Replacing the Pratt & Whitney J57 with the General Electric J79?

[Christopher Wang]

The following United States military aircrafts of the Cold War were equipped with the Pratt & Whitney J57 axial-flow turbojet engines:

• Convair F-102 Delta Dagger
• Douglas A3D Skywarrior
• McDonnell F-101 Voodoo
• North American F-100 Super Sabre
• Vought F-8 Crusader

Douglas, McDonnell, and Vought would later submit proposals based on their existing military aircraft designs which swapped out the Pratt & Whitney J57 for the General Electric J79:

• Douglas D-790 (Air-to-air variant armed with AAM-N-10 Eagles for fleet air defense)
• McDonnell F-101D/E (Proposed tactical fighter variants with variable ramp inlets)
• Vought V-1000 (Land-based export fighter offered to replace the F-5 Freedom Fighter)

Although none of these proposals ever made it past the blueprint stage, the designs seem to imply the technical feasibility of replacing the J57 with the J79.

Could other J57-powered military aircraft designs such as the F-100 Super Sabre and F-102 Delta Dagger be outfitted with the J79?

Hypothetically, could these J57- / J79-powered military aircraft designs also be upgraded with modern low-bypass turbofan engines such as the Pratt & Whitney PW1120 or General Electric F404?

 

[tomo pauk]

Could other J57-powered military aircraft designs such as the F-100 Super Sabre and F-102 Delta Dagger be outfitted with the J79?

They probably could. The J79 was lighter and shorter than J57, with almost an identical diameter. Some shuffling of ancillaries will happen, though. We can recall people were re-engining aircraft in the Cold War and beyond, like the Israelis did the engine installation from Dagger into Kfir, or the Americans with F-16, or when XF-104 became the F-104.
Should've been far easier job than stuffing the big Spey in the Phantoms.

Hypothetically, could these J57- / J79-powered military aircraft designs also be upgraded with modern low-bypass turbofan engines such as the Pratt & Whitney PW1120 or General Electric F404?

Such a thing was mooted - the 'Super Phantom' by Boeing and P&W, supposed to be powered by PW 1120. A silently shelved project.

 

[Christopher Wang]

Could other J57-powered military aircraft designs such as the F-100 Super Sabre and F-102 Delta Dagger be outfitted with the J79?

They probably could. The J79 was lighter and shorter than J57, with almost an identical diameter. Some shuffling of ancillaries will happen, though. We can recall people were re-engining aircraft in the Cold War and beyond, like the Israelis did the engine installation from Dagger into Kfir, or the Americans with F-16, or when XF-104 became the F-104.
Should've been far easier job than stuffing the big Spey in the Phantoms.

Hypothetically, could these J57- / J79-powered military aircraft designs also be upgraded with modern low-bypass turbofan engines such as the Pratt & Whitney PW1120 or General Electric F404?

Such a thing was mooted - the 'Super Phantom' by Boeing and P&W, supposed to be powered by PW 1120. A silently shelved project.

But the Super Phantom does illustrate that it is technically feasible to swap out the J57 or J79 for the PW1120 or F404?

 

[apparition13]

Yes. IAI flew them in a Phantom. I believe the plan mooted was to upgrade Israeli Phantoms with PW1120 engines when the Lavi program went forward since that was to use the PW1120, but when the Lavi was cancelled the economy of scale for purchasing PW1120s disappeared as did plans to install them on F-4s.

Article:
https://www.19fortyfive.com/2021/07...antom-the-killer-fighter-jet-that-never-flew/

Video, with dodgy music and narration, of some of the 1987 Paris Air Show demo of the PW1120 Phantom.

View: https://www.youtube.com/watch?v=aePzMZbImNU

 

[BlackBat242]

You forgot the Douglas F4D-1 Skyray (J57*)... 419 built, in-service from 16 April 1956 to 29 February 1964... including many aircraft carrier deployments across the world. http://www.joebaugher.com/navy_fighters/f6_2.html

There were two prototypes of the F4D-2N (redesignated F5D-1). This had a thinner wing and 8' longer fuselage (incorporating "area rule" design), significantly reducing drag (particularly in the trans-sonic and super-sonic regimes), and almost doubling internal fuel capacity. They were fitted with J57s for their flight tests, but this aircraft actually WAS planned to have the J57 replaced by the J79 - but the program was cancelled before this could be done.

