On this page:

1. Conductive Tethers 3: LeBRETON: A bare rotating conductive tether around Jupiter
2. YES2 SpaceMail: Young Engineers Satellite 2, launched in 2007
3. MARS-g: Manned Antecedent for Reduced and Simulated Gravity
4. CAPREE: A very precisely guided re-entry
5. TSE: A Tethered Sample Return Capability for Space Station
6. Tethered Satellites in the Earth Atmosphere
7. YES: Building an Experiment for Tethered Momentum Transfer From GTO
8. DUTETHER: The Tether Degradable By Ultra-Violet
9. Conductive Tethers 1: In-House Simulator Development for the METS project
10. Conductive Tethers 2: Long Term Stability and Plasma Chamber Tests
11. In-House Study: Tethers and Debris Mitigation
12. T-Series: Tethered upper stage for micro launcher
13. FAQ: Frequently Asked Questions
Have there been any successful tether missions? So a promising near term tether application is tethered sample return. How does that work? What are electrodynamic tethers and what is their purpose? Where does the current come from and where does it go? Is there any work on new missions? Are there any practical problems or hazards in the use of tethers?
14. Tether Links

LeBRETON: A bare rotating conductive tether around Jupiter

Delta-Utec has successfully concluded a small study towards innovative use of a bare tether around Jupiter. The main question being: can a tether system be used to the advantage around Jupiter, already with near-term technology?

For this Delta-Utec performed analytical analysis, and included the Jovian environment into our tether simulator ETBSim.
The primary innovative aspects studied were the self-stabilizing effect of the Lorentz-torque (which also yields significant momentum transfer Delta-V capability on the side) on a rotating bare tether, and neutral gas discharge, enabling a cathode with simple hardware.

Two fundamentally advantageous mission opportunities are identified that are advantageous with such a system: Low Jupiter Orbiter mission combined with Low Velocity Jovian Atmospheric Entry Probe, as well as a short-lived power-generating orbiter in highly elliptic orbit. Download the paper by clicking on the image! (IAF Bremen '03)

YES2: Be part of the 2nd YOUNG ENGINEERS SATELLITE!

In April 2002, Delta-Utec started for ESA Education Office the 2nd Young Engineers Satellite YES2 SpaceMail, a small lightweight re-entry capsule that deorbited in 2007 from LEO (Foton-M3) by tethered momentum transfer (SpaceMail). This educational project provided students from all over Europe with a spectacular hands-on experience. The innovative technologies, the bright tether in orbit (apparent size 10 times that of the Moon!) and the landing make it extremely visible and attractive. Visit the blog at YES2 website for more information.

MARS-g: Manned Antecedent for Reduced and Simulated Gravity

There is as yet only one way to send humans to Mars and have them arrive fit enough to explore its surface: have them travel there under artificial gravity using a rotating tethered system. One side of the 1 km tether is attached to a spent stage, the other side to the manned module. In order to demonstrate the (SIMULATED gravity) technology, stability and study possible physiological side-effects of such a tethered system, Delta-Utec has recently proposed to develop a roadway to MARS-g. A manned precursor will eventually fly in LEO that can also be used to understand REDUCED gravity of a planetary surface and its implications on physiology and operations.

CAPREE: A very precisely guided re-entry

Delta-Utec is providing the tether input to the CAPREE project: a guided capsule that will contain an end-to-end guidance algorithm employing e.g. GPS and aerofoil to land with only meters accuracy.

TSE: A TETHERED SAMPLE RETURN CAPABILITY FOR SPACE STATION

Delta-Utec is part of a European consortium that will perform a tethered re-entry demonstration mission for the International Space Station application. This mission is called TSE (Tether System Experiment). The baseline control law for this mission is the two stage deployment as described in the StarTrack study. The authoritative 1995 IAF paper can now be downloaded.

Bristol Aerospace TSE capsule, Delta-Utec tether and Noordwijk Space Expo Space Station model.

