I Explain the orbital physics of the JAXA Tether experiment?

[1oldman2]

First up, this backs Sophiecentaurs point about the economics driven aspect regarding deorbiting spacecraft with tethers. As the Kessler Syndrome evolves you will see an increased interest in the removal of debris (particularly propellant free methods such as Tethers) the more commercialized space becomes the more expenses and profit come into play.
http://spacenews.com/nasas-interest-in-removal-of-orbital-debris-limited-to-tech-demos/
"NASA adopted a policy in June 2014 to support development of orbital debris removal technology but not of operational systems. Specifically, the space agency backs projects with Technology Readiness Levels (TRL) 1 through 4, which means NASA’s support for projects ends once components or prototypes work in a laboratory setting."

What I see from a chronological viewpoint concerning Tethers is that the science has been worked out for some time now, it's just waiting on the development and implementation.
(http://www.tethers.com/)

http://www.eucass-proceedings.eu/articles/eucass/abs/2013/01/eucass4p763/eucass4p763.html
http://www.eucass-proceedings.eu/articles/eucass/abs/2009/01/eucass1p495/eucass1p495.html
https://arxiv.org/abs/1404.7430
https://arxiv.org/abs/1403.2221
https://arxiv.org/abs/1401.5968
https://arxiv.org/abs/1603.05563
https://trs.jpl.nasa.gov/bitstream/handle/2014/13405/01-2311.pdf?...1
https://trs.jpl.nasa.gov/bitstream/handle/2014/10112/02-2046.pdf?...
www.nasa.gov/centers/marshall/pdf/100415main_momentum.pdf
www.nasa.gov/centers/marshall/pdf/115871main_MXER_TS.pdf
https://www.nasa.gov/directorates/spacetech/strg/mueterthies.html

 

[sophiecentaur]

nothing has been successfully engineered, tested or achieved

Just wait for the first junk collision with a brand new satellite or a space station. It will suddenly become high priority. Same as with near miss asteroids.

 

[mfb]

The French lost a military satellite after 1 year.
Iridium lost an operational satellite.

Some more satellite losses could have been from space debris, but it is hard to find the cause of failure if the spacecraft just stops sending data.

 

[sophiecentaur]

The French lost a military satellite after 1 year.
Iridium lost an operational satellite.

Some more satellite losses could have been from space debris, but it is hard to find the cause of failure if the spacecraft just stops sending data.

It would not be difficult to make all satellites transmit an alarm signal to warn of an impact and the resulting spin. I believe there's already an omni antenna on most satellites to help regain control under some emergencies.
The problems experienced with the old Olympus DBS satellite were put down to a possible impact with one PV panel. It lost a lot of power and started spinning. Control was regained but the lifetime was badly compromised due to the need to waste fuel keeping its attitude correct. There will be loads of other such stories.

Why not have satellites deploy a massive 'braking chute' to slow them down at the end of life? I mean massive, of course, but it would only need to be gossamer thin and produce fractions of a Newton of braking force. Size would scale with satellite size to some extent - the advantage would be with the smaller satellites, as with falling insects and elephants.

 

[mfb]

If space debris hits a solar cell, you might have a chance to understand what happened. If it hits some critical component of the satellite (damaging the main computer, the antenna mount, or ruining the integrity of the whole structure), there is nothing you can do.

A braking sail is a complex and massive system, it would make collisions with space debris more likely (potentially breaking this up into smaller pieces and/or producing more pieces from the sail), and it is unclear if that would be better than lowering perigee with thrusters.

 

[sophiecentaur]

A braking sail is a complex and massive system, it would make collisions with space debris more likely (potentially breaking this up into smaller pieces and/or producing more pieces from the sail),

Yes, the possibility is there but the pieces of sail would be very flimsy and, just as with a spider's web (not complicated or expensive and would deploy like a balloon with a 'puff' of gas), the structure would be very tolerant of local damage. Is there a know upper limit of possible contact speeds? The probability of a strike would be proportional to the area so a factor of 10000 or more would be involved, I guess. (How many more zeros there?)
But I have a feeling that a braking sail would be cheaper (?).

