Wait, I thought the J57 started out as the XT-45 which was a twin-spool turboprop, which was then reconfigured to pure-jet configuration instead.
Well, it did but the XT-45 was an evolutionary step. P & W was looking for a replacement for the large piston engine and had decided to develop 3 products, a small turboprop, a large turbo jet and and a large truboprop in 1945. The large turbojet was a single spool 6 to 1 pressure ratio engine (JT3-6) of 7500lbs but it was never run. It morphed into the two spool XT-45 (PT-4 also never run) and then by loosing the propeller section/parts it was turned into the two spool JT3-8 (X-176) which was first run in the late spring/early summer of 1949. This projected started work on the JT3-10 and about the same time as the JT3-8 ran work started on the JT3-A with the wasp waist. Please note that most of these projects overlapped to a considerable extent. Work on the XT-45 didn't stop when the JT3-8 started. It stopped when the JT3-10 started and the work on the JT3-8 didn't stop until over a year after the JT3A was started. The JT3A was the J57.
JT stands for jet engine, the number 3 stands for the design number (3rd jet engine) and the -6, -8, -10 stands for the pressure ratio. PT stands for turboprop. These are P &W designations while J57 and T45 are government designations.
I was talking about the engine itself, not the plane...
Low frontal area got a lot of "play or press" in the 1930s with the V-12 vrs radial engine arguments. A bit over blown as a sitting pilot (even with legs out stretched) has more frontal area than most V-12 engines although he does have to have his head (and shoulders) above the engine. But then some of these aircraft only carried 100-180 gallons of fuel. Once you switched to jets that need hundreds of gallons per engine just to stay in the air for a couple of hours the fuselage became so fat that small differences in diameter are irrelevant.
On long range jet fighters it gets worse. For an extreme example
1150 gallons of internal fuel. J33 engines about 50 in diameter. or 13.9 sq ft frontal area each. Changing to J35s of 37.5 in D and 7.7sq ft would obviously have helped
some. going to Westinghouse J34s of 24in diameter and 3.1 sqft with that fat fuselage?
On single engine aircraft the fat centrifugal engines could be hidden inside the fuselage of most planes that held around 400 gallons or more of fuel,
I would note that even the Folland Gnat used an engine of 32.4 in diameter and 5.7sqft frontal area and carried 240 gallons of internal fuel.
two seat trainer may have carried less?
as a counter point you have the Vought F6U Pirate, J34 24in diameter engine but 370 gallons of internal fuel
tip tanks held 140 each so the fat fuselage may not be all due to the internal fuel.
As I understand it, the roots of the J34 were in the J30: The J30 was designed as an auxiliary powerplant at first, because the US Navy did think turbojets were suitable for operating off a carrier-deck. They eventually had a change of heart and built, what would become, the FH-1 Phantom.
Since the engine was fairly small at first, they needed six to eight engines: They realized it would be a dumb idea to have a fighter powered by eight engines, so they began bulking up the engine and also let the engine manufacturer have a role in the design process. What previously required six-to-eight engines, now required four, and eventually two.
There had been a thought of putting one large engine in the aircraft (presumably something J34 sized), but McDonnell did not wish to go through with it because they'd already worked out the basic fuselage configuration and the fuel-tank layout.
Mostly right but there may be some confusion as to which engine/s were intended for the 6-8 engine layouts. The J30 ( Westinghouse 19A=19in diameter) was giving 1135lbs on the first test run so 8 such engines seems a bit like overkill. Westinghouse did build and run a 9.5in diameter engine (the Yankee 9.5 or J32) which gave around 275lbs of thrust and
may have had drawings for an 11.5in and/or another intermediate size. No actual work (metal cut/shaped) being done on these sizes. The 19A was changed the 19B when the compressor went from 6 stages to 10 stages.
There was some excess enthusiasm for a short period of time during WW II for jet engines as (over simplified) they thought that such a simple basic engine, only one moving part
could be scaled up or down with relative ease to suit different applications. after a few years of laborious development that idea went out the window and only returned
after they got a much firmer grasp on materials, manufacturing techniques, and airflow through the compressor combustion chambers and turbines. Even in the 70s and 80s engines were sometimes adjusted in power by adding/subtracting compressor stages rather than changing diameters. At any rate Westinghouse found they had quite enough on their plate just getting the A19 to run properly. Combustion problems lead to uneven temperatures around the combustion chamber lead to short life of the combustion chamber and turbine. This lead to restrictions in performance and while the FH-1 Phantom was supposed to have a 41,000ft ceiling it rarely operated over 30,000ft. Range was less than book figures and it is unknown if it ever reached it's book speed of 479mph at sea level.