Avon engines

[Yankee_Aviator]

My understanding is that SNECMA was weak at combustion chambers and other "hots" parts, and the solution was their partnership with General Electric over the CFM56. That way they snatched the F101 core, and it probably helped a lot when making the M88.

This is an impression I've had for awhile based on the ATAR's mediocre performance, always wondered if there was any truth to it, or just an opinion formed based on casual observations. Anyways, I wonder if a partnership with another engine company earlier on (say RR or MTU) could have helped squeeze more power out of the ATAR without increasing its weight. Again, at this point I just want to know if there is some design or materials change SNECMA could have hypothetically made to improve the ATAR 9 family's T/W (and thus the Mirage III's T/W)

 

[Archibald]

You've been told repeatedly ATAR performance was NOT mediocre...

 

[GTX]

You've been told repeatedly ATAR performance was NOT mediocre...

Indeed, with only 3600+ produced...

 

[kaiserbill]

The American engines would have required much redesign of the rear fuselage; the Aussies' opinion of them was not overly high in other respects.

I'm aware that the Kfir suffered from stability (aft CG) issues due to the J79 being shorter and heavier (more weight positioned further aft in the fuselage) and the additional cooling equipment required for it ate into the performance advantages the J79 offered. At this point I just want to know if there's anything SNECMA could have done to improve the ATAR's thrust output without increasing its weight, I want to know if they could have done anything to improve the Mirage III/F1's thrust to weight ratio.

South Africa did work on an improved ATAR 9k50.
The two major goals were:
1. Increased thrust.
2. Weight reduction.

From what has been intimated, an increase in thrust was achievable ( about 10%), but the weight reduction goals (not stated) were not achieved, at least when the project was shelved.

At that stage (late 80's), a low bypass turbofan was the better solution.

 

[BlackBat242]

I suppose I had failed to take into account another well-known possible issue with the Avon...

Taking a second look at the engine stats I have, I see that the series 300 Avons had an airflow-mass requirement of 170-175 lb/sec... while the ATAR 9C had only a 150 lb/sec requirement.

As the Mirage III's intakes were sized to provide the proper airflow-mass for the ATAR 9C, and having them unnecessarily large would increase drag... perhaps the Avon's disappointing performance could be simply due to the test aircraft's intakes having NOT been enlarged in cross-section, thus slightly starving the Avon of needed airflow-mass? Or if they were enlarged, did that significantly increase the drag?


It was the need to enlarge the intakes on the British Phantoms to satisfy the higher airflow-mass requirements of the Spey (200-210 lb/sec vs 170 lb/sec for the J79) that increased the airframe drag of the Queen's Phantoms so significantly, after all.

 

[steelpillow]

As the Mirage III's intakes were sized to provide the proper airflow-mass for the ATAR 9C, and having them unnecessarily large would increase drag... perhaps the Avon's disappointing performance could be simply due to the test aircraft's intakes having NOT been enlarged in cross-section, thus slightly starving the Avon of needed airflow-mass? Or if they were enlarged, did that significantly increase the drag?

It was the need to enlarge the intakes on the British Phantoms to satisfy the higher airflow-mass requirements of the Spey (200-210 lb/sec vs 170 lb/sec for the J79) that increased the airframe drag of the Queen's Phantoms so significantly, after all.

You would have to dig out the Dassault archives to find out if they modified the intake geometry for the Avon. It had moveable shock cones, so maybe they just adjusted the positioning. Then again, you would have to compare the altitudes, temperatures and airspeeds for which the airflow numbers were given. I am sure that R-R and the Aussies would have been aware of any such issue and would have taken it all into account.

My understanding of the RN Phantoms was that the Speys hung down at the back, and this was what stuffed the area ruling. But maybe it was a double-whammy.

 

[Archibald]

I suppose I had failed to take into account another well-known possible issue with the Avon...

Taking a second look at the engine stats I have, I see that the series 300 Avons had an airflow-mass requirement of 170-175 lb/sec... while the ATAR 9C had only a 150 lb/sec requirement.

As the Mirage III's intakes were sized to provide the proper airflow-mass for the ATAR 9C, and having them unnecessarily large would increase drag... perhaps the Avon's disappointing performance could be simply due to the test aircraft's intakes having NOT been enlarged in cross-section, thus slightly starving the Avon of needed airflow-mass? Or if they were enlarged, did that significantly increase the drag?


It was the need to enlarge the intakes on the British Phantoms to satisfy the higher airflow-mass requirements of the Spey (200-210 lb/sec vs 170 lb/sec for the J79) that increased the airframe drag of the Queen's Phantoms so significantly, after all.

Well if you are looking at a F-4K-like, "bloated by a big turbofan" Mirage III - try the III-T. It swallowed a Pratt JTF-10 / TF104 / TF106 but paid the price to it.

Another example is the Kfir. Digesting a J79 was a bit more easier but still caused a lot of issues.

Compared to the two above, never heard the Avon III-O suffered similar miseries.

A good case could be made the Mirage III / V / F1 engine bay upper growth limit is somewhere between Avon and J79.
- M53-2 was specifically designed to fit an Atar engine bay
- Avon fits barely
- J79 busted at the seams
- TF104 / 106 were clearly oversized
Asides from the specially designed M53, most of the 1960's era early tubofans (Spey, TF10/30) also busted that limit. A F404, M88 or EJ200 probably would not, but the Mirage III by this point of time was old stuff. The F1 could have benefited from them, however. But it missed the M53 train and scorned the M45 one.

 

[BlackBat242]

Its less about the size of the engine and more about the size of the intake.

Enlarging the intake cross-section to provide the greater airflow-mass changes the aerodynamics of the airframe along the whole fuselage between the intake mouth and the engine face.

The F404 and the F414 have the same engine body & afterburner diameter... but the diameter of the opening in the engine faces are different: the F404's is 27.7" and the F414's is 30.6".

The intake ducting all the way along is similarly larger for the Superhornet than for the Hornet, affecting the fuselage aerodynamic shaping.

F404 airflow-mass 142-146 lb/sec
F414 airflow-mass 169 lb/sec.

 

[steelpillow]

Its less about the size of the engine and more about the size of the intake.

and of course the intake is sized not just for the engine but for the speed and altitude at which the engine power is to be optimised. A low-altitude attack plane will have a smaller intake than a high-altitude interceptor, even with the same engine. This is the wonder of variable-geometry intakes, as standard on the Dassault Mirage.

 

[BlackBat242]

Its less about the size of the engine and more about the size of the intake.

and of course the intake is sized not just for the engine but for the speed and altitude at which the engine power is to be optimised. A low-altitude attack plane will have a smaller intake than a high-altitude interceptor, even with the same engine. This is the wonder of variable-geometry intakes, as standard on the Dassault Mirage.

The primary function of a variable-geometry intake is to slow the incoming air to subsonic speeds before it reaches the first row of turbine blades, as supersonic air does not work well entering a turbine.

Yes, most variable-geometry intakes do restrict the size of the opening to reduce the incoming airflow-mass at lower altitudes/speeds as well - but the intake itself must be sized to allow the proper amount of total airflow-mass to enter and pass through... having a variable-geometry intake will not magically compensate for a too-small intake cross-section.

 

[steelpillow]

having a variable-geometry intake will not magically compensate for a too-small intake cross-section.

I think we would all like to see the comparative figures used by Dassault in designing their conversion job. However, I am inclined to follow the Australian view that they knew what they were doing. Do you have any evidential basis for questioning their competence?