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GE J79, 8/15 vs 10/17 variants, why the change in nozzle design?


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This didn't seem appropriate for Jet Modeling or Research Corner.

 

What drove GE to design the new exhaust nozzle for the later J79 engines? (F-4J/E/S) What technical benefit was there? Did the longer feathers work better with the F-4's aerodynamics? If so, why did the F-104 get the new nozzle as well? It appears both have a similarly shaped constrictor segment.

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17 hours ago, airmechaja said:

I believe the -17's needed more cooling secondary air from the intake so the new nozzles were developed.

 

Hmm...I could understand the later power powerful engine needing more bleed air pumped through the nozzle, but from this link,

http://tailspintopics.blogspot.com/2012/12/j79-exhaust-nozzles.html

It appears the -15 nozzle has a more direct path for the bleed air than the -17.

This question concerns theorycrafting a Whiff. I'm envisioning that the Kfir with a J79-15 would be easier to fair the fuselage down to the nozzle, and thus more aerodynamic in military power. (Kfir had more drag in military power because of the gap between the nozzle lip and the fuselage, with the nozzle fully bloomed for wet thrust they were close enough it didn't matter much.) Is it simply that Israel didn't have the TDP for the -15, only the -17 engines? If there's a good reason then that's $20 I don't need to spend on F-4B nozzles to dry fit and adapt into this build.

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We are not talking about bleed air in this case. Bleed air comes generally from the compressor section. The air I am talking about surrounds the outside of the entire engine which keeps the outside of the engine cooler preventing it from melting the air frame. The answer you are looking for is beyond my scope. A GE scientist could help.

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Here's how I always saw it. It's been many, many years since I studied it, so the explanation might be rough.

 

For best thrust, a jet engine needs a convergent-divergent ('con-di') nozzle. It accelerates the air to Mach 1 at the narrowest part (note that Mach 1 for the local exhaust air temperature is different than Mach 1 for the air in which the aircraft flies), and then a funny thing happens: when the nozzle diverges, it keeps accelerating the air. Note that this only happens in a supersonic flow, in a subsonic flow the air would decelerate again. The more speed the exhaust air has, the more thrust, so it's very worthwhile to have a good convergent-divergent nozzle.

 

The short J79 nozzle is an 'ejector' type nozzle, that uses a secondary air flow around the engine to create the divergent part of the nozzle. The long J79 nozzle is a true 'con-di' nozzle. The short one is a compromise solution, but simpler and lighter. True 'con-di' nozzles appeared later on the stage of jet engines worldwide, maybe they were a big design challenge, or a material selection challenge.

 

Rob

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@Rob, that makes sense. This far really my best exposure to the J79 nozzles has been modles and the -17 kit nozzles and Wolfpack resin have all given me the impression of being the 'ejector' style described. Actually, even some YouTube videos by AgentJayZ that shows the detail of the nozzle still depict a significant step between the actual constrictor and what look like the outer feathers. I've seen that page you linked, I actually linked it earlier I'm the thread lol. But it only describes the what without the why. 

 

Seeing as how at least on wikipedia's article there is not a huge difference between the -15 and -17 afterburning thrust rating, this change mostly affected dry thrust?

Edited by TheGloriousTachikoma
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3 hours ago, TheGloriousTachikoma said:

Seeing as how at least on wikipedia's article there is not a huge difference between the -15 and -17 afterburning thrust rating, this change mostly affected dry thrust?

 

That is an interesting question. I would think it's true, if I think about the RNLAF F-16 F100 nozzle that I saw in operation so many times, 30 years ago. At full dry thrust, the nozzle is closed nearly all the way, therefore the 'con-di' effect must be the largest. When AB was selected, the nozzle opened completely, leaving little 'con-di' effect.

 

BTW, the same engine in the F-15, without the 'turkey feathers', shows the con-di shape extremely well.

 

Rob

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4 hours ago, Rob de Bie said:

 

That is an interesting question. I would think it's true, if I think about the RNLAF F-16 F100 nozzle that I saw in operation so many times, 30 years ago. At full dry thrust, the nozzle is closed nearly all the way, therefore the 'con-di' effect must be the largest. When AB was selected, the nozzle opened completely, leaving little 'con-di' effect.

 

BTW, the same engine in the F-15, without the 'turkey feathers', shows the con-di shape extremely well.

 

Rob

 

So it took a little extra searching but it seems the -15 was rated at 10,900lb dry and the -17 is rated at 11,800lb. So there's almost a thousand pounds of thrust difference. I wonder if there were other changes between the two engines besides the exhaust nozzle? (that was rhetorical)

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  • 2 weeks later...

It's not your original question, but I can't help posting this video that demonstrates the fantastic howling of the 'short nozzle' J79:

 

https://www.youtube.com/watch?v=vCEhhRoJOaw&ab_channel=AgentJayZ

 

It brings me right back to Volkel AFB in the early eighties, with three squadrons of 104s flying their pants off. My uncle was a J79 mechanic on base.

 

I once heard the Volkel 104s taking off at a distance of 36 kilometers away, at the fence of RAFG Laarbruch. We could hear the engine check that each a/c performed on the runway: full mil, back to idle for some seconds, full mil, brakes off, afterburner.

 

Rob

Edited by Rob de Bie
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11 hours ago, Rob de Bie said:

It's not you original question, but I can't help posting this video that demonstrates the fantastic howling of the 'short nozzle' J79:

 

https://www.youtube.com/watch?v=vCEhhRoJOaw&ab_channel=AgentJayZ

 

It brings me right back to Volkel AFB in the early eighties, with three squadrons of 104s flying their pants off. My uncle was a J79 mechanic on base.

 

I once heard the Volkel 104s taking off at a distance of 36 kilometers away, at the fence of RAFG Laarbruch. We could hear the engine check that each a/c performed on the runway: full mil, back to idle for some seconds, full mil, brakes off, afterburner.

 

Rob

 

That is a chilling, cataclysmic sound. 😍

 

I ordered some -15 nozzles a week back for experimentation. I'm not liking how these Mirage F1s look with -17 nozzles poking out the back (far enough back not to incur the same aerodynamic penalty the Kfir suffers) and they will squat on the tail with the elevators installed. The -15 nozzles should put the burner cans a bit further forward and maybe I get away without using nose weights.

 

Could the continued use of an ejector nozzle be a reason why the M53 was...not up to the same standard as its contemporaries, or would a turbofan's gross surplus of bypass air make up for this? Or was Snecma really just a bunch of decrepit Krauts who needed to cheat off P&W's notes to get new ideas about turbofan design?

Edited by TheGloriousTachikoma
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