Tether propulsion systems are proposals to use long, very strong cables (known as
tetherA tether is a cord that anchors something movable to a stationary point. There are a number of applications for tethers, but the primary use is limiting the movement of animals.-Uses for tethers:...
s) to change the velocity of
spacecraftA spacecraft is a craft or machine designed for spaceflight. On a sub-orbital spaceflight, a spacecraft enters space then returns to the Earth. For an orbital spaceflight, a spacecraft enters a closed orbit around the planetary body. Spacecraft used for human spaceflight carry people on board as...
. The tethers may be used to initiate launch, complete launch, or alter the orbit of a
spacecraftA spacecraft is a craft or machine designed for spaceflight. On a sub-orbital spaceflight, a spacecraft enters space then returns to the Earth. For an orbital spaceflight, a spacecraft enters a closed orbit around the planetary body. Spacecraft used for human spaceflight carry people on board as...
.
SpaceflightSpaceflight is the use of space technology to achieve the flight of spacecraft into and through outer space.Spaceflight is used in space exploration, and also in commercial activities like space tourism and satellite telecommunications...
using this form of
spacecraft propulsionSpacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by exhausting a gas from the...
may be significantly less expensive than spaceflight using
rocket engineA rocket engine or simply "rocket" is a jet engine[Rocket Propulsion Elements; 7th edition- chapter 1] that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s.
Some current tether designs use crystalline plastics such as
ultra high molecular weight polyethyleneUltra high molecular weight polyethylene , also known as high-modulus polyethylene or high-performance polyethylene , is a subset of the thermoplastic polyethylene. It has extremely long chains, with molecular weight numbering in the millions, usually between 2 and 6 million...
,
aramidAramid fibers are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic rated body armor fabric, and as an asbestos substitute. The name is a shortened form of "aromatic polyamide"...
or carbon fiber. A possible future material would be
carbon nanotubeCarbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 28,000,000:1, which is significantly larger than any other material...
s, which have a theoretical
tensile strengthTensile strength is indicated by the maxima of a stress-strain curve and, in general, indicates when necking will occur. As it is an intensive property, its value does not depend on the size of the test specimen...
of at least 60
GPaThis is a tabulated listing of the orders of magnitude in relation to pressure expressed in pascals....
.
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 ArtsutanovYuri N. Artsutanov is a Russian engineer born in Leningrad. A graduate of Leningrad Technological Institute, he is best known for being one of the pioneers of the idea of space elevator ....
, wrote in greater detail about a tensile cable to be deployed from a geosynchronous satellite in
Komsomolskaya PravdaKomsomolskaya Pravda is a Russian tabloid newspaper. It was the All-Union newspaper of the Soviet Union and an official organ of the Central Committee of the Komsomol between 1925 and 1991...
(July 31, 1960).
Jerome PearsonJerome Pearson is an American engineer and space scientist best known for his work on Space elevators and Lunar space elevator. He is president of STAR, Inc., and has developed aircraft and spacecraft technology for the United States Air Force, DARPA, and NASA...
explored synchronous tethers further, and in particular analysed the lunar elevator that can go through the L1 and L2 points.
Hans MoravecHans Moravec is an adjunct faculty member at the Robotics Institute of Carnegie Mellon University. He is known for his work on robotics, artificial intelligence, and writings on the impact of technology. Moravec also is a futurist with many of his publications and predictions focusing on...
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.
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.
Building materials
Materials proposed include
KevlarKevlar is the registered trademark for a light, strong para-aramid synthetic fiber, related to other aramids such as Nomex and Technora.Developed at DuPont in 1965 by Stephanie Kwolek it was first commercially used in the early 1970s as a replacement for steel in racing tires...
,
ultra high molecular weight polyethyleneUltra high molecular weight polyethylene , also known as high-modulus polyethylene or high-performance polyethylene , is a subset of the thermoplastic polyethylene. It has extremely long chains, with molecular weight numbering in the millions, usually between 2 and 6 million...
, carbon nanotubes,
M5 fiberM5 fiber is a high-strength synthetic fiber first developed by, among others, Dr. Doetze Sikkema at the Dutch chemical firm Akzo Nobel. Currently, it is being produced by the United States Magellan chemical company.-Preparation:...
, and
diamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
.
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 1
g (10 m/s²).
Potential tether/elevator materials
| Material |
Density
ρ
(kg/m³) |
Stress Limit
σ
(GPa) |
Char.
Length
Lc=σ/ρg, (km) |
Specific
Velocity
Vs=√(σ/ρ), (km/s) |
Char.
Velocity
Vc=√(2σ/ρ), (km/s) |
| Single-wall carbon nanotubes (laboratory measurements) |
2266 |
50 |
2200 |
4.7 |
6.6 |
| Aramid Aramid fibers are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic rated body armor fabric, and as an asbestos substitute. The name is a shortened form of "aromatic polyamide"...
