totm nov 2023 SpaceX Discussion Thread

[sevenperforce]

Here's a gif of the ITS landing leg deployment if you wanna compare.

[StrandedonEarth]

 

[mikegarrison]

5 hours ago, JoeSchmuckatelli said:

That landing sequence is not quite what I would expect - there is a lot of lateral movement for something so heavy.  I would have expected every frame to be moved up one spot with the final frame before landing being directly above the landed photo. 

Keep in mind that they have a really tough problem.

Their aerodynamic control authority only works when they are sideways. As soon as they start the flip, they are committed to 100% propulsive control. And like with any Kerbal suicide burn, that's a thing with very little margin. Burn too long and you run out of fuel and crash. Burn too much and you start going up. Burn too little or too late and you crash.

At least with Falcon the grid fins keep working almost all the way down to landing (although they lose control authority as the velocity decreases).

I think Starship is always going to be fundamentally locked into this almost-no-margin "do or die" pradigm. It's one reason I tend to wonder if it will ever actually be safe enough for people.

[tater]

Regarding the leg failure, does anyone know what the mechanism is? Hydraulic, I presume? Do they mechanically lock, or is that also hydraulic (if they are deployed hydraulically)?

What else is hydraulic? Engine gimbal?

Is there a chance the system had a transient pressure drop because of the change to a 3 engine flip, then rotating the 2 shut down out of the way, vs 2 dropping to 1, and somehow the deployment or locking was not to spec?

[RealKerbal3x]

This is an image from Maezawa's facebook page (apparently, I found it on reddit). It shows the landing legs a little bit more clearly, though I still can't tell whether they're F9-style flip-out legs or ITS-esque feet.

Just now, tater said:

Regarding the leg failure, does anyone know what the mechanism is? Hydraulic, I presume? Do they mechanically lock, or is that also hydraulic (if they are deployed hydraulically)?

What else is hydraulic? Engine gimbal?

Is there a chance the system had a transient pressure drop because of the change to a 3 engine flip, then rotating the 2 shut down out of the way, vs 2 dropping to 1, and somehow the deployment or locking was not to spec?

I vaguely remember reading at some point that they were mainly extended via gravity. I don't really know.

[kerbiloid]

Spoiler

They need longer legs.

 

[tater]

9 minutes ago, RealKerbal3x said:

This is an image from Maezawa's facebook page (apparently, I found it on reddit). It shows the landing legs a little bit more clearly, though I still can't tell whether they're F9-style flip-out legs or ITS-esque feet.

They can flip out to leeward, no problem.

The "center" engine for the flip is the +Z engine, right?

      O
O        O

The top one when falling horizontally.  That top side is the leeward side, and as it flips, the leeward side then is lower WRT the ground than the windward side, as it burns to reduce the flip overcorrect.

So far, so good. It seems like if it contacts any legs first, they would be on the leeward side.

If the legs flipped down on that side—as "outrigger" legs, the belly legs could come down closer to the skirt line. It's asymmetrical, but I'm not sure that's a problem.

Edited March 4, 2021 by tater

[JoeSchmuckatelli]

1 hour ago, mikegarrison said:

At least with Falcon the grid fins keep working almost all the way down to landing (although they lose control authority as the velocity decreases).

How much below terminal do the grid fins keep Falcon? 

 

Would a pair of pop-out fins perpendicular to the forward fins (even if merely passive) help with the final moments once flipped? 

 

Nvm - that would screw with the weight /COM. 

 

Rotatable fins? 

Edited March 4, 2021 by JoeSchmuckatelli

[mikegarrison]

Just now, JoeSchmuckatelli said:

How much below terminal do the grid fins keep Falcon? 

 

Would a pair of pop-out fins perpendicular to the forward fins (even if merely passive) help with the final moments once flipped? 

I've got no idea what the answer is to either of those questions, and I'm not going to just guess and pretend I know.

[JoeSchmuckatelli]

3 minutes ago, mikegarrison said:

I've got no idea what the answer is to either of those questions, and I'm not going to just guess and pretend I know.

Yeah - as I think about it, once they're lighting the rockets, they really don't need some other aerodynamic system trying to compete.  Certainly would not help on moon or Mars 

 

I strike the question 

Edited March 4, 2021 by JoeSchmuckatelli

[sevenperforce]

1 hour ago, RealKerbal3x said:

This is an image from Maezawa's facebook page (apparently, I found it on reddit). It shows the landing legs a little bit more clearly, though I still can't tell whether they're F9-style flip-out legs or ITS-esque feet.

