Adour versus J85

[tomo pauk]

Does anyone know why the Adour was so darned heavy?

 

[tomo pauk]

Is it?

If it is, then it's likely because it's a scaled down version of the original RB.172 which was closer to RB.153 or RB.199 or M.45 in size.

Two J85s were lighter than one Adour (both reheated versions) by 200 kg/450 lbs; single non-reheated Adour weighted almost as much as two reheated J85s. One old-tech Avon weighted 300 kg less than the two Adours, while making same thrust as these two combined.
Yes, I'm comparing turbofans vs. turbojets, here, but anyway the weight/thrust discrepancy is amazing.

We can take a look at the RB.199, too. Weighted 20% more than the reheated Adour (976 vs. 810 kg) while making 80-100% more on reheat and 35-50 % 'dry'.

The old Bristol Orpheus also had an amazing thrust to weight ratio, perhaps it is too bad it wasn't developed into at a 'leaky turbojet' version? Development of the Pegasus shows the untapped potential of the Orpheus IMO.

 

[overscan (PaulMM)]

Turbofans are always going to be heavier than turbojets. The benefit is increased fuel efficiency.

J85 was designed for light weight and initially short duration disposable use. It had high thrust to weight but also high specific fuel consumption. Orpheus was somewhat similar.

Adour was designed to be low risk, rugged, reliable, and reasonably efficient on fuel. T/W ratio was about 4.75 which isn't far off the Spey (5:1). Some versions had limited reheat only.

J85 reached 8:1 T/W ratio with later versions but was a very different engine.

The Adour was initially designed well within the limits of technology available when its specification was formulated. The aim was to produce a relatively simple and rugged design, with modest operating temperatures, capable of performing reliably under the demanding conditions required for training and for the low level strike role.

[..]
The Adour engine cycle was chosen to meet the demanding requirements of ground attack and advanced training. A low bypass ratio of 0.75 to 1 was chosen to combine a small frontal area with the good fuel economy of the turbofan, and to provide the capability for substantial thrust increases through afterburning. Also, the traditional turbofan thrust lapse rate with altitude is minimized at this bypass ratio, providing good climb performance. Moderate operating temperatures and pressures were specified and combined with rugged construction to provide good reliability, high resistance to FOD, and considerable potential for thrust growth. Substantial improvements in maintainability were provided by grouping all major accessories on the underside to provide easy access, and by adopting modular construction.

Source: The Rolls-Royce/Turbomeca Adour The Optimum Trainer Engine
MICHAEL FERGUSON
Rolls-Royce Inc. Washington, D.C.

 

[tomo pauk]

Turbofans are always going to be heavier than turbojets. The benefit is increased fuel efficiency.

J85 was designed for light weight and initially short duration disposable use. It had high thrust to weight but also high specific fuel consumption. Orpheus was somewhat similar.

J85 reached 8:1 T/W ratio with later versions but was a very different engine.

The J85s powering manned aircraft were produced for 'normal' service lives, and still were very light.
J85-GE-13, that powered the F-5A, was making 4080 lb of thrust at A/B, weighting 579 lbs; T/W ratio of 7:1. 1st flight of the F-5A happened 9 years before Jaguar's 1st flight.

Yes, turbofans will have better fuel efficiency. Unfortunately, heavier engines will drive up the weight and size of aircraft designed around them, weight and size tend to eat up the fuel savings provided by engine type. Growth in size and weight also increases the price of an aircraft, not a good thing when commercial success means a lot. Take-off runs are also increased.

The Adour was initially designed well within the limits of technology available when its specification was formulated. The aim was to produce a relatively simple and rugged design, with modest operating temperatures, capable of performing reliably under the demanding conditions required for training and for the low level strike role.

[..]
The Adour engine cycle was chosen to meet the demanding requirements of ground attack and advanced training. A low bypass ratio of 0.75 to 1 was chosen to combine a small frontal area with the good fuel economy of the turbofan, and to provide the capability for substantial thrust increases through afterburning. Also, the traditional turbofan thrust lapse rate with altitude is minimized at this bypass ratio, providing good climb performance. Moderate operating temperatures and pressures were specified and combined with rugged construction to provide good reliability, high resistance to FOD, and considerable potential for thrust growth. Substantial improvements in maintainability were provided by grouping all major accessories on the underside to provide easy access, and by adopting modular construction.

