WHAT IS SPACE TETHERS? :-
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.
A space tether is a long cable used to couple spacecraft to each other or to other masses, such as a spent booster rocket, space station, or an asteroid. Space tethers are usually made of thin strands of high-strength fibers or conducting wires. The tether can provide a mechanical connection between two space objects that enables the transfer of energy and momentum from one object to the other, and as a result they can be used to provide space propulsion without consuming propellant. Additionally, conductive space tethers can interact with the Earth's magnetic field and ionospheric plasma to generate thrust or drag forces without expending propellant. Most current tether designs use crystalline plastics such as Spectra. A possible future material would be carbon nanotubes, which have theoretical strengths up to 100GPa.
Classification:-
Tethers can be classified into four ways:-
1) Momentum exchange space tethers
2) Electrodynamic space tethers
3) Tethers system for flying
4) HIVOLT tethers
Ø 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).
Ø Beanstalks or Space Elevators:
A beanstalk 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. An Earth beanstalk would be at the limit of current known material strength (2004). Mars and Lunar beanstalks can be done with modern-day materials however.
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Momentum Exchange Space Tethers:
A momentum exchange tether is a long thin cable used to couple two objects in space together so that one transfers momentum and energy to the other. A tether is deployed by pushing one object up or down from the other. Once the two objects are separated by enough distance, the difference in the gravitational force at the two locations will cause the objects to be "pulled" apart. This is called the "gravity gradient force". The tether can then be let out at a controlled rate, pulled by the tension caused by the gravity gradient force. Once the tether is deployed, if there are no other forces on the tether it will have an equilibrium orientation that is aligned vertically. There are a number of different concepts for momentum exchange using tethers. Some general categories are:
A stationary tether is one that connects two masses together and remains at constant length, except, of course, for deployment and retrieval. A stationary tether could drag a payload through the upper atmosphere of a planet and lower payloads to the surface of an asteroid. If the tether is conducting and is moving through electric or magnetic fields, then it can be used as a generator to provide electrical power, or as a motor to provide propulsion. If the tether and its masses are orbiting a massive body, then typically the system will be gravity gradient stabilized, with the tether pointed along the radius vector to the massive body. Thus, although the tether is stationary in the orbital reference frame, it is really rotating once per orbit in inertial space, and so is a slowly rotating bolo.
A bolo is a long rotating cable anywhere in space that is used as a "momentum-energy bank". It could be used to "catch" a payload coming from any given direction (in its plane of rotation) at any given speed (less than its maximum tip speed), and then some time later, "launch" the payload off in some other direction at some other speed. A gravity gradient stabilized bolo orbiting some planet has the property that if the tether is cut, then one-half an orbit later, the separation distance between the two masses is seven times larger than the initial separation. This can be used to deorbit the lower mass, or throw the upper mass to a rendezvous or to escape.
A "rotovator" is a long bolo in low orbit around a planet (or moon) that is used as a giant elevator to reach down from space to lift payloads from a planet or to deposit payloads onto a planet. To reach the surface of the planet, the orbital altitude should be equal to half the length of the rotating cable. By proper adjustment of the cable rotation period to the orbital period of the center of mass of the cable , the relative velocity of the planetary surface and the tip of the cable can be made zero at the time of touchdown, allowing for easy payload transfer. A half-rotation later, the payload is at the top of the trajectory with a cable tip velocity that is twice the orbital velocity. Although present day material strengths do not allow the construction of rotovators around Earth or the major planets, they can be built for Mars, Mercury, and most moons, especially including Earth's Moon.
Momentum-Exchange/Electrodynamic-Reboost Tethers :
The concept of combining momentum-exchange tether principles with electrodynamic tether propulsion techniques to create a capability for transporting payloads from low Earth orbit, without using propellant, was originated in the late 1980's by Dr. Robert Hoyt of TUI. In a "Momentum-Exchange/Electrodynamic-Reboost (MXER) tether system, a long, thin, high-strength cable is deployed in orbit and set into rotation around a massive central body. If the tether facility is placed in an elliptical orbit and its rotation is timed so that the tether will be oriented vertically below the central body and swinging backwards when the facility reaches perigee, then a grapple assembly located at the tether tip can rendezvous with and acquire a payload moving in a lower orbit, as illustrated below.
Half a rotation later, the tether will release the payload, tossing it into a higher energy orbit. This concept is termed a momentum-exchange tether because when the tether picks up and throws the payload, it transfers some of its orbital energy and momentum to the payload. Because the MXER tether facility's orbit drops when it boosts the payload, its orbital energy must be restored if it is to boost additional payloads. The tether facility's orbit can be restored without consuming propellant by reboosting with electrodynamic tether propulsion.
Ø Electrodynamic Tethers:
An electrodynamic tether is essentially a long conducting wire extended from a spacecraft. The gravity gradient field (also known as the "tidal force") will tend to orient the tether in a vertical position. If the tether is orbiting around the Earth, it will be crossing the Earth's magnetic field lines at orbital velocity (7-8 km/s!). The motion of the conductor across the magnetic field induces a voltage along the length of the tether. This voltage can be up to several hundred volts per kilometer.
