I Explain the orbital physics of the JAXA Tether experiment?

[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.

 

[sophiecentaur]

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.

No. Neither I nor anyone else is saying that. There is an equilibrium position that the trailing object will assume.That will be at a slightly greater distance from the Earth from the parent object. If you read the links that have been quoted on this thread, it is all explained. You are trying to reach an understanding of this arm waving and that is not a reliable way of working. If you consider the tension on the tether, there is an equilibrium condition with the coupled mass at a greater radius but with the same period as the parent.
(It's not Rocket Science lol because it's all passive)

 

[SteveO33]

Interesting thoughts about orbits. I had always assumed that the tether-end would actually be propelled into a different orbit - by standard means - Hohmann transfer orbit and so on. Now the idea that Janus proposed that the Tether experimenters wouldn't do that, but instead would just change to an elliptical orbit is confusing to me - especially since the tether-end is still flexibly attached to the tether-source. Seems like it could make a mess or things?

Have been continuing to search for more tether items and found this undated website entry..
http://www.daviddarling.info/encyclopedia/S/space_tether.html
It describes three types of tethers... (1) Momentum Exchange Tether, (2) Electrodynamics Tether, and (3) Hybrid Tether (combination of both). Might be getting somewhere. The site also has some interesting links to other missions - like SEDS (Small Expendable-tether Deployer System). Will track some of those down. Assuming they are not from the early days (60's-70's), they could contain real experiments.

 

[sophiecentaur]

but instead would just change to an elliptical orbit is confusing to me

I don't understand how that would happen if there is no disturbance of the CM of the two tethered masses but if the smaller mass is actually propelled to a new position, the momentum transfer could alter the CM orbit. Not too hard to correct for, though.

 

[mfb]

Interesting thoughts about orbits. I had always assumed that the tether-end would actually be propelled into a different orbit - by standard means - Hohmann transfer orbit and so on. Now the idea that Janus proposed that the Tether experimenters wouldn't do that, but instead would just change to an elliptical orbit is confusing to me - especially since the tether-end is still flexibly attached to the tether-source. Seems like it could make a mess or things?

Consider the ISS. Different parts of it are at different heights above Earth. So what? It rotates once per orbit to keep its orientation relative to Earth stable. It feels tiny tension in its modules in up/down direction from tidal gravity. The upper modules don't "move forward" and the lower modules don't "move back".

A tether is not that different. The tether system rotates once per orbit to keep the tether vertical. Nothing moves forward or backward, the system is stable.

 

[sophiecentaur]

Nothing moves forward or backward, the system is stable.

. . . . except if one end of the tether is put in place by careless use of thrusters. That would alter the orbit.

I had always assumed that the tether-end would actually be propelled into a different orbit - by standard means - Hohmann transfer orbit and so on

But that wouldn't achieve what's needed. The orbit of the ensemble must not be changed (for the purpose of the tether operation)

 

[SteveO33]

There seems to be several "camps" in the discussion. MFB thinks the tether structure should be considered as a rigid body like ISS (even though the tether is completely flexible). Sophiecentaur thinks the tether will go into the desired orbital configuration and stay there (even though the tether is completely flexible). And me, who thinks the flexible nature of the tether is what makes it so intriguing - by far the largest (longest) structures ever deployed in space and it could hold the secret to "free" lower orbital propulsion - if only there was agreement on how it would/should behave...
There also remains the simple question, if the physics are well known, and since tethers have been worked on for 20+ years, why hasn't the science translated into some standard working products. And seeing how far the space programs of many countries have progressed in those 20+ years, it is really difficult to accept, "space is hard". Beginning to suspect that "tethers are hard, too hard" because all the physics/science isn't settled. So engineers design experiments and spacecraft missions based on incomplete science, and the missions fail costing lots of money without return.
In the current day, only JAXA seems to be working on a tether solution - which just failed - will be interesting to see if they continue. NASA seems to have abandoned tethers altogether. Now that could be because they don't want to spend (waste?) any more money, or it could be that they know that tethers don't behave well (predictably), or it could be that they know that tethers don't really do what everyone hoped they would do, or a little of all?

 

[sophiecentaur]

There seems to be several "camps" in the discussion.

As far as I can see, there are no 'camps' involved in the views of mfb and myself. mfb has merely pointed out that the various parts of an orbiting body of finite size are experiencing different values of g but orbiting at the same rate. That is precisely what I am saying in a different way. The fact is that, once it has settled down, the tether ensemble could be rigid or flexible. Nothing will alter the relative positions of the two ends. Despite what you seem to be implying, there is nothing that can 'turn up' about the basic mechanics of the tether system which will change things. It is a fact that a successful working system hasn't been established just implies that there are still problems in getting the deployment right. If you look at the very small forces that are involved and the fact that nothing that can be done on Earth to test such a system, it is not too surprising that there has not been enough time to experiment with an in-orbit system. Having a crew, dedicated to deploying such a tether manually over many hours (days?) is probably beyond the scope of any missions so far and it's that, rather than the "incomplete Science" that you are suggesting.
Also, twenty years is not a relevant figure. What counts in such matters is how much design time and experimental measurements have been carried out.

 

[SteveO33]

Found some interesting baseline material in of all places - Wikipedia.
https://en.wikipedia.org/wiki/Space_tether
The entries are far-ranging from the topics of our discussions to the more scientific science-fiction - tether elevators, etc (and I don't say that negatively, because a lot of concepts can start as science fiction-like, but end up as reality). They also contain a number of what appears to be qualify references and links.
I've also reached out to the JAXA team to see if they would be willing to participate in our blog. Wouldn't that be cool...
Continuing to dig for some of the documented results of the tether missions of the last centaury (1980's - 1990's).

