totm nov 2023 SpaceX Discussion Thread

[tater]

 

10 minutes after the DIVH stream starts. Busy night.

[AckSed]

Question: How viable would it be to launch a reduced Starlink constellation around the moon? Assume 10 V2 Mini and an additional custom laser-link hub that can connect with the Starlinks around Earth. How much bandwidth could it enable and could other satellites and landers connect to them?

[tater]

 

 

 

[sevenperforce]

1 hour ago, AckSed said:

Question: How viable would it be to launch a reduced Starlink constellation around the moon? Assume 10 V2 Mini and an additional custom laser-link hub that can connect with the Starlinks around Earth. How much bandwidth could it enable and could other satellites and landers connect to them?

The first immediate issue I can see is that the magnetic rods used to desaturate the reaction control wheels on the Starlink bus wouldn't work in cislunar space because Earth's magnetic field is too weak out there. They'd have to add a monopropellant system for control wheel desaturation.

Also, I'm not entirely sure that ten satellites will be enough. The short-range phased-array antenna means they stay at around 550 km, which as you can see only gets you about 4.2e6 km² of surface area coverage per satellite:

4.2e6 km² is roughly 1/9 of the surface area of the moon, but that doesn't mean nine satellites will do the trick, because circles don't lay seamlessly on the surface area of a sphere. You would need at least twelve in a dodecahedral orientation to get total coverage. And even then, you've got another problem: satellites aren't statites, so they have to move, and their orientation relative to each other is only consistent insofar as they have the same period and inclination.

There's a 1977 paper with a mathematical formula to determine the minimum number of satellites for a given orbit radius, but it's paywalled and I don't have access. My guess is that you're going to need somewhere in the neighborhood of fifteen satellites to provide complete coverage and twenty to provide some sort of backup functionality.

[darthgently]

26 minutes ago, sevenperforce said:

The first immediate issue I can see is that the magnetic rods used to desaturate the reaction control wheels on the Starlink bus wouldn't work in cislunar space because Earth's magnetic field is too weak out there. They'd have to add a monopropellant system for control wheel desaturation.

Also, I'm not entirely sure that ten satellites will be enough. The short-range phased-array antenna means they stay at around 550 km, which as you can see only gets you about 4.2e6 km² of surface area coverage per satellite:

4.2e6 km² is roughly 1/9 of the surface area of the moon, but that doesn't mean nine satellites will do the trick, because circles don't lay seamlessly on the surface area of a sphere. You would need at least twelve in a dodecahedral orientation to get total coverage. And even then, you've got another problem: satellites aren't statites, so they have to move, and their orientation relative to each other is only consistent insofar as they have the same period and inclination.

There's a 1977 paper with a mathematical formula to determine the minimum number of satellites for a given orbit radius, but it's paywalled and I don't have access. My guess is that you're going to need somewhere in the neighborhood of fifteen satellites to provide complete coverage and twenty to provide some sort of backup functionality.

If the sats were laser linked into a mesh and we are willing to have some blackout periods fewer sats could work better than nothing

That is a good catch about the desaturation

[JoeSchmuckatelli]

33 minutes ago, sevenperforce said:

You would need at least twelve in a dodecahedral orientation to get total coverage

What about a pair of relay satellites at Earth L1 and L2? 

They'd have full view of 'the dark side' for most of the orbit (while Earth based covers the face we are used to seeing).  Only during the period when the moon is perpendicular to the sun-earth line would you (maybe) lose some coverage of the part furthest from the earth. 

[sevenperforce]

2 minutes ago, darthgently said:

If the sats were laser linked into a mesh and we are willing to have some blackout periods fewer sats could work better than nothing

Fortunately, 550 km is high enough above the lunar surface to minimize the problems with mascons which destabilize satellites in low lunar orbit. The image I rendered above is to scale.

The Artemis program is targeting the lunar poles, so if you are willing to have some blackout periods they should be closer to the lunar equator. So high-inclination or even true polar orbits are good. Seven satellites in a coplanar polar orbit, phased equally, would ensure that all satellites have line of sight to each other and that you always have at least two satellites overhead and at least three satellites with line-of-sight to Earth.