In an "electrodynamic tether drag" system, such as the Terminator Tether, the tether can be used to reduce the orbit of the spacecraft to which it is attached. If the system has a means for collecting electrons from the ionospheric plasma at one end of the tether and expelling them back into the plasma at the other end of the tether, the voltage can drive a current along the tether. This current will, in turn, interact with the Earth's magnetic field to cause a Lorentz JXB force which will oppose the motion of the tether and whatever it is attached to. This "electrodynamic drag force" will decrease the orbit of the tether and its host spacecraft. Essentially, the tether converts the orbital energy of the host spacecraft into electrical power, which is dissipated as ohmic heating in the tether.
Principle of electrodynamic tether propulsion
In an "electrodynamic propulsion" system, the tether can be used to boost the orbit of the spacecraft. If a power supply is added to the tether system and used to drive current in the direction opposite to that which it normally wants to flow, the tether can "push" against the Earth's magnetic field to raise the spacecraft's orbit. The major advantage of this technique compared to other space propulsion systems is that it doesn't require any propellant. It uses the Earth's magnetic field as its "reaction mass." By eliminating the need to launch large amounts of propellant into orbit, electrodynamic tethers can greatly reduce the cost of in-space propulsion.
Tether Transport Architectures:
Ø Cislunar Tether Transportation System:
Under a Phase I NIAC effort, Hoyt and Uphoff refined the LEO to Lunar system design to account for the full three-dimensional orbital mechanics of the Earth-Moon system, proposing a "Cislunar Tether Transportation System." This architecture would use one tether in elliptical, equatorial Earth orbit to toss payloads to minimum-energy lunar transfer orbits, where a second tether, called a "Lunavator™" would catch them and deliver them to the lunar surface. The total mass of the tether system could be as small as 27 times the mass of the payloads it could transport.
The Cislunar Tether Transport System
(1) A payload is launched into a LEO holding orbit; (2) A Tether Boost Facility in elliptical, equatorial Earth orbit picks up the payload (3) and tosses it (4) into a lunar transfer trajectory. When it nears the Moon, (5), a Lunavator Tether (6) captures it and delivers it to the lunar surface.
Ø "Mars-Earth Rapid Interplanetary Tether Transport (MERITT)":
Figure of MERITT
The same NIAC effort also resulted in a preliminary design by Forward and Nordley for a "Mars-Earth Rapid Interplanetary Tether Transport (MERITT)" sys-tem capable of transporting payloads on rapid trajectories between Earth and Mars.
Ø The Lunavator:
Moravec investigated whether it was possible to design a rotovator for the Earth and other planets. Unfortunately, his analysis showed that unless very strong tether materials can be found, the mass ratio of a rotovator for the Earth or other large planet becomes too high to be practical to build. This is unfortunate, because otherwise, it would be possible to build tether transport systems that would allow travel between the surface of the Earth and the surface of any solid body in the solar system without requiring fuel, provided the mass flow in toward the Sun slightly exceeds the mass flow outward. Moravec did find that rotovators were feasible for the Moon and other small airless bodies. Rotovators could be designed that would touch down from one to n times per orbit, but the tether mass was minimum for a rotovator that had a total length one-third the diameter of the body, was in an orbit with an altitude of one-sixth the diameter of the body, and rotated so that it touched down six times per orbit. In a later, unpublished paper, Moravec designed a rotovator made of the Dupont fiber Kevlar for use around the moon.
Ø The Cable Catapult:
Another concept developed by Dr. Forward of Tethers Unlimited is the Cable Catapult System. In this system, a very long tether is used as a launch rail. A long tether is extended in space and pointed towards the target. The payload is attached to a linear motor powered by an external electrical source, and the linear motor "climbs" the tether, accelerating the payload up to launch speed. At the launch point, the payload is released to travel on to the destination while the linear motor is decelerated to a halt on a shorter section of tether. This concept has the potential to enable launch velocities 30x the characteristic speed of the tether material. With advanced materials, launch velocities of 30-100 km/s may be possible, enabling interplanetary travel with durations of months rather than years.
The Forward Cable Catapult System Concept
Tether Systems for Formation Flying:
Clusters of small spacecraft flying in formation may provide revolutionary capabilities for a wide range of applications, including interferometric astronomy for investigation of the structure of the cosmos, synthetic aperture radar for environmental studies, and military surveillance missions. Due to the nature of orbital mechanics, however, a group of satellites in orbit will tend to drift away from each other. Consequently, to hold the spacecraft in formation requires some form of propulsion. For some applications, rockets or electric thrusters for formation flying is an acceptable solution, but for many applications the propellant requirements would be prohibitive. TUI is currently working with a NASA-Goddard led team of scientists to develop small, lightweight tether systems that will enable satellite clusters to fly in formation for long durations without expending any propellant.
