo and E.C. Lorenzini Third Edition December 1997]
* [http://www.niac.usra.edu/files/library/meetings/fellows/nov99/355Bogar.pdf NASA IAC report on orbital systems]
* [http://www.frc.ri.cmu.edu/~hpm/project.archive/general.articles/1987/skyhook.ltx Hans Moravec's thoughts on skyhooks, tethers, rotovators etc.]
* [http://spacetethers.com/ SpaceTethers.com, space tether simulator applet]
* [http://www.npr.org/templates/story/story.php?storyId=9574513 Npr.org - Space Tethers: Slinging Objects in Orbit?]
* [http://www.esa.int/esaED/SEMVJ1AATME_highereducation_0.html ESA - The YES2 project]
* [http://www.esa.int/esaHS/SEMG3ZU681F_index_0.html ESA - Students test 'space postal service' during Foton mission]