We're yet to encounter a manufacturer that will criticize his product.
This (my bold):
"Also, the traditional turbofan thrust lapse rate with altitude is minimized at this bypass ratio, providing good climb performance."

Climb performance is directly related to the thrust/weight ratio, a thing where neither Adour nor Jaguar excelled.
Also this:
"and considerable potential for thrust growth."

Part of that potential was realized after few redesigns during the last 55 years. Calling it 'considerable' is a bit too much.

 

[BLACK_MAMBA]

The J85s powering manned aircraft were produced for 'normal' service lives, and still were very light.
J85-GE-13, that powered the F-5A, was making 4080 lb of thrust at A/B, weighting 579 lbs; T/W ratio of 7:1. 1st flight of the F-5A happened 9 years before Jaguar's 1st flight.

Yes, turbofans will have better fuel efficiency. Unfortunately, heavier engines will drive up the weight and size of aircraft designed around them, weight and size tend to eat up the fuel savings provided by engine type. Growth in size and weight also increases the price of an aircraft, not a good thing when commercial success means a lot. Take-off runs are also increased.

So more than enough time for any competant engineering team to have noticed that they were complete idiots in wanting to use a new turbofan - or it actually provided them with benefits to validate using it?

Nearly all the engine manufacturers by the late 60's to the mid 70's started working almost exclusively on turbofans for new designs while only updating older turbojets as technology advanced (there will probably be single exceptions). So either the turbofan has benefits over turbojets, or the thousands of engineers were just wrong in their understanding of the advantages.

 

[BLACK_MAMBA]

??

If a small jurbojet and airframe really was obvious an advantage of a design path 9 years is more than enough time for Rolls Royce, Turbomeca, Breguet and BAC to have taken notice and develop their engine and airframe concepts accordingly. The fact that their design was thrust into a role it was never really designed for as its primary mission is another story all together.

 

[tomo pauk]

If a small jurbojet and airframe really was obvious an advantage of a design path 9 years is more than enough time for Rolls Royce, Turbomeca, Breguet and BAC to have taken notice and develop their engine and airframe concepts accordingly.

I'd prefer a design powered by turbofans of better T/W ratio. Or by the 'leaky turbojet' sibling to the Orpheus.

The fact that their design was thrust into a role it was never really designed for as its primary mission is another story all together.

By French in the early 1960s it was designated for the roles of "École de Combat et d'Appui Tactique", or literary "Trainer for combat and for tactical support". Lugging bombs, rockets and guns was in it's description.

 

[BLACK_MAMBA]

By French in the early 1960s it was designated for the roles of "École de Combat et d'Appui Tactique", or literary "Trainer for combat and for tactical support". Lugging bombs, rockets and guns was in it's description.

Hency why I stated "primary mission". A armed trainer is very different in requirement to a dedicated strike platform which the jaguar became upon entering service. Look at the Hawk. Good combat trainer with tactical support capability, in no way a dedicated strike platform. Poor comparison in terms of size and power yes, but combat trainer usually means comparatively light loads - not lots of large heavy bombs and tanks on long missions with lots of extra ECM toys to try and protect you. The latter will always be a compromise if starting with an advanced jet trainer as your primary mission.

 

[tomo pauk]

Poor comparison in terms of size and power yes, but combat trainer usually means comparatively light loads - not lots of large heavy bombs and tanks on long missions with lots of extra ECM toys to try and protect you. The latter will always be a compromise if starting with an advanced jet trainer as your primary mission.

Please note the "d'Appui Tactique" (tactical support) half of the job description.

 

[BLACK_MAMBA]

Please note the "d'Appui Tactique" (tactical support) half of the job description.

Most armed trainers can provide tactical support if required. Both the Hawk and Alpha Jet were designed with such a secondary mission if Europe had to go to war. Unless the actual requirements were comparable to those of a dedicated strike platform (which I highly doubt given how the program evolved but do correct me if I'm wrong) a trainer with weapons capability would have surficed for that role.

A project title and the requirements listed under that title can be very different. The one is often given as marketing and can be overstated.

 

[Archibald]

That "appui tactique" is closer from what is called Close Air Support in english. And this is different from the long range strike role the Jaguar drifted toward, according to the many aborted British programs: TSR-2, F-111, then waiting for the Tornado.
A trainer can do CAS, but will have difficulty doing long range strike. For example, the Jaguar trainer official successor in France - with Germany - was pushed by this country toward the CAS mission. But an Alphajet would never do what the French Jaguars did in Africa or in GW1.