, Ltd. polybenzoxazole fiber (Zylon PBO) |
1560 |
5.8 |
380 |
1.9 |
2.7 |
| Toray carbon fiber (T1000G) |
1810 |
6.4 |
360 |
1.9 |
2.7 |
| Magellan honeycomb polymer M5 (planned values) |
1700 |
9.5 |
570 |
2.4 |
3.3 |
| Magellan honeycomb polymer M5 (existing) |
1700 |
5.7 |
340 |
1.8 |
2.6 |
| Honeywell extended chain polyethylene fiber (Spectra Spectra are conditions or values that vary over a continuum.Spectra may also refer to:* Sally Spectra, a fictional character on The Bold and the Beautiful* Kia Spectra, a car developed by Kia Motors from 2000-present...
2000) |
970 |
3.0 |
316 |
1.8 |
2.5 |
| DuPont Aramid fiber (Kevlar 49) |
1440 |
3.6 |
255 |
1.6 |
2.2 |
| Specialty materials e.g. silicon carbide |
3000 |
5.9 |
199 |
1.4 |
2.0 |
AluminiumAluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
(6061 T6) |
2700 |
0.276 |
10. |
0.32 |
0.45 |
Shape
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
HoytetherThe Hoytether is a trademarked name of for a novel topology for a space tether, consisting of a lattice of strands, arranged in a circular cross-section with redundancy to handle potential damage from space debris and micrometeoroids....
; 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.
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
satelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
on the other.
Tidal forceThe tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force exerted on one body by a second body is not constant across its diameter...
s 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 forceIn classical mechanics, centrifugal force is an outward force associated with curved motion, that is, rotation about some center...
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 fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
such as around the Earth or
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
, a conducting rotovator can be configured as an
electrodynamic tetherElectrodynamic tethers are long conducting wires, such as the one deployed from the tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy...
. This can either be used as a
dynamo-In Engineering:* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field-Europe:...
, 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
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
ing 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 currentIn alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again...
s).
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
momentumIn classical mechanics, momentum is the product of the mass and velocity of an object . For more accurate measures of momentum, see the section "modern definitions of momentum" on this page...
and
angular momentumAngular momentum is a quantity that is useful in describing the rotational state of a physical system. For a rigid body rotating around an axis of symmetry , the angular momentum can be expressed as the product of the body's moment of inertia and its angular velocity...
is changed, and this costs
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
that must be recouped. The idea is that the recharge could be done with some form of energy (for instance
solar panelA photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells, also known as solar cells. The photovoltaic module, known more commonly as the solar panel, is then used as a component in a larger photovoltaic system to offer electricity for commercial and...
s generating
currentElectric current can mean, depending on the context, a flow of electric charge or the rate of flow of electric charge ....
for
electromagnetic propulsionElectromagnetic propulsion uses the concepts and applications of electromagnets in order to propel an object. It includes the class of experiments and theories that have been examined for the development of advanced aerospace propulsion systems...
) 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
rotor and
elevatorAn elevator or lift is a vertical transport vehicle that efficiently moves people or goods between floors of a building...
. 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
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
and possibly
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
), 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.
Rotovators can thus be charged by
momentumIn classical mechanics, momentum is the product of the mass and velocity of an object . For more accurate measures of momentum, see the section "modern definitions of momentum" on this page...
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
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
and
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
. 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 systemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
.
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-12Mach number is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance...
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 motorA linear motor or linear induction motor is an alternating current electric motor that has had its stator "unrolled" so that instead of producing a torque it produces a linear force along its length...
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 "
skyhookSkyhooks are a class of cable based techniques for lifting payloads to high altitudes and speeds. The name skyhook is a reference to an imaginary hook that hangs from the sky....
" since it appears to be "hooked onto the sky". This term was introduced relating to satellites and orbital mechanics by the Italian scientist
Giuseppe ColomboGiuseppe Colombo , better known by his nickname Bepi Colombo, was an Italian scientist, mathematician and engineer at the University of Padua, Italy .- Mercury :He is best known for his work on the planet Mercury, and it was his calculations on how to get a...
. 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
MachMach number is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance...
-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 ForwardRobert Lull Forward — known as Robert L. Forward — was an American physicist and science fiction writer...
and Dr.
Robert P. HoytDr. Robert P. Hoyt is a physicist and engineer who is famous for his invention of the Hoytether. He also originated the MXER Tether concept, which combines momentum-exchange techniques with electrodynamic reboost propulsion to enable a bolo tether system to serve as a fully-reusable in-space upper...
) has called this approach "Tether Launch Assist".
Space elevator (beanstalk)
A beanstalk (a type of
space elevatorA space elevator is a proposed structure designed to transport material from a celestial body's surface into space. Many variants have been proposed, all of which involve traveling along a fixed structure instead of using rocket powered space launch...
) 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 momentumAngular momentum is a quantity that is useful in describing the rotational state of a physical system. For a rigid body rotating around an axis of symmetry , the angular momentum can be expressed as the product of the body's moment of inertia and its angular velocity...
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 beanstalksA lunar space elevator is a proposed cable running from the surface of the Moon into space.It is similar in concept to the better known Earth space elevator idea...
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 eventAn impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Impact events have been a plot and background element in science fiction since knowledge of real impacts became established in the scientific mainstream.-Sizes and...
s 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.