They're the same shape on the lee side and the windward side, so presumably they have a common design. The only way for the windward version to get out of the shielded fairing would be to extend downward first. So clearly they cannot be hinged at the skirt as with the F9 legs. 

So they may extend down and then out like the ITS legs. Or, they may extend down and then out, similar to the legs on New Shepard. 

[CatastrophicFailure]

2 hours ago, mikegarrison said:

Burn too much and you start going up.

Also keep in mind that Starship can hover on a single engine, maybe two, so it’s not a full-on suicide burn. They are, however, very propellant limited right now, AFAIK these SNs cannot lift off with full tanks, no VacRaps, so that itself also greatly lowers their margins. The whole system will probably work better as, well, a whole system, including...

 

1 hour ago, JoeSchmuckatelli said:

Would a pair of pop-out fins perpendicular to the forward fins (even if merely passive) help with the final moments once flipped? 

We haven’t heard anything for a while, other than it’s been temporarily back-burnered, but SpaceX did/does have plans for hot-gas methalox RCS thrusters, which would greatly help with control during the flip. Now, hot-gas thrusters, a greater fuel load at landing ignition, and a whole bunch more flights to tweak the interactions of all of the above, now that the general concept has been proven, and things will look a lot less squirrelly. 

[tater]

1 hour ago, sevenperforce said:

They're the same shape on the lee side and the windward side, so presumably they have a common design. The only way for the windward version to get out of the shielded fairing would be to extend downward first. So clearly they cannot be hinged at the skirt as with the F9 legs. 

Not exactly. The belly side shows a single bulge across the whole bottom, vs two smaller ones to leeward.

(or did in the other new renders up thread)

Strike that, I swore they did, but I might have been doing a side by side with the old renders the other night, they look the same now.

Meanwhile in Starbase, TX, apparently SN11 heads to the pad Monday.

[sevenperforce]

1 hour ago, tater said:

Not exactly. The belly side shows a single bulge across the whole bottom, vs two smaller ones to leeward.

(or did in the other new renders up thread)

Strike that, I swore they did, but I might have been doing a side by side with the old renders the other night, they look the same now.

Some good notional renders from Twitter:

The fold-out legs a la Falcon 9 would be the simplest, sure, but they're also simply not possible because that would involve (a) a seam in the heat shield, and (b) the entire vehicle resting on the freaking heat shield tiles. 

His pop-out legs a la New Shepard are also a nonstarter because of the heat shield seam issue and because of load paths.

So the straight-down solution seems straightforward. It's also the most capable of auto-leveling and shock absorption.

ITS-style legs that descend straight down and then fold outward are another possibility.

[tater]

16 minutes ago, sevenperforce said:

The fold-out legs a la Falcon 9 would be the simplest, sure, but they're also simply not possible because that would involve (a) a seam in the heat shield, and (b) the entire vehicle resting on the freaking heat shield tiles. 

Those all have a seam (unless one is a cut-away diagram). The only way not to is the ITS style. It's not a problem to do F9 type if they don't have to be symmetric windward/leeward (leeward F9, others not).

Quote

His pop-out legs a la New Shepard are also a nonstarter because of the heat shield seam issue and because of load paths.

Yeah.

Quote

So the straight-down solution seems straightforward. It's also the most capable of auto-leveling and shock absorption.

This one:

Still has a seam, though. Or is that supposed to be a cut away diagram the the gear are sliding underneath the cover?

Quote

ITS-style legs that descend straight down and then fold outward are another possibility.

Seems like a better idea, particularly if it can get a better stance on rough terrain.

[sevenperforce]

20 minutes ago, tater said:

This one:

Still has a seam, though. Or is that supposed to be a cut away diagram the the gear are sliding underneath the cover?

I think this is the leeward view. The heat shield would cover the leg completely. 

20 minutes ago, tater said:

Seems like a better idea, particularly if it can get a better stance on rough terrain.

In three dimensions, there might be room to do something interesting. Two pistons that descend vertically and one that protrudes through the skirt (perhaps from the thrust puck) to the furthest extent of the foot.

[tater]

It's a fascinating problem. I'd like to see the lunar version go very wide stance to be honest.

heck, initial Mars versions as well, though yesterday's hard landing was surprisingly stable minus legs at all (unless the ~3 that deployed stabbed into the concrete like lawn darts ).

