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Wait a sec... there’s an assumption the whole stack is built the same. What if the ship is titanium and the booster is composite, or vice versa? Would there be any obvious advantages there? They’ve put so much effort into composites, it seems odd that they would completely abandon them, especially with so many new bits on F9B5. 

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Metal (even steel is possible, maybe) might be more heavy for a given strength than carbon fiber composite, but if the CFC has to then have additional heat shielding added, maybe it's a wash. Make it out of thermally conductive material, and have no heat shield at all. That's... delightfully counter-intuitive.

This is what the solution was for Dynasoar, right?

 

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Just now, CatastrophicFailure said:

Wait a sec... there’s an assumption the whole stack is built the same. What if the ship is titanium and the booster is composite, or vice versa? Would there be any obvious advantages there? They’ve put so much effort into composites, it seems odd that they would completely abandon them, especially with so many new bits on F9B5. 

I thought about that, it would be a good design compromise if you had the booster metal and the ship composite (mass ratio is more important on the ship I think) but then they have to design two completely different methods, and you couldn't really use the same equipment for metal and composite.

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If they have tools for 9m metal, they might as well do it with metal.

People need to dump the "they have tooling for CFC tanks" thing, I think. Sunk cost fallacy. Less agile companies would stick with a design, but this could be like the Merlin. A tough call that serves them well in the long term. They either sell the tooling, or come up with something else they can do with it.

 

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24 minutes ago, tater said:

If they have tools for 9m metal, they might as well do it with metal.

People need to dump the "they have tooling for CFC tanks" thing, I think. Sunk cost fallacy. Less agile companies would stick with a design, but this could be like the Merlin. A tough call that serves them well in the long term. They either sell the tooling, or come up with something else they can do with it.

  

Plot twist: Jeff Bezos buys the tooling.

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https://www.reddit.com/r/spacex/comments/a4eo12/first_grid_fin_has_been_removed_from_the_3x_flown/

Those gridfins look almost new although they were reused 3 times. Titanium would be a good candidate for a strong metall that can also resist high temperatures, but since Elon is talking about a "heavy" metal i can also imagine something nickel based, it could resist even higher temperatures. But even with the extremly low balistic coefficient of an almost empty starship i just cant imagine it reentering from interplanetary velocitys on just metall as a heatshield...

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10 minutes ago, Elthy said:

Those gridfins look almost new although they were reused 3 times.

To be fair, if your control surfaces looks wrong after the first attempt then it's probably not a good idea to :

- Fly it in the way you flew it

- Fly it ever again.

6 hours ago, CatastrophicFailure said:

so... is that four ElonWeeks or four week weeks?

Call it 4 "after they had done the papers" weeks

Edited by YNM
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Its happending! Woop woop!

Didn't the X-33 also switch from CFC to a metal which would work fine, but got cancelled because a they specifically wanted a CFC fuel tank?

Luckily starship doesn't have any limits.

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8 hours ago, Ultimate Steve said:

reduced payload (AGAIN!)

Not again, it's 100+ tons BECAUSE of this change. Elon didn't talk about it at Dear Moon presentation, but knew about the changes. That plus no vacuum Raptors reduced the payload from 150 to 100t.

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3 hours ago, NSEP said:

Its happending! Woop woop!

Didn't the X-33 also switch from CFC to a metal which would work fine, but got cancelled because a they specifically wanted a CFC fuel tank?

Luckily starship doesn't have any limits.

Not quite. The X-33 was going for a full on SSTO, and as a result they needed to be extremely lightweight to get to orbit. Composite fuel tanks were one of the ways to do this, but they could get a fuel tank that wouldn't crack when filled (I'm not entirely sure why the struggled). Without the composite fuel tank, it couldn't get any payload into orbit, so it was abandoned after being used to test some VTVL techniques. 

Edited by MinimumSky5
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11 hours ago, tater said:

This is what the solution was for Dynasoar, right?

The Dynasor used Rene 41 and molybdenum. The original Spiral plan VN5AP, namely Si2Mo-coated niobium; BOR-2 tried a single panel, and it burnt right through. By BOR-4 the TPS was a Buran replica (RCC, silica tiles), minus the evaporation-cooled movable wings, too thin for the tiles.

I strongly suspect the Shuttle transitioning from Rene 41 to RCC+silica tiles was not an accident either.

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46 minutes ago, Jaff said:

Can those silica tiles not be formed as 1 bit tile/heat shield to cover a specific area? Like say the bottom of a starship? 

One word: expansion stress of the underlying vehicle structure. That silica material is fragile as all heck, too, so it needs to be easily replaceable, which a unipiece solution isn't.

A similar situation basically buried the Su-27KM project and its high-rigidity carbon fiber wing - zero reparability.

Spoiler

su27-6.jpg

 

Edited by DDE
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5 hours ago, sh1pman said:

Not again, it's 100+ tons BECAUSE of this change. Elon didn't talk about it at Dear Moon presentation, but knew about the changes. That plus no vacuum Raptors reduced the payload from 150 to 100t.

Are you sure? Because he showed off the carbon fiber stuff pretty heavily at that presentation.

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5 hours ago, MinimumSky5 said:

Not quite. The X-33 was going for a full on SSTO, and as a result they needed to be extremely lightweight to get to orbit. Composite fuel tanks were one of the ways to do this, but they could get a fuel tank that wouldn't crack when filled (I'm not entirely sure why the struggled). Without the composite fuel tank, it couldn't get any payload into orbit, so it was abandoned after being used to test some VTVL techniques. 

X-33 was a tech demonstration and would never go to orbit. That said they had trouble with cryogenic composite tanks (thermal stress), and the unconventional tank geometry didn’t help. Moving to metal would make the tanks work but they became too heavy. And that’s before the performance of the aerospikes is considered (worse than SSMEs in many regards). X-33 never flew, and neither did Venturestar. If only the DC-X was developed instead...

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6 hours ago, MinimumSky5 said:

Without the composite fuel tank, it couldn't get any payload into orbit, so it was abandoned after being used to test some VTVL techniques. 

What @Bill Phil said, you’re thinking the DC-X, here, I think, which was wonderfully successful at demonstrating the technology until a balky landing gear killed it. SpaceX no doubt learned a lot from it, too. That one could have been something. 

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59 minutes ago, CatastrophicFailure said:

What @Bill Phil said, you’re thinking the DC-X, here, I think, which was wonderfully successful at demonstrating the technology until a balky landing gear killed it. SpaceX no doubt learned a lot from it, too. That one could have been something. 

DC-X was a test article. A fairly successful one at that. “Not invented here” killed DC-X once NASA gained control of the program. Instead NASA began development of X-33 and Venturestar... which couldn’t even have a successful demonstration.

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1 hour ago, Bill Phil said:

DC-X was a test article. A fairly successful one at that. “Not invented here” killed DC-X once NASA gained control of the program. Instead NASA began development of X-33 and Venturestar... which couldn’t even have a successful demonstration.

“Not invented here” is the greatest killer of spaceflight programs.

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My guess is that the booster is still composite bc they can check up on it after every flight and it is in a much less severe heating environment, while the spaceship is titanium or some new alloy.  The spaceship needs to be much better proven than the booster because it has much longer missions and is more important to cree safety.  By still doing the composite booster they gain experience allowing future spaceships to be made in composites, and the dry mass of the booster still matters a lot because it does RTLS, which costs several km/s of dv.

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