Application:
ED Tether Applications:
Propellant less Propulsion for LEO Spacecraft
ED tether systems can provide propellant less propulsion for spacecraft operating in low Earth orbit. Because the tether system does not consume propellant, it can provide very large delta-V's with a very small total mass, dramatically reducing costs for missions that involve delta-V hungry maneuvers such as formation flying, low-altitude station keeping, orbit rising, and end-of-mission deorbit. TUI is developing several ED tether products.
Ø Electrodynamic Reboost of the International Space Station:
The International Space Station (ISS) will experience a small but constant aerodynamic drag force as it moves through the thin upper reaches of the Earth's atmosphere. This drag force will cause the station's orbit to decay. NASA currently plans to launch several large rockets every year to carry fuel up to the station so that it can reboost its orbit, which will be very costly.
Tethers Unlimited, Inc. has helped NASA to explore the potential for using electrodynamic tether propulsion to maintain the orbit of the ISS. By using excess power generated by the ISS's solar panels to drive current through a conducting tether, a tether reboost system could counteract the drag forces or even raise the station's orbit. NASA and TUI's studies revealed that such a tether reboost system could potentially save up to $2 billion over the first ten years of the station's operation.
High-Voltage Orbiting Long Tether (HiVOLT):
The HiVOLT System Concept for Radiation Belt Remediation
The space radiation environment presents a significant impediment to both human and robotic exploration and development of space. The Earth’s magnetic field traps high energy charged particles generated by cosmic rays, solar storms, and other processes, forming the “Van Allen” belts. The high fluxes of energetic particles in the radiation belts will rapidly damage electronic and biological systems in these regions unless extraordinary and expensive measures are taken to harden or shield against these particles. Even with hardening measures, the lifetime and reliability of space systems is often limited by the steady degradation caused by very energetic particles. TUI is currently investigating a novel concept for remediating the radiation belts to improve the safety and reliability of manned and unmanned missions in Earth orbit. The High Voltage Orbiting Long Tether (HiVOLT) System, illustrated in Figure 1 below, will utilize long, lightweight, conducting structures deployed in the radiation belts and charged to very high voltages to scatter the energetic radiation particles, causing them to leave the radiation belts. Preliminary analyses indicate that a HiVOLT System can reduce the MeV particle flux in the inner electron belt to 1% of its natural levels within about half a year.
A Space Tether Experiment:
The space tether experiment, a joint venture of the US and Italy, called for a scientific payload--a large, spherical satellite--to be deployed from the US space shuttle at the end of a conducting cable (tether) 20 km (12.5 miles) long. The idea was to let the shuttle drag the tether across the Earth's magnetic field, producing one part of a dynamo circuit. The return current, from the shuttle to the payload, would flow in the Earth's ionosphere, which also conducted electricity, even though not as well as the wire.
An earlier tether experiment ended prematurely when problems arose with the deploying mechanism, but the one on February 25, 1996, began as planned, unrolling mile after mile of tether while the observed dynamo current grew at the predicted rate. The deployment was almost complete when the unexpected happened: the tether suddenly broke and its end whipped away into space in great wavy wiggles. The satellite payload at the far end of the tether remained linked by radio and was tracked for a while, but the tether experiment itself was over.
It took a considerable amount of detective work to figure out what had happened. Back on Earth the frayed end of the tether aboard the space shuttle was examined, and pieces of the cable were tested in a vacuum chamber. The nature of the break suggested it was not caused by excessive tension, but rather that an electric current had melted the tether.
The electric conductor of the tether was a copper braid wound around a nylon string. It was encased in Teflon-like insulation, with an outer cover of Kevlar, a tough plastic also used in bullet-proof vests, all this inside a nylon sheath. The culprit turned out to be the innermost core, made of a porous material which, during its manufacture, trapped many bubbles of air, at atmospheric pressure. Later vacuum-chamber experiments suggested that the unwinding of the reel uncovered pinholes in the insulation. That in it would not have caused a major problem, because the ionosphere around the tether, under normal circumstance, was too rarefied to divert much of the current. However, the air trapped in the insulation changed that. As it bubbled out of the pinholes, the high voltage ("electric pressure") of the nearby tether, about 3500 volts, converted it into plasma, a relatively dense one and therefore a much better conductor of electricity.
The instruments aboard the tether satellite showed that this plasma diverted through the pinhole about 1 ampere, a current comparable to that of a 100-watt bulb (but at 3500 volts!); to the metal of the shuttle and from there to the ionospheric return circuit. That current was enough to melt the cable. However, many of the scientific experiments had already begun during deployment and yielded good data. And the break itself, though unfortunate, added an unscheduled experiment to the mission, one which highlighted the risks and complexities of operating scientific equipment in space.
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 correct this. A 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, very high flying, very supersonic 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 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 surpress most vibrations by changing speed and direction. The climber can also repair or augment a tether by spinning more strands.
· 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.
· An electrically conductive tether can act as an antenna for EMP (electromagnetic pulse) -- a strong, but brief pulse of radio energy.
Conclusion:
By using this technology, we can conserve energy which can be a step ahead in solving energy crisis. Though it is a small step, it can make a huge difference. It has the potential to make space travel significantly cheaper.
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