totm nov 2023 SpaceX Discussion Thread

[RealKerbal3x]

Just now, Hannu2 said:

Of course they are tested as well as they can. But it seem there have been problems with underperforming Raptors in SN9 and SN10. Something may be going on and they have to find a solution. I hope I am wrong and they had some sporadic problems easy to fix, of course.

Raptor is a comparatively immature engine right now, and a complex one at that. It's a test program so it's a good thing that they're finding issues now and not when they're flying commercial payloads or, worse, crew.

2 minutes ago, Hannu2 said:

That strange flight profile may also be engine testing. They ascent very slowly and burn engines several minutes. At apoapsis they almost hover a minute or two with very small velocity. If it was just aerodynamic test they could ascend probably in less than a minute. I do not see other reasons for such profile than get important test minutes for engines at (almost) realistic flight conditions.

 That's probably a part of it, but it also looks like they don't want to break the sound barrier during ascent, or put too much stress on the airframe. 

[Hannu2]

2 minutes 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. 

 

Also - am I wrong to assume SX can recycle all the steel recovered from the past SNs? 

Scrap steel is not suitable for building of high strength structures anymore, but it is very pure stainless steel (compared to many other scrapped machines which have significant part of mass from other materials) and very effectively recycled to new steel in steel factory.

[sevenperforce]

What are we thinking in terms of actual mechanism?

My best guess is that when the legs failed to lock out properly, it came down hard on the engine bells and damaged the thrust puck. The impact probably ruptured the methane downcomer, allowing the remaining liquid methane from the methane header tank to mix with residuals in the main oxygen tank. The oxygen tank was basically a fuel-air bomb. Of course, no big deal...until the fire outside the skirt made its way underneath and up to the thrust puck. And then, kablooey.

[JoeSchmuckatelli]

What was crazy about the explosion was that it lifted itself a full body length (or more) into the air. 

[SOXBLOX]

Wait, so, it landed? And then exploded? I just got here...

[RealKerbal3x]

Just now, SOXBLOX said:

Wait, so, it landed? And then exploded? I just got here...

Yeah, an engine underperformed, so it landed in one piece, albeit hard. It seemed like the damage caused the propellants to ignite about 8 minutes later and boom.

[sevenperforce]

10 minutes ago, JoeSchmuckatelli said:

What was crazy about the explosion was that it lifted itself a full body length (or more) into the air. 

Well it basically was another rocket engine. The ruptured thrust puck probably acted like a nozzle throat.

4 minutes ago, RealKerbal3x said:

Yeah, an engine underperformed, so it landed in one piece, albeit hard. It seemed like the damage caused the propellants to ignite about 8 minutes later and boom.

Do we have word of Elon that there was landing engine underperformance? It looked like a landing leg issue to me. 

[RealKerbal3x]

Just now, sevenperforce said:

Do we have word of Elon that there was landing engine underperformance? It looked like a landing leg issue to me. 

I don't think there was any official confirmation that the engine underperformed but it sure looked like it. The landing legs failed to deploy properly but that doesn't account for how fast the touchdown looked.

[tater]

17 minutes ago, sevenperforce said:

What are we thinking in terms of actual mechanism?

My best guess is that when the legs failed to lock out properly, it came down hard on the engine bells and damaged the thrust puck. The impact probably ruptured the methane downcomer, allowing the remaining liquid methane from the methane header tank to mix with residuals in the main oxygen tank. The oxygen tank was basically a fuel-air bomb. Of course, no big deal...until the fire outside the skirt made its way underneath and up to the thrust puck. And then, kablooey.

Yeah, and possibly more complex than that. Landing on the lend nominally engages the crush cores as needed, and some asymmetry in landing is likely designed in, they might have even simulated some legs undeployed. The dangling legs, however were not just undeployed, but possibly hanging down lower at the moment of impact, which could result in tearing the skirt, or even breaking them and throwing them up into the engines.

The bells are pretty close to the skirt edge, though (height off the ground distance), right?

[sevenperforce]

34 minutes ago, RealKerbal3x said:

I don't think there was any official confirmation that the engine underperformed but it sure looked like it. The landing legs failed to deploy properly but that doesn't account for how fast the touchdown looked.

I guess we could compare the descent rate between SN10 and SN5/6 to get a feel for it. 

I don't think they have any "change your thrust rate by X if the landing legs don't fully deploy" programming. So if it shut down the engine as if the landing legs had deployed, that alone would have dropped it a meter or so (however long the legs are).

36 minutes ago, tater said:

Yeah, and possibly more complex than that. Landing on the lend nominally engages the crush cores as needed, and some asymmetry in landing is likely designed in, they might have even simulated some legs undeployed. The dangling legs, however were not just undeployed, but possibly hanging down lower at the moment of impact, which could result in tearing the skirt, or even breaking them and throwing them up into the engines.

The bells are pretty close to the skirt edge, though (height off the ground distance), right?

