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BFR Developments

Yazata Offline
(Apr 3, 2020 08:35 PM)confused2 Wrote: That (the fail) was horrible to watch.
ComicfromPlanet9 Wrote:@Geopilot8h
This is what happened to the empty sealed bottom tank as the top tank cooled it.
Sounds like the most plausible I've seen. It did look (to me) like negative pressure in the lower tank. Maybe just starting to put liquid gas into a tank of warm air without opening the sucking relief valve and - foop. 
If it was empty and simply collapsed under the weight of the top tank -surely not.

Hi C2.

There's been a great deal of discussion about that among the engineers on nsf. Your theory was championed by some. But the consensus seems to be converging on a different theory, summarized very succinctly by 'cdebuhr':

"A lot of chatter again about how SN3 imploded due to negative gauge pressure in the LOX tank.  This was discussed extensively up-thread, but in short, this was not a partial vacuum induced implosion.  The video evidence is completely inconsistent with the LOX tank at anytime reaching below-ambient absolute pressure (i.e., negative gauge pressure).  The LOX tank was venting continuously before and during the failure, right up until the sides finally ripped open due to failure of the metal.  The venting was slowing, indicating the pressure was falling, but at no point did it stop.  At some point there was insufficient pressure in the LOX tank to support the overlying weight in the LCH4 tank, and the sidewalls buckled.  Why and how this condition was achieved is the main outstanding question here.  As the sidewalls buckled, the pressure in the now shortened LOX tank increased as the volume decreased, as shown by the increased venting from the LOX vent port, which actually halted the collapse for a second or two.  Once the tank wall breached, the game was over, and the collapse continued without further hindrance."

Another of the engineers did some calculations based on the known volume of the CH4 tank and the density of liquid nitrogen. If full of liquid nitrogen, the CH4 tank would have weighed as much as 500 tons.

Others pointed out that the tanks are designed to support the weight of the top half of the rocket, its payload and structure, even when the tanks are empty. (Filled with air at atmospheric pressure.) The top of the complete rocket is estimated at roughly 150 tons of payload, plus maybe 50 tons of structure. 200 tons total. So it looks like the weight of the CH4 tank full of liquid nitrogen exceeded that design specification by up to 300 tons.

(It's extreme rocket engineering, that's for sure.)

The lower LOX tank, when pressurized properly, could hold it and was successfully holding it for an extended period prior to the failure. But this theory suggests that the LOX tank started losing its pressurization for some unknown (to outsiders, I bet SpaceX knows) reason, so that the 300 ton overload buckled the LOX tank walls. The engineers note that the tank held together (albeit crumpled) for a second or more, before rupturing. That's a testament to the improved weld quality, perhaps.

The finger-of-blame, among outsiders at least, is pointing at the valves, perhaps the LOX tank relief valve in particular. If it was stuck open or couldn't close fully, then tank pressure could have dropped below red-line levels. Elon had tweeted earlier that they were having valve trouble.

With Mk.1 and Sn.1 we saw how pressurizing the tanks can cause explosions. So the question arises, why pressurize them at all? What we just saw is the reason why. Tanks have to be light enough to get up into space. But they have to support huge mechanical loads. So since the early days of rocketry, the solution has been to pressurize them like balloons.
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confused2 Offline
Should not have happened! Another weld failure would just have been another of those but another spectacular fail!
I am reminded of the normal course of any project-
1. Enthusiasm,
2. Disillusionment,
3. Panic,
4. Search for the guilty,
5. Punishment of the innocent
6. Praise and honor for the nonparticipants.
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Yazata Offline
Elon's tweeting...

Somebody asked, "Elon, it seemed like CH4 tank was filled and the LOX tank which was not filled collapsed because of the weight."

Elon replied, "Pretty much. Good news is that this was a test configuration error, rather than a design or build mistake. not enough pressure in the LOX tank ullage to maintain stability with a heavy load in the CH4 tank. This was done with n2"

https://twitter.com/elonmusk/status/1246677676733104130

"There are redundant pressure control valves. It's a new system and Sn.3 was simply commanded wrong. Rockets are hard."

https://twitter.com/elonmusk/status/1246696253074915328

Photo of the three raptor engines intended for Sn.3 and now for Sn.4. The photo was evidently taken inside one of the three Giant Tents. 

https://twitter.com/elonmusk/status/1246701851585794048

(Photo by Elon)


[Image: EU0suUeXQAIZgUl?format=jpg&name=small]
[Image: EU0suUeXQAIZgUl?format=jpg&name=small]

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C C Offline
(Apr 5, 2020 08:08 PM)Yazata Wrote: ... Elon replied, "Pretty much. Good news is that this was a test configuration error, rather than a design or build mistake. not enough pressure in the LOX tank ullage to maintain stability with a heavy load in the CH4 tank. This was done with n2" [...]"There are redundant pressure control valves. It's a new system and Sn.3 was simply commanded wrong. Rockets are hard."

Sheesh, these over-complicated tin cans. Back in the Franklin expedition days, they were so simple that it was once thought the bad soldering had driven them insane from lead poisoning.
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Yazata Offline
(Apr 4, 2020 11:35 PM)confused2 Wrote: Should not have happened! Another weld failure would just have been another of those but another spectacular fail!
I am reminded of the normal course of any project-
1. Enthusiasm,
2. Disillusionment,
3. Panic,
4. Search for the guilty,
5. Punishment of the innocent
6. Praise and honor for the nonparticipants.

