Old designs with modern engines

[Spino]

I like that idea, you'd have half an Phantom in one step. 

In so many ways 

Think about it.  F3H has a single pilot, F-4 has a pilot plus a RIO.  F3H can carry AIM-7s and AIM-9s, but only half as many missiles as the F-4 total (4x AIM-7 or 4x AIM-9 or 2x AIM-7 and 2x AIM-9).  Give it a J79 engine and it goes from being a transonic interceptor with a pitifully low power to weight ratio to (probably) a reasonably supersonic fleet air defense platform with decent acceleration and exactly half as many J79s as the F-4!  I bet an F3H with a J79 in place of the J71 could have made Mach 1.15 in level flight fairly easily (pretty much exactly half of what the F-4 could do, incidentally  ).

[jcf]

Hear me out: F3H-2N Demon with J79 engine in place of the J71.  The J79 is pretty much the same diameter as the J71 (just a tad smaller in fact), and it's shorter, lighter, and delivers a LOT more thrust.

The biggest issue would be the increase in mass flow requirements, which means that
it's likely that the intakes would have to be enlarged.

The length of the engine varies between the short afterburner and long afterburner
subtypes. The 284.5" J71-A-2 of the F3H has the long afterburner, the short afterburner
types varied between ~191 and 208 inches overall. This is why the J71 in the F3H is so
much longer and heavier than a J79. It's 420 pounds heavier than the short afterburner
J71-A-4.

[Spino]

The biggest issue would be the increase in mass flow requirements, which means that
it's likely that the intakes would have to be enlarged.

The length of the engine varies between the short afterburner and long afterburner
subtypes. The 284.5" J71-A-2 of the F3H has the long afterburner, the short afterburner
types varied between ~191 and 208 inches overall. This is why the J71 in the F3H is so
much longer and heavier than a J79. It's 420 pounds heavier than the short afterburner
J71-A-4.

According to a NASA report on the J71, its mass flow rate is 160lb/s.  The J79-GE-17 (used on the F-4E) needs 170lb/s, so a greater mass flow rate would be required, but I'm not entirely sure it's enough to warrant enlargement of the intakes.  It's only 6.25% more than what was already required for the J71.  With that being said, the intake design on the F3H supposedly wasn't great, especially in terms of pressure recovery, so a redesign of the intakes might have been in order anyway.

Here's the link to the NASA report in case anyone is interested: https://ntrs.nasa.gov/api/citations/19930089217/downloads/19930089217.pdf

[kerick]

One F-101 Voodoo actually had J79 engines installed. Known as the NF-101A, one F-101A was used by General Electric for testing of the General Electric J79 engine. The F-101D and E were proposed J79 versions but never built.
The Voodoo had the Pratt & Whitney J57-P-55 afterburning turbojet engines, 11,990 lbf (53.3 kN) dry thrust, 16,900 lbf (75 kN) with afterburner while the J79 had 11,870 lbf (52.8 kN) dry thrust and 17,900 lbf (80 kN) with afterburner. This upgrade would be pretty easy. The J79 afterburners would just hang out the back end.

[jcf]

One F-101 Voodoo actually had J79 engines installed. Known as the NF-101A, one F-101A was used by General Electric for testing of the General Electric J79 engine. The F-101D and E were proposed J79 versions but never built.
The Voodoo had the Pratt & Whitney J57-P-55 afterburning turbojet engines, 11,990 lbf (53.3 kN) dry thrust, 16,900 lbf (75 kN) with afterburner while the J79 had 11,870 lbf (52.8 kN) dry thrust and 17,900 lbf (80 kN) with afterburner. This upgrade would be pretty easy. The J79 afterburners would just hang out the back end.

The NF-101A J79 installation pretty much just looks like the J57-P-55 of the JF-101A and F-101B with its extended afterburners.

[Spino]

What was the point of swapping out the J57s for J79s on the F-101?  Less weight and a bit more thrust?  Or better high speed performance?

[Weaver]

What was the point of swapping out the J57s for J79s on the F-101?  Less weight and a bit more thrust?  Or better high speed performance?

It was to flight test the J-79s. It would indeed have less weight and more thrust, plus significantly lower fuel consumption as well.

