Starlink Thread (split from SpaceX)

[Nothalogh]

On 12/9/2020 at 3:38 PM, sevenperforce said:

I'm wondering whether they have enough dV to do an orbital injection at Mars.

I'm imagining this as an interplanetary shotgun blast, of several hundred, from a Superheavy booster with an expendable upper stage launching directly into a transfer trajectory.
I suspect you've been watching too much Stratzenblitz.

[YNM]

On 12/10/2020 at 3:38 AM, sevenperforce said:

I'm wondering whether they have enough dV to do an orbital injection at Mars.

Don't think we've ever done ion orbit injection. Think Dawn used the normal thrusters for entering Ceres and Vesta. For Mars you can always aerobrake for assist, though.

Edited December 11, 2020 by YNM

[sevenperforce]

1 hour ago, YNM said:

Don't think we've ever done ion orbit injection. Think Dawn used the normal thrusters for entering Ceres and Vesta. For Mars you can always aerobrake for assist, though.

No, Dawn used ions at both Ceres and Vesta. However, their gravity wells are very shallow, and Dawn had already done a lot of the spiraling necessary to match orbit.

Starlink sats could use aerobraking passes at Mars to circularize, but aerocapture would be a little more challenging. When you only have one pass you have to go hella low.

Another issue with Starlink is that it has no way to unload its reaction wheels once it is out of Earth’s magnetosphere. 

[YNM]

17 minutes ago, sevenperforce said:

Dawn used ions at both Ceres and Vesta. However, their gravity wells are very shallow, and Dawn had already done a lot of the spiraling necessary to match orbit.

Ah alright. There was an RCS system but the dV they had is much smaller than the ion system. (found this paper that gave the dV amounts on Dawn for both the ion and RCS system, apparently 11 km/s vs. only 10 m/s.)

 

Maybe if you have careful planning, you could insert Starlink into Martian orbit. But think 3.6 km/s is on the low side. The lower the thrust, the more dV you need to expend.

Starlink system on Mars would be really helpful for surface communications...

Edited December 11, 2020 by YNM

[tater]

Put a fairing around a group of them (with its own solar and thrusters), and aerobrake, then pop the fairing and let the Starlinks go.

[sevenperforce]

42 minutes ago, YNM said:

Ah alright. There was an RCS system but the dV they had is much smaller than the ion system. (found this paper that gave the dV amounts on Dawn for both the ion and RCS system, apparently 11 km/s vs. only 10 m/s.)

Yeah, the RCS was really only to unload Dawn's reaction wheels. The mission-ending failure happened when one of the reaction wheels seized and so it had to begin using RCS for pointing in that axis. Which, as I noted, is a problem for Starlink. Starlink uses a magnetic rod to unload reaction wheels, pushing against Earth's magnetosphere, but it can't do that outside of Earth orbit. Mars has no magnetic field. And I don't believe the sun's magnetic field is strong enough either...it's a lot stronger than Earth's, of course, but it's also a lot farther away. Earth's magnetic field drops from 0.5 Gauss at the surface to about 0.31 Gauss at an orbital altitude of 1,110 km, which is where the highest band of Starlink sats will operate. In contrast, even though the sun's magnetic field is twice as strong as Earth's at its surface, it drops to just over ten billionths of a Gauss at 1 AU, and even less at Mars.

42 minutes ago, YNM said:

Maybe if you have careful planning, you could insert Starlink into Martian orbit. But think 3.6 km/s is on the low side. The lower the thrust, the more dV you need to expend.

Well, the trick is the capture burn. To get from a Mars transfer orbit to a Mars capture orbit you need only 900 m/s...you need another 1400 m/s to circularize. There's no direct conversion from high-thrust to low-thrust transfers, but I know that if you're at EML-1, you can get a high-thrust transfer to GEO for 1,380 m/s and to LLO for 640 m/s. In contrast, a low-thrust transfer for those two maneuvers would cost you up to 1,750 m/s and up to 800 m/s, respectively. So if we assume the low-thrust penalty is on the order of 26%, that would mean a spiraling-out low-thrust capture at Mars would cost under 1,200 m/s, which is well within the capabilities of Starlink. Then it could circularize using successive aerobraking passes.

1 hour ago, tater said:

Put a fairing around a group of them (with its own solar and thrusters), and aerobrake, then pop the fairing and let the Starlinks go.

If you do an aerocapture like this and enter a very eccentric Martian orbit, the cost of inclination changes to the individual Starlinks would be negligible. You could deliver a full constellation in a single mission.