Research and development that will be performed on the International Space Station (ISS) in the near future, will create a need for frequent analyses of specimens and samples produced in the micro-gravity environment. Therefore we need Space Mail: a frequent sample return capability by small re-entry capsules. Because of safety, cost and achievable landing site accuracy it is proposed to deploy the capsule on a 35 km tether, instead of using a rocket engine and 'throw' it accurately to the Earth as shown in the picture. Such a system saves fuel on the capsule, but also gives the Space Station a small reboost that it can very well use, to make up for orbital energy lost by atmospheric drag.

Contrary to common believe tether missions have been successful in the recent past: just look at the great data and imagery obtained from Oedipus, SEDS 1, SEDS 2, TiPS, PMG & TSS-1R. See links at the bottom of this page.

For TSE integrated feedback tests were performed on a test rig with deployer hardware and real-time simulator in the loop. Testing on Earth cannot get more realistic than this. The hardware was extremely simple to be as reliable as possible: a simple inaccurate friction brake and only length measurement for feedback. The non-linear Energy Feedback proved to accurately control the critical and very sensitive first stage of a StarTrack deployment. A decimeter per second velocity error or a few centinewtons friction error could be lethal. To challenge fate even more, an ejection velocity error of -3% was applied. But the deployment to a 3400 m vertical was tight on the spot: about 10 m and 0.5 degree error! A perfect basis for a fast and reliable second stage and accurate capsule landing on Earth. These astonishing results should silence many critics...

YES: BUILDING AN EXPERIMENT FOR TETHERED MOMENTUM TRANSFER FROM GTO

Prof. Dr. W.J. Ockels:
"The engineer of the future will be the one that shows: Creativity, Initiative and Responsibility."

Delta-Utec and a group of enthusiastic young engineers proposed to build a small, 200 kg, satellite: YES that was to demonstrate a tethered momentum transfer in GTO for a small satellite, TORI (after an inspired musician). It was soon decided to combine the YES satellite with other ESA/ESTEC experiments in a robust experiment box called TEAMSAT that could be flown on the 2^nd Ariane 5 qualification flight. Apart from a tether experiment, YES carried many new technologies. Delta-Utec was responsible for delivery of this multi-technology experiment, which was built by Delta-Utec stagiaires & engineers, ESTEC young graduates and ESA professionals using a hands-on, end-to-end approach. YES was financed by ESA, the Dutch government and Delta-Utec.
Unfortunately the tether part of the mission was cancelled and a combination of problems caused part of the other experiments planned not to be executed. However, the development of YES featured a series of firsts and successes:

* the longest tether ever in space,

* first tether in GTO and in elliptic orbit,

* the first failsafe tether in space (Carroll caduceus),

* the first absorptive tether application (dampening of ejection shock),

* the fastest concept-to-construction time for a functional free flying satellite ever (6 months),

* first free flying satellite built on location at ESA/ESTEC in Noordwijk,

* first successful flight of Scintillating Fibre radiation measurement technology,

* demonstration of many new technologies for use in space (ESA's OBDH asynchronous VC, Delta-Utec's JORIS! flight computer with tether control H/W, PC-104 with QuickCam),

* low cost due to large involvement of students and trainees, the hands-on approach, short leading time plus associated great enthusiasm, use of engineering models and reflight of space hardware, both granted by ESA, gifts of space H/W for maiden flights, use of commercial technologies and support from industry,

* important trendsetter in the evolving space debris movement: first experiment to be cancelled by the ESA Steering Committee for being a potential space debris source.

YES as (probably) seen by the TEAM cameras (left: AVS, right: VTS) in the apogee following ejection, at several 100 meters distance

DUTETHER: THE TETHER DEGRADABLE BY ULTRA-VIOLET

The reason that the YES tether experiment was cancelled, was the collision risk that the YES tether could have introduced in case of experiment failure. Depending on the launch time, the tether lifetime was estimated between 2 months and tens of years. It became clear that no good model exists for determination of tether lifetime in long lived orbit such as that of the Ariane 5 upper stage. The idea was born to develop a tether that will completely degrade by ultra-violet when the nominal mission has been executed.

