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I Explain the orbital physics of the JAXA Tether experiment?

  1. Jan 31, 2017 #1
    How is the JAXA tether experiment supposed to work in space? I hear the words, "it will act as a drag and cause early de-orbit". However, the tether is orbiting with the satellite (ATV) - same orbit, same speed (velocity), but with less mass and less drag profile than the Host Transfer Vehicle. So how does the tether "unfurl", how does it "change orbit" and how does it slow down the larger mass, larger drag profile vehicle? Thx.
     
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  3. Jan 31, 2017 #2

    mfb

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    The cable has certainly more surface area per mass than satellites, but that is not the point. They want to induce a current in the cable and de-orbit via interaction with the geomagnetic field.
    Here is an animation, unfortunately just in Japanese, but with nice arrows drawn to show the current and forces (starting 4:30).
     
  4. Jan 31, 2017 #3
    The point is that the two masses connected by a tether are NOT in quite the same orbit, but BOTH are constrained to the same angular velocity. The higher elevation orbit mass would normally (free fall) have less angular velocity, and the lower elevation orbit mass would normally (free fall) have greater angular velocity. This means that the upper mass is being pulled forward and the lower mass is being pulled back... the result is tension in the tether. The two tethered masses experience a small acceleration toward each other because their orbital angular speeds without the tether would be greater for the lower and less for the higher.
    Because the upper mass is pulling back and the lower pulling forward, the line of the tether slants so that the lower mass is ahead of the higher with respect to the earth surface (this was not really clear in the video; the tether is short).

    So one utility of a tether is to create an "artificial gravity" at each mass with the upper pointing down/in and the lower pointing up/out... to get a substantial effect the tether needs to be on the order of hundreds of kilometers. Docking at the tether mid-point where there is no effect is easy and distribution of supplies to either end is simply letting the stuff slide "down" the tether to either end.

    The tether (and the masses) in low earth orbit are circling the earth every 45 minutes and cutting a lot of magnetic field lines. With a tether comprised of a thousand conductive strands of hundreds of kilometers of individual length, the tether acts as a dynamo. Discharging the tether will raise or lower the orbital elevation of the center of mass of the whole system, depending on whether you discharge the upper or lower end of the tether.

    Being able with sufficient planning to swing the system down low enough to intercept boosters and tanks discarded from launch vehicles (at about 90 miles altitude) would be a nice thing since these still have some fuel in them and can be used structurally for various things in orbit (like constructing structures at the ends of the tether itself...)
     
  5. Feb 1, 2017 #4

    sophiecentaur

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    I need some confirmation here about how it really works. Changing the orbit of the space junk by hanging it on another ship will involve some Energy transfer. The circulation of charge via the released electrons and their capture on the tether will require some energy input but the energy for braking, due to the motor effect of the current through the conduction loop of electron gas plus tether is greater(? I have a nagging issue with Lenz's Law here). How much energy needs to be supplied with this method, compared with the equivalent energy required from a simple retro thruster?
     
  6. Feb 1, 2017 #5

    mfb

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    Thrusters need fuel, which limits the size of the satellite you can de-orbit. Electromagnetic braking just needs solar cells and time.
     
  7. Feb 1, 2017 #6

    sophiecentaur

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    OK, thanks. I wonder what the area is for the current loop that's involved. Ideally, an electron gun would be firing electrons at some optimal angle to make this area as big as possible. The Video doesn't seem to commit itself about that but I imagine they expect a significant number of electrons to make it to the far end of the tether.
     
  8. Feb 1, 2017 #7
    Okay, looked at the animation... very interesting.. appears to show the tether is extending into a higher orbit (not sure how it got there?)... which I assume would have a longer and slower orbital period. So, the "higher" end of the tether would fall "behind", and the ATV-end of the tether, being in a lower, shorter and faster orbital period, would mover "ahead". The net affect would seem to be, the higher end would provide a pulling tension on the ATV and visa versa. Now what happens? Biggest mass wins? EMF wins? Tether is stretched and breaks, or tether is heated by EMF and melts? Or ATV and tether settle into the same obit? Or EMF drags them both down? Seems to be a lot of competing forces.
     
  9. Feb 1, 2017 #8

    sophiecentaur

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    What 'wins' is the drag force on the current loop moving through the Earths Magnetic field and dissipating energy in the loop resistance.
    It's disappointing that the actual numbers don't seem to be readily available. It would be very interesting to know what braking force could be achieved. That is the bottom line, after all. If the braking process were to take many months then the amount of junk shifted would hardly make the operation worth while. It strikes me that an alternative, triangular tether arrangement could be used to connect three suitable pieces of junk and let them get on with it. They would need to be arranged in a suitable orientation ( I think it could be made self stable). No Electron gun would need to be involved.
     
  10. Feb 1, 2017 #9
    Thx sophiecentaur
    The hope does appear clear from the experiment that EMF will win. Just don't see how. Once the tether is out, won't orbital differences be felt within one or two orbits. And isn't that time on the order of 1-3 hour duration? And wouldn't that cause some kind of orbital "rationalization" of the entire ATV-Tether structure? Can they really remain is different altitude orbits?
     
  11. Feb 2, 2017 #10

    sophiecentaur

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    What does that mean? It is not a Competition. If you provide a suitable path for an induced current to flow, induction will happen when the magnetic flux lines are cut and the resulting current will dissipate power in the circuit - producing a braking effect. The same would apply with a loop of wire. The two ends of the tether are kept apart because of fairly straightforward orbital Physics and the tether remains under tension, with the two ends at different orbital heights.
    You're referring to the equilibrium position of the two ends (?)- which is stable. Post #3 explains the mechanism.
     
