And that's why they are testing it.
If they pumped it until it explodes it would be a kind of test (also called experiment) where it makes sense.
If you design something with a margin of safety, then pump it to working pressure and it blows, then it means you made a mistake. If you made a mistake it shows you don't know what is necessary to design a rocket (yet).
Seriously, they don't build skyscrapers or bridges and leave for 5 days to see if they topple to prove the design was correct.
They gave up on carbon fiber awhile ago.
*I think they still use small composite overwrapped pressure vessels for fluids they need less of than CH4 and O2, just referring to giving up on carbon for the main tanks.
My understanding is that the skin is the pressure vessel. The reason for going with stainless steel at all is because it maintains integrity at high temperature. Carbon fiber does not. So a stainless steel combined heatsink and structure can maintain integrity at 1000C or whatever, while carbon fiber+ceramic heatshield will need to hold temperatures below 400C or whatever.
And even though the fiber structure is much, much lighter, the amount of heat protection required is massive. And the stainless steel "cheats" by performing double duty. And you get a second free lunch because the heat protection requirements are slimmer.
It wouldn't be the first time I've been wrong today though. Nor the second.
Edit: "So in summary. SpaceX chose stainless steel over carbon composites because it’s about as light, it can handle higher temperatures which means less heat shield is need, which then makes it lighter, it reflects heat which means even less heat shield which again makes it even lighter, it’ll be cheaper and quicker to build AND it’ll look FREAKING AWESOME."
AFAIK everybody is.
_Lifting_ that fuel is NOT CHEAP. See also, the rocket equation.
But we're essentially arguing whether max payload per launch or payload efficiency to orbit is more important.
So yes, fuel is cheap.
With one and done you have a larger mass fraction to work with. Reusable spacecraft operate on much tighter constraints. Every single wasted pound in Space X’s design likely costs them 100’s of thousands of dollars in lost profit over time.
Reusable means cost savings, but it has a dramatic reduction in cargo capacity. This means more trips to get the same mass into orbit, which means mire fuel directly used and every extra lb in the design is then carried up multiple times while further requiring more trips.
Or, equivalently, in one trip with a bigger rocket you have left over from another launch, because you didn't ditch it.
Which again demonstrates reusability is really expensive, though clearly not always as expensive as a new rocket.
Remember while the first stage is significantly more valuable, it's not just a question of fuel the upper stage is destroyed with both approaches. So, combined with significantly lower payloads the cost per kg does not drop as much as you might think.
It doesn't make fuel cheap, let alone very cheap.
Actually another significant part of SpaceX business is modern engineering and modern mean of production. They used to launch without recoverability for some high orbit payloads and they were competitively priced for that too.
SpaceX's greatest contribution has been to look at rocketry from the point of view of economics of running a rocket launching business as opposed to a "per launch cost plus" model realizing that fuel cost for launching a rocket is close to trivial compared to the facrication cost of making a rocket.
So their entire design philosophy is around reusability and reusability seems to push costs down low enough that you can get more payload up by simply doing more launches. While SpaceX is famous for not filing patents and instead protecting their IP using the trade secret approach and fabricating everything in-house, you can bet they'll be operating with more engineering headroom to get reliability and reusability.
Evidence for their bet having been right is the fact that they have grabbed virtually 100% of commercial and quasi-governmental launches and some part of US government launches. Other players in this space now almost entirely depend on defence contracts or national prestige contracts to survive.
Any rocket HAS to be very close to the limits of what materials used allow, and not the +50% margin of error you routinely seen in other fields. What you say about SpaceX is true, but only relative to the rocket industry as a whole. E.g. SpaceX uses cheap, available materials and simple designs over exotic composites and complex mechanisms. But they still operate close to the margins, as they have to in order to have any payload capacity at all. In fact, the move to steel probably reduced their mass margins even further.
EDIT: Actually maybe I shouldn't have used a car as an example: https://www.youtube.com/watch?v=pJdrlWR-yFM
"Have you got a spare billion dollars?"
"No. That's why we came to you."
Yeah, sure SpaceX doesn't know what's necessary to design a rocket.
Perhaps a better analogy is carving a violin top. Skyscrapers and bridges can be overbuilt without destroying their utility, but a violin has to be as thin as possible without breaking if you want it to function well as a violin.
https://www.krgv.com/news/spacex-extends-buyout-offer-deadli...
Suppose you bought a used car, and the brakes failed before you could drive it off the dealer's lot. The dealer graciously fixes the problem, and points out the great news - that nothing catastrophic happened, and that the problem is now fixed!
You would, of course, start wondering - what other unexpected failures are going to take place, after you drive out of the dealership. Maybe nothing. Maybe something life-threatening.
If the failure was a surprise for everyone involved, I'd be worried - because other surprises might not get caught in testing. If it was a 'Well, we're not sure what would happen', I'd be less worried.
“SUCCESS! We now now max pressure the tank can hold and the damage a rupture might do.”
But they have learned to be more careful with the welds going forward. Though, admittedly, they only used panel welding on the first prototype and have already switched it up for the second one.
Don't get too excited about the damage, in space it does not matter much, the crew is screwed anyway.
A bit before that it is supposed to do a manned moon flyby in 2023.
New York to Shanghai slightly more than economy ticket by 4/2028:
https://www.vox.com/2018/4/11/17227036/flight-spacex-gwynne-...
Moon 2023:
https://www.scientificamerican.com/article/spacex-plans-to-f...