That said, the reason Starlink satellites are so low is to reduce latency, and latency isn't as big of an issue on the moon because you don't necessarily need lightning-fast internet speeds (you're already dealing with a two-second delay in comms with Earth).

1 minute ago, JoeSchmuckatelli said:

What about a pair of relay satellites at Earth L1 and L2? 

They'd have full view of 'the dark side' for most of the orbit (while Earth based covers the face we are used to seeing).  Only during the period when the moon is perpendicular to the sun-earth line would you (maybe) lose some coverage of the part furthest from the earth. 

Too far. Latency is already bad enough between Earth and the moon; you don't want to have the signals travel an additional 3 million kilometers between pings.

[JoeSchmuckatelli]

Yeah, 10 second delays are annoying! 

   

 

 

                                             Buffering 

 

 

 

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Edited June 22, 2023 by JoeSchmuckatelli

[tater]

 

[tater]

live

[tater]

203rd landing.

Nominal orbit

[RCgothic]

Starship/Superheavy changing to hot staging. 10% payload increase. That'd be up to 165t to LEO Ref.

 

 

Booster is getting longer for the adaptor and shielding required. Ready for flight in about 6 weeks (Elon time).

 

 

Edited June 24, 2023 by RCgothic

[RCgothic]

Notably the ability to hot stage potentially allows a zero zero abort.

[tater]

1 hour ago, RCgothic said:

Booster is getting longer for the adaptor and shielding required. Ready for flight in about 6 weeks (Elon time).

That section of the booster has already been seen, built, actually. Weeks ago.

Can't find the picture of the actual part—but these are old speculations based on the actual ring section already built.

 

[RCgothic]

Yes I recall that speculation now. Good call!

[tater]

From a month ago:

 

With Raptor 3, and a 10% increase on top, we're at what now for an expended stack? Over 300t, right? Boostback for F9 is a ~40% hit. Likely lower for SH as no entry burn. I'm wondering about the stats on an expended ship, but recovered booster. Alternatly, that margin can cover for adding legs back if required.

[Royalswissarmyknife]

When I thought Starship couldn't carry more

Guess I was wrong

[CatastrophicFailure]

3 hours ago, RCgothic said:

Starship/Superheavy changing to hot staging. 10% payload increase. That'd be up to 165t to LEO Ref.

This makes me smile.  Completely validates that I keep regressing to this in my RSS campaign. Just so much simpler.

Spoiler

 

1 hour ago, Royalswissarmyknife said:

When I thought Starship couldn't carry more

Guess I was wrong

Just wait til you see Starship Heavy... 

[Nuke]

hot staging on top of a stage with fuel in it for landing just seems like a bad idea.

[Royalswissarmyknife]

What Booster and Ship is Assumed to have Hot staging Capabilitys? 

[tomf]

How would hot staging give a 10% payload improvement? I can't think of any way that the difference isn't trivial.

[Brotoro]

Hot staging Starship? That's quite a shift in thinking. I wonder if they will only ignite a few of the Starship engines at first to decrease blast on the booster, then kick in the other engines after separation. 

[CastleKSide]

59 minutes ago, tomf said:

How would hot staging give a 10% payload improvement? I can't think of any way that the difference isn't trivial.

Perhaps by having continuous acceleration? Not sure but things tend to slow down in the time it takes to stage, which means you need to waste fuel speeding back up again. Im speculating from my KSP knowledge on this o if anyone knows more feel free to correct me.

[KSK]

Are they going to install one of those upside down shower plates from the launch pad on top of the booster, to keep it cool?

I’m with  @Nuke on this one - I can’t see how this goes well for a reusable first stage.

After the launchpad demolition snafu, I’m no longer quite as persuaded by SpaceX’s wild ideas.

[darthgently]

3 hours ago, tomf said:

How would hot staging give a 10% payload improvement? I can't think of any way that the difference isn't trivial.

Maybe they are losing parts and complexity related to stage 2 ullage? Not sure

But if they can maintain continuous thrust it would make a difference.  Try it in KSP, you'll see