[cpgxiii]

I'd say the F-111 is a particularly odd case, given the vastly different initial requirements involved. To be clear, everything that made the F-111 a great long-range interdictor for the USAF would have have also made for a great long-range interceptor and strike platform for the USN. It wouldn't have made a good air superiority fighter, and the experience over Vietnam made it clear that this capability was still very much necessary, and there was nowhere near enough budget (or, more critically, carrier deck and hangar space) for the USN to operate both an air superiority fighter and a dedicated long-range interceptor.

There is a very long list of could-have-been multi-service aircraft, though. If you look at the number of ground-based operators of the F-18, clearly the USAF could have been satisfied with it as well. The USN probably could have been satisfied by a navalized F-22 derivative (the story of 1990s/early 2000s procurement is complicated), the USMC definitely could have been satisfied by a navalized AH-64 rather than developing the AH-1Z, etc.

The services are just very resistant to ever needing to compromise on procurement issues unless Congress and the DoD make it clear that they have to. The USN feels their needs are special, and that the USAF would dominate any shared procurement and force them to compromise too much, while the USAF feels like every pound added for carrier operation is a direct affront. Neither view is entirely wrong - the development delays and compromises of a navalized platform like the Dassault Rafale are another good example of the costs of shared development - but the simple reality of modern aircraft development costs and defense budgets means joint platforms are here to stay.

[greedo]

The USN didn't need an air-superiority fighter when they were working on the F-111B, they needed a fleet defense fighter capable of lifting a huge radar and missile set. When the terrible engines in the B model gave them an out, they took it and moved forward with the Tomcat (which used the same engines and weapons set). Little did they realize the TF-30 would remain a terrible engine for so long, and replacing it with F110s would take almost two decades.

I'm not sure that the F-22 would have ever worked for the USN either. I think that its stealth coating are just too fragile for a marine environment. And the USMC could never afford the AH-64.

[cpgxiii]

The engines certainly gave the USN the out it wanted, but the thing that truly killed the F-111B was the need for a dogfight-capable fleet fighter. There was no way they could have afforded two separate fighter development programs at the time, and Grumman had just the design they wanted ready to jump to.

The NATF program probably would have worked out fine, albeit expensively. There has always been speculation that part of the selection of the F-22 over the F-23 was because the F-22 was considered more suitable for a navalized version - and the comparatively small design differences between the F-35A and F-35C designed later by the same group suggest that a similar amount of work would have been involved in navalizing the F-22. Coatings would have been an issue in the 90s, but would largely have been solved by an realistic service entry date in the late 2000s. What killed the NATF was post-cold war budget reductions and shortsighted policymakers, not technical challenges.

The story of a navalized AH-64 is an equally strange saga. To hear the USN and USMC tell it, you would think it impossible to operate the AH-64 from a ship, yet the RN has done so extensively with relatively minimal modifications to the airframe. Given the small production run of the AH-1Z/UH-1Y program, it's questionable if much was saved. Certainly if you look at foreign customers, the capability/price of the AH-64 has been much more appealing than the AH-1Z.

[greedo]

Regarding the Sea Apache, it really would have been a huge redesign. Deleting the 30mm Chain Gun, moving the landing gear to the wing stubs, adding a nose mounted radar, and Harpoon/Sidewinder capability. Now some of these requirements were Navy specific, but some would have been required had it been selected just for the USMC.

I think that even an unmodified Apache would have been just too expensive for the always cash-strapped Corps. Plus the airframe and engine commonality with the UH1s they flew simplified their logistics tremendously. It's always a gamble comparing unit costs since so much is either hidden or excluded, but the latest info I found showed a new build AH-1Z was around $30M in 2018, while a new build AH-64 is currently around $52M. And I'm pretty sure the Corp has simply been rebuilding older airframes to the new "Viper" standard. (Turns out most were rebuilds, but some were new construction.)

Regarding the RN flying Apaches, is this from large decked ships, or from anything with a pad? I know the Merlin had a neat winch type thing to help it land in bad weather, and I just wonder how much more likely a rollover would be on a small decked ship versus something like HMS Ocean etc.

[cpgxiii]

I think the Sea Apache design was doomed to failure, potentially deliberately, by including so many modifications to the base design. A reasonably scoped modification for the USMC would have been the standard increased corrosion resistance, folding main rotor, tail wheel moved forwards, and folding tail fin+rotor (i.e. all the same modifications made to the base S-70 design for shipboard use).

The cost question is a very tricky one, hence why I used the comparison of foreign orders between the two types as an assessment of overall value. My understanding is that more AH-1Z/UH-1Y airframes were new builds than was originally planned (hardly a surprise), which strongly suggests that airframe reuse did not save as much in production as hoped. The strongest argument for the AH-1Z cost-wise was the commonality with UH-1Y, but had the USMC considered switching entirely to AH-64 + SH-60 derivatives instead I don't think that argument would have been as strong (after all, the whole rest of the USN operates SH-60 variants, including the ships that operate USMC AH-1/UH-1s).

The RN has flown WAH-64s from their large deck ships, so roughly equivalent size-wise to anything that routinely operates AH-1Zs. The only important modification in the WAH-64 design is a folding rotor to reduce storage size, but that doesn't affect takeoff/landing abilities. With some minor changes like pulling the tail gear forwards (e.g. as on the {S,M}H-60{B,F,R}), you could fit an Apache on anything that can handle a Seahawk (i.e. basically everything with a pad). Notably, the AH-64 wheelbase is not that different from the spacing of the skids on an AH-1, so it's not going to be hugely different in terms of stability on a large deck ship. There's also been a fair number of training exercises between US army AH-64 units and USN ships, so shipboard Apache operations aren't exactly foreign to the US either.