 

[mfb]

Everything faster than 11 km/s is not in orbit, so the absolute upper limit for a collision of two orbiting objects is about 22 km/s. Highly eccentric orbits are rare, so typically both particles will be in roughly circular orbits, leading to a maximal collision speed of about 15 km/s. Apart from a few military satellites, nearly everything is launched eastward, on polar orbits or somewhere in between. A collision between an equatorial and a polar orbit leads to an impact speed of about 11 km/s. Collisions between other objects will often lead to lower collision speeds. All those are way above the speed of sound in the colliding objects - the speed doesn't really matter, it is a hypervelocity collision anyway.

 

[SteveO33]

has not actually received the level of funding needed for success

this backs Sophiecentaurs point about the economics driven aspect

Hi guys, sorry I've been away. Can we pick it up with "economics". Good thoughts and believe you're correct, but we should also consider the investment side. How much would you invest in a Tether Experiment let alone a Tether Service? JAXA just invested ?? millions and got a return of "zero". If I was a government or private enterprise, I would want more science and "earthly" experimentation first. Is anyone looking at these? Science would revolve around three areas (1) separate from EMF, what are the Newtonian orbital mechanics of long tethers and long semi-articulated orbital bodies, (2) separate from orbital mechanics, what are the physics of the electromagnetic forces at work for an 18,000 MPH earth-directed tether, deep space directed tether, or orbital trailing tether, and (3) how do the dynamics of the ionosphere contribute or detract to the situation (maybe tethers will only work in lower orbit and not in synchronous orbit). Lastly, once the science is proposed, theorized, suggested, or otherwise, do some Earth bound testing.

 

[sophiecentaur]

Lastly, once the science is proposed, theorized, suggested, or otherwise, do some Earth bound testing.

What Orbital experiment can be carried out on the surface of the Earth - apart from computer simulations, of course (which are what the designs are based on)?
I don't know of a craft that can spend much time in the Ionosphere for experiments to be carried out and any tether would not be for ionospheric use.

 

[SteveO33]

Here's an experiment: spin a long wire at 18,000 MPH though a magnetic field and see what happens. Can really do that on earth... must appropriately modify..

Ionosphere: the layer of the Earth's atmosphere that contains a high concentration of ions and free electrons and is able to reflect radio waves. It lies above the mesosphere and extends from about 50 to 600 miles (80 to 1,000 km) above the Earth's surface.

 

[sophiecentaur]

high concentration of ions and free electrons

I was forgetting the F layer, which is around LEO height. But, with electron densities no higher than 10⁶ per cm³, it isn't surprising that the experimental system has to use its own extra electron source. The issue is that, at heights where the drag is low enough to sustain almost continuous orbiting, the electron density will also be very low so it sort of makes sense that the electromagnetic forces will also be very low. The effect of that density of electrons on Radio Waves is another thing but even them it is only high enough (F layer) to work at HF frequencies (max) at a glancing angle. The mean free path is so large that there is little energy loss in that medium and the recombination time takes it right through the night.

spin a long wire at 18,000 MPH though a magnetic field and see what happens.

You only need to look at a car alternator to have proof that there is an induced current and that the load on the alternator takes rotational energy from it (vastly different Powers involved, but no difference in principle). I don't think there needs to be much doubt about the figures that have been used in the plans for an electrical tether - the only thing that could be in doubt could be the effective swept area of the looping electrons from the electron source on the vehicle.
The main thing is probably the practical problems involved with setting up the hundreds of metres of tether. I guess it would involve steering the vehicles at both ends to get them in position. The total mass of the tether equipment would need to be minimal to keep the cost reasonable enough to deal with thousands of items of junk.

Give me a death dealing laser cannon, any day!

 

[mfb]

There is no tether that would survive spinning at 18,000 mph. And there is nothing we could learn from such an experiment.

 

[sophiecentaur]

There is no tether that would survive spinning at 18,000 mph. And there is nothing we could learn from such an experiment.

I guess that a very broad electron beam in a large vacuum container could achieve a very small scale version of that proposal. You can obtain electron speeds of more or less anything you want up to small fractions of c. But to what end?