CAS is not MRI.

 

[Archibald]

That debate underlines how the Jaguar project ddrifted between 1964 and 1969. The Adour (like the Larzac after it) was created as small turbofan for a trainer, and that trainer was the original Jaguar.
Jaguar kept the smallish Adours even if it drifted toward the attack mission. Still, the Adour was a subscale variant of the RB.172, which was the engine of the true strike platform in the May 17th 1965 anglo-french agreement: the AFVG.
A case could be make that when the AFVG was screwed, Jaguar could have had one RB.172 instead of two Adours. But it was too far long.

So yes, the Jaguar was a weird compromise: a supersonic trainer that had drifted toward a long range strike platform but still kept smallish trainer engines.

 

[tomo pauk]

A case could be make that when the AFVG was screwed, Jaguar could have had one RB.172 instead of two Adours. But it was too far long.

Is there some good data about the RB.172 online?

 

[Archibald]

Also known as M45 on the french side... and it is absurdly difficult, indeed, to find any data about that one.

 

[overscan (PaulMM)]

G

J85 reached 8:1 T/W ratio with later versions but was a very different engine.

The J85s powering manned aircraft were produced for 'normal' service lives, and still were very light.
J85-GE-13, that powered the F-5A, was making 4080 lb of thrust at A/B, weighting 579 lbs; T/W ratio of 7:1. 1st flight of the F-5A happened 9 years before Jaguar's 1st flight.

2 x J85-13 weighed 1,195lb and produced 8,160lb thrust in afterburning, which is 6.8 thrust/weight ratio, with SFC 1.26 in dry thrust
1 x Adour Mk 102 weighed 1,552lb and produced 7305lb thrust in afterburning, which is 4.7 t/w ratio, with SFC 0.75 in dry thrust
1 x Adour Mk 104 weighed 1572lb and produced 8040lb thrust in afterburning, which is 5.1 t/w ratio, with SFC 0.75 in dry thrust

So for 377 extra pounds, you get similar maximum thrust, but your dry SFC is so much lower that it will make a big difference for range in a strike aircraft mission. You'll actually save weight on the fuel needed for the mission.

For a short range lightweight air-air fighter, the J85 is a better option.
For Jaguar, the Adour is a better option.

 

[tomo pauk]

2 x J85-13 weighed 1,195lb and produced 8,160lb thrust in afterburning, which is 6.8 thrust/weight ratio, with SFC 1.26 in dry thrust
1 x Adour Mk 102 weighed 1,552lb and produced 7305lb thrust in afterburning, which is 4.7 t/w ratio, with SFC 0.75 in dry thrust
1 x Adour Mk 104 weighed 1572lb and produced 8040lb thrust in afterburning, which is 5.1 t/w ratio, with SFC 0.75 in dry thrust

You are indeed right wtr. the wight of the -31.
2 x J85-21 weighted 1278 lbs, thrust in A/B of 10000 lbs, the t/w ratio of 7.8.

So for 377 extra pounds, you get similar maximum thrust, but your dry SFC is so much lower that it will make a big difference for range in a strike aircraft mission. You'll actually save weight on the fuel needed for the mission.

There are two engines on a Jaguar. Fuel savings will not happen (especially vs. the -13), since those will consume more due to the greater thrust and needing to power a draggier and heavier aircraft. Jaguar carried a lot of fuel.

For a short range lightweight air-air fighter, the J85 is a better option.
For Jaguar, the Adour is a better option.

A 'not Jaguar' designed around two J85s can been smaller and lighter than the historical Jaguar, if not exactly the size & weight of F-5.

 

[overscan (PaulMM)]

That is literally not how specific fuel consumption works. It is a dimensionless measure, showing the efficiency of converting fuel energy into thrust. J85 is great at producing lots of thrust but it does it inefficiently. Adour is less good at generating high levels of thrust but does it much more efficiently. A modern turbofan like the F404 or F100 basically has the thrust-to-weight of a J85-21 combined with fuel efficient operation like an Adour, but it required lots of expensive research and engineering, massive increases in operating temperature and exotic materials.