[jcf]

The J79 was much shorter than the J57 so it sticking way out the back like that is a bit
puzzling. The JT3C-7, non-afterburning civil J57, is 155" long, the J57-P-55 is 252" long,
so 8' of the difference is the afterburner. The J79-GE-17 is 208" long, between the two,
so either it was mounted farther aft or GE stuck a really long afterburner on the engines
when they mounted them in the airframe.

[Spino]

The J79 was much shorter than the J57 so it sticking way out the back like that is a bit
puzzling. The JT3C-7, non-afterburning civil J57, is 155" long, the J57-P-55 is 252" long,
so 8' of the difference is the afterburner. The J79-GE-17 is 208" long, between the two,
so either it was mounted farther aft or GE stuck a really long afterburner on the engines
when they mounted them in the airframe.

They may have been mounted farther back to preserve CG location.  In a way it's too bad USAF didn't actually order J79-powered F-101s, with the right equipment fit they could have been comparable to the F-4 in many respects.  Oh wait, Mac the Knife would have had none of that!  Still, I can't help but wonder about an F-101 with twin J79s and a combination of AIM-9s and AIM-7s.  Could probably have kept a couple of the guns too, although maneuverability may have been worse than even the F-4 so dogfighting would likely not be a great idea.

[Spino]

Back to my F3H idea for a second, a J79-powered Demon might have fared rather well in Vietnam honestly.  The F3H already had a low enough wing loading that it could give an F-8 a hard time in a slow-speed fight (although only to the extent that its criminally underpowered engine would allow), and it had really good cockpit visibility and onboard guns.  That combination plus AIM-7 capability and of course Sidewinders could have made it pretty good against MiG-17s, although it would have been no match for a MiG-21 even with a J79 in its tail.

[kerick]

The J79 was much shorter than the J57 so it sticking way out the back like that is a bit
puzzling. The JT3C-7, non-afterburning civil J57, is 155" long, the J57-P-55 is 252" long,
so 8' of the difference is the afterburner. The J79-GE-17 is 208" long, between the two,
so either it was mounted farther aft or GE stuck a really long afterburner on the engines
when they mounted them in the airframe.

They may have been mounted farther back to preserve CG location.  In a way it's too bad USAF didn't actually order J79-powered F-101s, with the right equipment fit they could have been comparable to the F-4 in many respects.  Oh wait, Mac the Knife would have had none of that!  Still, I can't help but wonder about an F-101 with twin J79s and a combination of AIM-9s and AIM-7s.  Could probably have kept a couple of the guns too, although maneuverability may have been worse than even the F-4 so dogfighting would likely not be a great idea.

If I remember correctly the F-101 had a tendency to pitch up violently with little warning that killed a number of experienced pilots. Perhaps the two seaters didn't suffer this as badly as I haven't heard of it being a problem for the CAF. That could be one reason for going straight to the F-4.

Here's my version of an upgraded Voodoo; https://www.whatifmodellers.com/index.php?topic=39666.msg685679#msg685679

Now the J-79 in the Demon would be a great upgrade!

[Rick Lowe]

If I remember correctly the F-101 had a tendency to pitch up violently with little warning that killed a number of experienced pilots. Perhaps the two seaters didn't suffer this as badly as I haven't heard of it being a problem for the CAF. That could be one reason for going straight to the F-4.

Stretch the F-101s cockpit, but keep a single pilot.
Make the extra room for avionics/fuel/whatever.

[jcf]

The J79 was much shorter than the J57 so it sticking way out the back like that is a bit
puzzling. The JT3C-7, non-afterburning civil J57, is 155" long, the J57-P-55 is 252" long,
so 8' of the difference is the afterburner. The J79-GE-17 is 208" long, between the two,
so either it was mounted farther aft or GE stuck a really long afterburner on the engines
when they mounted them in the airframe.

They may have been mounted farther back to preserve CG location.  In a way it's too bad USAF didn't actually order J79-powered F-101s, with the right equipment fit they could have been comparable to the F-4 in many respects.  Oh wait, Mac the Knife would have had none of that!  Still, I can't help but wonder about an F-101 with twin J79s and a combination of AIM-9s and AIM-7s.  Could probably have kept a couple of the guns too, although maneuverability may have been worse than even the F-4 so dogfighting would likely not be a great idea.

