I was very excited when I read that ULA had decided to bring elements of the ACES upper stage forward and begin work on Centaur V. This will be a 5.4 meter diameter stage, likely use the BE-3U engine, and not feature the poorly acronymed but technically wonderful Integrated Vehicle Fluids (IVF).
I was a never a fan of the awkward, iterative development plan that was first shown with the Vulcan announcement. It focused early development on the first stage to remove the dependency on the RD-180 engines, but kept the upper stage at a 4 meter diameter, keeping Centaur unchanged until it was replaced by ACES. Bringing forward a few critical items from ACES to Centaur V is a brilliant move by Tory Bruno. The most important thing it does is increase the lift capacity of the first generation Vulcan, so it can carry all of the national security payloads. This means that as soon as it is qualified and in operation, ULA can immediately retire both the Atlas V and Delta IV families of rockets. Having to support 2 completely different, enormous rockets places a huge strain on ULA's ability to produce rockets in high quantities, and now that SpaceX has ended their monopoly on national security payloads, they can't afford that anymore. This is a good move.
But I wanted to offer a different development path that could potentially get Vulcan flying a little quicker for a lower cost, and still satisfy their need to replace the Delta-IV heavy. Instead of investing in new tooling and re-qualifying the main propellant tanks of centaur to a new diameter, they could instead choose to keep the 3.05 meter diameter and stretch the tanks to length to meet the payload requirements needed to keep the national security payloads flying. Vulcan is expected to have the capability to place about 40,000 kg in LEO in its final version, but to replace the Delta-IV heavy, you only have to be able to lift 28,790 kg. With this assumption in mind, I tried to figure out what a stretch Centaur V would look like.
Obviously, the engineers at ULA must have considered this, weighed it against their options, and rejected the plan for any number of reasons. They have far more insight into their engineering bandwidth, manufacturing capability, actual performance and mass data, and future business climate than I do. A lot of them are outright better engineers than me too. That said, its fun to engage in a little speculation.
To make these estimations I started with some assumptions backed up with some hasty research about Centaur and the BE-3 engine. I plugged some numbers in to a python script and generated some lengths for what a stretched Centaur would look like given a target Delta-V and mass. I put the code up on github, so play around with it and see what you can come up with, or tell me what errors I made.