Bespoke EE Lightning replacement 1977?

[Riain]

A world where Britain buys twice as many Lightnings as OTL by 1965 to cover both fighter roles and Ground Attack/Recce roles is a world where the RAF doesn't have either the Phantom or Jaguar in service by 1977 when the Lightning fleet needs to be replaced. In this world Britain is still a strong and potentially independent producer of first rate combat aircraft, having just completed TSR2 and Harrier (P1127) production.

In this world, starting project definition in the 2nd half of the 60s looking for an IOC date of 1975 what does Britain build to replace it's large Lightning fleet, in the order of ~200 aircraft for 15 squadrons give or take with OCU, Trials and other cats and dogs units?

What would it look like; would it be big like the F14/15, medium like the Viggen/Tornado, or small like Mirage F1 or 2000 and F16?
Would it have fixed wing and conventional tail, delta, canard or swing wing?
What engines would it use?
Would it be built with partner nations, if so which ones?
Would it sell internationally, if so to which countries?
What weapons will it carry and at what range and speed?

In order to clarify, the RN has it's Phantoms and Buccaneers both of which will last until 1990 so they are not a customer and buying 'off the shelf' is not an option because Britain is 'the shelf'.

 

[EAF602Whizz]

A world where Britain buys twice as many Lightnings as OTL by 1965 to cover both fighter roles and Ground Attack/Recce roles is a world where the RAF doesn't have either the Phantom or Jaguar in service by 1977 when the Lightning fleet needs to be replaced. In this world Britain is still a strong and potentially independent producer of first rate combat aircraft, having just completed TSR2 and Harrier (P1127) production.

In this world, starting project definition in the 2nd half of the 60s looking for an IOC date of 1975 what does Britain build to replace it's large Lightning fleet, in the order of ~200 aircraft for 15 squadrons give or take with OCU, Trials and other cats and dogs units?

What would it look like; would it be big like the F14/15, medium like the Viggen/Tornado, or small like Mirage F1 or 2000 and F16?
Would it have fixed wing and conventional tail, delta, canard or swing wing?
What engines would it use?
Would it be built with partner nations, if so which ones?
Would it sell internationally, if so to which countries?
What weapons will it carry and at what range and speed?

In order to clarify, the RN has it's Phantoms and Buccaneers both of which will last until 1990 so they are not a customer and buying 'off the shelf' is not an option because Britain is 'the shelf'.

I think it would be medium sized, similar to the F18. The RAF hasn't had much interest in light fighter concepts unfortunately and large fighter aircraft like the F14 are probably too expensive for 1970's Britain. I also think that it would be a fixed wing, conventional tail design like the contemporary types being tested in the US and Soviet Union. Engines are an interesting question. Here it might follow the MRCA route of Italy, Germany and the UK collaborating on the programme. It would probably be similar to Tornado in the sense that it would meet similar criteria and find roughly the same market. Basically a Tornado with fixed wings and with the same armaments and avionics and weapon suite. Sitting somewhere between the Tornado and the concepts that eventually produced the Typhoon. Could it be designed primarily as an air to air platform from the outset rather than as a bomber that does air to air?

 

[Riain]

I think it would be medium sized, similar to the F18. The RAF hasn't had much interest in light fighter concepts unfortunately and large fighter aircraft like the F14 are probably too expensive for 1970's Britain. I also think that it would be a fixed wing, conventional tail design like the contemporary types being tested in the US and Soviet Union. Engines are an interesting question. Here it might follow the MRCA route of Italy, Germany and the UK collaborating on the programme. It would probably be similar to Tornado in the sense that it would meet similar criteria and find roughly the same market. Basically a Tornado with fixed wings and with the same armaments and avionics and weapon suite. Sitting somewhere between the Tornado and the concepts that eventually produced the Typhoon. Could it be designed primarily as an air to air platform from the outset rather than as a bomber that does air to air?

I'm inclined to agree about the size, but by the mid 70s a medium fighter can do most of what the F14/15 could so that's not a problem . Apparently in 1965 the AFVG was to replace the Lightning, when the RAF had options for 110 F111s, but when that reduced to 50 the AFVG was morphed into a strike aircraft to compliment to it. When the French pulled out the UKVG was the basis for the NATO common strike aircraft MoU.

If the requirement was for a fighter I'd imagine that the AFVG/UKVG could be specced suitably. In 1967 the requirement for the F14 was to be able to outmanouvre the Mig 19, 19 & 21, so a VG fighter can be agile enough.

