totm nov 2023 SpaceX Discussion Thread

[tater]

 

 

[tater]

 

[Beccab]

2 hours ago, tater said:

 

This has been discovered to be the Booster 8 (!) thrust puck, and given that the previous boosters thrust pucks have already been spotted it confirms that the switch from 29 to 33 engines is part of the S25 series upgrades. Perhaps S25 could be the first fully operative (chomper door and payload) variant?

Edited October 6, 2021 by Beccab

[Deddly]

Practical Engineering just released a fascinating video giving insight into a problem SpaceX had with their launch pad, and I have a whole new level of respect for anyone able to actually land on another planet/moon! 

 

[kerbiloid]

That's why proper designs set the legs wide, rather than try to land a standing pencil.

[Deddly]

2 hours ago, kerbiloid said:

That's why proper designs set the legs wide, rather than try to land a standing pencil.

How does the width of the legs help avoid the problems of landing on regolith? 

[Beccab]

6 minutes ago, Deddly said:

How does the width of the legs help avoid the problems of landing on regolith? 

It doesn't, in fact the very wide Luna 23 still tipped over during or shortly after landing


Wide legs aren't as important as good control and careful planning

Edited October 6, 2021 by Beccab

[adsii1970]

4 minutes ago, Deddly said:

How does the width of the legs help avoid the problems of landing on regolith? 

Distributes the weight of the craft over a wider area of the surface and lowers the center of gravity of the lander. This increases the stability and reduces the chances the lander will topple over. If we are talking about a Mars or even a lunar landing, it would suck to strand a bunch of astronauts because the lander toppled over.

This is why the Apollo lander was designed as it was. At the time, NACA/NASA believed the surface of the Moon was covered in roughly a foot of lunar dust. They believed the wide, concave landing pads at the end of the legs, the wide span of the legs, and the low center of gravity. In the end, they learned from the Apollo 11 mission the dust wasn't as thick as they expected it to be.

[kerbiloid]

15 minutes ago, Deddly said:

How does the width of the legs help avoid the problems of landing on regolith? 

Different roughness of the surface.

A flat concrete against pits, stones, and pits&stones covered with unstable ground.

The roughness amplitude /  leg span ratio should be kept in appropriate range to keep in range the tilt angle of the landed ship.

12 minutes ago, Beccab said:

It doesn't

It does.

That's why rovers are low and flat and use outriggers

17 minutes ago, Beccab said:

It doesn't, in fact the very wide Luna 23 still tipped over during or shortly after landing

If it was a pencil, it would just fall aside.

18 minutes ago, Beccab said:

good control and careful planning

When you have concrete-flat areas wider than the leg span.

[KSK]

I always thought that the leg span was more to do with providing a pivot point that’s further from the centre of gravity, than weight distribution or lowering that centre of gravity.

To tip a lander over, its centre of gravity needs to be moved beyond  the point it’s pivoting  about - which will be one of the legs.

For a lander with a high centre of gravity and narrow leg span, it doesn’t need to tip very far before the CoG is over the pivot leg. For a lander with a low centre of gravity and a wide leg span, it needs to tip much further for that to happen.

Compare a pencil balanced on its blunt end to a can of paint. It’s quite possible that the centre of gravity for both will be at the same height - but it’ll still be a lot easier to knock the pencil over!

But maybe we’re just saying the same thing in different words.

Edited October 6, 2021 by KSK

[kerbiloid]

5 minutes ago, KSK said:

I always thought that the leg span was more to do with providing a pivot point that’s further from the centre of gravity, than weight distribution or lowering that centre of gravity.

Yes, and CoM of a pencil is higher, so it needs even wider legs than a legged ball.

Upd.

Also, unlike the flat and horizontal concrete,  the ground roughness cause a tilt in any case.

So, it's important to provide the H_CoM / W ratio safe.

Can the Starship shape have same CoM relative height as LEM? Unlikely.

So, the only way to do that is to widen the legs.

[JoeSchmuckatelli]

I think you guys are talking around each other.  The pencil can stand if you have an active controller keeping it upright, but will fall if it's just a passive stick. 

Traditional design says that the ship should be stable when turned completely off.  Hence low COG / wide legs.  

I'm not sure you can turn SS Moon / Mars completely off w/o legs... It is likely to need a big gyro and stability system running at all times 

[KSK]

I agree @kerbiloid but the difference in stability might be smaller between Lunar Starship and the LEM might not be quite as much as it first appears.

According to Wikipedia, the LEM diameter was about 14 feet without landing gear, Judging from pictures alone, I estimate that the diameter with landing gear extended is about double that. That’s still less than the Starship tank diameter.

Then consider that a landed Starship still has quite a bit of propellant on board in its main tanks since it needs to get to lunar orbit and do the TEI burn home. That’s going to lower the CoG quite a bit. 
 

Is ‘quite a bit’ the same as ‘enough for safety’? That I don’t know.

