totm nov 2023 SpaceX Discussion Thread

[sevenperforce]

8 minutes ago, mikegarrison said:

Well they weren't "always full" this time, were they?

Elon said the problem was pressurization, not gas-gulping.

Here's a new leg design that I think I really like.

It's the only way I can think of to get the clearance that the Raptors need, maximize the extension of the legs, and avoid any heat shield seams.

1. At leg deployment, the leg slides down, guided by a "shoe" that wraps around it. The pneumatic pistons on the outside deploy.
2. Once the leg has reached maximum extension, it is locked in place and the "shoe" rotates outward until the leg contacts the pneumatic pistons.
3. As the tip of the leg touches the ground, it applies upward pressure to the pneumatic pistons, causing them to compress. If the ground is uneven, the pressure setting in each piston can be adjusted to self-level.
4. The "heel" contacts the ground to bear the static load at the same time as the pneumatic pistons absorb all dynamic load and reach minimum stroke.

This way you avoid having both a sliding and rotating member. The only challenge is figuring out how to actuate the sliding action within the rotating shoe.

[Deddly]

@sevenperforce I might be a little out of touch here, but why does Starship need to widen its stance? It's 9 m in diameter. It feels like it should be difficult for it to topple over, especially if auto-levelling is built in anyway. So my question is, why not just...

 

Or why not have landing legs slightly lower down than depicted here and forego the need to extend them at all? "The best part is no part"

[Entropian]

Not sevenperforce, but here's my answer:

Basically, the aspect ratio is large.  It doesn't matter how wide the base is if the height scales with it.  Imagine the Eiffel Tower without its four legs.  I'm pretty sure it would fall over quickly, as it's next to a riverbed and is already tilting.  The same goes for Starship.  The aspect ratio is still high, so it has a lower angle tolerance.

EDIT: This is too vague.  I'm setting up an image now.

Edited December 16, 2020 by Entropian

[Nightside]

4 minutes ago, Deddly said:

@sevenperforce I might be a little out of touch here, but why does Starship need to widen its stance? It's 9 m in diameter. It feels like it should be difficult for it to topple over, especially if auto-levelling is built in anyway. So my question is, why not just...

 

This is a good point. A wider stance certainly gives a better margin of error on horizontal movement that would cause tipping.  It would be interesting to know how high the center of mass is at landing.

That said, they seem to have mastered cancelling their horizontal velocity (as long as they can also cancel their vertical velocity). If they cannot cancel their vertical velocity then nothing else matters! 

 

[Entropian]

Ok, basically this:

With the red vector as the gravitational force and the yellow dot for the CoM.  (my photoshop is terrible)

[Nightside]

3 minutes ago, Entropian said:

Ok, basically this:

With the red vector as the gravitational force and the yellow dot for the CoM.  (my photoshop is terrible)

Yep. But do we expect the CoM to really be that high? What is the expected dry mass and payload these days?

[Deddly]

Well that does make sense, of course, but with self-levelling feet I don't see the problem

[Entropian]

No way should it be that high.  I was exaggerating it so that the effect would be more obvious.

Just now, Deddly said:

Well that does make sense, of course, but with self-levelling feet I don't see the problem

Problem is, the self-levelling feet have an angle limit.  If there's too much horizontal velocity or it's tilted too far, the feet won't have enough freedom to guide it back upright.

[sevenperforce]

Just now, Deddly said:

Well that does make sense, of course, but with self-levelling feet I don't see the problem

Elon has said they need a wider stance, so evidently he thinks there's a problem.

Remember that Elon wants to put people on Starship. So it needs to be able to have every advantage for contingency landings. It needs to be able to land on one engine, on two engines, at an angle, in the wind, on uneven surfaces...everything. So the ability to have a wide, shock-absorbing stance is critical.

[tater]

Spoiler

 

[cubinator]

We all know what happens when you fly a fuel tank that was dropped...

[zolotiyeruki]

1 hour ago, Nightside said:

Yep. But do we expect the CoM to really be that high? What is the expected dry mass and payload these days?

Keep in mind, though, that regardless of the use case on earth, the eventual goal is to land these on Mars, then refuel and return.  Depending on the mass of the payload up front, filling the fuel tanks may shift the CoM towards the nose (or toward the rear!).

Speaking of which, was there a mass simulator in the nose of SN-8, or was it just a hollow shell?