Douglas F4D-2N Skyray/F5D-1 Skylancer

* The aircraft was designed to use the Westinghouse J40 - but designer Ed Heinemann was dubious about this in-development "uber-engine" and designed the engine bay to take a larger engine. The two prototypes initially flew with J33s, and later got developmental versions of the J40 - but that engine was eventually cancelled, and the decision was made to use the J57 in the production Skyrays. http://www.joebaugher.com/navy_fighters/f6_1.html

 

[SSgtC]

In theory, sure. You could follow a path of J57-J79-F404 (or PW1120). But eventually, you hit a point of diminishing returns. How much life are you really getting out of a 30+ year old fighter with a new engine?

 

[BLACK_MAMBA]

In theory, sure. You could follow a path of J57-J79-F404 (or PW1120). But eventually, you hit a point of diminishing returns. How much life are you really getting out of a 30+ year old fighter with a new engine?

Most countries seemed to rather extend the service life of a fighter with an avionics upgrade, rather than engines. German F-4F, Israeli Kurnass, Greek Phantoms, Spanish F1M etc. All aircraft where service lives were extended through avionics as that is where the real gains lie. Performance is always welcome but a better radar & weapons are more economic ways of prolonging useful service life.

The Super Phantom also had the issue that Lavi fell (PW1120 gone) and it sort of competed in MDD and the Hornet's world! Why spend big bucks for an avionics and engine upgrade if you could either - just do avionics (cheaper than doing both) or buy a new aircraft and gain an extra 10 - 20 useful service years over an upgraded type.

As a side note - I often wondered if the Phantom could take the F404, but then you really need to start asking if a Hornet isn't a better choice...

 

[1635yankee]

One problem with replacing the J57 with the J79 is that the first generations of the J79 were, reputedly, quite susceptible to FOD. After all, a Luftwaffe F-104 was taken down by a dumpling. The J57, as a twin spool engine, didn't need variable IGV or compressor stators. In other words, if the J57 was replaced by the early variants of the J79 the aircraft would likely have significantly lower availability or higher maintenance requirements. Of course, later variants of the J79 did improve, but then the J57 didn't stand still in that regard. In another thread, I posted the USAF loss rates due to engine failures, and the J79-powered F-104 had about 70% greater loss rate due to engine problems as did the J57-powered F-100. I think it's telling that the only single-J79 aircraft to see US service was the F-104. The Israeli-developed J79 powered derivatives of the Mirage III were fielded about 20 years after the J79 entered service.

Also, if you want to replace the J57, you'd want to replace them with an engine that is significantly more powerful, at least 14,500 lbst or so without afterburning and 20,000 lbst afterburning.

 

[apparition13]

Since this is secretprojects, my ideal engine upgrade path for this class of engine would be J57/J79 -> (cancelled) RB.106 (20k lbs afterburner, which could get some of these aircraft over unity) -> PW1120 (turbofan, so more range) and then -> (proposed but not developed) EJ270, for 17/27 thousand pounds of thrust, preferably with thrust vectoring, just to tic all the boxes.

Assuming, as I've sometimes read, that the projected EJ270 somewhat larger, so it would better fit the space and deal with weight balance issues. Just think of it - 54000 lbs of thrust in an IAI/Boeing Super-Phantom, preferably with Boeing's conformal weapons and fuel pallet. Or an F-104 with 60% more thrust and TV, although I would certainly want the Dogfighter mod Lockheed proposed in the late 70s that enlarged the wing and moved the horizontal tail down to the fuselage. Fuel might become an issue though.

 

[1635yankee]

Since this is secretprojects, my ideal engine upgrade path for this class of engine would be J57/J79 -> (cancelled) RB.106 (20k lbs afterburner, which could get some of these aircraft over unity) -> PW1120 (turbofan, so more range) and then -> (proposed but not developed) EJ270, for 17/27 thousand pounds of thrust, preferably with thrust vectoring, just to tic all the boxes.