I don't think 200 would be enough to start full development, Britain would need at least 1 development partner and maybe even an export customer.

 

[Peg Leg Pom]

A world where Britain buys twice as many Lightnings as OTL by 1965 to cover both fighter roles and Ground Attack/Recce roles is a world where the RAF doesn't have either the Phantom or Jaguar in service by 1977 when the Lightning fleet needs to be replaced. In this world Britain is still a strong and potentially independent producer of first rate combat aircraft, having just completed TSR2 and Harrier (P1127) production.

In this world, starting project definition in the 2nd half of the 60s looking for an IOC date of 1975 what does Britain build to replace it's large Lightning fleet, in the order of ~200 aircraft for 15 squadrons give or take with OCU, Trials and other cats and dogs units?

What would it look like; would it be big like the F14/15, medium like the Viggen/Tornado, or small like Mirage F1 or 2000 and F16?
Would it have fixed wing and conventional tail, delta, canard or swing wing?
What engines would it use?
Would it be built with partner nations, if so which ones?
Would it sell internationally, if so to which countries?
What weapons will it carry and at what range and speed?

In order to clarify, the RN has it's Phantoms and Buccaneers both of which will last until 1990 so they are not a customer and buying 'off the shelf' is not an option because Britain is 'the shelf'.

The obvious answer to this is that Britain develops this https://en.wikipedia.org/wiki/BAC/Dassault_AFVG entirely on its own.

 

[tomo pauk]

The Viggen equivalent in weight, size and number of engines. Whether a canard-delta or a 'classic' wing - depending on the taste, both work. The best radar available, 2-4 Skyflash + 4 Sidewinder. 'Raised' cockpit, at least comparable with what Lighting had, single-piece windscreen. 'Combat Tree' system, plus IRST. Spey engine, unless RR whips up something much more modern.

Alternatively, choose a 2-engined design looking like baby F-14 with fixed wing, or what became MiG-29 (but with bigger spine for bigger fuel tanks, like the MiG-29M/K had); I'd avoid the draggy wing like the F-18 used, since now fuselage produces a loft of lift by itself. Engine choice is easier this way vs. 1-engined job, since it can use RB.199s.

 

[CV12Hornet]

With the cancellation of the TSR-2 and P.1154 IOTL, the RAF was left with three major capability gaps: home long-range air defense, for which the short-legged Lightning was becoming increasingly incapable of and was covered by first the F-4K and then the Tornado ADV; deep strike and interdiction, which was eventually filled by the Tornado after the cancellation of the F-111K and with the Buccaneer filling in on an interim basis; and close air support, which was filled by first the F-4M and then the Jaguar.

TTL the deep strike role is covered by the TSR-2, while close air support will probably be filled by a combination of Harriers and Buccaneers in the absence of the Jaguar. That means this aircraft will be filling the Phantom role in home air defense, and probably should be covering the air defense duties in Germany, with a requirement for 10 frontline squadrons leading to around 250 aircraft.

What would it look like; would it be big like the F14/15, medium like the Viggen/Tornado, or small like Mirage F1 or 2000 and F16?

Tornado/Eagle sized (Yes, the two are about the same size). The range requirements for CAP operations over the North Sea are enormous: 2 hours 560-740 kilometers from base. That's something even the Tomcat would have trouble with, with the caveat that I have no idea what armament and drop tank loads the Tornado needed to achieve that kind of interception range. Regardless, you're going to need a large aircraft to carry all that fuel. You just don't need something the size of the Tomcat, because you don't need a radar the size of the AWG-9 nor missiles the size of the Phoenix.

Of course, there is the temptation to do two aircraft, one a heavier interceptor for the North Sea coverage and a smaller single-engine aircraft for operations in Europe and Southern England, but I don't think the Brits can shoulder the development costs of two aircraft, especially with all the competition in both categories in the overseas export market.

Would it have fixed wing and conventional tail, delta, canard or swing wing?

I'd expect a swing wing in this era, especially with the obvious connections to the AFVG project.

What engines would it use?

For something the size of the Eagle or Tornado, and is expected to tangle with Soviet fighters over Germany, I think the most likely option is a development of the Harrier's Pegasus engine. The Pegasus 11 was due to enter service in 1974 and it could deliver 21,000 lbs of dry thrust; strip out the thrust vector components and add an afterburner and you have an engine comparable to the F100.

