Author Topic: Starlink derived debris deorbiting.  (Read 19218 times)

Offline speedevil

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Starlink derived debris deorbiting.
« on: 06/05/2019 10:56 am »
As mentioned in the tug thread, the starlink bus appears likely to be capable of handling ~250kg of propellant, leading to a delta-v of 10+km/s.

The plausible available delta-v for  Starlink, is 1200-4000m/s for the 1200, and 350km layer.

What changes would need to be done to the bus so as to make it work for debris removal, and what capability does it have now?

The most obvious way is crude moderate velocity impact. Hit the satellite you want to deorbit, and transfer enough momentum.
This will in most cases be a very bad idea.

Without any modifications, the existing ion engine can be used to orient and apply force to debris.
This is approximately 200mN, or 100mN if it has to stationkeep on the debris.

This is quite a small force, but over a week is 100KNs. This is enough to despin even very large stages, for example a 9 ton 11m long Zenit 2 stage spinning end-over-end at 10 revolutions per minute has a total impulse required to despin of the order of 80kNs. Even with some inefficiency added due to standoff distances and dispersion, this is hardly a prohibitive time.

Once fully despun in all axes, a gentle approach at a couple of milimeters a second, followed by attaching a small magnet or glue patch and then retreating to a few meters beginning tow would more than double available thrust as it does not now have to stationkeep.

Continuing with the Zenit 2 as an example, following the ~week despinning, the stages have altitudes of around 800km, requiring around 250m/s to get down -  around 250kg of Krypton, or 125kg, if it was directly attached.

Elon has directly mentioned in the StarKicker tweet that 250kg or so of propellant can be used in starlink derived probes, so I don't think it's unreasonable to assume this is plausible. (Even if the Starlink satelites

There are 200 tons of Zenit stages in a common inclination, one launch of 22 Starlink class tug satellites is enough to deorbit them all.

A several newton thruster with a small tank of hypergols would considerably reduce concerns about crossing critical altitudes rapidly.

More near-term.
Very little propellant compared to the grand designs above is required to deorbit your completely failed neighbor in a constellation promptly, or indeed second stage debris.

See also this thread:
https://forum.nasaspaceflight.com/index.php?topic=48222.0 (Q: How to design a satellite for orbital debris collection ...)
« Last Edit: 06/10/2019 07:39 pm by Lar »

Online RDMM2081

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Re: Starlink derived debris deorbiting.
« Reply #1 on: 06/05/2019 03:59 pm »
Sounds interesting, one clarification question I have is:

During your hypothetical de-spin of the Zenit 2 stage, are you suggesting to keep the StarLink bus flying in formation with the Zenit 2, and using the exhaust of the ion drive to perform the de-spin maneuver without physically contacting the spinning Zenit 2 stage?  Then once the Zenit 2 is despun, moving in for the attach and de-orbit?

I think that's what you mean and it sounds like a very cool approach!

Offline envy887

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Re: Starlink derived debris deorbiting.
« Reply #2 on: 06/05/2019 04:18 pm »
Might it not be easier to harpoon it near the center of rotation and put a slip bearing on the harpoon line so the rotation isn't transmitted to the tug?

Offline speedevil

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Re: Starlink derived debris deorbiting.
« Reply #3 on: 06/06/2019 11:14 am »
Sounds interesting, one clarification question I have is:

During your hypothetical de-spin of the Zenit 2 stage, are you suggesting to keep the StarLink bus flying in formation with the Zenit 2, and using the exhaust of the ion drive to perform the de-spin maneuver without physically contacting the spinning Zenit 2 stage?  Then once the Zenit 2 is despun, moving in for the attach and de-orbit?

I think that's what you mean and it sounds like a very cool approach!

Yes, exactly that.

With a chemical engine, this is a prohibitive approach that uses a _lot_ of fuel, and optimising makes sense.
Also with a chemical engine, your thrust is so high that you may spall large amounts off, or are using an even more inefficient engine raising the costs.


Offline speedevil

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Re: Starlink derived debris deorbiting.
« Reply #4 on: 06/06/2019 11:16 am »
Might it not be easier to harpoon it near the center of rotation and put a slip bearing on the harpoon line so the rotation isn't transmitted to the tug?

If you can precisely model the interactions of the whole system and line as it is spinning and perhaps tumbling, yes.
This seems nontrivial.
If a valid alternative is standing off just enough so that any possible tumble can't hit you and just waiting a week.

Offline Bynaus

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Re: Starlink derived debris deorbiting.
« Reply #5 on: 06/06/2019 12:37 pm »
But wouldn't the Starlink move away from the spinning piece of debris? Even worse, the debris would take up the momentum and move away from the Starlink as well. The lower the distance between Starlink and the debris, the lower the momentum that can be transferred. This seems like a very self-limitinig process.
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Offline speedevil

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Re: Starlink derived debris deorbiting.
« Reply #6 on: 06/06/2019 12:45 pm »
But wouldn't the Starlink move away from the spinning piece of debris? Even worse, the debris would take up the momentum and move away from the Starlink as well. The lower the distance between Starlink and the debris, the lower the momentum that can be transferred. This seems like a very self-limitinig process.
Mostly accounted for above:
Quote from: SpeedEvil
This is approximately 200mN, or 100mN if it has to stationkeep on the debris.