Setting the J79s further back would be very complicated as it would require numerous structural changes, the frames where the engines attach in particular, and extending the intake trunking. A long tailpipe and ballasting would be the most logical way to do it.

[Weaver]

The J79 was much shorter than the J57 so it sticking way out the back like that is a bit
puzzling. The JT3C-7, non-afterburning civil J57, is 155" long, the J57-P-55 is 252" long,
so 8' of the difference is the afterburner. The J79-GE-17 is 208" long, between the two,
so either it was mounted farther aft or GE stuck a really long afterburner on the engines
when they mounted them in the airframe.

They may have been mounted farther back to preserve CG location.  In a way it's too bad USAF didn't actually order J79-powered F-101s, with the right equipment fit they could have been comparable to the F-4 in many respects.  Oh wait, Mac the Knife would have had none of that!  Still, I can't help but wonder about an F-101 with twin J79s and a combination of AIM-9s and AIM-7s.  Could probably have kept a couple of the guns too, although maneuverability may have been worse than even the F-4 so dogfighting would likely not be a great idea.

If I remember correctly the F-101 had a tendency to pitch up violently with little warning that killed a number of experienced pilots. Perhaps the two seaters didn't suffer this as badly as I haven't heard of it being a problem for the CAF. That could be one reason for going straight to the F-4.

Here's my version of an upgraded Voodoo; https://www.whatifmodellers.com/index.php?topic=39666.msg685679#msg685679

Now the J-79 in the Demon would be a great upgrade!

IIRC (which I may not) wasn't the F-101 fitted with an early form of fly-by-wire pitch stabilising system to mitigate that problem?

The F-101 had a built-in ability to replace the ammo tanks with an extra fuel tank for ferry purposes and this was done in service on a number of occasions. The main problem with a multi-role F-101 would be pylons. It only had three close together under the fuselage and none under the very narrow-chord wings. Even if the wings are strong enough to mount a couple of low-rated outboard pylons (which I doubt), that's still all you get because the undercarriage bays take up a big chunk of the lower surface. I suppose you could fit a single-seater with the rotating missile door from the F-101B, but that then costs you the guns and limits you to Falcons and Genies: the door's too short for Sidewinders or Sparrows.

[jcf]

The J79 was much shorter than the J57 so it sticking way out the back like that is a bit
puzzling. The JT3C-7, non-afterburning civil J57, is 155" long, the J57-P-55 is 252" long,
so 8' of the difference is the afterburner. The J79-GE-17 is 208" long, between the two,
so either it was mounted farther aft or GE stuck a really long afterburner on the engines
when they mounted them in the airframe.

They may have been mounted farther back to preserve CG location.  In a way it's too bad USAF didn't actually order J79-powered F-101s, with the right equipment fit they could have been comparable to the F-4 in many respects.  Oh wait, Mac the Knife would have had none of that!  Still, I can't help but wonder about an F-101 with twin J79s and a combination of AIM-9s and AIM-7s.  Could probably have kept a couple of the guns too, although maneuverability may have been worse than even the F-4 so dogfighting would likely not be a great idea.

If I remember correctly the F-101 had a tendency to pitch up violently with little warning that killed a number of experienced pilots. Perhaps the two seaters didn't suffer this as badly as I haven't heard of it being a problem for the CAF. That could be one reason for going straight to the F-4.

Here's my version of an upgraded Voodoo; https://www.whatifmodellers.com/index.php?topic=39666.msg685679#msg685679

Now the J-79 in the Demon would be a great upgrade!

IIRC (which I may not) wasn't the F-101 fitted with an early form of fly-by-wire pitch stabilising system to mitigate that problem?

The F-101 had a built-in ability to replace the ammo tanks with an extra fuel tank for ferry purposes and this was done in service on a number of occasions. The main problem with a multi-role F-101 would be pylons. It only had three close together under the fuselage and none under the very narrow-chord wings. Even if the wings are strong enough to mount a couple of low-rated outboard pylons (which I doubt), that's still all you get because the undercarriage bays take up a big chunk of the lower surface. I suppose you could fit a single-seater with the rotating missile door from the F-101B, but that then costs you the guns and limits you to Falcons and Genies: the door's too short for Sidewinders or Sparrows.

Since when is a Genie a limitation?