Delta-Utec is now investigating the feasibility of developing such a material, in its DUtether project.

Samples tested in termal vacuum only


Samples tested under VUV conditions: chemistry is initiated. Our task is to enhance the chemistry sufficiently for full degradation in 6 months.

A DUTether material could open up a whole scope of new applications:

Many challenges will have to be conquered.

• The tether lifetime versus mission time has to be tuned perfectly. Furthermore one would not want the tether just to fall apart into brittle fragments, as in this way, the combined risk of all the particles may even increase the total debris risk. Degradation should be to a level where the particles can no longer harm other satellites.
• The tether should perform well in space. In terms of stiffness, strength, smoothness and thermal & vacuum environment, requirements are high.
• The tether has to be fabricated, transported, wound, stored and tested on Earth. During all these processes, the tether characteristics should not degrade. It should only be sensitive to the specific space environment.

Today the materials that are considered for most momentum transfer missions are DYNEEMA and/or Kevlar. Although they do degrade under UV, the effect is way too slow and incomplete for a true DUtether application. Delta-Utec has performed a technology availability study and as a result has granted a contract to DSM Research in the Netherlands to select and investigate an initial set of candidate materials. A world-wide search for material candidates was pursued. UV testing was performed at the ESA/ESTEC facilities in Noordwijk, the Netherlands. Measurements on mass loss, spectrum (photoinitiation) and gel-forming (cross-linking) were performed. A second phase detailed the focus on some of the materials that seemed most promising. Chemical mixes were prepared with a selection of photo-initiators and a second test cycle was performed. Progress was made and a better understanding achieved, but the project is as yet "sleeping" and waiting for further funding.

Delta-Utec particular interest is to initiate a new scope of possible applications. We are therefore flexible on the issue of product rights and patents if we can co-operate with you.

Preliminary results presented at the IAF in Amsterdam can be downloaded here.

CONDUCTIVE TETHERS 1: IN-HOUSE SIMULATOR DEVELOPMENT FOR METS PROJECT

Together with CNES delta-utec performed a feasibility stufy to of use tether momentum-transfer to replace the circularisation and de-orbiting functions of a micro-launcher upper stage. As an integrated launch system, the T-Series concept was found to be a low cost, simple method of achieving a circular orbit with an expected success rate of 96%, and simultaneously de-orbiting the spent solid stage to a safe impact site with ~100% success.

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FAQs:

Have there been any successful tether missions?

Haddock is annoyed by his tether! Courtesy Herge.

YES! In fact most tether missions were successful, although they did not always get media attention. The most simple and lightweight technologies (deployment only, friction braking) have been proven very reliable.

TSS (l) and YES2 (r) tethers compared

Right now, a 4 km, 2mm tether (TiPS) is orbiting for 6 years, beating all survival estimates based on ground based dust particle impact tests. TiPS can be seen with the naked eye from the ground as a line ("moving Jedi-sword") the size of the Moon. Most of the previous tether missions were witnessed by surprised people on the ground and many UFO reports came in! An overview of some missions:

Gemini 11 & 12:
The first tether missions! Both Geminis were tethered to their Agena upper stage. Already in the 60s, rotating and MANNED! They created a milligee of artificial gravity and discovered the awkward dynamics. The control by humans was seen as a way to simplify the system rather than complicate it! Click the image for a spectacular movie (with useful voice-over commentary) (1.5 MB QuickTime).

SEDS-1, SEDS-2:
Designed by Tether Applications, both missions deployed 20 km of Spectra tether (a high-tech polymer). SEDS-2 proved that one can accurately deploy a tether to a stable vertical position by feedback control with a simple friction brake.