When operating in dry thrust, a J85 type engine burns about 68% more fuel than an Adour for the same amount of thrust. Now, its possible that most efficient cruise SFC is a little different, but the broad ratio will be the same.

If you wanted to fly the same distance in economic cruise, the J85-engined aircraft would need a lot more fuel, the weight of which (and the structure to carry it) would almost certainly outweigh the weight and drag penalty of the heavier Adour.

This analysis is simplified, but the point is the J85 engine isn't magic, and the Adour isn't an incompetent design - they are each optimised for different things.

Tthings are never as simple as 'armchair analysis' suggests.

 

[KHambsch]

Is it?

If it is, then it's likely because it's a scaled down version of the original RB.172 which was closer to RB.153 or RB.199 or M.45 in size.

Two J85s were lighter than one Adour (both reheated versions) by 200 kg/450 lbs; single non-reheated Adour weighted almost as much as two reheated J85s. One old-tech Avon weighted 300 kg less than the two Adours, while making same thrust as these two combined.
Yes, I'm comparing turbofans vs. turbojets, here, but anyway the weight/thrust discrepancy is amazing.

We can take a look at the RB.199, too. Weighted 20% more than the reheated Adour (976 vs. 810 kg) while making 80-100% more on reheat and 35-50 % 'dry'.

The old Bristol Orpheus also had an amazing thrust to weight ratio, perhaps it is too bad it wasn't developed into at a 'leaky turbojet' version? Development of the Pegasus shows the untapped potential of the Orpheus IMO.

Guess the trade space was filled where weight vs SFC was concerned....did size go into the calculus of the design?

 

[KHambsch]

Is it?

If it is, then it's likely because it's a scaled down version of the original RB.172 which was closer to RB.153 or RB.199 or M.45 in size.

Two J85s were lighter than one Adour (both reheated versions) by 200 kg/450 lbs; single non-reheated Adour weighted almost as much as two reheated J85s. One old-tech Avon weighted 300 kg less than the two Adours, while making same thrust as these two combined.
Yes, I'm comparing turbofans vs. turbojets, here, but anyway the weight/thrust discrepancy is amazing.

We can take a look at the RB.199, too. Weighted 20% more than the reheated Adour (976 vs. 810 kg) while making 80-100% more on reheat and 35-50 % 'dry'.

The old Bristol Orpheus also had an amazing thrust to weight ratio, perhaps it is too bad it wasn't developed into at a 'leaky turbojet' version? Development of the Pegasus shows the untapped potential of the Orpheus IMO.

Guess the trade space was filled where weight vs SFC was concerned....did size go into the calculus of the design? Yes, while heavier, they are not much larger in diameter ~5-6 cm and a good deal shorter with afterburner. Looking at the SFC, Adour is .8 something and J-85 is 1.2 something. That is what approx 33% more efficient?!.

Am going to make an overt edit....correction to the above turbomecca adour is over all 5 cm longer, but dry thrust 6,000 lbs vs j85 3,600 lbs
There is only ~1000 lbs difference with reheat: j85 nearly doubling thrust to 7,000 lbs to Adours 8,000 lbs.

 

[tomo pauk]

Tthings are never as simple as 'armchair analysis' suggests.

This is why I've brought out the aircraft's weight, size and drag - those are of interest for anyone making an analysis. A bigger and heavier engine will drive these up, and it will drive up the fuel spent for a mile traveled.

 

[red admiral]

When comparing the Thrust of turbofans and turbojets you need to be careful of what you are comparing and where in the flight envelope.

Actually Turbofans generally have better Thrust/weight ratio because the fan produces additional thrust for not much weight. But this difference reduces with speed and altitude.

Or you could compare thrust/ mass flow rate in which case the turbojet is better. Simply the turbofan is bigger and requires a larger intake, more drag, mass etc.

But for this thread, Adour is relatively heavy. Unclear why. The accessories are also massive when you look at them

 

[CV12Hornet]

This is why I've brought out the aircraft's weight, size and drag - those are of interest for anyone making an analysis. A bigger and heavier engine will drive these up, and it will drive up the fuel spent for a mile traveled.

Except that while the Jaguar carries 64% more fuel internally* (4200 liters versus 2560) it has almost four times the combat radius on that fuel load (440nm versus 120nm hi-lo-hi, no tanks). The greater weight and drag of the Jaguar sure seems like it mattered a lot less in the range department compared to the Adour's far lower fuel consumption.

