American Super Fighter: The XP-75

[SpicyJuan11]

Hello, as I know next to nothing on expirimental US fighters during World War II, could somebody please shed some light on the XP-75 Eagle for me, specifically why it was cancelled? It seeks like a killer aircraft to me Many thanks.

 

[wuzak]

It was designed as a long range fighter but by the time it flew the P-51B was already doing the job.

Fisher P-75 Eagle

 

[GregP]

Not one of our brightest bulbs ...

 

[SpicyJuan11]

Not one of our brightest bulbs ...

What do you mean? Did it not have good performance?

 

[SpicyJuan11]

It was designed as a long range fighter but by the time it flew the P-51B was already doing the job.

Fisher P-75 Eagle

Thanks for the link But didn't the U.S. have the capacity to make multiple fighters of the same role?

 

[Shortround6]

The US did. You have to careful comparing "manufacturer's estimates" to actual performance. In late 1944 you not only had the P-51 doing an excellent job but you had the P-47N in the works. You also had the late model P-38s doing a good job in the Pacific. A 4th fighter type that didn't really bring much to the table wasn't needed.
As an indication of which way things were going, less than 3 months after the P-75 was canceled North American Aviation got a contract to build 1000 P-80 jets as a second source to Lockheed. This contract was canceled right after V-J day but building large numbers of a "NEW" piston engine fighter was doubtful.

 

[gjs238]

At that point in time, wasn't the US considering, if not already in the process of, consolidating fighter types under development?

 

[GregP]

It's performance fell short of estimates, as you might expect by taking completed assemblies made for other aircraft and cobbling them together into an aircraft. The redesign showed some promise ... for a plane to be used in, maybe 1942 - 1943, but by the time it was ready and flying well, jets were out and the handwriting was on the wall.

It never did quite makes estimated performance and I don't know of any service aircrft that ever did well with a W-24 type engine made of Siamese V-12s. Some will say the He 177 did well. I disagree. It showed promise when they separated the engines and made it into a 4-engine aircraft.

While it was a twin, it never did manage to be effective in numbers. By "effective," I mean it was never able to swing the tide of an operation or battle from defeat to victory, AFAIK. It DID manage to drop some bombs and they did mostly cure the catch-fire problem, but it was not enough to change anything of import.

 

[drgondog]

The XP-75 was so close to purchase by Material Command leadership that listened to USN advisors stating that it was desirable to "have an optimal cruise at speeds near the bombers'.

Barney Giles is credited as the person designated by Arnold to 'get a fighter capable of escorting AAF bombers to the target' and that the P-38 was deemed to slow, P-47 not enough range. The P-51B was very promising but not quite enough range and the P-75 was at that time deemed a possible solution.

Col Mark Bradley, respected engineer/pilot had solved a lot of P-47 start up mechanical issues and improved the range with unpressurized external CL tanks. He was a strong supporter of the P-51B with the Merlin and had recently flown the XP-75 at Wright Pat. He considered bot a 'dog' as well as dangerous to fly. He appealed to General Orval to slow down the procurement of 2000 P-75s until he could take his idea of putting an extra '200 gallons' into the P-51B. Generals Orval and Echols gave him permission to take the idea to Dutch Kindleberger.

Bradley and Barney Giles met with NAA, were told that the gear would not support an extra 200 but would 85 do?

It did and the P-75, 'designed per USN advice that it needed to fly long way but not too fast' died a grisly death.

Source Forged In Fire b Dewitt Coop

 

[Shortround6]

The P-47N also managed another 184 gallons inside the airplane.

 

[drgondog]

The difference between the P-51B and the P-47D/N was that the long range fix was developed by NAA and installations started within 30 days of initial P-51B combat operations, whereas it took 14 months for the P-47C through D-22 to increase fuselage fuel by 60 gallons in the P-47D-25 and nearly two years for the wet wing P-47N

 

[Shortround6]

True but for a good part of those 14 months the P-47 had no water injection and tooth-pic props. P-51 may have had an element of luck going for it. Room to put the fuel and with the fitting of the heavier Merlin engines a CG position that allowed for it. Sticking a 60-85 gallon tank in the same place in an Allison powered Mustang could have been very exciting for the pilot. new underwear at the very least.

In any case with almost 200 extra gallons over and above the larger fuselage tanks in the P-47 the need to build something like the P-75 certainly diminished. The Mustang just went from strength to strength the rear fuselage tank and bigger drop tanks.

With 3 fighters capable of doing long range escort already in production (not saying they were all equal) the need for a 4th was rather small.

 

[drgondog]

I agree - but I do have a little confusion regarding Republic taking so long to get the extra 60 gallons internally that didn't seem to require much engineering or tooling changes - and the issue was obvious when the P-47C was introduced into the ETO before the first production P-51B flew.

Colonel Bradley was the ETO engineering guy under Cass Hough that fixed the radio interference, got the plumbing fixed for C/L drop tank - the came home and went to work solving the Merlin P-51 ultra long range issue..

 

[kool kitty89]

I agree - but I do have a little confusion regarding Republic taking so long to get the extra 60 gallons internally that didn't seem to require much engineering or tooling changes - and the issue was obvious when the P-47C was introduced into the ETO before the first production P-51B flew.