31 minutes ago, cubinator said:

We all know what happens when you fly a fuel tank that was dropped...

As I recall, it wasn't a dropped tank, it was a change in the voltage spec for some instrument or tank heater.

[Clamp-o-Tron]

30 minutes ago, cubinator said:

We all know what happens when you fly a fuel tank that was dropped...

My personal theory is that the downcomer is actually just a really big stirring stick for the LOX tank, driven by the turbopumps. Low header tank pressure in SN8 wasn’t actually the problem. They intended to make a 2-1 landing burn, and did so, but failed to take into account that the stirrer would move a lot slower, causing a massive explosion between the LOX main tank and the CH4 header just as it hit the ground. (It was traveling at just the right speed. It looked very fast because of the camera angle, field of view, daylight savings, difference between the speed of light and sound, suboptimal video compression, axial tilt of the sun, Jupiter in Pisces, gravitational influence of Alpha Centauri A, weird perspective, and the Brazilian Senate) The green stuff was just a crapload of TEA-TEB Elon injected to mess with us. (/s of course)

[Nightside]

1 hour ago, zolotiyeruki said:

Keep in mind, though, that regardless of the use case on earth, the eventual goal is to land these on Mars, then refuel and return.  Depending on the mass of the payload up front, filling the fuel tanks may shift the CoM towards the nose (or toward the rear!).

Speaking of which, was there a mass simulator in the nose of SN-8, or was it just a hollow shell?

As I recall, it wasn't a dropped tank, it was a change in the voltage spec for some instrument or tank heater.

Do we know how much that mass simulator weighs?

[sevenperforce]

1 hour ago, zolotiyeruki said:

Keep in mind, though, that regardless of the use case on earth, the eventual goal is to land these on Mars, then refuel and return.  Depending on the mass of the payload up front, filling the fuel tanks may shift the CoM towards the nose (or toward the rear!).

Speaking of which, was there a mass simulator in the nose of SN-8, or was it just a hollow shell?

No mass simulator in SN8. The mass sims for SN5 and SN6 were intended to substitute for the COM moment of the fairing and LOX header. They were not simulating a payload.

In other news, here's one additional possible render. It has an "ankle", a "heel", a "toe", a "sole", and a "shin".

 

The ankle rotates the shoe, the toe slides through the shoe, the ankle locks against the shin, and the sole makes contact, distributing force between the heel and the toe. The load path goes from the toe to the shin and from the heel to the ankle.

[JoeSchmuckatelli]

If there is vectoring... Do you have clearance? 

[tater]

Live in about 12 hours.

 

[mikegarrison]

41 minutes ago, JoeSchmuckatelli said:

If there is vectoring... Do you have clearance? 

Roger.

[tater]

Surely you have clearance.

[StrandedonEarth]

Roger, roger! Checking  the vector, Victor! Yes we have clearance and don’t call me Shirley!

Edited December 17, 2020 by StrandedonEarth

[sevenperforce]

3 hours ago, JoeSchmuckatelli said:

If there is vectoring... Do you have clearance? 

Clearance shouldn't be a problem. Depends on piston stroke and stuff, but you can definitely make it fit.

[Brotoro]

The flight of Starship SN8 inspired me to make a 3D-printed 1/100 scale Starship. So far I have the airframe printed (and currently held together with tape), and I haven't built the internals yet (other than the centering rings) that will allow it to fly. It will need a considerable amount of nose weight and the addition of at least one clear acrylic fin to make it stable. I put it on my shelf of mostly 1/100 scale models for comparison. The Starship is 90 mm in diameter (3.54 inches), while the 1/100 scale Saturn V behind it is 100 mm in diameter.

 

[Scotius]

Add a SuperHeavy, and it will rival Saturn in height too

[James Kerman]

Welcome to the forum @bearnard1244, I didn't have to pay for it so the landing was a huge success for me! Also, your post has been merged into our dedicated SpaceX discussion thread.

[RealKerbal3x]

59 minutes ago, bearnard1244 said:

As probably all of you guys have seen the launch of the new rocket by Space X Starship. Was the result of that launch successful. I knot, that the landing part was not the aim of the mission but still, it spoilt a bit the whole picture of the test.

95% of the test objectives were completed, so the flight was a success. It wasn't really expected to get as far as it did (the landing was really a nice to have), so hopefully with the next prototype they can stick the landing.