* A number which, incidentally, is about the same as the ratio of their empty weights, so no shenanigans with fuel fractions going on.

 

[tomo pauk]

Except that while the Jaguar carries 64% more fuel internally* (4200 liters versus 2560) it has almost four times the combat radius on that fuel load (440nm versus 120nm hi-lo-hi, no tanks).

What would be the loads for these radii?

If we're already going by Wikipedia, for the Jaguar the ferry range with 3 drop tanks was supposed to be 1027 nmi "with full internal and external tanks", while the combat radius with external fuel is 760 nmi? Making the aircraft go 760 nmi in one distance, reduce the altitude, drop the bombs, climb while returning to base for another 760 nmi for total of 1520 nmi?? Almost 500 nmi greater distance covered while making a combat sortie than it is the ferry distance.

 

[CV12Hornet]

What would be the loads for these radii?

If we're already going by Wikipedia, for the Jaguar the ferry range with 3 drop tanks was supposed to be 1027 nmi "with full internal and external tanks", while the combat radius with external fuel is 760 nmi? Making the aircraft go 760 nmi in one distance, reduce the altitude, drop the bombs, climb while returning to base for another 760 nmi for total of 1520 nmi?? Almost 500 nmi greater distance covered while making a combat sortie than it is the ferry distance.

The balance of secondary sources point to the ferry range being the incorrect number: AirVectors backs up the combat radius, as do multiple other online sources, and this gives the ferry range as 3524 kilometers.

As for loads, again based on AirVectors information the most likely load Jaguar would be carrying two AS.30L missiles, an Atlis II sensor pod, and two chaff pods, for a total warload of approximately 3000 pounds. All other loadouts show use of fuel tanks.

Which, yes, is a fair bit less than the listed warload for the F-5, but not so much to account for the entire massive difference in range.

 

[tomo pauk]

The balance of secondary sources point to the ferry range being the incorrect number: AirVectors backs up the combat radius, as do multiple other online sources, and this gives the ferry range as 3524 kilometers.

As for loads, again based on AirVectors information the most likely load Jaguar would be carrying two AS.30L missiles, an Atlis II sensor pod, and two chaff pods, for a total warload of approximately 3000 pounds. All other loadouts show use of fuel tanks.

Which, yes, is a fair bit less than the listed warload for the F-5, but not so much to account for the entire massive difference in range.

Thank you for sifting through the sources.
About the ferry range:
If the empty drop tanks are retained on the F-5E, it can go 2565 km; starts with 5860L of fuel. Jaguar has a 40% greater range (3524 km), achieved with 7800L, that is 33% more fuel.
5860/2565=~2.28 L/km for the F-5E. 7800/3524 = ~2.21L km for the Jag. Difference of 3.5%.

 

[1635yankee]

One other issue that hasn't been addressed is that there may be significant differences in the two engines resistance to things like birdstrikes, which would be a very real issue in the low-level operations the Jaguar was frequently tasked with, and FOD, especially from operating off ill-prepared runways. I do not know how these engines compare, and without access to actual test and service data, it may not be possible to make the comparison.

 

[KHambsch]

One other issue that hasn't been addressed is that there may be significant differences in the two engines resistance to things like birdstrikes, which would be a very real issue in the low-level operations the Jaguar was frequently tasked with, and FOD, especially from operating off ill-prepared runways. I do not know how these engines compare, and without access to actual test and service data, it may not be possible to make the comparison.

So the trade space(s) is/are full, up and down the ladder....did we include politics? : )

 

[1635yankee]

One other issue that hasn't been addressed is that there may be significant differences in the two engines resistance to things like birdstrikes, which would be a very real issue in the low-level operations the Jaguar was frequently tasked with, and FOD, especially from operating off ill-prepared runways. I do not know how these engines compare, and without access to actual test and service data, it may not be possible to make the comparison.

So the trade space(s) is/are full, up and down the ladder....did we include politics? : )

Do you actually have an answer to the question?