TiPS:
The longest living tether in space, 4 km, orbiting at 1000 km, used to study lifetime and dynamics. It was discovered that the librations are strongly dampened by internal friction. The images below were taken from the ground and are part of a video showing the deployment. Find TiPS in the nightsky via Heavens Above!

PMG:
Plasma Motor Generator: a 500 m electrodynamic tether that proved both power generation and propellantless thrusting.

TSS1R:
A 20 km electrodynamic tether (by far the most complex and heavy): succesfully deployed from the Space Shuttle and proved the potential to create very large electrical power (many amperes and 1000s of Volts). An abundance of plasma science was collected. The cut of the tether because of a shortage that went through the 10 year old isolation material got a lot of media attention and shaped the image of tethers in space for most public as well as space experts.

Click image to download mpeg (812 kB) of TSS tether cut and recoil.

Oedipus, Charge:
A multitude of Canadian and Japanese sounding rockets succesfully deployed electrodynamic tethers on sounding rockets (upto 1 km in length), doing measurements on axially spinning tether systems and plasma research.

Some missions failed, they were perhaps too complex: TSS1 (Heavy tether in Space Shuttle with large reel system. Tether got stuck), ATeX (Flat tape, reeled out at very low velocity. Software protection stopped deployment), YES (Rotating tether system in long lived GTO orbit. Deployment was not initiated because of potential collision hazard due to off-nominal orbit).

So a promising near term tether application is tethered sample return. How does that work?
A sample return mission of a capsule from Space Station needs about 120 m/s deceleration to go from its initial circular orbit at 400 km altitude to a landing site on Earth, where it is e.g. captured by a helicopter when drifting on its parachute. This velocity or "Delta-V" is obtained by two effects:

1. The swing of the tether (40 m/s): the tether will be deployed downward to a large angle with respect to the vertical, controlled by a closed loop braking device. After deployment, gravity gradient forces will pull the tethered system to the vertical: a swing is initiated. Near the vertical, the tether is cut.

2. Gravity gradient (80 m/s): when the tether is vertical, the capsule and Space Station are forced by their mechanical connection to orbit Earth in the same period, namely the period of their center of mass. Space Station will be slightly above this center, the capsule considerably below it. Because the gravity is stronger when closer to Earth, a higher velocity is required there to remain in a circular orbit [e.g. the Space Shuttle (400 km high) orbits Earth at 8 km/s in 90 minutes, the Moon (400.000 km high) requires a month and moves at 1 km/s]. So the capsule is forced by the tether to move too slow for the circular orbit that it's in. When the tether is cut, there is no more force that keeps the capsule in its circular orbit. It will 'fall' into a new orbit, with apogee (highest point) being the point of release and perigee (lowest point) being some 13 times the tether length (say 500 km) CLOSER to Earth. If the initial orbit altitude is lower than that (Space Station 400 km), the Earth itself is in the way for the orbiting capsule to reach its perigee. The capsule enters the atmosphere and is decelerated enormously by air drag. This is what is called re-entry.
Thanks to the tether no rocket engine is required. The tether in TSE is 7 kg and 35 km. It remains attached to the capsule and burns in the atmosphere. So it is safe and light and doesn't require storage of a handful of capsules with fuel tanks on the manned Space Station. Also, whereas teh capsule is re-entered, the Space Station comes into an orbit with a slightly higher apogee, making up for some of its continuous drag loss. The fuel profit of a tether in Space Station is therefore automatically doubled! A capsule for this purpose has already been built in the USA that weighs as little as 7 kg, with 35 kg payload capability!

What are electrodynamic tethers and what is their purpose?
Electrodynamic tethers are several kms long and can be used for two purposes:

1. power and/or drag generation,
2. thrust generation.

In case of thrust generation, a voltage of several hundreds of volts needs to be applied to drive a current down the tether. The interaction with the magnetic field will create a Lorentz-force of about 0.5-1 N that will raise the orbit of the system. The electric power source can be most conveniently: solar energy: solar energy is converted into orbital energy. If no potential difference is applied to overrule the induced field of about 100 V/km, a current will flow in the opposite direction, causing a drag force: orbital energy is converted into electric energy, that can actually be stored in batteries or dissipated in resistors.

