Explain the orbital physics of the JAXA Tether experiment?

[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'.