Yes, I believe they are very close -- probably only 20 cm or so. 

I remember looking at the closeup on the SpaceX feed and thinking it was odd how close the flaperons seemed to the ground:

For any fans of Doctor Who we have this lovely comment by John Insprucker:

GERONIMO!

We need better legs, now. But it does look like better legs may be in the works. From the DearMoon update:

Definitely a different design than before. How large are these exactly? It looks like they have reduced to just four legs, mounted externally. But they are REALLY wide now.

 

1 hour ago, tater said:

The bells are pretty close to the skirt edge, though (height off the ground distance), right?

Wreckage photos show that the Raptors are at least not completely crumpled.

Spoiler

 

[StrandedonEarth]

 

[JoeSchmuckatelli]

15 minutes ago, StrandedonEarth said:

 

That wins Post of the Day! 

1 hour ago, sevenperforce said:

Well it basically was another rocket engine. The ruptured thrust puck probably acted like a nozzle throat

Or like the recoil on a cannon.  

 

I find the skirt acting like an impromptu rocket bell somewhat likely, but don't know enough about whether the thrust puck could have done it.  You are thinking that combustion happened higher in the stack?  

[sevenperforce]

5 minutes ago, JoeSchmuckatelli said:

Or like the recoil on a cannon.  

I find the skirt acting like an impromptu rocket bell somewhat likely, but don't know enough about whether the thrust puck could have done it.  You are thinking that combustion happened higher in the stack?  

I'm not saying it was properly choked or anything. But yes, if you look at slow-motion captures it definitely appears the bulk of the explosion happened in the LOX tank.

 

[tater]

Look at the leeward bulges. Leg changes.

[kerbiloid]

Next step.
 

Spoiler

 

(Offtopic)

Btw, look how close is this to the Canadarm grappling fixture.

 

[Elthy]

Maybe those are large flaps that fold outwards (hinge on the skirt). Those large, flat areas should be easier to shield from reentry than more prominent/protruding thinner legs like on the old renders.

[cubinator]

Looks like it could be more like the old ITS landing feet.

[JoeSchmuckatelli]

18 minutes ago, Elthy said:

Maybe those are large flaps that fold outwards (hinge on the skirt). Those large, flat areas should be easier to shield from reentry than more prominent/protruding thinner legs like on the old renders.

I would hinge the leading edge, let it pop out 10-20 deg. with a telescoping, shockabsorbing 'leg' inside (= much wider base).

[sevenperforce]

More evidence that the engines did not contact the ground at touchdown:

If they had, there would be vertical crumple damage around the mouth of the engine bell.

[sevenperforce]

If they are more like the old ITS legs:

 

[JIMMY_the_DOG]

What's the difference between ITS and starship?

[sevenperforce]

1 minute ago, JIMMY_the_DOG said:

What's the difference between ITS and starship?

ITS was going to be 12 meters, carbon-fiber, with no wings, and primarily intended for Martian colonization.

Of course they had no budget for this and so they scaled back and built Starship. Originally designed to be carbon-fiber with PICA-X heat shielding as well, they switched to stainless steel and ceramic tiles because it works better and is a lot cheaper and easier to build. They added the flaperons to control attitude. Unlike ITS, Starship has valid use cases for LEO and cislunar activities.

Here's the image from the 2016 IAC showing those giant landing legs:

 

[Delay]

4 hours ago, RealKerbal3x said:

the damage caused the propellants to ignite about 8 minutes later and boom.

That's 8 minutes longer than I expected given the earlier abort!

[YNM]

4 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.

It's fairly empty when it's only about to land. Large movement is what I expected. Plus they fired all three engines right from when it was parallel to the ground, and had only the engine thrust mostly to maneuver.

53 minutes ago, sevenperforce said:

If they had, there would be vertical crumple damage around the mouth of the engine bell.

That's only 1 engine out of 3... Touching might've been light on the forces since the side walls are still supporting some weight.

[mikegarrison]

4 hours ago, sevenperforce said:

What are we thinking in terms of actual mechanism?

My best guess is that when the legs failed to lock out properly, it came down hard on the engine bells and damaged the thrust puck. The impact probably ruptured the methane downcomer, allowing the remaining liquid methane from the methane header tank to mix with residuals in the main oxygen tank. The oxygen tank was basically a fuel-air bomb. Of course, no big deal...until the fire outside the skirt made its way underneath and up to the thrust puck. And then, kablooey.

If you go back about 5 or 10 or whatever pages, you'll see that I pointed out that the fire suppression system tailed off and ended. (Either they shut it off or they ran out of stored water.) Everything sat there for about another 5 seconds, then BOOM!

I still strongly suspect that there was an ongoing fire the entire time inside the engine compartment. But all that water was cooling everything down. Once that ended, temperatures rose inside and something failed. Whatever it was only needed the the fuel/air ratio to reach the right (ie. explosive) ratio, and then it exploded.