Why did first generation jets with fuselage mounted engines have such long jet pipes?

[z42]

If one looks at drawings of first generation jet fighters with the engine mounted internally in the fuselage, like the F-86, one thing that strikes me a bit strange is that the engine is mounted quite centrally, with a long exhaust pipe going from the back of the engine to the tail of the aircraft.

Similar for same generation USSR fighters like Mig-15 and Mig-17.

Or a very early French prototype using Jumo 004 engine:

One could argue this is for balance, but, well, just move the wings backward? Which is what happened for the next generation, like Mig-21 or Mirage III:

 

[bf109xxl]

Jumo 004

R13-300 (MiG-21MF engine)

AFAIK, long exhaust nozzles were necessary to prevent incomplete expansion of the jet stream. The exhaust nozzle of the R13-300 looks quite long, by the way. Compare with the non-afterburn modification (R95Sh):

 

[GrauGeist]

First generation jet engines had a much lower thrust-to-weight ratio, so they had to be mounted where it would benefit the aircraft's CoG.

Even the He178 had it's HeS3 mounted near the center of the airframe.

 

[SplitRz]

If one looks at drawings of first generation jet fighters with the engine mounted internally in the fuselage, like the F-86, one thing that strikes me a bit strange is that the engine is mounted quite centrally, with a long exhaust pipe going from the back of the engine to the tail of the aircraft. View attachment 742025

Similar for same generation USSR fighters like Mig-15 and Mig-17.

Or a very early French prototype using Jumo 004 engine: View attachment 742026

One could argue this is for balance, but, well, just move the wings backward? Which is what happened for the next generation, like Mig-21 or Mirage III: View attachment 742027

True first gen jet fighters generally had very SHORT jet pipes! Think ME262, Gloster meteor, De Havilland Vampire. It was a deliberate part of the designs to mount the jets on the wings or within the nacelle of the twin boom to keep that jet pipe as short as possible to limit thrust loss. The aircraft already had marginal power as it was.

On the Vampire in particular:

"The low power output of the early jet engines had meant that only twin-engined aircraft designs were considered to be practical; as more powerful jet engines were quickly developed, particularly Halford's H.1 (later known as the de Havilland Goblin), the practicalities of single-engined jet fighter were soon realised.[4] de Havilland was approached to produce an airframe for the H.1 as insurance against Germany using jet bombers against Britain; this was considered more important than de Havilland's suggestion of a high-speed jet bomber.[4][5] Its first design, the DH.99, was set out in a brochure dated 6 June 1941; it was an all-metal, twin-boom, tricycle undercarriage aircraft armed with four cannon. The use of a twin boom enabled the jet pipe to be kept relatively short, which avoided the power loss that would have occurred if a long pipe was used, as would have been necessary in a conventional fuselage. It also put the tailplane clear of interference from the exhaust."

 

[z42]

Jumo 004
View attachment 742041

R13-300 (MiG-21MF engine)
View attachment 742042
AFAIK, long exhaust nozzles were necessary to prevent incomplete expansion of the jet stream. The exhaust nozzle of the R13-300 looks quite long, by the way. Compare with the non-afterburn modification (R95Sh):
View attachment 742043

I'm not sure I understand the point about incomplete expansion? If you want to change the expansion of the jet stream, you need a divergent or convergent nozzle. A straight pipe like on the F-86 does nothing except eats thrust via skin friction, no? Or well, given the engine is where it is, it prevents the tail of the plane from catching on fire, which one presumes is useful.

But yes, good point about afterburning engines, obviously they need sort of a jet pipe for the afterburner.

 

[Engineman]

The very early jets were not powerful and the main aim seems likely to have been CofG and weight saving. If that put the engine in the middle of the fuselage, they usually had a long jetpipe to the tail.
Afterburners need a certain length for full combustion. Short afterburners need good design.

Eng

 

[z42]

First generation jet engines had a much lower thrust-to-weight ratio, so they had to be mounted where it would benefit the aircraft's CoG.

Even the He178 had it's HeS3 mounted near the center of the airframe.

Yes, but it seems while the next generation fighters had engines with significantly improved thrust/weight, they didn't use this improvement to install lighter weight engines, but rather to go faster. If you compare the weight of the engine as a fraction of gross weight of the plane, you'll see it's fairly similar in F-86, Mig-21 and Mirage III (I didn't do an exhaustive check, just a few representative aircraft). Still the later two aircraft managed to place the engines at the rear end of the airframe (well, they had afterburners so the engines were longer than the F-86 one, but still..).

 

[GrauGeist]

The MiG-21's engine had a thrust-to-weight ratio of 5.6 with a weight of 2,600 pounds, the MiG-15's engine was 3.1 with a weight just under 2,000 pounds.

So you can see in this comparison, the MiG-21's engine was almost twice the power while only being a little over a quarter ton heavier.

With the advent of more powerful engines, the engineers were able to move the engines further back.

As I noted earlier, the first-gen jet engines dictated a narrow area where they could be mounted in order to preserve CoG. Even the Me262 required modifications to it's wings in order to keep the weight of the Jumos within that zone.

 

[z42]

With the advent of more powerful engines, the engineers were able to move the engines further back.

I don't understand how more powerful engines would allow you to disregard CoG considerations.

Just by eyeballing, on the newer generation as they moved the engines further back, they also moved the center of lift backwards. So the question is, why couldn't they have done the same on earlier generation aircraft?