Then again, it might be easier to develop a wholly new engine that's essentially a larger RB.199, but for this you want something in the F100 class.

Would it be built with partner nations, if so which ones?

France, despite their desire for a similar interceptor aircraft, are unlikely given how opposed Dassault was to participating. Without the Tornado project they might be able to talk Italy into contributing; Italy was working on the F-104S at the time and this could replace it, with a potential strike variant a la the Strike Eagle down the line to fully cover the gap left by the Tornado. And who knows, maybe they get the foot in the door for the Japanese program that led to the F-15J. I think that one's a long shot, though.

Would it sell internationally, if so to which countries?

The Saudis seem a prime candidate to buy the aircraft, and with the aircraft being more Eagle-like than the Tornado ADV could very well buy more in place of their early Eagle purchases. If a strike variant can be developed that opens even more doors: Germany, Australia, and Spain all spring to mind as potential candidates.

What weapons will it carry and at what range and speed?

I've gone over the range above, but the Mach 2.2 of the Tornado ADV and its 4 Skyflash/4 Sidewinder armament is likely to be replicated as well for the air defense version.

Speaking of, I would expect four variants: two-seat interceptor, single-seat fighter, two-seat deep strike, and two-seat EW.

 

[Riain]

The obvious answer to this is that Britain develops this https://en.wikipedia.org/wiki/BAC/Dassault_AFVG entirely on its own.

View attachment 609739

I'm inclined to agree and the original 1965 requirement was for a Lightning replacement, but I think with a requirement of only 200 it is beyond the financial capability of Britain alone.

 

[Riain]

The Viggen equivalent in weight, size and number of engines. Whether a canard-delta or a 'classic' wing - depending on the taste, both work. The best radar available, 2-4 Skyflash + 4 Sidewinder. 'Raised' cockpit, at least comparable with what Lighting had, single-piece windscreen. 'Combat Tree' system, plus IRST. Spey engine, unless RR whips up something much more modern.

Alternatively, choose a 2-engined design looking like baby F-14 with fixed wing, or what became MiG-29 (but with bigger spine for bigger fuel tanks, like the MiG-29M/K had); I'd avoid the draggy wing like the F-18 used, since now fuselage produces a loft of lift by itself. Engine choice is easier this way vs. 1-engined job, since it can use RB.199s.

I like the Viggen, it more or less fits the stike frame and the Swedes wanted to use the RR Medway before it was cancelled. However the initial version was a dedicated strike aircraft and the fighter version was a separate development that didn't reach IOC until 1979 which is 5 years too late.

As for the 2 engine option, the UKVG looks iike a good bet although RB199 analogues seem a to lack thrust for a 45,000lbs fighter but by 1975 the Spey was a step behind what was possible.

 

[Peg Leg Pom]

I'm inclined to agree and the original 1965 requirement was for a Lightning replacement, but I think with a requirement of only 200 it is beyond the financial capability of Britain alone.

If they can get others to commit to buying it then it's doable, even if you have to sub contract some of the work. The thing is Britain has to keep control of the project not have it end up in international committee hell, which just adds cost.

 

[tomo pauk]

Tornado/Eagle sized (Yes, the two are about the same size). The range requirements for CAP operations over the North Sea are enormous: 2 hours 560-740 kilometers from base. That's something even the Tomcat would have trouble with, with the caveat that I have no idea what armament and drop tank loads the Tornado needed to achieve that kind of interception range. Regardless, you're going to need a large aircraft to carry all that fuel. You just don't need something the size of the Tomcat, because you don't need a radar the size of the AWG-9 nor missiles the size of the Phoenix.

Of course, there is the temptation to do two aircraft, one a heavier interceptor for the North Sea coverage and a smaller single-engine aircraft for operations in Europe and Southern England, but I don't think the Brits can shoulder the development costs of two aircraft, especially with all the competition in both categories in the overseas export market.

I'd expect a swing wing in this era, especially with the obvious connections to the AFVG project.

For something the size of the Eagle or Tornado, and is expected to tangle with Soviet fighters over Germany, I think the most likely option is a development of the Harrier's Pegasus engine. The Pegasus 11 was due to enter service in 1974 and it could deliver 21,000 lbs of dry thrust; strip out the thrust vector components and add an afterburner and you have an engine comparable to the F100.