Electrodynamic tethers start to really pay off if you use them for prolonged period of times, e.g. to balance the aerodynamic drag of Space Station or to have a 'travelling servicing station' that connects to old satellites and deorbits them, then picks up a fresh satellite to reorbit that.

Where does the current come from and where does it go?
Electrons need to be collected from the plasma, so a contactor surface is required. This can be a large ball at the end of the tether, but a more elegant and efficient concept is to use a bare Aluminum tether that itself serves as a collecting surface.
In case of a hypothetical mission to cancel out air drag for Space Station, the tether is deployed downward and since it is bare, it serves as a collector itself with a contact surface to the ionospheric plasma of some tens of square meters. The potential difference between tether and plasma that attracts the electrons out of the plasma into the tether is built up of two main components:

1. the 'Hall'-effect (or alternatively: 'induction of the second kind') of the tether with its electrons speeding through the (basically static) plasma, this is called EMF, a field of about 100 volts drop (plasma with respect to tether) per kilometer of increasing altitude.
2. A positive potential difference (with respect to the plasma) added by a source located on the upper mass (Space Station in this case) minus the impedance losses of the current that is created (and which is flowing downward)

The electrons are emitted by a plasma contactor at Space Station. They travel down some hundreds of km following the magnetic field lines and bounce (measurably!) between ionospheric boundary layers, but never find their way back to the tether. At every reflection some energy is converted and finally they disappear in the background plasma again. This closes the loop.

Is there any work on new missions?
YES2, carrying 30 km of mechanical tether for the SpaceMail application, is built and will be deployed from 300 km altitude in 2007.

Are there any practical problems or hazards in the use of tethers?
Tethers do not just 'curl' up in space as randomly as they do on Earth, because of the overruling power of the gravity gradient, which has the strong tendency to nicely stretch out a tether vertically. That's why they can be so long in space.
Deployment of tethers has been succesfully demonstrated, and studies have shown that tether deployment can be controlled very accurately. No active reeling is required to deploy a tether: after initiation of the deploymenty by ejection springs, a tether will be simply drawn from a spool by the gravity gradient, where the spool is placed sideways for along-axis low-drag low-inertia deployment, such as in some fishing gear.
Meteoroid risks can also be contained with multi-wire or tape designs, although practical data for such tethers in space is not yet available.
Electrodynamic tethers are a bit more complicated, because they involve large currents and voltages. Near-term mechanical tether applications (sample return, atmospheric research) generally last hours to days, whereas electrodynamic applications are for months or years. Whereas mechanical tethers are strongly forced by gravity to remain within the orbit plane, electrodynamic tethers have out-of-plane components in their dynamics caused by the Earth's magnetic field. Also the day-night effects on magnetic field, ionosphere and temperature cause oscillations of the tether that are a greater challenge to control. Simulators such as the one developed by Delta-Utec will help solve these problems.
Tethers can be a collision risk for other satellites, because of their size. The risky part of any orbit is the time spent below 2500 km altitude where satellite density is the highest. Tethers accidentally left in space under 400 km will quickly re-enter because of the large air drag over mass ratio. Tethers with a perigee upto ~800 km and a (very) high apogee can re-enter within a few months due to the pressure of sunlight (!), provided that their orbit is oriented in more or less the right direction (this should be a launch requirement). Even higher applications should be either inherently safe, otherwise avoided. Such safety can be achieved by mission design, or material design: a material that evaporates after its use would be an elegant solution (DUtether).
Concerning threats to human life, tethers will actually contribute to safety: they can be used to decrease debris in space, and with tethers, there is less need for fuel and fuel storage on the manned Space Station.

 

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TETHER LINKS

For interesting tether links we refer to:

Tethers Unlimited

Tether Applications


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