 

[Hood]

Jane's of 1990-91 has the following range data for the Jaguar:

Internal fuel 4,200 litres (1,109.7 US gal; 924 Imp gal), up to three 1,200 litre (317 US gal; 264 Imp gal) drop-tanks. Refuelling probe.
Attack radius internal fuel only hi-lo-hi 460 nm (825 km; 530 miles); lo-lo-lo 290 nm (537 km; 334 miles)
Attack radius with external fuel hi-lo-hi 760 nm (1,408 km; 875 miles); lo-lo-lo 495 nm (917 km; 570 miles)
Ferry range with external fuel 1,902 nm (3,524 km; 2,190 miles)

 

[Archibald]

Just for the record, France stretched that to 10h20 minutes, 10 000 km (and counting) with countless air to air refueling, obviously.

 

[Magnum_Force]

Below is a screenshot from The HAWK Story pdf - it has figures for both the Adour and J85, as well as others:

 

[MadRat]

It is not about simply having a more efficient motor. Liquid fuels have high energy density. You want to be able to convert that fuel to energy quickly. At other times you want to use the bare minimum. Turbofans are fuel efficient compared to turbojets when similarly sized, but turbojets generally convert fuel to energy faster for similar sized motors. Turbofans used like turbojets isnt ideal because of the high drag. Turbojets cannot typically offer anywhere near the turbofans efficiency even though they have lower drag.

They've had liftjets in the 20+ twr range for 5 decades. I'm surprised they couldnt find a way to run a turbofan in cruise mode while liftjet type engines are concealed. When high speeds are needed you flip out the liftjets and run the turbofan with a simpler, less fuel hungry afterburner.

Jets like Jaguar didnt need both Adours for the whole mission. One motor slightly larger than the Adour and the liftjet style motors could have allowed a higher fineness ratio overall, using only one motor running at a better power rating for the bulk of flight. Takeoffs and combat then would depend on high twr motors to maximum burn fuel at the right time. While these motors may be deadweight when not in use, some of these motors were only around 200 pounds in weight putting out thrust comparable to the J85 wet thrust.

 

[H_K]

Jets like Jaguar didnt need both Adours for the whole mission.

I think experience showed that the Jaguar needed both Adours when cruising at altitude… during air-to-air refueling it was not uncommon to have to light up partial reheat on one engine to offset the drag at high incidence!

There’s also a story of a ferry flight by a Jaguar and a Etendard, where the Jaguar burned so much fuel in reheat to make up for the Adours low thrust that it went fuel critical before the Etendard, despite the later’s supposedly inefficient Atar turbojet…

 

[Magnum_Force]

It is not about simply having a more efficient motor. Liquid fuels have high energy density. You want to be able to convert that fuel to energy quickly. At other times you want to use the bare minimum. Turbofans are fuel efficient compared to turbojets when similarly sized, but turbojets generally convert fuel to energy faster for similar sized motors. Turbofans used like turbojets isnt ideal because of the high drag. Turbojets cannot typically offer anywhere near the turbofans efficiency even though they have lower drag.

They've had liftjets in the 20+ twr range for 5 decades. I'm surprised they couldnt find a way to run a turbofan in cruise mode while liftjet type engines are concealed. When high speeds are needed you flip out the liftjets and run the turbofan with a simpler, less fuel hungry afterburner.

Jets like Jaguar didnt need both Adours for the whole mission. One motor slightly larger than the Adour and the liftjet style motors could have allowed a higher fineness ratio overall, using only one motor running at a better power rating for the bulk of flight. Takeoffs and combat then would depend on high twr motors to maximum burn fuel at the right time. While these motors may be deadweight when not in use, some of these motors were only around 200 pounds in weight putting out thrust comparable to the J85 wet thrust.

This has been done in the civil sector, namely the Trident 3B airliner which had a extra jet in the tail for take off / climb in certain hot/high conditions. It used a modified lift engine jet - from Wikipedia
"Trident 3B, used, in addition to its three Rolls-Royce Spey turbofan engines, a centrally mounted RB.162-86 which was used for takeoff and climb in the hot prevailing conditions of the Mediterranean area. The 'boost' engine was shut down for cruising flight. Some conversion was needed for the change from vertical to horizontal installation. With the RB.162 fitted the Trident 3B had a 15% increase in thrust over the earlier variants for an engine weight penalty of only 5%"

 

[Archibald]

It is not about simply having a more efficient motor. Liquid fuels have high energy density. You want to be able to convert that fuel to energy quickly. At other times you want to use the bare minimum. Turbofans are fuel efficient compared to turbojets when similarly sized, but turbojets generally convert fuel to energy faster for similar sized motors. Turbofans used like turbojets isnt ideal because of the high drag. Turbojets cannot typically offer anywhere near the turbofans efficiency even though they have lower drag.