I am implicitly assuming that the Starlink derived tug is a small fraction of the mass of the thing to be moved, so there is not an additional correction needed for the target moving away.

Yes, you probably want to connect in some manner once the target is safely despun, but the amount of thrust needed to despin even a very large fast rotating satellite is not significant, and it would significantly simplify things if instead of having to harpoon a fast rotating craft precisely on the axis, it is basically stationary.

For debris much lighter than the starlink craft, there is little reason to tether, as the majority of the thrust goes into keeping up with the target as you push it away.

Offline envy887

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Re: Starlink derived debris deorbiting.
« Reply #7 on: 06/06/2019 01:08 pm »
But wouldn't the Starlink move away from the spinning piece of debris? Even worse, the debris would take up the momentum and move away from the Starlink as well. The lower the distance between Starlink and the debris, the lower the momentum that can be transferred. This seems like a very self-limitinig process.
Mostly accounted for above:
Quote from: SpeedEvil
This is approximately 200mN, or 100mN if it has to stationkeep on the debris.

I am implicitly assuming that the Starlink derived tug is a small fraction of the mass of the thing to be moved, so there is not an additional correction needed for the target moving away.

Yes, you probably want to connect in some manner once the target is safely despun, but the amount of thrust needed to despin even a very large fast rotating satellite is not significant, and it would significantly simplify things if instead of having to harpoon a fast rotating craft precisely on the axis, it is basically stationary.

For debris much lighter than the starlink craft, there is little reason to tether, as the majority of the thrust goes into keeping up with the target as you push it away.

Why waste time and propellant despinning it if you could simply slow it down and deorbit it directly with the ion exhaust while it spins?

And how coherent is the ion exhaust? Is most of the stream going to hit the target from, say, 50 m away? 10 m?

Offline speedevil

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Re: Starlink derived debris deorbiting.
« Reply #8 on: 06/06/2019 01:11 pm »
Why waste time and propellant despinning it if you could simply slow it down and deorbit it directly with the ion exhaust while it spins?

And how coherent is the ion exhaust? Is most of the stream going to hit the target from, say, 50 m away? 10 m?
It would be somewhat easier to target if it is not spinning, and the despin part is nearly trivial delta-v wise anyway, so it doesn't really matter which way.

Ten degrees is mentioned in places, thirty in others.
It's going to need to be quite close.

Offline Ludus

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Re: Starlink derived debris deorbiting.
« Reply #9 on: 06/06/2019 08:07 pm »
Is there a Starlink bus derived approach for small debris too?

Offline Lar

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Re: Starlink derived debris deorbiting.
« Reply #10 on: 06/06/2019 08:24 pm »
Is there a Starlink bus derived approach for small debris too?
Sounds like stationkeep with them, just ahead in their orbit, and bathe the debris in thruster exhaust while also firing in the opposite direction to stay put. The debris gradually decelerates, so you fire slightly more than exactly equal, or something like that. As it decelerates it is also going to change altitude so it's probably more complex than I make it. But the basic idea is bathe it in exhaust, which will have an effect due to impingement.

That's my guess

This actually has a lot of legs, I think. you just have to know where it is.
« Last Edit: 06/06/2019 08:25 pm by Lar »
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
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Offline acsawdey

Re: Starlink derived debris deorbiting.
« Reply #11 on: 06/06/2019 08:52 pm »
Is there a Starlink bus derived approach for small debris too?
Sounds like stationkeep with them, just ahead in their orbit, and bathe the debris in thruster exhaust while also firing in the opposite direction to stay put. The debris gradually decelerates, so you fire slightly more than exactly equal, or something like that. As it decelerates it is also going to change altitude so it's probably more complex than I make it. But the basic idea is bathe it in exhaust, which will have an effect due to impingement.

That's my guess

This actually has a lot of legs, I think. you just have to know where it is.

On the subject of knowing where it is ... Could be as simple as a camera and a LED strobe so you can image the target somewhat independent of lighting conditions. If you know the divergence angle of the ion exhaust, simply position the debris tug so that the target mostly covers that solid angle. You don't actually need to know the size of the target nor the absolute distance, just that you are close enough to push it, and that it is in the right position relative to the tug (based on orbital velocity vector).

Offline speedevil

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Re: Starlink derived debris deorbiting.
« Reply #12 on: 06/06/2019 09:44 pm »
On the subject of knowing where it is ... Could be as simple as a camera and a LED strobe so you can image the target somewhat independent of lighting conditions. If you know the divergence angle of the ion exhaust, simply position the debris tug so that the target mostly covers that solid angle. You don't actually need to know the size of the target nor the absolute distance, just that you are close enough to push it, and that it is in the right position relative to the tug (based on orbital velocity vector).