 

[SteveO33]

no tether that would survive spinning at 18,000 mph

Okay, agree... so three things...
(1) On orbit, the tether is orbiting at that velocity, and if it is 1-2 km long, there must be a fair amount of tensile force along the tether. Can it be calculated?
(2) We must modify the earthly test model... Consider this, an enterprise hard disk drive spinning at 15,000 RPM is going ~170 MPH at the outer rim (assume ~1 foot circumference, 15,000 feet per minute, ~ 2.8 miles per minute or ~170 MPH).. So, the 18,000 MPH velocity through the Earth's magnetic field is seriously beyond (>100x) that scale and probably beyond what we could test? Are the properties all linearly scalar? Probably in theory, but how about in application? So how much current is generated? To modify the tether experiment for earth, how about we spin the magnetic field (without reversing polls) and hold the tether stationary. What would be the scalar sizes for earthly testing?
(3) NASA appears to be abandoning tethers. See: Synchronized Position Hold, Engage, Reorient, Experimental Satellites (SPHERES) - 12.15.16 - ...de-orbit malfunctioning or defunct spacecraft . (https://www.nasa.gov/mission_pages/station/research/experiments/311.html)
ps. I did like the Death Ray suggestion... as long as it doesn't create more junk..

 

[sophiecentaur]

(1) On orbit, the tether is orbiting at that velocity, and if it is 1-2 km long, there must be a fair amount of tensile force along the tether. Can it be calculated?

Why would you think that? It is traveling at the same speed as everything that's attached to it except for the tiny difference due to its 'once per orbit' rotation. There is no significant resistance from the atmosphere up there (which is why the scheme is being proposed). Where would other stresses come from, apart from the difference between the 'weight' forces experienced by the masses at both ends? The conditions are referred to as Microgravity with good reason.

 

[mfb]

Can it be calculated?

Yes of course. Tidal gravity is tiny. The tension is enough to make the tether straight (or oscillate in some way), but not nearly enough to challenge the material.

 

[SteveO33]

Why would you think that?

Well, I say that about tensile forces because "it depends", which is part of the lack of a "design" for tether deployment and de-orbit configurations.
1. If the tether is deployed in the same orbital plane - which I doubt - then I agree there would only be the force of the EMF - very small? If fact, I would suppose that deployment would be pretty difficult because the lengthy tether may not have sufficient force to "stretch it out" and keep it that way. And it should be deployed "trailing" the deployment craft to be a very slight "drag" on the deployment craft. If deployed "forward" it could be a mess.
2. However, if the tether is deployed perpendicular (z-axis?) to the orbit of the deployment craft, then there is a 1-2km tether which has its end-point in a lower or higher orbit than the deployment craft. The far end of the tether, will either orbit faster or slower than the deployment craft. Secondly, there will be the EMF forces which will be "pulling" along the length of the tether. These two forces should create a tensile force along the length of the tether which ultimately should help to de-orbit the target object - in the JAXA test case - the ATV.
3. From the discussions so far, my conclusion is that the tether should be deployed perpendicular and into a higher orbit away from the earth.

 

[mfb]

The far end of the tether, will either orbit faster or slower than the deployment craft.

Not necessarily, it depends on the deployment mechanism and time. Rotating arrangements can be interesting as well.

in the JAXA test case - the ATV.

The tether was never planned to de-orbit the ATV. It was supposed to test forces for a while, afterwards the ATV would have used its thrusters to de-orbit (that happened now after the failed test).

 

[sophiecentaur]

the force of the EMF

This phrase occurs in several places in this thread. What exactly does it mean? EMF is not a force so I don't see its place in a discussion about forces.

If the tether is deployed in the same orbital plane - which I doubt

There is only one orbit possible with a given natural period in a given plane. If you try to deploy the two craft 'side by side' they will be in different planes which will intersect, making the tether go slack (or have a collision) twice every orbit. If they are in the same plane and have the same period, there will be no tension. For the system to work, the craft need to be at different levels in the same plane. The two craft could move like a bola but the force will act, alternately to speed up the orbit and slow it down - not what's needed.

ps. I did like the Death Ray suggestion... as long as it doesn't create more junk.