They've had liftjets in the 20+ twr range for 5 decades. I'm surprised they couldnt find a way to run a turbofan in cruise mode while liftjet type engines are concealed. When high speeds are needed you flip out the liftjets and run the turbofan with a simpler, less fuel hungry afterburner.

Jets like Jaguar didnt need both Adours for the whole mission. One motor slightly larger than the Adour and the liftjet style motors could have allowed a higher fineness ratio overall, using only one motor running at a better power rating for the bulk of flight. Takeoffs and combat then would depend on high twr motors to maximum burn fuel at the right time. While these motors may be deadweight when not in use, some of these motors were only around 200 pounds in weight putting out thrust comparable to the J85 wet thrust.

This has been done in the civil sector, namely the Trident 3B airliner which had a extra jet in the tail for take off / climb in certain hot/high conditions. It used a modified lift engine jet - from Wikipedia
"Trident 3B, used, in addition to its three Rolls-Royce Spey turbofan engines, a centrally mounted RB.162-86 which was used for takeoff and climb in the hot prevailing conditions of the Mediterranean area. The 'boost' engine was shut down for cruising flight. Some conversion was needed for the change from vertical to horizontal installation. With the RB.162 fitted the Trident 3B had a 15% increase in thrust over the earlier variants for an engine weight penalty of only 5%"

Wait, whaaaaaaat ? Only you british excentrics could do that ! In passing, it's amazing to think RB.162 got out of the VSTOL morasse... through an airliner ! 26 Trident 3B were build.

 

[MadRat]

That is surreal. Interesting demonstration of RB.162 utility. 5,000 pounds of thrust in <300 pounds at 18.5:1 twr in 1962.

Now can they do that on say, an interceptor?

 

[Archibald]

"Surreal", that's the word. Counting the APU, that's a five turbines airliner !

Can't help thinking: typical 1960's solution. A brute-force approach from the days oil was dirt cheap, with noise and pollution no real problems... Don't you dare to try that in 2023. (imagine a 777 with a CFM56 in the tail, DC-10 or Tristar style)

More seriously, never thought a VSTOL lift jet like RB.162 could find application on an airliner and that way. The 1960's airliners way of using lift jets was the usual VSTOL whackiness, with dozens of engines used like some kind of magical or antigravity devices. Noise, NOx pollution, CO2 emissions, peak oil ? what are you talking about ?

By this point I'm left wondering whether somebody proposed a similar scheme except with a rocket in place of the liftjet, same place: booster in the tail.

 

[Archibald]

According to Wikipedia, this was (yet another) desperate move related to a (yet another dumbarse aerospace) decision by the British government.

https://en.wikipedia.org/wiki/Rolls-Royce_RB.162

In 1966 British European Airways (BEA) had a requirement for an extended range aircraft to serve Mediterranean destinations. After a plan to operate a mixed fleet of Boeing 727 and 737 aircraft was not approved by the British Government[4] Hawker Siddeley offered BEA a stretched and improved performance version of the Trident that they were already operating. This variant, the Trident 3B, used, in addition to its three Rolls-Royce Spey turbofan engines, a centrally mounted RB.162-86 which was used for takeoff and climb in the hot prevailing conditions of the Mediterranean area. The 'boost' engine was shut down for cruising flight. Some conversion was needed for the change from vertical to horizontal installation. With the RB.162 fitted the Trident 3B had a 15% increase in thrust over the earlier variants for an engine weight penalty of only 5%.

 

[Archibald]

I've checked the internet for moar RB-162-86 and another result blew my mind. Not a Mirage III-V nor a Trident airliner but... the Space Shuttle.
Capcomespace.net, good as usual, mentions it

1970

Makes a lot of sense. The Shuttle flyback & ferry engines were a massive deadweight, so the compact and very high T/W RB-162-86 was probably very welcome.
Lift jets were specifically design to fit in very small niche, inside vehicle, and thrust a lot above their weight. Good for a VSTOL, but also of interest for a weight limited Space Shuttle...

"RB-162-86" site:ntrs.nasa.gov - Google Search

Also "A Shuttle chronology" a damn fine NASA history series from 1987. Volume 3, page 210 of the pdf.