I like the "Pegasus-based afterburning engine" idea very much. Swing wing by early 1970s wasn't the only game in town, specifically I'd avoid it. Eagle was much bigger than IDS Tornado (so was the F-14), though the ADV Tornado upped the length by 10-15%.

I like the Viggen, it more or less fits the stike frame and the Swedes wanted to use the RR Medway before it was cancelled. However the initial version was a dedicated strike aircraft and the fighter version was a separate development that didn't reach IOC until 1979 which is 5 years too late.

As for the 2 engine option, the UKVG looks iike a good bet although RB199 analogues seem a to lack thrust for a 45,000lbs fighter but by 1975 the Spey was a step behind what was possible.

Not sure whether we want the Lighting replacement by 1977, or for 1975 - the title and 1st post specify two different years.

After carefuly reading my post (English is not my 1st language, I can barely understand it, let alone write in English), one can note that I've said 'The Viggen equivalent in weight, size and number of engines', not 'RAF has a 100% copy of Viggen and that's it'. Nothing prevents the British designing the equivalent of Viggen in weight, size and number of engines to have canted air intakes, not to be canard-delta, and/or to have the raised cockpit.
WIth regard to the engines - yes, RB.199 are not the best, however the dedicated fighter with 1 crew member and no swing wing is to be smaller and lighter than the Tornado ADV, and with better power to weight ratio than the F-18A or C in 1980s.
I'd advise start the job on the EJ 200 equivalent ASAP, though.

 

[Riain]

With the cancellation of the TSR-2 and P.1154 IOTL, the RAF was left with three major capability gaps: home long-range air defense, for which the short-legged Lightning was becoming increasingly incapable of and was covered by first the F-4K and then the Tornado ADV; deep strike and interdiction, which was eventually filled by the Tornado after the cancellation of the F-111K and with the Buccaneer filling in on an interim basis; and close air support, which was filled by first the F-4M and then the Jaguar.

TTL the deep strike role is covered by the TSR-2, while close air support will probably be filled by a combination of Harriers and Buccaneers in the absence of the Jaguar. That means this aircraft will be filling the Phantom role in home air defense, and probably should be covering the air defense duties in Germany, with a requirement for 10 frontline squadrons leading to around 250 aircraft.

Tornado/Eagle sized (Yes, the two are about the same size). The range requirements for CAP operations over the North Sea are enormous: 2 hours 560-740 kilometers from base. That's something even the Tomcat would have trouble with, with the caveat that I have no idea what armament and drop tank loads the Tornado needed to achieve that kind of interception range. Regardless, you're going to need a large aircraft to carry all that fuel. You just don't need something the size of the Tomcat, because you don't need a radar the size of the AWG-9 nor missiles the size of the Phoenix.

Of course, there is the temptation to do two aircraft, one a heavier interceptor for the North Sea coverage and a smaller single-engine aircraft for operations in Europe and Southern England, but I don't think the Brits can shoulder the development costs of two aircraft, especially with all the competition in both categories in the overseas export market.

I'd expect a swing wing in this era, especially with the obvious connections to the AFVG project.

For something the size of the Eagle or Tornado, and is expected to tangle with Soviet fighters over Germany, I think the most likely option is a development of the Harrier's Pegasus engine. The Pegasus 11 was due to enter service in 1974 and it could deliver 21,000 lbs of dry thrust; strip out the thrust vector components and add an afterburner and you have an engine comparable to the F100.

Then again, it might be easier to develop a wholly new engine that's essentially a larger RB.199, but for this you want something in the F100 class.

France, despite their desire for a similar interceptor aircraft, are unlikely given how opposed Dassault was to participating. Without the Tornado project they might be able to talk Italy into contributing; Italy was working on the F-104S at the time and this could replace it, with a potential strike variant a la the Strike Eagle down the line to fully cover the gap left by the Tornado. And who knows, maybe they get the foot in the door for the Japanese program that led to the F-15J. I think that one's a long shot, though.

The Saudis seem a prime candidate to buy the aircraft, and with the aircraft being more Eagle-like than the Tornado ADV could very well buy more in place of their early Eagle purchases. If a strike variant can be developed that opens even more doors: Germany, Australia, and Spain all spring to mind as potential candidates.

I've gone over the range above, but the Mach 2.2 of the Tornado ADV and its 4 Skyflash/4 Sidewinder armament is likely to be replicated as well for the air defense version.