Assuming, as I've sometimes read, that the projected EJ270 somewhat larger, so it would better fit the space and deal with weight balance issues. Just think of it - 54000 lbs of thrust in an IAI/Boeing Super-Phantom, preferably with Boeing's conformal weapons and fuel pallet. Or an F-104 with 60% more thrust and TV, although I would certainly want the Dogfighter mod Lockheed proposed in the late 70s that enlarged the wing and moved the horizontal tail down to the fuselage. Fuel might become an issue though.

The Lockheed Lancer proposal.

 

[Archibald]

One problem with replacing the J57 with the J79 is that the first generations of the J79 were, reputedly, quite susceptible to FOD. After all, a Luftwaffe F-104 was taken down by a dumpling. The J57, as a twin spool engine, didn't need variable IGV or compressor stators. In other words, if the J57 was replaced by the early variants of the J79 the aircraft would likely have significantly lower availability or higher maintenance requirements. Of course, later variants of the J79 did improve, but then the J57 didn't stand still in that regard. In another thread, I posted the USAF loss rates due to engine failures, and the J79-powered F-104 had about 70% greater loss rate due to engine problems as did the J57-powered F-100. I think it's telling that the only single-J79 aircraft to see US service was the F-104. The Israeli-developed J79 powered derivatives of the Mirage III were fielded about 20 years after the J79 entered service.

Also, if you want to replace the J57, you'd want to replace them with an engine that is significantly more powerful, at least 14,500 lbst or so without afterburning and 20,000 lbst afterburning.

The Atar might have been a dated relic for way too long, but compared to that early J79 misery, it worked like a clock, was rugged, reliable, even in the worst possible environments.

I would say the Avon was probably the best of both worlds.

 

[Christopher Wang]

I'm bumping this thread by restating my question: Is it technically feasible to swap out the Pratt & Whitney J57 for the General Electric J79?

Is it also possible to replace the Pratt & Whitney J57 axial-flow turbojet with a modern low-bypass turbofan such as the Pratt & Whitney PW1120 or General Electric F404?

 

[SSgtC]

I'm bumping this thread by restating my question: Is it technically feasible to swap out the Pratt & Whitney J57 for the General Electric J79?

Is it also possible to replace the Pratt & Whitney J57 axial-flow turbojet with a modern low-bypass turbofan such as the Pratt & Whitney PW1120 or General Electric F404?

Whether it's feasible from a technical standpoint, I'm not sure. The J57 is a big engine for a pure turbojet. Whereas the J79 is much more compact. For example, the J79 is some 5 FEET shorter than the J57, 1.38 inches narrower, and some 1,300 pounds lighter while coming very close in thrust: 18,000 pounds for the J57-P-20 and 17,859 for the J79-GE-10. So physically the swap can be done. But it could also result in some serious weight and balance issues when replacing the J57.

The F404 is smaller even than the J79. It's 4.5 feet shorter and just over 3 inches narrower. It's also another 1,600 pounds lighter (or nearly 3,000 pounds lighter than the J57). Again, it can physically fit, but weight and balance is an even bigger concern than it was with the J79.

The PW1120 was built specifically as a replacement for the J79 and to fit in a Phantom with only minimal modifications to the inlet and mounting hardware. So it will definitely fit. Again, weight and balance could be an issue, the 1120 is more than 2,000 pounds lighter than the J57.