[YNM]

34 minutes ago, sevenperforce said:

To get from a Mars transfer orbit to a Mars capture orbit you need only 900 m/s...you need another 1400 m/s to circularize.

...

So if we assume the low-thrust penalty is on the order of 26%, that would mean a spiraling-out low-thrust capture at Mars would cost under 1,200 m/s, which is well within the capabilities of Starlink. Then it could circularize using successive aerobraking passes.

MRO uses aerobraking for circularization. If anything I'd imagine with a single Starlink batch being sent over to Mars you'd want to either space out and / or change the orbital plane (either the inclination or the LAN/RAAN) while in the initial elliptical capture orbit. Also I suppose aerobraking will be more effective with a lighter satellite. We obviously haven't talked about stationkeeping here once you're on the intended orbit, but given Mars' smaller size and thinner atmosphere you probably wouldn't spend too much on the stationkeeping.

If the requirements are low enough then maybe you can change some of the krypton tank with normal hydrazine thrusters... I don't know how much ion engines they have but it could be cut down to two (if they have more than two).

Sole question remaining is the shade tech - would be really sad to finally get to a place with larger stellar parallax and ruin it !

Edited December 11, 2020 by YNM

[sevenperforce]

13 minutes ago, YNM said:

MRO uses aerobraking for circularization. If anything I'd imagine with a single Starlink batch being sent over to Mars you'd want to either space out and / or change the orbital plane (either the inclination or the LAN/RAAN) while in the initial elliptical capture orbit.

Yes, absolutely. It would cost very little dV. Then aerobrake to circularize. MRO used aerobraking for circularization but it used its six large hydrazine monopropellant thrusters to provide all the dV for the initial eccentric orbital insertion burn.

13 minutes ago, YNM said:

If the requirements are low enough then maybe you can change some of the krypton tank with normal hydrazine thrusters... I don't know how much ion engines they have but it could be cut down to two (if they have more than two).

The Starlink satellites only have one ion engine. They have three reaction wheels. They would need chemical thrusters to unload the reaction wheels once they were out of LEO, though.

[tater]

34 minutes ago, sevenperforce said:

If you do an aerocapture like this and enter a very eccentric Martian orbit, the cost of inclination changes to the individual Starlinks would be negligible. You could deliver a full constellation in a single mission.

That was my thought, particularly if they were using Starship.

[YNM]

13 minutes ago, sevenperforce said:

The Starlink satellites only have one ion engine. They have three reaction wheels. They would need chemical thrusters to unload the reaction wheels once they were out of LEO, though.

I see. Well maybe the sats would need to be somewhat bigger than they currently are...

48 minutes ago, sevenperforce said:

If you do an aerocapture like this and enter a very eccentric Martian orbit, the cost of inclination changes to the individual Starlinks would be negligible. You could deliver a full constellation in a single mission.

Ahh I see now if you have Starship. That'd work pretty grand, slowly releasing the sats with each plane changes. We could even dump the ion engines honestly since the bigger concern is attitude control rather than orbit stationkeeping. (unless someone worked out a small yet powerful enough ion thruster that can do the job.)

Edited December 11, 2020 by YNM

[sevenperforce]

6 minutes ago, YNM said:

I see. Well maybe the sats would need to be somewhat bigger than they currently are...

We could even dump the ion engines honestly since the bigger concern is attitude control rather than orbit stationkeeping. (unless someone worked out a small yet powerful enough ion thruster that can do the job.)

Dawn's three ion engines were gimbaled, which helped a little bit with attitude control and allowed for some assistance in unloading the reaction wheels. It carried 45.6 kg of hydrazine and it was nearly five times as heavy as a Starlink. So presumably a modified starlink with hydrazine thrusters for reaction wheel loading would only need around 10 kg of hydrazine.

Or even less. I misspoke earlier -- the hydrazine thrusters were used for part of the orbital insertion at Vesta.

It definitely needs the ion engine, though. The whole spacecraft is built around those engines.

11 minutes ago, YNM said:

That'd work pretty grand, slowly releasing the sats with each plane changes.

Nah, you wouldn't get any plane changes from the mothership. You'd simply use the mothership for the aerocapture into eccentric orbit, then release all the Starlinks so they could each do their own inclination changes independently.

[YNM]

10 minutes ago, sevenperforce said:

the hydrazine thrusters were used for part of the orbital insertion at Vesta.

Well they said that it can be used for low-dV point changes. But given the dV amounts each engine type was able to give out I suppose most of the work was indeed done by the ion engines rather than RCS, unlike (say) Hayabusa (and Hayabusa2) which much like Rosetta had to maneuver around a very-low-gravity object (while Rosetta did entered a few different stable orbits around 67P/C-G they formed a much smaller part of the observation mission).

