The problem is the cats on the Essex class. The C11 cat struggled to get a Phantom into the air when it was hot and the aircraft was heavy. Adding a few thousand pounds of thrust (and deleting a couple thousand pounds of weight) helps that problem immeasurably. The USN had a possible fix for it (essentially an F-4K with the J79 and without the blown flaps) that would allow the C11 to launch a fully loaded Phantom at 90° temperature. But I don't see the Navy wanting a special model of Phantom just for the Essex. New engines are useful for every model.
Edit: In the TL, the Navy will also have zero interest in upgrading the Phantom too much and endangering the F-14 program. New engines (that are very close to "off the shelf") can be sold as a limited, though still useful, upgrade to keep the fleet viable until they are phased out of service.
Hello. Longtime lurker, got lots of information from here; on this, I can chip in a few bits of random information I've collected over the years, maybe something is helpful for your scenario:
- Coral Sea and FDR operated F-4s (B, N, J and S) with their C-11-1 catapults. They had the problem with hot/heavy, but it seems that was not a big deal.
- Due to the already mentionned size/fuel demands, you'd probably have to compromise on the nr of F-4s, 24 would be very demanding on an Essex.
- The "F-4K with the J79" - the extended 40 inch nose wheel strut would probably suffice to solve the takeoff problem. The RN report on the F-4K has data on the (probable?) testing of the F-4B with the extension, and it is worth 10-12 kts or about 5000 lbs takeoff weight.
- Does anybody know why the USN did not use the extension? Heat problems should be less than with the Spey, and even that was soluble with the JBDs as on Ark Royal. I can only guess, two aspects seem plausible:
-- The C-11 catapult would be phased out with the retirement of the Essexes and the SCB-101.66 refits of the Midways. That FDR and Coral Sea soldiered on with the old cats was not planned; also, both would be decommissioned in the late 70s anyway, the reprieve for Coral Sea was another unplanned development.
-- Caygill, Phantom from the Cockpit, quotes from the F-4K testing that weight would be limited to about 10x540 lb bombs, giving all up about 50k lbs due to "aircraft weight, centre of gravity and balance problems". The catapult limit of USN F-4s was usually 56k lbs; this may be a downside of the high AoA?
- There was a plan to reopen the production line of the F-4J about 1973 for the Marine Corps, somehow related to the F-14 contract problems. The idea went nowhere as the upfront costs were too high. (Here, starting p 395:
https://books.google.at/books?id=s522AAAAIAAJ&pg=RA2-PA397&dq="f-4j"+"appropriations"+1974&hl=de&sa=X&ved=2ahUKEwjRwYjcseDqAhUox4sKHd7fDPgQ6AEwAHoECAMQAg#v=onepage&q&f=false ). But you could depart with the project going through, or more money for the CILOP phantoms (like new engines) as a compromise to fill a few extra decks, with a certain delay for the F-14.
- Most turbofans would require a higher air intake than the J79; that was the case with they Spey, and would also be so for example with the M53. Even the PW1120 required about 4 % more (
https://asmedigitalcollection.asme....V002T02A008/2395305/v002t02a008-84-gt-230.pdf ), so probably some change to the air inlets necessary, too.