Hi C2.

This is one of the reasons why I like SpaceX. If it was NASA, if there was a succession of failures like this, they would have started with #'s 2 and 3. Then they would have started investigations, Congress would have probably held investigations as well. The failures would have had to have been somebody's fault, and heads would have to roll. (After which, the committees would pat themselves on the back for a job well done.)

So as a result, NASA has grown extremely risk-averse. They actually successfully shot men to the Moon in 1969, only 12 years after the first tiny satellite went up in 1957. But they seem to me to have lost the "right stuff" after management started fearing that any failure would mean the end of their careers. So all we see now are safe and sane projects that cost untold billions over budget and go years behind schedule, until every possibility of failure is banished. (It's already been nine years since a human has been launched into space on an American rocket by NASA.)

Compare that to SpaceX, essentially owned by one man, who not only is answerable only to himself, he's also Enthusiasm in the flesh. So when a failure happens, they skip all the disfunctional bits, number's 2 through 6, and just recycle back to #1 again. I guess that the danger there is crazy 'damn-the-torpedoes' foolhardiness, but it's sure fun to watch. Reminds me of the old NASA.

It's Elon's money after all, and he's not likely to launch humans on his Starship until it has lots of unmanned flights successfully done. As long as his test flights are out over the Gulf of Mexico so that pieces of ships that flake out in flight don't fall back down on locals, the failures are basically his business.

'Mars or bust'.
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Yazata Offline
Despite just losing Sn.3, Sn.4 is already coming along nicely. There are several nosecones to choose from, the thrust dome is well underway, the top dome and the common bulkhead are nearing completion.

Here's a interesting photo by Elon showing the common bulkhead with the spherical CH4 Header embedded in it. The circular hole will be covered by a cap, but until then it provides an access port so that welders can get in to weld all the plumbing fixtures in place.
 

https://twitter.com/elonmusk/status/1247056590340947969

Elon's photo doesn't do justice to the size of this thing. You could put a car inside the "little" header tank.

The header tanks are there to maintain a steady fuel flow to the engines, even during extreme maneuvers. Imagine the main fuel tank is mostly empty and the vehicle pulls a maneuver that sloshes the remaining fuel. Rocket engines don't like it when they suddenly are fed gas when they are expecting liquid. They tend to quit and can be fatally damaged. So the engines feed off the header which keeps itself full from the main fuel tank, the bottom inner surface which is shown here.


[Image: EU5vWYcUUAAN0GZ?format=jpg&name=large]
[Image: EU5vWYcUUAAN0GZ?format=jpg&name=large]

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Yazata Offline
When Elon says that he wants to crank out Starships by mass-production, he isn't kidding. Here we are, just 8 days after the demise of Sn.3 and the pieces of Sn.4 are already being assembled. Here's Mary's video of what happened today, showing some serious spaceship building action. It looks like they can already build them at a rate of better than one a month. (They just have to get them to work...)

It's kind of fun to watch the workers rush around like insects in this speeded up view. (When Elon says work fast, they work fast.)

It's also apparent that parts for future Starships are already accumulating. Towards the end there's a look inside one of the Giant Tents that reveals several bulkheads and domes being completed that won't be needed for the current version.

And components of a really large crane, either Big Berry or a relative, is on the scene and is being readied. Still no announcements of road closures.


https://www.youtube-nocookie.com/embed/Kaay1HFezss
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Yazata Offline
They are nearing the point of having SN4's structure completed.

The big event today (Easter) was the flipping of the thrust dome structure (pink in the diagram in the tweet below). This was constructed convex side up, but fits onto the rocket convex side down, so it has to be flipped. (And it's big.) Stacking the rest of the tank structure atop it is next up.

The new very big crane is attracting some interest. It isn't a top-of-the-line Liebherr like the earlier giant cranes. It's a different make and apparently not quite as large. Those familiar with its specs say that while it can lift the tank section and is tall enough to place the nose fairing atop it, it isn't powerful enough to lift the full stack of both at once, a completed Starship. So there's speculation that they may plan to move the tanks and the nose to the launch area separately and integrate them there. That way they won't have to lift and move the whole thing.

What's more, the new crane looks very shiny and new. So there's questions about whether SpaceX bought it themselves rather than renting it. Considering the pace of construction, they probably need a big crane on site full time.

Here's Raphael Adamy's diagram of where we stand as of today. (Click on the image to get the whole thing.) Note the little human figure at the lower left for scale.

https://twitter.com/fael097/status/1249083944223596544
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Yazata Offline
Assembly of the new crane is completed and it is indeed a giant. It looks to my eye to be the equal of the earlier giant cranes, so maybe it isn't smaller after all.

And in more 'giant' news, large crates and metal parts have been trucked in with the Sprung label on them. Dimensions on the labels confirm that it's the makings of a fourth giant tent. It isn't clear where it will go. The spot that seemed to be prepared for it next to the other three giant tents is already taken by a smaller big-top tent.

Starlinks and giant tents are both things that Elon can't get enough of.
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