The answer with all of these is the same: it depends. The engines will all physically fit. But beyond that, you'd need someone with advanced degrees in aeronautics to tell you if it was technically feasible or not

 

[1635yankee]

Did your order go to l

The following United States military aircrafts of the Cold War were equipped with the Pratt & Whitney J57 axial-flow turbojet engines:

• Convair F-102 Delta Dagger
• Douglas A3D Skywarrior
• McDonnell F-101 Voodoo
• North American F-100 Super Sabre
• Vought F-8 Crusader

Douglas, McDonnell, and Vought would later submit proposals based on their existing military aircraft designs which swapped out the Pratt & Whitney J57 for the General Electric J79:

• Douglas D-790 (Air-to-air variant armed with AAM-N-10 Eagles for fleet air defense)
• McDonnell F-101D/E (Proposed tactical fighter variants with variable ramp inlets)
• Vought V-1000 (Land-based export fighter offered to replace the F-5 Freedom Fighter)

Although none of these proposals ever made it past the blueprint stage, the designs seem to imply the technical feasibility of replacing the J57 with the J79.

Could other J57-powered military aircraft designs such as the F-100 Super Sabre and F-102 Delta Dagger be outfitted with the J79?

Hypothetically, could these J57- / J79-powered military aircraft designs also be upgraded with modern low-bypass turbofan engines such as the Pratt & Whitney PW1120 or General Electric F404?

The unofficial motto of aeronautical engineers everywhere: "bring money; we can do anything." The questions are "why?" and "does it make economic sense?"

 

[1635yankee]

The "why?" is very important. The J57 was robust and reliable, two things the J79 was not early in its career. Some variants were also more powerful than contemporary J79s, which would also mitigate against its use instead of the J57. By the time it could make sense to drop a J79 in place of a J57, the aircraft was probably obsolescent, and the effort wouldn't be worthwhile.

 

[Christopher Wang]

The general consensus on this thread seems to be that it is possible to swap out the Pratt & Whitney J57 for the General Electric J79, but it would require addressing weight and balance issues.

 

[1635yankee]

The general consensus on this thread seems to be that it is possible to swap out the Pratt & Whitney J57 for the General Electric J79, but it would require addressing weight and balance issues.

That would be correct. It would also need some more justification than "P&WA bad, GE good."

 

[aim9xray]

There is also a possible need for structural alterations in either the engine(s) or the aircraft for the location of the engine mounts.

 

[pathology_doc]

In theory, sure. You could follow a path of J57-J79-F404 (or PW1120). But eventually, you hit a point of diminishing returns. How much life are you really getting out of a 30+ year old fighter with a new engine?

Given that the last of the B-52s will probably be approaching a hundred years old when they finally retire, quite a lot. Imagine being the crew that gets to fly the last mission, in an airplane that's old enough to be their great grandfather.

 

[SSgtC]

In theory, sure. You could follow a path of J57-J79-F404 (or PW1120). But eventually, you hit a point of diminishing returns. How much life are you really getting out of a 30+ year old fighter with a new engine?

Given that the last of the B-52s will probably be approaching a hundred years old when they finally retire, quite a lot. Imagine being the crew that gets to fly the last mission, in an airplane that's old enough to be their great grandfather.

Not exactly the same thing. The B-52 has been rewinged, reskined, had the flight deck ripped out and replaced, the engines have already been replaced once before. I mean, at this point, the B-52 fleet is kinda like George Washington's axe. The axehead has been replaced twice and the handle has been replaced 4 times, but that is George Washington's axe.

 

[pathology_doc]

In theory, sure. You could follow a path of J57-J79-F404 (or PW1120). But eventually, you hit a point of diminishing returns. How much life are you really getting out of a 30+ year old fighter with a new engine?

Given that the last of the B-52s will probably be approaching a hundred years old when they finally retire, quite a lot. Imagine being the crew that gets to fly the last mission, in an airplane that's old enough to be their great grandfather.

Not exactly the same thing. The B-52 has been rewinged, reskined, had the flight deck ripped out and replaced, the engines have already been replaced once before. I mean, at this point, the B-52 fleet is kinda like George Washington's axe. The axehead has been replaced twice and the handle has been replaced 4 times, but that is George Washington's axe.

Yeah, I am imagining a similar thing happening. Certainly not for max-intensity first-world combat, but there are places where such an aircraft would still be valuable - viz. the WW2-era Skyraider still being relevant in Vietnam.

 

[1635yankee]

In theory, sure. You could follow a path of J57-J79-F404 (or PW1120). But eventually, you hit a point of diminishing returns. How much life are you really getting out of a 30+ year old fighter with a new engine?