[magnemoe]

50 minutes ago, Beccab said:

It doesn't, in fact the very wide Luna 23 still tipped over during or shortly after landing


Wide legs aren't as important as good control and careful planning

My guess is that the lower tanks was pretty empty then landing while the accent stage was full, putting the CoM pretty high. 
But yes moonship will land based on much better maps and with very high accuracy, might well using lidar to survey the landing zone before doing the deorbit burn, as an fail save I guess it will do an abort burn during landing if one leg landed in fluffy dust or broke. and your tilt got to high. else you want the thing to be level for crew comfort and to unload heavy stuff like the drill rig, 

[tater]

I think the primary concern on an unprepared surface is not tipping, but the plume-regolith interaction deforming the previously flat surface. Lunar regolith is in fact pretty compacted (there was some difficulty driving flagpoles, core sample tools, etc into the surface), but a powerful rocket engine will none the less dig a deep hole.

The landing engine concept for the current iteration of LSS deals with this nicely, though apparently SpaceX thinks they might not need to do that (per Musk).

 

[kerbiloid]

12 minutes ago, JoeSchmuckatelli said:

The pencil can stand if you have an active controller keeping it upright, but will fall if it's just a passive stick. 

1. Any controller has mass, and the lighter it is - the less it's effective.

2. See the touchdown of Falcons, and imagine the gravity is six times lower. It would bounce like Philae from comet.

14 minutes ago, JoeSchmuckatelli said:

Traditional design says that the ship should be stable when turned completely off.  Hence low COG / wide legs.  

I'm not sure you can turn SS Moon / Mars completely off w/o legs... It is likely to need a big gyro and stability system running at all times 

A gyro is overheavy, eats a lot, and is another source of an overturn, because conflicts with reaction forces of the ground.
Also it requires periodical stops and RCS adjustment of the orientation, to release the accumulated stresses.

It's relatively appropriate in space, but definitely not an option for ground.

14 minutes ago, KSK said:

Then consider that a landed Starship still has quite a bit of propellant on board in its main tanks

One tank above another, and they arer vertical and each has its fuel (for ~3.5 km/s delta-V, btw). 
So, CoM is much higher than Luna or LEM.

14 minutes ago, magnemoe said:

But yes moonship will land based on much better maps and with very high accuracy

The accuracy doesn't help when you land on a raw ground, tilted, covered with pits, heaps, stones, and areas of unstable ground.

Only a proper geometry helps.

See the battle tank proportions. It's for wildlands.

[sevenperforce]

1 hour ago, adsii1970 said:

This is why the Apollo lander was designed as it was. At the time, NACA/NASA believed the surface of the Moon was covered in roughly a foot of lunar dust. They believed the wide, concave landing pads at the end of the legs, the wide span of the legs, and the low center of gravity. In the end, they learned from the Apollo 11 mission the dust wasn't as thick as they expected it to be.

Minor quibble: by the time of Apollo 11, NASA knew for sure that the moon dust wasn't a foot thick. The unmanned Surveyor landings gave them enough data to establish that the dust was compacted.

The larger concern for the landings were the possibility of coming in with a non-negligible horizontal velocity component and the associated tipover. 

[kerbiloid]

A backup option.

Spoiler

Sharpen it and make spin.

For same money you get artificial gravity.

 

Edited October 6, 2021 by kerbiloid

[adsii1970]

12 minutes ago, sevenperforce said:

Minor quibble: by the time of Apollo 11, NASA knew for sure that the moon dust wasn't a foot thick. The unmanned Surveyor landings gave them enough data to establish that the dust was compacted.

There was still a fear of what was under the dust - was it all solid rock or a mixture that could support the weight of a lander:
https://www.icr.org/article/moon-dust-solar-system/

Unrelated reading:
https://www.hq.nasa.gov/alsj/TP-2006-213726.pdf

[Beccab]

Thread about the Crew 3 press conference, posting the most important ones

 

Edited October 6, 2021 by Beccab

[sevenperforce]

1 hour ago, adsii1970 said:

1 hour ago, sevenperforce said:

Minor quibble: by the time of Apollo 11, NASA knew for sure that the moon dust wasn't a foot thick. The unmanned Surveyor landings gave them enough data to establish that the dust was compacted.

There was still a fear of what was under the dust - was it all solid rock or a mixture that could support the weight of a lander:
https://www.icr.org/article/moon-dust-solar-system/

ICR is known to be a particularly unreliable source.

(The reason I happen to be knowledgeable on the moon dust issue is that I used to work with that and related groups, back in my anti-science days, and this was a common bone of contention.) 

[Beccab]

Some Starship updates:

 

Road closures have changed: new ones for likely starship testing on Monday and Tuesday, and tomorrow an intermittent road closure possibly for GSE-8

B5 had two of its methane tank sections stacked together, out of four total for that tank

Lots of stuff happened yesteday too:

 

 

[zolotiyeruki]

I love the fact that Dragon has red and green navigation lights, just like an airplane.

[Terwin]

6 minutes ago, zolotiyeruki said:

I love the fact that Dragon has red and green navigation lights, just like an airplane.

I would not be surprised if they were obeying the same law....

[GuessingEveryDay]

6 minutes ago, zolotiyeruki said:

I love the fact that Dragon has red and green navigation lights, just like an airplane.

It's time to play "WWISTDP?!" (Which Way is the Dragon Pointing™️)