Ah well, it would need to disintegrate the junk into tiny pieces which would be small enough for the atmospheric drag forces to be effective. Unfortunately, the speeds of some of the bits could be a hazard to passing craft unless the bits were if molecular size. But the total energy needed would be greater than what simple retro rockets would need. There could be some advantages in that the 'herding' vehicle would not need to manoeuvre close to the junk, just using the focussed beam to reach the targets from a steadily orbiting vehicle.
Where are the Vogons, with their tractor beams, when you need them?

 

[SteveO33]

The tension is enough to make the tether straight (or oscillate in some way), but not nearly enough to challenge the material.

Regarding tension... Would expect the tether to contain a non-conducting connecting support element and then a conducting bare wire which would not have to bear the forces of the physical source to end connection. The non-conducting component hopefully would not be taunt - this would have to be carefully controlled by the deployment of the tether-end component. Any slight miss calculation which adds more tension would cause a mess in obit. Suspect that the tether length component should be kept fairly loose even to the point that the tether would not be straight during deployment. When the tether-end is sent into higher orbit, the orbital period would be longer, and therefore it would lag behind the tether-source. With a very long tether length which is highly flexible, the tether length could even be curved with the tether-source being ahead and the tether-end being behind in orbit.

Regarding challenging the material... Most tether attempts have resulted in unsuccessful deployments due to tether "failure"... it broke! So, something is challenging the material... either the non-conducting physical connection or the conducting wire connection... Physical - too much force - or electrical - more current/voltage than expected - or both?

 

[SteveO33]

For the system to work, the craft need to be at different levels in the same plane.

Totally agree, a "sideways" tether deployment is out... And wouldn't expect "sideways" to be the plan... Would expect orbital deployment to depend on the goal of the tether mission. A higher orbited tether-end would be a be a drag (lower my source-end orbit), and a lower orbited tether-end would be an acceleration (raise my source-end orbit) provided that this lower-end was not dragging too much into the upper atmosphere. Could even see a "trailing" in orbit deployment when no orbital change is desired, but perhaps the electrical charges in the tether are harvested for use as power on the source-end satellite.

For tether deployment, would expect that the tether-end has a propulsive component such that the tether-end component will actually pull the tether length away from the tether-source. The propulsive component would ever so slightly pull the tether length into higher, lower or trailing orbit. During this deployment the tether length would be kept fairly slack to the point of the tether length even being curved up or curved down during deployment. Once the tether-end component reaches the target orbital position, the tether would eventually become taut due to the differences in orbital period. A trailing deployment would require a more active positional management since the orbital periods would be the same - a very slightly lower orbit could possibly keep the tether taut and provide sufficient acceleration force to overcome the drag of the electromagnetic forces generated by the current in the tether wire.

 

[sophiecentaur]

@Steve: We seem to be largely in agreement about this but I should still like you to tell me about this EMF force.

 

[mfb]

Any slight miss calculation which adds more tension would cause a mess in obit.

Why? Tension is low anyway. There is no need to add support structure.
If there is no tension at all: Then you get a mess from cables moving in uncontrolled ways.

When the tether-end is sent into higher orbit, the orbital period would be longer, and therefore it would lag behind the tether-source.

This is wrong, as has been explained several times. You cannot assume a force-free orbit with a force acting. That doesn't work together.

Most tether attempts have resulted in unsuccessful deployments due to tether "failure"... it broke!

From tension while extended? Reference?

 

[SteveO33]

Why?

and other questions...
First comment "Why": For a multi-mile tether length, you can't assume that the tether will just deploy by itself under low tension. It would have to be carefully designed to be so. When the propelled tether-end component started to propel away from the tether-source, if it is under tension, it will pull the tether-source satellite - not good. The tether-source needs to unreel the tether at approximately the same rate as the tether-end propulsion is pulling the tether. The tether-end propulsion component would be pulling ever so carefully as it moved into a higher (or other) orbit.

