There's a lot of mystery right now about what vehicle/configuration they will actually use for the first orbital launch attempt. SN21 and 22 now are basically built, and while both were started a ways back it accelerated on them lately and there hasn't been a lot of test fires/proofing for SN20, which may just be a prop. I think they also had some issues with their new tank farm on the methane side as they haven't even tried to fill/proof it all the way yet.
See here:
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Given that Starship S20 effectively completed qualification testing with three successful static fires in December 2021 and a fourth in early January 2022 and has been seemingly ready to fly ever since, its Super Heavy booster readiness not ship readiness that appears to be holding SpaceX back. Perhaps because of pad readiness issues, SpaceX has yet to perform a single Super Heavy static fire test or even a less risky wet dress rehearsal at the orbital launch site. As such, it's hard to say why SpaceX has suddenly decided to finish Ship 22 instead of focusing on a newer version of Starship (S24) and Super Heavy (B7) both of which are expected to debut upgrades.
It's possible that Ship 22 is being completed merely as practice for the Starbase workforce, who have gone half a year without fully assembling another ship prototype, but then there would have been no reason not to install Ship 21's nose on Ship 21's tank section instead of withholding it for Ship 22. Ship 22 could also be a replacement for Ship 21 if appearances are misleading and SpaceX uncovered issues with the older prototype during testing but again, no booster is ready to launch either ship.
Regardless of the outcome or purpose of Ship 22, seeing any new Starship prototype completed is an exciting and interesting change of pace after half a year of following the windy paths of Ship 20, Booster 5, and Ship 21 to their uncertain goals.
My guess/swag is that they are hesitant to do much more fabrication of iterative prototypes until they do finally get one off the pad (or a pad, whether in BC or Florida). I wouldn't think it would matter too much to them as they plan to expend at least the first couple anyway, but why build more until the final configuration (with tanks and raptor 2's) for the next phase is at least confirmed internally? SN20 with raptor 1's and without the total number of engines/stretched tanks etc. planned for actual use down the road may just be a practice toy for the cameras/cranes to work with.
They must internally have a fairly complicated flowchart of objectives, evolving toward a final design and roughly using something like the below;
![](https://forum.nasaspaceflight.com/assets/55358.0/2085623.jpg)
Elon talked in his speech about Raptor 2 challenges; this stuff has to be resolved before they can really spec/build a ship to use it, imho (also, kind of humorous that this is exactly the type of stress rocketlab wants to avoid in their new/next engine).
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The only remaining issue that we are aware of is melting the chamber. So, that thing really wants to melt, you know. It's got like on the order of a gigawatt of heat, so it pretty hot. Like, a gigawatt is, like, what a nuclear power plant produces, so it really is desperately trying to melt at any point in time. So, we've got - You know, we're flowing an immense amount of cryogenic fuel to cool the chamber in the channels. We have head-end film cooling. We've got throat film cooling. And we're just trying to get the exact sort of balance between head-end film cooling and throat film cooling to not melt the chamber. I think we're pretty close. Like, we have a couple of engines in the stand that have, I think, 700 or 800 seconds of operation and several start cycles, so it's looking positive, but that's the remaining issue; melting the chamber.