Speaking of, I would expect four variants: two-seat interceptor, single-seat fighter, two-seat deep strike, and two-seat EW.

In this TL CAS is done by Lightnings, which get more development than OTL, and are produced instead of the Hunter FGA9/FR10 conversions. The RAF doesn't get the Buccaneer, the RN is keeping theirs and the TSR2 is replacing the Canberras and some V bombers. They also get the P1127 Harrier a touch earlier.

I suppose the Tornado is the same size as the F15, but with all that power it is in a different performance class. IOTL with the cancellation of the conventional carriers the Tornado ADV also had to patrol right out over the GIUK Gap in 4 hour sorties, however ITTL the RN still has it's own fighters so the requirement isn't quite so drastic. However a 3 hour sortie time over the North Sea and southern Norway, operations in the Near and Far East as well as hard turning and burning from bases in western West Germany would be required. 4 Big AAMs would be the minimum armament, perhaps Sparrows or developments of Red Top but I'd imagine they'd be supplemented by a dogfighting missile like Taildog/SRAAM.

I agree with Swing Wing, it was very trendy at the time and F100 class engines. Is a Spey development good enough in 1975? It would save a lot on development costs and the Alison TF41 version already has 14,500-15,000lbs dry thrust compared to the ~12,000lbs dry of the Mk202/203 so with that as the base it should produce F100 levels of power in afterburner.

Italy is a potential minor development partner, but the F104S reached IOC in 1971 which is a touch early. I don't know much about the Italian Air Force, can they wait until 1977 to get their new fighter? I agree that Saudi Arabia might be an export customer, as too might Australia and Canada who were early adopters of the Hornet, but I'd think Britain would need a big customer in the bag to take on development without a partner.

 

[Riain]

If they can get others to commit to buying it then it's doable, even if you have to sub contract some of the work. The thing is Britain has to keep control of the project not have it end up in international committee hell, which just adds cost.

I agree, they need a decent bit more than a mere 50% so they don't end up with OTL Jaguar where the partner had their own planes as competition or Tornado where Germany got 42.5% of the workshare based on 600 planes but then dropped it's order to 334. For example if they sign Italy up as a partner for 150 planes with a 40% share and they drop down to say 80 planes they sill cover up to ~30% of the development bill and provide a launch customer.

 

[Riain]

Not sure whether we want the Lighting replacement by 1977, or for 1975 - the title and 1st post specify two different years.

After carefuly reading my post (English is not my 1st language, I can barely understand it, let alone write in English), one can note that I've said 'The Viggen equivalent in weight, size and number of engines', not 'RAF has a 100% copy of Viggen and that's it'. Nothing prevents the British designing the equivalent of Viggen in weight, size and number of engines to have canted air intakes, not to be canard-delta, and/or to have the raised cockpit.
WIth regard to the engines - yes, RB.199 are not the best, however the dedicated fighter with 1 crew member and no swing wing is to be smaller and lighter than the Tornado ADV, and with better power to weight ratio than the F-18A or C in 1980s.
I'd advise start the job on the EJ 200 equivalent ASAP, though.

In the early/mid 60s the British planned to support the Lightning until 1977, so it needs to be on its way out by then. IOTL the Jaguar reached fighting squadrons in 1974, so GR sqns could hand their Phantoms to Fighter sqns who could get rid of their Lightnings, by 1977 this process was complete. We need the new fighter to be in squadron and wing service by 1977, not starting to trickle off the production line, it needs to be more Jaguar schedule rather than Tornado schedule, hence the 1975 date for initial Operational Capability: the first Squadron.

I'm on board with the idea that Britain develops the Viggen with Sweden, as long as the fighter version is available in 1975 rather than 1979 as per OTL. However duplicating the Viggen more or less with only 200 planned is beyond Britain's financial (but not technical) resources to go it alone.

If a Spey sized engine can fit into a Tornado sized aircraft that has been designed to dogfight from the outset that's a good combination.

 

[CV12Hornet]

I like the "Pegasus-based afterburning engine" idea very much. Swing wing by early 1970s wasn't the only game in town, specifically I'd avoid it. Eagle was much bigger than IDS Tornado (so was the F-14), though the ADV Tornado upped the length by 10-15%.

It really isn't. The empty weights are damn near identical.

