This answer to What is the difference between the Vulcan 562 configuration and the Vulcan 5H2 ("Heavy") configuration? explains that the 2nd stage nozzles were modified by extending them, which appears to allow for more expansion (they almost touch in the drawing) and links to https://www.ulalaunch.com/docs/default-source/rockets/vulcancentaur.pdf?sfvrsn=10d7f58f_2

I don't see any additional distance in the interstage area, they could have been this long from the beginning.

That makes me wonder how improving the expansion for a vacuum engine could have been an afterthought in this case. Why not make them longer from the beginning?

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    $\begingroup$ The RL10CX is not just a larger nozzle on an RL10C. It allegedly has an entirely 3D printed injector and chamber. $\endgroup$ – Russell Borogove Aug 22 at 16:58
  • $\begingroup$ @RussellBorogove I wonder if that made the longer, wider nozzle possible, or if they could have made it this long with the original technology and just didn't? $\endgroup$ – uhoh Aug 22 at 22:35
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    $\begingroup$ There's a large sliding-nozzle-extension version (RL10B-2) used on the Delta upper stage -- sliding so as to not need the extra long interstage that Vulcan seems to use -- but that's a single-engine stage. The real question to me is how they intend to gimbal a pair of large nozzle engines in yaw without banging them together. (Historically Centaur has come in both single-engine and double-engine versions.) $\endgroup$ – Russell Borogove Aug 22 at 23:57
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    $\begingroup$ I don't know if the RL10CX is expected to take longer to flight-ready than the rest of the Vulcan; another possibility that occurred to me is that with SpaceX taking customers away from Atlas, ULA may have a backlog of old RL10s they need to expend... $\endgroup$ – Russell Borogove Aug 23 at 0:30
  • $\begingroup$ @RussellBorogove if they both gimbaled the same way would that solve that problem? $\endgroup$ – uhoh Aug 23 at 11:04

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