[sevenperforce]

7 minutes ago, tater said:

It's a fascinating problem. I'd like to see the lunar version go very wide stance to be honest.

heck, initial Mars versions as well, though yesterday's hard landing was surprisingly stable minus legs at all (unless the ~3 that deployed stabbed into the concrete like lawn darts ).

I think the spreading of the aft flaps helped with stability actually.

Here's how a three-piston ITS-style solution might work.

 

[SOXBLOX]

I watched the LabPadre and Everyday Astronaut footage. I wonder, how much pressure is that fuel and LOX under? Cause SS was really thrown into the air.

[tater]

9 minutes ago, sevenperforce said:

Here's how a three-piston ITS-style solution might work.

For the narrow stance versions, I like that one a lot. But I mean a WIDE stance.

[sevenperforce]

10 minutes ago, SOXBLOX said:

I watched the LabPadre and Everyday Astronaut footage. I wonder, how much pressure is that fuel and LOX under? Cause SS was really thrown into the air.

It was the kaboom that launched it in the air, not the 3 bar of ullage pressure. The kaboom was probably 100 bar or more.

2 minutes ago, tater said:

For the narrow stance versions, I like that one a lot. But I mean a WIDE stance.

 

Spoiler

 

Beautiful, but for Starship definitely a non-starter.

Over at NSF people are discussing launch abort systems for Starship, which has always been an issue because despite the vehicle's excellent payload, the nose section is SO large that a functioning abort system would really cut into your mass budget.

And then someone came up with what has to be the best solution I have heard yet.

Quote

What if you had launch escape in the form of hybrid puller rockets in the nose which drew from the LOX header tank but were otherwise made of a storable solid rubber fuel?

That's.......tremendous.

Rubber or plasticized petrol for the fuel, the existing LOX header tank for the oxidizer, pyro torches for instant ignition. Hybrids have remarkably good TWR and can use differential throttling, vectored nozzles, or both to provide pointing. Detcord just above the top of the methane tank for separation. It separates with its forward flaps, which assist in maintaining a heading until escape engine shutoff.

Multiple large chutes deploy from explosively-jettisoned panels under the LOX head tank and the aft of the fairing provides several meters of crumple zone for a survivable, if unpleasant, landing.

Not good enough for P2P, of course.......but absolutely good enough for a 10-person crew.

[tater]

16 minutes ago, sevenperforce said:

Beautiful, but for Starship definitely a non-starter.

Sorry, I meant Lunar Starship.

[Brotoro]

Will the final versions of Starship have the LOX header tank in the tip of the nose, or is that just an arrangement for the protypes?

[RyanRising]

22 minutes ago, Brotoro said:

Will the final versions of Starship have the LOX header tank in the tip of the nose, or is that just an arrangement for the protypes?

I think it was up there to pull the centre of mass towards the nose a bit, so if that's right then you'd still want it for at least cargo and refuelling Starships that land empty. But I don't have any sources for that.

 

56 minutes ago, sevenperforce said:

It was the kaboom that launched it in the air, not the 3 bar of ullage pressure. The kaboom was probably 100 bar or more.

Beautiful, but for Starship definitely a non-starter.

Over at NSF people are discussing launch abort systems for Starship, which has always been an issue because despite the vehicle's excellent payload, the nose section is SO large that a functioning abort system would really cut into your mass budget.

And then someone came up with what has to be the best solution I have heard yet.

"What if you had launch escape in the form of hybrid puller rockets in the nose which drew from the LOX header tank but were otherwise made of a storable solid rubber fuel? "

That's.......tremendous.

Rubber or plasticized petrol for the fuel, the existing LOX header tank for the oxidizer, pyro torches for instant ignition. Hybrids have remarkably good TWR and can use differential throttling, vectored nozzles, or both to provide pointing. Detcord just above the top of the methane tank for separation. It separates with its forward flaps, which assist in maintaining a heading until escape engine shutoff.

Multiple large chutes deploy from explosively-jettisoned panels under the LOX head tank and the aft of the fairing provides several meters of crumple zone for a survivable, if unpleasant, landing.

Not good enough for P2P, of course.......but absolutely good enough for a 10-person crew.

Ooooh, I love that. Quick, someone tell me why it's unworkable!

[tater]

For a crew version, I think the payload penalty of a LAS is totally unconcerning.

How many people at once does one need to send to space? Use the abort-capable version to send people to LEO. Dock with a crew vehicle in orbit, and take them to the Moon (wherever). The crew vehicle can aerobrake to LEO, discharge passengers into the abort-capable version, then do EDL.

At least until they get some 100s or 1000s of consecutive, perfect landings.