 

[sophiecentaur]

Found some interesting baseline material in of all places - Wikipedia.
https://en.wikipedia.org/wiki/Space_tether

Shame that the first picture there (Artist's Impression) shows the tether 'vertical' / radial. A bad start, I think.

 

[SteveO33]

shows the tether 'vertical' / radial

Interestingly, that does seem to be the preferred deployment... I've just come across some very interesting material from Shuttle Mission STS-75. It was a Tether Experimental Mission and it appears it was deployed in that profile.
Also, I did hear back from the JAXA KITE team. Currently, they are too occupied with analysis of the failure in their last mission to participate, but they did appreciate hearing of our interest.

 

[sophiecentaur]

Interestingly, that does seem to be the preferred deployment.

How can that be a stable situation when the natural orbit periods at the two levels are different? But I see you use the word "deployment" so perhaps you are referring just to the initial condition. If that's the case, I still see the possibility of a long term 'pendulum' swinging of the (undamped) tether. But I suppose that an electrical tether could dissipate that energy due to resistance.
It's good that you have been in contact with the JAXA KITE team.

 

[SteveO33]

How can that be a stable situation

I had wondered the same... and that orbital observation is correct... It appears that the lower end is dominated by centripetal forces of gravity - it's pulling forward... the upper end is in a lower gravitational gradient and not as dominant so it drags... End forces are tangential and along the radii, but the one along the tether isn't?
And as we had discussed, if you un-tether the two objects, the one in lower orbit, will have a faster orbital period (having to circumscribe less distance per orbit) than the upper one which is in a slightly higher orbit (will have a slightly slower orbital period having to circumscribe slightly more distance per orbit).

But some how the EMF (electromotive force) does help stabilize...
Consider: in one of the Non-Conducting Tether experiment of old, they did observe a "librating" (oscillating) condition... but later experiments with Conductive Tether's do not mention this... The one thing they do mention is voltages and currents far in excess (3x) of what was predicted by calculations - just getting into this area. That aside, agree, the EMF resistance along the entire tether length could be providing the necessary damping of the two body (ends) system - probably doesn't need much to dampen any oscillatory forces caused by the deployment which would allow it to settle nicely into a stable state. Still suspect that over a really long time, the two body system would settle into a single orbit around the center of mass of the two, but EMF would prevail before this and cause it to deorbit.

 

[mfb]

If you un-tether the objects in the stable vertical configuration, both will follow an eccentric orbit. The upper one will have its perigee at the point of disconnection (because it is moving faster than the speed of a circular orbit there), the lower one will have its apogee there (because it is moving slower than the speed of a circular orbit there).

But some how the EMF (electromotive force) does help stabilize...

A vertical tether is stable without any electromagnetic forces.

 

[sophiecentaur]

If you un-tether the objects in the stable vertical configuration, both will follow an eccentric orbit. The upper one will have its perigee at the point of disconnection (because it is moving faster than the speed of a circular orbit there), the lower one will have its apogee there (because it is moving slower than the speed of a circular orbit there).A vertical tether is stable without any electromagnetic forces.

I can appreciate that the two ends will leave a circular orbit of the tether breaks because there will be a change in force imparted to each end as the tension is removed.
having thought about it further, it now seems reasonable that a radial tether can be stable - in fact the only way for the orbit to be circular is if there is a constant radial resultant force from gravity plus the tension. For that to happen, and gravity always acts radially, so the tension should also be radial. I now have a problem with the idea of a 'trailing' tether. Any electromagnetic braking force would then cause the tether to trail and produce a change in orbit, with the resultant being no longer radial.

 

[mfb]

A non-radial tether needs a position-dependent force to keep its orientation. Unless it is exactly aligned with the flight direction, then it is in an unstable equilibrium.

 

[sophiecentaur]

So we are still left with the question about what is going wrong with the experiments. I guess no one wants to spend more or less the whole cost of a launch on this sort of project, on the grounds that not everyone is convinced that it's worth doing in the first place. They don't do "nice if" experiments in Space.
I guess the Moon Elevator could turn out to make tethers more popular. (So lucky that the Moon happens to be locked to Earth).

 

[SteveO33]

Ah, not exactly buying that whole Moon Elevator thing... it still makes good sci-fi...
But, I did find a gem of a real video on YouTube...
This shows the Tether deployment that was part of STS-75 way back in 1996...! Confirmed a bunch of things for me...
Check out that curve (they call it a bow) in the tether... yeah...
Thrusters on the Tether-end satellite to move to a higher orbit and to 'pull' the tether from the Tether-source.. yeah...
Perpendicular / orbitally-rearward deployment.. yeah...
Clearly NASA put a lot of money and effort put into this experiment...
Very, very long tether deployed - 19-20 KM...
Lots of power (their term) generated... couple of KW's...
Tether broke - actually looks more like it was electrically fried (melted)... at the near end...
And last, but not least - much more power (3x) than was expected/predicted and ultimately, too unpredictable in behavior..
Now I see why NASA is not doing tethers anymore...
I'm probably going to put Space Tethers into my Unsolved Mysteries folder along with "how do clouds hold up tons of rain moisture"...

 

[sophiecentaur]

"how do clouds hold up tons of rain moisture"...

An H₂O molecule has much less mass than an O₂ or N₂ molecule so water vapour is 'lighter than air'. The condensed water droplets are held up by convection until they have accumulated sufficient weight to drop out as rain.
Air is 'not a sponge, holding water'.