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0 pointspeeters6y ago0 comments

I'm not in that area, but I struggle to find an analogue of a large pressurized tank on Earth where mass is the dominant cost factor.

You might have equally big tanks on Earth, under equal pressures, but it will be more cost-effective to just increase the material used on the shell of the tank to solve your problems. Whereas in space, every kg that can theoretically be removed from the launch vehicle is worth investing in, even if it raises the cost of engineering and materials.

So yes it seems reasonable to me that these are groundbreaking challenges.

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dexen6y ago

>where mass is the dominant cost factor.

The compounding importance of mass is one factor here, but only half the story. The other half is how SpaceX' quick iterations and ability to retire vehicles early allows ample room to experiment and also to shave off safety factors.

In the more mundane aerospace engineering mass is quite important, but the manufacturing, servicing, longevity etc. are pretty important considerations too. A typical airplane is designed for manufacture of some 200 ... 2000 units and to remain in use for 1 ... 3 decades. This, together with the usual requirement for human flight ratings, ends up dictating rather high safety margins for designs.

Contrast that with SpaceX' vehicles which are supposed to perform a dozen unmanned flights each at most, and where one-way mission of a vehicle is a perfectly feasible strategy for testing while still making decent revenue.

Quick iterations get shit done in aerospace. Among historical examples, that's how MacCready's Gossamer Albatross[1] record-breaker came to be, where other teams failed.

[1] https://en.wikipedia.org/wiki/MacCready_Gossamer_Albatross

m4rtink6y ago

>I'm not in that area, but I struggle to find an analogue of a large pressurized tank on Earth where mass is the dominant cost factor.

If you stretch the definition of "tank" and "on Earth" then modern pressurized airliner cabins definitely qualify.

dexen6y ago

>modern pressurized airliner cabins

It's a good reference point, even if there are some key differences worth mentioning.

First up, an airliner fuselage is specced for tens of thousands of pressure cycles; needs to sustain the wear & tear, accumulated stress, and distortion. The rocket needs to handle maybe a dozen pressure cycles at most, and it is feasible to only handle one cycle for first few experimental vehicles.

Secondly, the airliner fuselage has much more complex shape, due to conflicting needs for aerodynamics, attachment of wings & control surfaces, etc.. This, together with varying materials and large openings concentrates stresses in key points. The rocket tank is as close to a big dumb uniform pipe as you can get. Even better, a rocket may rely on internal pressure for structural rigidity, while airliner needs to handle depressurization.

Lastly, the Starship is designed to survive re-entry, thus heat resistance is a major concern there; not anymore for an airliner ever since the Concorde SSTO got retired.

aguyfromnb6y ago

>So yes it seems reasonable to me that these are groundbreaking challenges.

Isn't weld strength something engineers would know/understand in advance, though? How would it come as a surprise that the welds would fail under pressure?

It seems strange to me that Musk could be spending so much time working on Starship, and somehow a "badly designed, badly built, and badly checked" rocket makes it to the platform and explodes.

zchrykng6y ago

Welding by hand is both an art and a science. There is going to be variability in the quality of the weld, which will introduce variability into the strength. Machines can be far more precise and consistent compared to humans.

They probably designed their tolerances assuming perfect welds and then didn't have them.

aguyfromnb6y ago

>They probably designed their tolerances assuming perfect welds and then didn't have them.

This sounds like terrible engineering practice.

2 more replies

grey-area6y ago

There are additional constraints not found in most welding projects. They can’t just make the material and weld thicker/heavier as that would impact the rocket performance.

So it has to be as thin and fragile as possible but no thinner. That makes it very hard to get right, and the welds have to be done to very fine tolerances on thin material, you could say it is rocket science.

aguyfromnb6y ago

>They can’t just make the material and weld thicker/heavier as that would impact the rocket performance.

I understand. But the stresses that materials and welds can withstand are not unknown variables. We're talking about a pressurized tank here. You aren't going to build a rocket (designed to carry people) that has tolerances so tight that it's touch and go whether it explodes as soon as it's pressurized.

1014046y ago

They are currently not primarily working on the rocket, but in the production process.

j / k navigate · click thread line to collapse

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dexen6y ago

>where mass is the dominant cost factor.

Quick iterations get shit done in aerospace. Among historical examples, that's how MacCready's Gossamer Albatross[1] record-breaker came to be, where other teams failed.

[1] https://en.wikipedia.org/wiki/MacCready_Gossamer_Albatross

m4rtink6y ago

>I'm not in that area, but I struggle to find an analogue of a large pressurized tank on Earth where mass is the dominant cost factor.

If you stretch the definition of "tank" and "on Earth" then modern pressurized airliner cabins definitely qualify.

dexen6y ago

>modern pressurized airliner cabins

It's a good reference point, even if there are some key differences worth mentioning.

Lastly, the Starship is designed to survive re-entry, thus heat resistance is a major concern there; not anymore for an airliner ever since the Concorde SSTO got retired.

aguyfromnb6y ago

>So yes it seems reasonable to me that these are groundbreaking challenges.

Isn't weld strength something engineers would know/understand in advance, though? How would it come as a surprise that the welds would fail under pressure?

It seems strange to me that Musk could be spending so much time working on Starship, and somehow a "badly designed, badly built, and badly checked" rocket makes it to the platform and explodes.

zchrykng6y ago

They probably designed their tolerances assuming perfect welds and then didn't have them.

aguyfromnb6y ago

>They probably designed their tolerances assuming perfect welds and then didn't have them.

This sounds like terrible engineering practice.

2 more replies

grey-area6y ago

There are additional constraints not found in most welding projects. They can’t just make the material and weld thicker/heavier as that would impact the rocket performance.

aguyfromnb6y ago

>They can’t just make the material and weld thicker/heavier as that would impact the rocket performance.

1014046y ago

They are currently not primarily working on the rocket, but in the production process.

j / k navigate · click thread line to collapse