Second comment "This is wrong": I don't think so. As long as you are within Earth's gravitation field, a higher orbit has a longer period. Period. Those periods go from about 90 min to about 24 hours as you move out to a geo-sync orbit. How else could it work? Can you sight any example of a satellite in higher orbit somehow being the same or shorter orbital period?

Third comment "From tension": That is the question? Did the tethers break from tension or electrical overload or material failure or what? For the 10-15 years of experimentation, there hasn't been an opportunity to analyze any experimental materials - they're sort of "lost in space".

 

[mfb]

You can unreel the tether slowly, keeping a low but non-zero tension in it the whole time. Based on orbital mechanics, the deployment can be done within a single orbit.

As long as you are within Earth's gravitation field, a higher orbit has a longer period. Period.

Not necessarily if there is a force on the object. An additional downwards force (e. g. from a tether!) will shorten the orbital period.

Can you sight any example of a satellite in higher orbit somehow being the same or shorter orbital period?

We don't have satellites attached to tethers right now. A space elevator shortly before getting connected to the ground would be a great example.

Third comment "From tension": That is the question? Did the tethers break from tension or electrical overload or material failure or what? For the 10-15 years of experimentation, there hasn't been an opportunity to analyze any experimental materials - they're sort of "lost in space".

If they broke due to other issues (or didn't break, e. g. with the Japanese attempt where it did not deploy at all), tension in the tethers was not the problem. Tension in the tethers can be very small, I don't see how it should be a problem and I don't see any evidence that it would be one.

 

[sophiecentaur]

You can unreel the tether slowly, keeping a low but non-zero tension in it the whole time.

How easy is this to initiate? The two craft start off at the same orbital position and will stay together - subject only to tiny perturbing forces. Would it not be better to use a thruster to get a separation between them that's enough to produce a useful tension? The tension is very small so the separation would need to be many (hundreds of?) metres before you could be sure of some stability. The direction and amount of thrust needed would be critical to avoid pendulum action of the long tether. This could keep going without some damping mechanism.

 

[mfb]

I'm not a tether engineer, but we can follow the JAXA design: The separation mechanism was supposed to give the object at the tether end an initial speed of 1 m/s. Based on the 20 kg end mass, we can unroll the tether and maintain 40 mN of tension for 500 seconds until the mass would stop in free space. During that time, the mass would move by 250 meters. But we are not in completely free space: At that distance, tidal gravity is 15 mN, so with our constant tension the mass will still move a bit (because acceleration was lower than 40 mN/20kg). We can reduce tension in the tether and slowly unroll further. At 700 meters, tension is 42 mN.

Just for mechanical stability during launch and so on you want the tether to be able to handle much more than those tiny tension values.

 

[SteveO33]

tell me about this EMF force

https://www.plasma-universe.com/Electromagnetic_force
Well, I was hoping that the Physicists in this blog would tell us about that... But my understanding is the Electromagnetic Force - EMF - is the force that plays multiple roles in this situation... (1) the Earth's magnetic field and the ionized particles interact with the tether wire which is being dragged at 18,000 MPH through that field to create a current in the wire - some standard stuff from generators/alternators, but don't know exactly about ionize particles, and (2) the current flowing in the tether wire creates a second magnetic field (the old right hand rule?) around the tether wire. Then, based on my readings of the expected behaviors by the tether planners and experimenters, the two magnetic fields - Earth's and the tether wire's - and their associated ionized particles are attracted to each other or somehow interact with each other - with a net affect of increased drag and slowing the orbital velocity of the tether wire which in turn causes a lowering of the orbit to cause or accelerate de-orbit. That's my understanding. For me, trying to work though some of those equations is way over my head.