Getting a large, pressurized belly tank into service sooner probably would have been even more useful than the added 60 gal tank (plus didn't have the problems with streamlining that the wing pylons -oddly- posed). A P-47C or early D with 200 gal flat (or long or conformal), pressurized belly drop tank should have helped a great deal and been getting better performance and fuel economy than the wing-pylon equipped models or ones with the added 60 gal tank. (not P-51 level range, but still a lot more useful especially early war) I forget if that flat belly tank used in the PTO was 200 US or IMP gallons, but I think it was US.

I'm not sure, but limiting the wing racks to carry only 75 gal or smaller tanks (or 500 lb bombs) might have allowed a more streamlined arrangement too. (or at least reduced the apparent difficulties) 200+75+75 gallons externally plus the added 60 gal internally along with water injection and the paddle props should have been enough to make it full-length escort worthy in the UTO)

 

[drgondog]

Getting a large, pressurized belly tank into service sooner probably would have been even more useful than the added 60 gal tank (plus didn't have the problems with streamlining that the wing pylons -oddly- posed). A P-47C or early D with 200 gal flat (or long or conformal), pressurized belly drop tank should have helped a great deal and been getting better performance and fuel economy than the wing-pylon equipped models or ones with the added 60 gal tank. (not P-51 level range, but still a lot more useful especially early war) I forget if that flat belly tank used in the PTO was 200 US or IMP gallons, but I think it was US.

For planning purposes, the internal fuel capacity was the foundation - and by necessity the most important factor - including PTO. The reason the F6F could not have been very effective as an escort (high altitude performance deficiencies versus Bf 109 aside) is that the F6F never had more than 250 gallons of internal fuel. No matter how big the external tank is, It is gone when entering combat, and all you have left to fight with at 3X fuel consumption, then return home is what you have left in the tanks. The P-47C through D-22 had 55 more gallons than F6F and couldn't go near far enough to provide medium to deep range escorts.

I'm not sure, but limiting the wing racks to carry only 75 gal or smaller tanks (or 500 lb bombs) might have allowed a more streamlined arrangement too. (or at least reduced the apparent difficulties) 200+75+75 gallons externally plus the added 60 gal internally along with water injection and the paddle props should have been enough to make it full-length escort worthy in the UTO)

When attacked the first things to 'go' are the "200+75=75 gallon tanks'. Having said that, increasing external load capability was a practical design goal - not decreasing the load. Bigger load=better short range tactical capability for CAS, and greater tactical footprint for longer range mission requirements. Offhand I don't know of a successful fighter that shrunk its load capability during development cycle

As an example - despite the dangers of aft CG in the aft 85 gallon tank in the P-51B/D, the SOP was to only burn 60 to restore excellent longitudinal stability - before switching to the externals. On a really long mission like a Shuttle to Russia or deep Oil targets like Ruhland, Brux, Stettin,etc many pilots burned only 25+- until they were comfortable 'enough' in the response.

 

[tomo pauk]

Getting a large, pressurized belly tank into service sooner probably would have been even more useful than the added 60 gal tank (plus didn't have the problems with streamlining that the wing pylons -oddly- posed). A P-47C or early D with 200 gal flat (or long or conformal), pressurized belly drop tank should have helped a great deal and been getting better performance and fuel economy than the wing-pylon equipped models or ones with the added 60 gal tank. (not P-51 level range, but still a lot more useful especially early war) I forget if that flat belly tank used in the PTO was 200 US or IMP gallons, but I think it was US.

There was no added 60 gal tank, the main tank gained in height to house now 270 gals of fuel for the late -D models (100 gals was carried in the auxiliary fuel tank, making 370 gals total of internal fuel). The 'flat' metal tank was produced in Brisbane, Australia, by Ford, in 1943 for the 5th Air Force - Gen. Kenney was agile as ever.
It is too bad that Republic tried to reinvent the wheel with their design of wing pilons - those at A-36 were already capable to carry 2 x 150 gal tanks, and installation on the P-38 (up to 2 x ~300) was also a no-fuss item.
The conformal 75 gal tank was also tried at Pacific, but it was too late since 370 gal -D models were available by then.

I'm not sure, but limiting the wing racks to carry only 75 gal or smaller tanks (or 500 lb bombs) might have allowed a more streamlined arrangement too. (or at least reduced the apparent difficulties) 200+75+75 gallons externally plus the added 60 gal internally along with water injection and the paddle props should have been enough to make it full-length escort worthy in the UTO)

The P-47D that had 305 gals internally and 300 externally was rated to 450 miles of escort radius by USAF (310 mph TAS cruise at 25000 ft, 15 min combat on mil power, 5 min on WER) , those with 370+300 (= late D models, also M) were rated to 600 miles

 

[Shortround6]

Once you have the fuel for take-off, climb to altitude, 15-20 minutes combat, and 20-30 minutes reserve covered, (which on the P-47D could be around 250 gallons) an extra 60 gallons (double what the older P-47 had without drop tanks) can extend the range rather nicely.

 

[kool kitty89]

I'd also meant that with the lower weight of the C and early D models as well as lack of wing pylons, the fuel efficiency for cruise should be a bit better. So 305 gal internal + 200 gal external should stretch a bit further than on the heavier, draggier late D models.

Shame they didn't just use Lockheed's pylon design though, or Lockheed drop tanks for that matter (both seemed pretty well streamlined). I know P-47s did end up using lockheed tanks in the Pacific later on. (doesn't really account for the pylon issues though)

The belly pylons added on the F4U seemed to be a bit more like what the P-38 used. Same for the large teardrop tanks. (though not actual P-38 tanks)