  12. Feb 2, 2017 #11

    mfb

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    Not necessarily.
    A vertical tether, where both ends orbit at the same angular velocity, is stable. It will be under tension, but nothing will move or pull forward or backward. You can have a rotating tether (more precisely: one that does not have bound rotation), but that is a different topic.
    Why? Having a satellite de-orbit within months instead of decades means the satellite will be gone soon. As long as the system works without continuous human input, a timescale of months is perfectly fine.
    You won't find three or even two larger pieces of debris in a suitable orbit. Every tiny difference in orbital elements will make such a structure impossible.
     
  13. Feb 2, 2017 #12

    sophiecentaur

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    It does tie up the disposal craft and its electron gun equipment for a long while, though. I was assuming that they would need to be re-usable, once the satellite orbit was sufficiently low.
    Is it not possible to 'shepherd' three objects into suitable orbits? I know this stuff is not intuitive so my idea may just be plain barmy.
     
  14. Feb 2, 2017 #13
    Tether Orbit Picture1.png
    Lots of good information, but I'm still stuck on the orbital mechanics side, so I put together a picture. The picture shows Orbit 1 of the ATV and notional Orbit 2 of the "end point of the tether". ATV Orbit 1 is lower altitude and faster orbital period. Tether End Orbit 2 is higher altitude and slower orbital period. So after an equal time period (as shown in the picture), the ATV in Orbit 1 is "ahead" of the Tether End in Orbit 2. And since the ATV wants to continue to go faster, but is tied to the Tether End, it will actually exert a force, F1, on the Tether End. Conversely, the Tether End in Orbit 2 does not want to go as fast as the ATV and so it will exert a force, F2, on the ATV. This is in addition to the previously discussed EMF forces. I'm suspecting that the orbital forces will be take a couple hours to be felt - tether must fully extend, etc. If the ATV-Tether was a rigid body, then center of mass and moment of inertia could apply, but they are not rigid at all. So my guess is that over time, the forces F1 and F2 would act the same as orbital attitude jets and would force the ATV and Tether End into a single orbital plane with the lower orbit, larger mass ATV being in the front and with the tether trailing behind. The final orbit, Orbit 3, would be very minimally higher than the original ATV Orbit 1.
     
  15. Feb 2, 2017 #14

    mfb

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    The satellite burns up with its payload. At least with the current plans.

    I guess you could take a single defunct satellite, connect the tether, then use that to change the orbit to catch another satellite, then find some way to establish all three tethers. Sounds complicated. And I'm not sure about the efficiency. The current system uses the homogeneous field to get a force. A closed current loop would get a net force from the inhomogeneous part of the field only.
    The orbital period is faster for an object in a free orbit. The satellite is not in a free orbit. You cannot use equations that assume no external forces (apart from gravity) to study a system where two things are connected by a tether.
     
  16. Feb 2, 2017 #15

    Janus

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    No. the forces involved will pull the tethers so that they point away from and towards the Earth. This is known as gravity-gradient stabilization or tidal stabilization.
    It has been used before to maintain satellites' orientation with respect to the Earth.
     
  17. Feb 2, 2017 #16
    Ah, interesting reference - gravity-gradient stabilization... So, the rigid example would seem to be achievable.. It's a fixed body... But the tether does not, and oddly enough, the success of tether experiments have been abysmal. In fact I could not find an example of a successful tether model - and from recent posts from JAXA, that may include their effort as well. Is it possible that the physics of the tethered version are not really known or perhaps flawed?
     
  18. Feb 3, 2017 #17

    mfb

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    What?
    This stabilization method works well. Many satellites use it, the ISS also uses it. You don't need a long tether if you just want to stabilize the orientation.

    All this is Newtonian mechanics, and known exactly for hundreds of years.
     
  19. Feb 3, 2017 #18
    Okay, I did not realize that ISS used tethers for stabilization. Do you have any references to successful tether-based stabilization?
     
  20. Feb 3, 2017 #19

    mfb

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    The ISS does not use tethers, but they have been discussed to increase the stability, and as propulsion method.
    Some slides

    I misremembered the ISS configuration. It is in an unstable equilibrium with respect to the gravity gradient.
     
  21. Feb 3, 2017 #20
    Now we're getting somewhere. Appreciate the slides MFB - very illuminating in many ways. Probably first is their age - 10-15 years old. I know the physics of this hasn't changed in that time, but I suspect that the "Tether" is much more an Applied Physics and Engineering challenge as opposed to any kind of theoretical discovery. And I'm guessing that the Applied Physics is pretty complex and largely unsolved since it's not on the ISS, and basically not in broad application (if really any at all?). Not to mention that the publicized Tether Test efforts have been largely unsuccessful. Second, and maybe this is where the Tether De-Orbit comes into play, all of NASA's slide-ware examples were using Tethers to gather charge to power Ion Engines to Boost or Change Inclination of the orbit. It also showed Upward, Downward and maybe even Sideways configuration of great mass - 1,000Kg and tremendous length - 50 Km, but none were De-Orbiters. So, perhaps if we accepted as you say, the Newtonian Physics of this is well established, and tried to figure out how to apply them to the Tether Challenge, maybe one of these Tether Experiments would actually work to de-orbit a dead satellite?
     
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