The target will also glow quite brightly when impacted by 16km/s ions.

Offline ArbitraryConstant

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Re: Starlink derived debris deorbiting.
« Reply #13 on: 06/06/2019 11:43 pm »
Sounds like stationkeep with them, just ahead in their orbit, and bathe the debris in thruster exhaust while also firing in the opposite direction to stay put. The debris gradually decelerates, so you fire slightly more than exactly equal, or something like that. As it decelerates it is also going to change altitude so it's probably more complex than I make it. But the basic idea is bathe it in exhaust, which will have an effect due to impingement.

That's my guess

This actually has a lot of legs, I think. you just have to know where it is.
Hm, that seems like it would be relatively inefficient if the exhaust plume spreads out.

Another idea might be a laser. Curiosity has a laser powerful enough to ablate rocks and it only has about a hundred watts of power so this seems easily within reach for the Starlink bus. All the Starlink deorbit rig would have to do is get close enough for a laser in vacuum to be able to be able to ablate material off the target. That would have the advantage that deorbit rig would not have to provide reaction mass, it would be ablated from the target as vapor and that would provide the reaction mass.

People have talked about ground-based laser brooms, but those need to be huge and use adaptive optics comparable to state of the art telescopes. I'm guessing a satellite based deorbit rig would be able to do it with commercially available laser diodes and relatively modest optics since they'd be operating at much closer range.

I also suspect it would be possible to use such a large constellation with lidar to detect much smaller debris objects than is currently possible with ground-based radar. The relatively modest impulse needed to deorbit these smaller objects would allow them to be targeted for deorbit during close encounters even if the debris' orbital parameters were nothing like the deorbit rig's, one good zap might knock tens of kilometers off their orbit.
« Last Edit: 06/06/2019 11:44 pm by ArbitraryConstant »

Offline Barley

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Re: Starlink derived debris deorbiting.
« Reply #14 on: 06/07/2019 12:20 am »
IIRC its more important to reduce the total mass in orbit than the number of fragments.  So it's best, and probably easiest, to go after the big ones -- before they become small thousands or tiny millions. 

While laser brooms may be fascinating, they won't help all that much while things like envisat are still up there,


Online Robotbeat

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Re: Starlink derived debris deorbiting.
« Reply #15 on: 06/07/2019 01:14 am »
In LEO, at least below 800km or so, the REALLY small stuff will tend to deorbit by themselves. So it makes more sense to go after the big stuff. And even in other areas, big stuff should be removed first before it generates more debris.
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Offline Lar

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Re: Starlink derived debris deorbiting.
« Reply #16 on: 06/07/2019 01:24 am »
Agreed. But I think this was for after. The big stuff is easy, unless it's spinning a lot, then it's medium difficulty. Just nudge it by pressing up against it.

The laser idea is also good if the debris is large enough to target.
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Online RDMM2081

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Re: Starlink derived debris deorbiting.
« Reply #17 on: 06/07/2019 02:33 am »
Completely honest question here and trying not to be hyperbolic, but what would the world/headlines have to say about "Elon Musk intends to operate a fleet of hundreds-thousands of space lasers".  Regulatorilly speaking, is there anything to prevent them from adding such a feature to their Starlink units already planned?  It seems like the FCC would have quite a lot to say about additional emissions, but is that just a paperwork hurdle, or would they have some legitimate ground to prevent the lasers?

Offline ZChris13

Re: Starlink derived debris deorbiting.
« Reply #18 on: 06/07/2019 05:52 am »
Completely honest question here and trying not to be hyperbolic, but what would the world/headlines have to say about "Elon Musk intends to operate a fleet of hundreds-thousands of space lasers".  Regulatorilly speaking, is there anything to prevent them from adding such a feature to their Starlink units already planned?  It seems like the FCC would have quite a lot to say about additional emissions, but is that just a paperwork hurdle, or would they have some legitimate ground to prevent the lasers?
Lasers are already planned for Starlink, because inter-satellite optical link means laser
However, that's a different beast from "weapons grade laser capable of burning a hole in other satellites" like is being suggested above.

Offline ArbitraryConstant

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Re: Starlink derived debris deorbiting.
« Reply #19 on: 06/07/2019 06:03 am »
Agreed. But I think this was for after.
Potentially concurrent, potentially after. Depends on how bad it gets before we do something. I agree in the current situation prioritizing larger objects would be better. Either way, the lidar concept would allow tracking for smaller debris than is currently possible and there is an important gap between what's currently trackable and what's big enough to cause damage.

The big stuff is easy, unless it's spinning a lot, then it's medium difficulty. Just nudge it by pressing up against it.
That may be tricky due to there not being any general method of evaluating where the center of mass would be from any particular direction of approach. For a contact-free approach like laser ablation or gravity tractor it would be much easier to automate, meaning it would be possible to launch many hundreds on a Starship and have them engage their targets automatically. They even have enough delta-v to handle a bit of plane change, especially if they're given a high apogee, so you can really get a lot of targets per launch. This seems like a good way to bring down the debris population quickly.

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