As I noted earlier, the first-gen jet engines dictated a narrow area where they could be mounted in order to preserve CoG. Even the Me262 required modifications to it's wings in order to keep the weight of the Jumos within that zone.

IIRC the problem with the Me262 was that the engines were heavier than expected, thus moving the CoG of the entire aircraft, so they had to modify the wing sweep to get the plane into balance.

But that doesn't mean you can't design an aircraft with a heavy weight at one end, but you have to take that into account during the design process. Single engine propeller aircraft, or for that matter newer jet fighters, being examples of this.

 

[Engineman]

The technology of Aerodynamics, Thermodynamics, Materials, Structures and Design etc, etc only progresses at a certain rate. Critical to Fast-Jet development has been the development of higher power to weight engines that allow the development of all the other design features and performance.
It is a bit like saying, why didn't George Stevenson build The Flying Scotsman instead of the Rocket, after all, it's just another steam engine.

Eng

 

[drgondog]

Pretty much locate engines near the CG for Jet aircraft. In the examples given above, the engine location for Delta wing is further aft to accomodate the aerodynamic center change from mid fuselage conventional wing design.

 

[z42]

The technology of Aerodynamics, Thermodynamics, Materials, Structures and Design etc, etc only progresses at a certain rate. Critical to Fast-Jet development has been the development of higher power to weight engines that allow the development of all the other design features and performance.
It is a bit like saying, why didn't George Stevenson build The Flying Scotsman instead of the Rocket, after all, it's just another steam engine.

Eng

Yes, that is exactly my question. Why did they need to place the engine close to the middle of the fuselage with a long jet pipe, instead of at the rear end like later ones? What understanding aerodynamics or something else prevented that?

Clearly they were aware of the disadvantages of a long jet pipe, see e.g. the quote above about the slightly peculiar layout of the Vampire.

I'm not asking why the F-86 didn't reach Mach 2 with a 130 kN engine like the F-16.

 

[Engineman]

Yes, that is exactly my question. Why did they need to place the engine close to the middle of the fuselage with a long jet pipe, instead of at the rear end like later ones? What understanding aerodynamics or something else prevented that?

Clearly they were aware of the disadvantages of a long jet pipe, see e.g. the quote above about the slightly peculiar layout of the Vampire.

I'm not asking why the F-86 didn't reach Mach 2 with a 130 kN engine like the F-16.

As I said, I think the greater power-to-weight engines made the development of rear mounted engines configuration possible and advantageous, but only as the technology of the whole machine became available.

Eng

 

[EwenS]

The Blackburn Buccaneer designed in the 1950s had the engines mounted ahead of the wing spar with exhaust at the rear of the wing.

 

[GrauGeist]

It comes down to how much thrust the aircraft's engine can produce.

If you have a powerful enough engine, you can make anything fly, like the F-104, which had a glide ratio that was comparable to a cinder block.

Early jet aircraft were completely reliant on airspeed for control authority unlike their prop-powered counterparts. With the much lower thrust-to-weight ratio, weight distribution was a critical factor for controlled flight.

Even small design features were critical, like the Me262's original design with conventional gear (tail dragger). It could not get airborn unless the brakes were applied at a certain speed to get the nose down, which evened the airflow over the wings, allowing to get airborn.

However, if the Me262 had been equipped with two GE TF37 engines instead of the Jumo004, this may not have been an issue: the TF37 weighs 735 pounds with a TtW ratio of 6.6 and produces 4,550 lb/ft of thrust while the Jumo004 weighs 1,585 pounds with a TtW ratio of 1.25 and produced 1,980 lb/ft of thrust.

And to touch back on the F-104 for a moment: a single F-104 engine's maximum thrust was the equivellant of three Me262s.

 

[pbehn]

Everything about the earliest jet engines was a question mark. They were inclined to vary hugely in size, weight and performance. If you mount the engine in the fuselage with the exhaust at the rear of the aircraft then the tail pipe length has to be long and variable. Other deigns took a different route.

 

[nuuumannn]

I don't understand how more powerful engines would allow you to disregard CoG considerations.

In later aircraft beyond the F-86, the nose of the aircraft is crammed full of avionics, radars and so forth, counterbalancing the weight of the engine so it can be placed further back.

 

[GreenKnight121]

The Blackburn Buccaneer designed in the 1950s had the engines mounted ahead of the wing spar with exhaust at the rear of the wing.

View attachment 742207


View attachment 742208

As did the A-6 Intruder (first flight 19 April 1960) - although the engine & pipe were actually under the spar:

Of course, there was another reason for the placement... the design was originally a STOL one - with tilting exhaust pipes. Only the first 4 prototypes had them, while the next 4 developmental aircraft had provisions for them, but fixed pipes. All production aircraft had no provisions for the tilt-pipes.

Fixed shorter pipes:

 

[SaparotRob]

I never knew that.

 

[elbmc1969]

If one looks at drawings of first generation jet fighters with the engine mounted internally in the fuselage, like the F-86, one thing that strikes me a bit strange is that the engine is mounted quite centrally, with a long exhaust pipe going from the back of the engine to the tail of the aircraft.

One could argue this is for balance, but, well, just move the wings backward? Which is what happened for the next generation, like Mig-21 or Mirage III:

Lack of all-flying tails? With the CoG and CoL so close to the elevators, they don't get much of a lever advantage. For aircraft without hydraulic boost, this would be even more if a problem.