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
micrometeoroidsA meteoroid is a sand to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor. If a meteoroid reaches the ground, it is then called a meteorite. Many meteors are part of a meteor shower...
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. HoytDr. Robert P. Hoyt is a physicist and engineer who is famous for his invention of the Hoytether. He also originated the MXER Tether concept, which combines momentum-exchange techniques with electrodynamic reboost propulsion to enable a bolo tether system to serve as a fully-reusable in-space upper...
patented an engineered circular net, such that a cut strand's strains would be redistributed automatically around the severed strand. This is called a
HoytetherThe Hoytether is a trademarked name of for a novel topology for a space tether, consisting of a lattice of strands, arranged in a circular cross-section with redundancy to handle potential damage from space debris and micrometeoroids....
. 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 (
KevlarKevlar is the registered trademark for a light, strong para-aramid synthetic fiber, related to other aramids such as Nomex and Technora.Developed at DuPont in 1965 by Stephanie Kwolek it was first commercially used in the early 1970s as a replacement for steel in racing tires...
and Spectra) permit rotovators to pluck masses from the surface of the Moon and
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
, a rotovator from these materials cannot lift from the surface of the Earth. In theory, high flying,
supersonicThe term supersonic is used to define a speed that is over the speed of sound . In dry air at 20 °C , the threshold value required for an object to be traveling at a supersonic speed is approximately 343 m/s, . Speeds greater than 5 times the speed of sound are often referred to as hypersonic...
(or
hypersonicIn aerodynamics, hypersonic speeds are those that are highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 and above...
) 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.
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 massThe center of mass of a system of particles is a specific point where, for many purposes, the system behaves as if its mass were concentrated there. The center of mass is a function only of the positions and masses of the particles that compose the system...
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.
SEDS-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.
MAST
The
MASTThe Multi-Application Survivable Tether experiment is an investigation designed to use picosatellite spacecraft connected by tethers to better understand the survivability of tethers in outer space. It was launched as a secondary payload on a Dnepr rocket on 17 April, 2007 as a part of the CubeSat...
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.
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.
STARS
The STARS mission, developed by the Kagawa Satellite Development Project at
Kagawa Universityis a national university in Takamatsu, Kagawa, Japan. The university was established in 1949 as a national university after the consolidation and reorganization of the Kagawa Normal School, the Kagawa Normal School for Youth and the Takamatsu College of Economics...
, was launched 23 January 2009 as a secondary payload aboard
H-IIAThe H-IIA is a family of Japanese liquid-fuelled rockets providing an expendable launch system for the purpose of launching satellites into geostationary orbit. It is manufactured by Mitsubishi Heavy Industries for the Japan Aerospace Exploration Agency, or JAXA. Launches occur at the Tanegashima...
flight 15, which also launched
GOSATThe Greenhouse Gases Observing Satellite or GOSAT, also known as , is an Earth observation satellite and the world's first satellite dedicated to greenhouse-gas-monitoring, which will be used to measure densities of carbon dioxide and methane from 56,000 locations on the Earth's atmosphere...
.
In fiction
The mechanics of tether propulsion are critical in resolving the climax of the book
The Descent of AnansiThe Descent of Anansi is a 1982 novel by Steven Barnes and Larry Niven. "It's the American Revolution all over again. But this time it's a ragtag band of space colonists vs. the United States...
by
Steven BarnesSteven Barnes is a science fiction writer, lecturer, creative consultant, and human performance technician. Barnes has written several episodes of The Outer Limits and Baywatch. He has also written the episode "Brief Candle" for Stargate SG-1 and the Andromeda episode "The Sum of its Parts"...
and
Larry NivenLaurence van Cott Niven is a US science fiction author. Perhaps his best-known work is Ringworld , which received Hugo, Locus, Ditmar, and Nebula awards. His work is primarily hard science fiction, using big science concepts and theoretical physics. It also often includes elements of detective...
.
In
Red Mars by
Kim Stanley RobinsonKim Stanley Robinson is an American science fiction writer known for his award-winning Mars trilogy. His work delves into ecological and sociological themes regularly, and many of his novels appear to be the direct result of his own scientific fascinations, such as the fifteen years of research...
, a Mars-based space elevator cable is built. It is used to accelerate colonization, and to begin integrating Mars and its resources into Earth's economy. Due to political unrest, the cable is sabotaged and collapses catastrophically.
See also
- Lofstrom launch loop - cable-like launch system
- Orbital ring
An Orbital Ring is a concept for a space elevator that consists of a ring in low earth orbit that rotates at slightly above orbital speed, that has fixed tethers hanging down to the ground.The structure is intended to be used for space launch....
- a ring around the Earth
- rope splicing
Rope splicing in ropework is the forming of a semi-permanent join between two ropes or two parts of the same rope by partly untwisting and then interweaving their strands. Splices can be used to form a stopper at the end of a line, to form a loop or an eye in a rope, or for joining two ropes together...
- how tethers can be joined and terminated
- Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by exhausting a gas from the...
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