10 minutes ago, sevenperforce said:

Dawn's three ion engines were gimbaled, which helped a little bit with attitude control and allowed for some assistance in unloading the reaction wheels.

Yeah, the two outer engines was mounted on a pivoting point I think. But I suppose something like this wouldn't work for the scale of Starlink satellites.

Or just throw away the ion engine, stick some tiny thrusters and change the krypton to good old hydrazine.

Edited December 11, 2020 by YNM

[sevenperforce]

3 minutes ago, YNM said:

Yeah, the two outer engines was mounted on a pivoting point I think. But I suppose something like this wouldn't work for the scale of Starlink satellites.

Or just stick some tiny thrusters and change the krypton to good old hydrazine.

I estimate the 260 kg spacecraft carries 33-60 kg of krypton, giving it 3600 m/s of dV. If it carried only hydrazine instead, it would have just 240 seconds of specific impulse which would give it 393 m/s of dV which is not nearly enough.

Starlink is designed around its ion engine.

[YNM]

13 minutes ago, sevenperforce said:

I estimate the 260 kg spacecraft carries 33-60 kg of krypton, giving it 3600 m/s of dV. If it carried only hydrazine instead, it would have just 240 seconds of specific impulse which would give it 393 m/s of dV which is not nearly enough.

Well, we'd have to send it over in a Starship... Initial capture done with the whole thing, then it'd get into the right inclination (for ejection). Perhaps the ejection process itself could be used to lower the periapse so it'd start to aerobrake (while the starship itself won't), then the Starship would shift over to a new LAN/RAAN, then do the same thing again, etc, etc . Each swarm would then distance itself apart in the orbital plane.

Sole other option is to send it already in the swarm state (good luck handling tens of smallsats at once around Mars all the way from the Earth) then use each sat's ion engine to capture then change plane then aerobrake and finally circularize (and distance apart). Sounds like a really difficult ground control ops unless you have fully autonomous sats (though this would be up in the valley of one of the other ventures perhaps).

I can't imagine sending a single stack (or any other amount of stack) of Starlink still in the stack form through the Earth-Mars transfer orbit and not enclosed in, or tug-ed with, another spacecraft. How would you communicate with it ? How would you carry out attitude control and trajectory corrections ?

Edited December 11, 2020 by YNM

[sevenperforce]

7 minutes ago, YNM said:

Perhaps the ejection process itself could be used to lower the periapse so it'd start to aerobrake (while the starship itself won't), then the Starship would shift over to a new LAN/RAAN, then do the same thing again, etc, etc . Each swarm would then distance itself apart in the orbital plane.

Starship doesn't have enough dV for that. The Starlinks do. You can just dump the entire swarm in one eccentric orbit and they can all head off in different directions.

7 minutes ago, YNM said:

I can't imagine sending a single stack of Starlink still in the stack form through the Earth-Mars transfer orbit and not enclosed in another spacecraft. How would you communicate with it ?

You'd definitely need a separate bus.

[YNM]

2 minutes ago, sevenperforce said:

Starship doesn't have enough dV for that. The Starlinks do.

I mean, maybe we could do a single-shot launch to Mars transfer ejection using Super Heavy, so Starship can just do the orbital insertion at Mars ? I mean this would leave some dV reserve on Starship perhaps ? Given the heat-resistance of Starship itself perhaps the Mars insertion can be part-aerobrake, part-insertion-burn. Would also limit the amount of sats you can carry per launch / vehicles as well though.

Also, since Mars is smaller we won't need as much sat, and perhaps we don't need such low latency either (like we're already so far away off Earth) so the constellation can be formed out of, idk, maybe a hundred or less or something. Honestly the question with larger satellite swarm around Mars is just "what do you do to dead satellites" since on Mars the planetary protection policy is not to crash onto the surface unless it was thoroughly cleaned beforehand. (would deffo change once we get there though.)

[Nightside]

1 hour ago, YNM said:

I mean, maybe we could do a single-shot launch to Mars transfer ejection using Super Heavy, so Starship can just do the orbital insertion at Mars ? I mean this would leave some dV reserve on Starship perhaps ? Given the heat-resistance of Starship itself perhaps the Mars insertion can be part-aerobrake, part-insertion-burn. Would also limit the amount of sats you can carry per launch / vehicles as well though.