I suppose the Tornado is the same size as the F15, but with all that power it is in a different performance class. IOTL with the cancellation of the conventional carriers the Tornado ADV also had to patrol right out over the GIUK Gap in 4 hour sorties, however ITTL the RN still has it's own fighters so the requirement isn't quite so drastic. However a 3 hour sortie time over the North Sea and southern Norway, operations in the Near and Far East as well as hard turning and burning from bases in western West Germany would be required. 4 Big AAMs would be the minimum armament, perhaps Sparrows or developments of Red Top but I'd imagine they'd be supplemented by a dogfighting missile like Taildog/SRAAM.

Mirroring my own thoughts. Still, even with the lesser range requirements 3-hour sortie time is pretty hefty range, especially when lugging four Sparrow-size missiles and a couple of short-range heatseekers. There are light and middleweight fighters in this time period that can do one or the other; there really isn't any that can do both that's not a heavy fighter like the Eagle.

I agree with Swing Wing, it was very trendy at the time and F100 class engines. Is a Spey development good enough in 1975? It would save a lot on development costs and the Alison TF41 version already has 14,500-15,000lbs dry thrust compared to the ~12,000lbs dry of the Mk202/203 so with that as the base it should produce F100 levels of power in afterburner.

I don't think so. The Spey's origins as a civilian engine made it less than optimal as a fighter engine, as it was unable to take the same compressor outlet temperatures as a military-grade engine. This prompted a reduced top speed in the F-4K, for example. This isn't nearly as bad a problem as the TF30's compressor stall problem, but it is a headache that any version of the Spey has to deal with.

Italy is a potential minor development partner, but the F104S reached IOC in 1971 which is a touch early. I don't know much about the Italian Air Force, can they wait until 1977 to get their new fighter? I agree that Saudi Arabia might be an export customer, as too might Australia and Canada who were early adopters of the Hornet, but I'd think Britain would need a big customer in the bag to take on development without a partner.

IOTL the Tornado originated from a study conducted between Germany, Italy, the Netherlands, Canada, and Belgium for an F-104 replacement, Britain joining in later. Belgium dropped out after France made them an offer they couldn't refuse for Mirage 5s. Canada dropped out due to political concerns, namely that the aircraft was too optimized for Western Europe operations. And the Netherlands pulled out after the Tornado got too sophisticated for their tastes.

I'd really love to know where I could find more information on Canada's decision, as that seems like the most likely of the lost partners to pick up. I suspect Canada's objection was related to the Tornado being a strike aircraft where Canada needed a NORAD interceptor, with also concerns about how much workshare would've gone to Germany and Italy. If so Canada seems a very likely partner in this scenario; without Germany as a senior partner more workshare can go to Canada and the fighter focus of this aircraft would mean it would be better suited to Canadian requirements.

I do agree that looking at the timing this is unlikely to get in on the F-104S buy. Pity.

 

[tomo pauk]

In the early/mid 60s the British planned to support the Lightning until 1977, so it needs to be on its way out by then. IOTL the Jaguar reached fighting squadrons in 1974, so GR sqns could hand their Phantoms to Fighter sqns who could get rid of their Lightnings, by 1977 this process was complete. We need the new fighter to be in squadron and wing service by 1977, not starting to trickle off the production line, it needs to be more Jaguar schedule rather than Tornado schedule, hence the 1975 date for initial Operational Capability: the first Squadron.

I'm on board with the idea that Britain develops the Viggen with Sweden, as long as the fighter version is available in 1975 rather than 1979 as per OTL. However duplicating the Viggen more or less with only 200 planned is beyond Britain's financial (but not technical) resources to go it alone.

If a Spey sized engine can fit into a Tornado sized aircraft that has been designed to dogfight from the outset that's a good combination.

British can offer the Swedes a deal: okay, we will do and re-do the things we need on a fighter version, while you will supply the bits and pieces that don't need the change. If both parties act swiftly and in co-operative manner (unlike how many times the things went with French), RAF can have a new fighter on time and on budget.
Spey-sized engine should fit on a Tornado-sized aircraft, two engines will not fit that easy. But then, the Spey-Phantoms were not that bigger than Tornado ADV...
As before - a fighter for second half of 1970s powered by one Spey nets the RAF the fighter at least as capable as Mirage 2000, F-16 (whether the 'classic' or XL, but with a good radar) or a Viggen - not too shabby at all. And you can sell it well abroad, making the RAF purchase cheaper.

 

[tomo pauk]

It really isn't. The empty weights are damn near identical.

I've commented the size, not weights.