Given that the last of the B-52s will probably be approaching a hundred years old when they finally retire, quite a lot. Imagine being the crew that gets to fly the last mission, in an airplane that's old enough to be their great grandfather.

Not exactly the same thing. The B-52 has been rewinged, reskined, had the flight deck ripped out and replaced, the engines have already been replaced once before. I mean, at this point, the B-52 fleet is kinda like George Washington's axe. The axehead has been replaced twice and the handle has been replaced 4 times, but that is George Washington's axe.

One other thing to remember is that, once low-altitude operations were ended, B-52s really have a pretty easy operational cycle, more akin to an airliner (or at least a military transport) than to a fighter or attack aircraft, and, like all military aircraft, they usually accrue both cycles and hours at a lower rate than commercial aircraft. I don't know specific expected numbers for the B-52, but I know the H-53's expected flight hours per year was about 360. I would be surprised if the highest-time B-52 had 40,000 flight hours, which is about a quarter of the highest-time jet liner in revenue service*.

____
* I'm quite aware that commercial aircraft have quite different operational profiles than military ones, but they also have much lower operational tempos. The highest time jet liner is about 30 years old with over 140,000 flight hours.

 

[Scott Kenny]

There is also a possible need for structural alterations in either the engine(s) or the aircraft for the location of the engine mounts.

That's honestly probably the single largest issue with swapping J57 for J79: where the engine mounts are.

Example: if the engine mounts are well forward on the J57 and more aft on the J79, then the lighter J79 will be well forward in the aircraft. That takes weight off the tail and would require some significant ballasting to keep CG in limits.

The ideal would be relatively aft mounts on the J57 and forward on the J79, so that the engine is as far aft as possible. This means that the change in weight is coming out of the middle of the aircraft relatively speaking, which means less ballast needed to keep CG in limits. Or maybe no ballast at all, if the stars align just right...

 

[BlackBat242]

Not exactly the same thing. The B-52 has been rewinged, reskined, had the flight deck ripped out and replaced, the engines have already been replaced once before. I mean, at this point, the B-52 fleet is kinda like George Washington's axe. The axehead has been replaced twice and the handle has been replaced 4 times, but that is George Washington's axe.

No B-52 has had its engines replaced with a different type.

The B-52A/B/C/D/E/F/G all had the J57 turbojet engine in different versions - and each kept the version they came off the assembly line with.

The B-52H had TF-33s (turbofan using the core of the J57) installed on the assembly line - and still have TF-33s on their wings.

Only the B-52Hs converted to B-52Js will have "had their engines replaced".

 

[aim9xray]

Not exactly... (the devil is in the outliers!)
A B-52A had all four outboard J57s (Engines 1,2 7 & 8) replaced by two J75s as a testbed.
A B-52G had all J57s replaced by TF33s as a testbed. The aircraft was later demodified and went to SAC as a standard, combat-ready B-52G.

And NASA's "Balls-Eight" (52-0008) flew with ten engines...

 

[Scott Kenny]

Not exactly... (the devil is in the outliers!)
A B-52A had all four outboard J57s (Engines 1,2 7 & 8) replaced by two J75s as a testbed.
A B-52G had all J57s replaced by TF33s as a testbed. The aircraft was later demodified and went to SAC as a standard, combat-ready B-52G.

I'm not willing to count testbeds as engine swaps. They're one-offs.

And NASA's "Balls-Eight" (52-0008) flew with ten engines...

What were the extra two engines, and where were they attached?

 

[Arjen]

Found here:

Balls_Eight_History_of_the_Boeing_NB-52B_Stratofortress_Mothership_eBook

For some time in the mid-1990s, the NB-52B flew as a ten-engine jet as it tested pollution reducing fuel additives with a pair of jet engines mounted under its bomb bay.

Two J85s.

 

[Scott Kenny]

Found here:

Balls_Eight_History_of_the_Boeing_NB-52B_Stratofortress_Mothership_eBook

Two J85s.

Thanks!