Would it not be better to use a thruster

Totally agree... I also said that in my Mar 7 post about the "propulsive tether-end component". So a thruster would fire to move the tether-end component into a higher orbit. That would pull the tether wire from the tether-source component (whether it is just freely unspooling or unreeled by the tether-source). As that tether-end component propels upward, isn't it changing orbit? And since Kepler says that the square of orbital period is directly proportional to the cube of the radius - therefore - with increasing radius, the orbital period is increasing. Since the orbit higher and the orbital period is longer, the far end of the tether will slowly but surely fall behind the source-end. So there might be three deployment profile stages (1) initially with a perpendicular thruster direction, a multi-mile (10+ miles ?) tether would start out as perpendicular to earth, (2) as the deployment continued and as the tether-end began to fall behind the tether-source the tether would go through a period of looking curved from source to end, and (3) as the tether-end continued to fall behind, the tether would be pulled straight. BTW, after Stage 3 is when I think things like orbital mechanics, tensile forces and EMF forces get interesting. Discuss later.
Here's my crude picture of what the Tether profile would look like at Stage 2:

 

[sophiecentaur]

.. But my understanding is the Electromagnetic Force - EMF - is the force that plays multiple roles in this situation..

The standard term EMF is used as an abbreviation for Electromotive Force, which is a Potential Difference, provided by a generator of any description. It would help if you used the conventional terms for these things. It has been confusing me (and possibly others) all through this thread.

Since the orbit higher and the orbital period is longer, the far end of the tether will slowly but surely fall behind the source-end.

This is not correct. The equilibrium position will be with the trailing end further from the Earth. The tether will be at an angle to the tangent of the orbits. You could put it in another way and say that the trailing end is constantly 'thrown outwards'. This is exactly what happens on a number of fairground rides (Chairoplanes?) where the chains that support the riders trail behind the horizontal rotating wheel an hang outwards. This YouTube link shows one in operation. You need to interpret the angles correctly because the view is not from along the axis.

 

[Janus]

https://www.plasma-universe.com/Electromagnetic_force
Totally agree... I also said that in my Mar 7 post about the "propulsive tether-end component". So a thruster would fire to move the tether-end component into a higher orbit. That would pull the tether wire from the tether-source component (whether it is just freely unspooling or unreeled by the tether-source). As that tether-end component propels upward, isn't it changing orbit? And since Kepler says that the square of orbital period is directly proportional to the cube of the radius - therefore - with increasing radius, the orbital period is increasing.

Here's the problem: You are trying to treat the object on the end on the tether as if it is free to assume the circular orbital speed at the altitude at which it is at on the end of the tether. This isn't the case. Just putting something at a higher or lower radius does not mean that it will automatically assume circular speed for that radius. It depends on how you put it there.( For example, you can launch a rocket so that its trajectory takes it to an altitude of 300 km, but once there, it will not automatically assume orbit. You would have to give it an additional velocity boost upon reaching that altitude or it will just fall back to Earth.)

Assume we start with a circular orbit, and then give the object a slight inward push and from there it was free to follow a free path(no tether). Then, as it falls inward, it will exchange gravitational potential for velocity and speed up. It will continue to do this until it reaches a perigee point, and begins to climb out again, losing speed as it does so. it will climb until it reaches apogee (which will be higher than its original starting point) before it begins to fall back into repeat the process. What you have basically done is put it into an elliptical orbit that crosses the original one.
Now assume the same situation, except you attach a tether that is being fed from a winch which is set to only feed at the original speed of the push. The object can not change its speed relative to the winch because the winch fixes its speed. And while the object is changing gravitational potential, this potential change is being taken up by the winch rather than by a change in speed of the object. If it tries to move forward faster than the winch, this will tend to swing it to a higher radius. But it can only do this if it gives up speed for altitude. But since it is already moving slower than it needs to maintain a circular orbit, it can't do this. Its natural tendency to to fall towards the Earth, and it will want to settle to the lowest possible point, which is when the tether is pointed at the Earth. It is moving slower than circular velocity for its new radius, and wants to fall in towards the Earth, but is prevented from doing so by the tether. (If you were to cut the tether at this point, the object would assume an elliptical orbit with its present altitude the apogee).
For the object on a tether being fed outward, the opposite is happening, it is moving faster than circular orbital velocity and wants to climb away but is prevented from doing so by the tether.

The difference between letting the object assume a free orbit and lowering it or raising it on the tether is equivalent to the difference between letting an object free fall from a height and lowering it slowly by a rope. After free falling a distance the object has picked up velocity, which it doesn't do if you lowered it by rope.