Dome to barrel weld made it to 7.1 bar, which is pretty good as ~6 bar is needed for orbital flight. With more precise parts & better welding conditions, we should reach ~8.5 bar, which is the 1.4 factor of safety needed for crewed flight.[i.e., 6 x 1.4]
A given tank pressure is needed to feed the engine turbopumps & pressure-stabilize/relieve compression load on the cylinder walls.
Everything is compressible, but liquid compression at these pressures is not significant. However, the tanks do expand under pressure, creating a bit more volume. Keeping propellant super cold has a big effect on density of ~10% in case of CH4.
Every tank under pressure is a balloon tank — it’s just question of degree. Starship tanks are not balloon tanks like Atlas in sense that they don’t collapse when depressurized on the ground.
SpaceXcentric has a nice video showing Starbopper breaking under pressure.
When I first saw this report, I was worried that it meant a delay in the Starship program, but I think in fact that the opposite is true. It's fascinating to see a company shows us all its "failures" like this. It makes their successes all the more impressive. And because the rocket will be re-usable, it can be much more comprehensively tested than single-use rockets.
However, the timetable has slipped. In July, I thought the first orbital tests would take place in late 2019/early 2020. End 2020 looks more likely now, though altitude tests to 20 km look possible over the next few months. Musk has made it clear that Starship can't get into orbit by itself, and needs the Super Heavy booster, and that won't even be built until the tank/bulkhead design is done for the Starship proper.
But the longer-term SpaceX/Mars timetable still looks doable, unless there is a disaster:
What will delay the timetable by 2 years will be SpaceX having to ditch stainless steel and go back to carbon-fibre composites.
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