Also, since Mars is smaller we won't need as much sat, and perhaps we don't need such low latency either (like we're already so far away off Earth) so the constellation can be formed out of, idk, maybe a hundred or less or something. Honestly the question with larger satellite swarm around Mars is just "what do you do to dead satellites" since on Mars the planetary protection policy is not to crash onto the surface unless it was thoroughly cleaned beforehand. (would deffo change once we get there though.)

I guess total number of satellites for an initial constellation will be dependent on how high the latitudes of interest are. A modest GPS network might be even more useful than ultra-low latency comms for complex robotic exploration and prospecting missions.

[YNM]

17 minutes ago, Nightside said:

A modest GPS network might be even more useful than ultra-low latency comms for complex robotic exploration and prospecting missions.

That would require an atomic clock however. Not sure if we can fit one of the currently employed design into the few tens of kg payload mass you can have on Starlink. There appears to be chip-sized atomic clocks though, not sure whether this would work as well or not.

Edited December 11, 2020 by YNM

[JoeSchmuckatelli]

Starlink question: presuming that the system works and places like rural America, Africa and other parts distant and beyond all get access to the internet... what happens when the satellites fly over a place that does not want people to access the service?

China, Russia and NK (not trying to be political) restrict access / information - and yet have neighbors like Europe, India, Japan and S. Korea that might want their people to have free access.  How does Starlink serve its customers without running afoul of some nation's definition of 'controlling its sovereign rights/airspace/infospace'?  

[tater]

7 minutes ago, JoeSchmuckatelli said:

Starlink question: presuming that the system works and places like rural America, Africa and other parts distant and beyond all get access to the internet... what happens when the satellites fly over a place that does not want people to access the service?

China, Russia and NK (not trying to be political) restrict access / information - and yet have neighbors like Europe, India, Japan and S. Korea that might want their people to have free access.  How does Starlink serve its customers without running afoul of some nation's definition of 'controlling its sovereign rights/airspace/infospace'?  

There is no "airspace" in orbit.

They can disallow selling the ground stations, or make owning one criminal, past that? Nothing they can do.

 

 

[StrandedonEarth]

4 minutes ago, tater said:

There is no "airspace" in orbit.

They can disallow selling the ground stations, or make owning one criminal, past that? Nothing they can do.

 

 

Pretty much my thoughts. But you can bet some top  NK officials will have their own Starlink terminals...

[JoeSchmuckatelli]

43 minutes ago, tater said:

There is no "airspace" in orbit.

They can disallow selling the ground stations, or make owning one criminal, past that? Nothing they can do.

 

 

Well - they could jam stuff flying 'overhead'.  Technically possible - but internationally legal?

This may not be the forum - and I really do want to avoid becoming political - but it does seem like a system like Starlink presents an information containment problem for nations that want to prevent people from access to the wider world.  That desire could cause them to try to interfere with the signals - which would have broader effects outside of their borders.

I just don't know enough about the 'law of space' to know whether there are any restraints on zapping commercial satellites.

 

 

EDIT:  Oh and I put 'airspace' in quotes to communicate the concept - and because SpaceSpace sounded weird.

Edited December 14, 2020 by JoeSchmuckatelli

[mikegarrison]

21 minutes ago, tater said:

There is no "airspace" in orbit.

They can disallow selling the ground stations, or make owning one criminal, past that? Nothing they can do.

Well, they could try to shoot them down or jam their signals or something. But that would probably be pretty futile.

Anyway, as far as I know there is no defined legal boundary for where "airspace" ends. Most countries in the world are signatories to the Outer Space Treaty, which says that outer space shall be free for exploration and use by all States. But I don't think it actually defines where outer space starts. And some countries are not parties to the treaty, but satellites fly over those countries anyway.

Very few countries have the ability to do anything about satellites that cross over their "airspace" anyway.

[kerbiloid]

The definition of Antarctica should be expanded to the any other such land, together with laws.

Then any solid part of Universe will be a part of Antarctica, regulated with well-known rules.

It will make the space much closer to the ground, as we can visually see where it begins.

And if ET appear and start to argue, we'll just show them an Earth map, where their planet is also a part of the wide-sense Antarctica.

[tater]

8 minutes ago, mikegarrison said:

Well, they could try to shoot them down or jam their signals or something. But that would probably be pretty futile.

Yeah, I think that ship sorta sailed with Sputnik. Way off topic, but I wonder what the Soviet reaction would have been to the US achieving orbit first? As it happened, the side more likely to complain (Ike has suggested "open skies" with aircraft as a precursor to the U2 overflights, after all) then overflew in orbit first—so now it was definitionally "OK."

Jamming is certainly a possibility.