Low Observeability of the F-135 engine

All about the Pratt & Whitney F135 and the (cancelled) General Electric/Rolls-Royce F136
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steve2267

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Unread post07 Dec 2016, 19:58

I am creating this thread for discussion of the Low Observable nature of the F-135 engine to avoid the sidetracking of the DT-III aboard USS America thread.

quote="SpudmanWP"]Is this a 1st public view of the radar blockers up the tailpipe???

Image[/quote]

spazsinbad wrote:Chain Me Up & that Rear End View (BLOCKED by Censors) fillum attached


I saw this a week or two ago in a (possibly different) video, but didn't take the time to try to screenshot the frame and post.

This is the first view up the tailpipe I've seen of the F135 engine. IMO we are seeing the aft end of the low pressure turbine. I think the term "radar blockers" is misleading. I can't think of sticking some fixed thing in the back end of a turbojet or turbofan engine that would "block radar" without also wreaking havoc on the flow qualities of the engine exhaust and hence the thermodynamics and the efficiency of the engine.

The objects extending radially from the conical hub at the center of the engine appear consistent with vanes such as you see on an old fashioned wind mill on a farm. That is, they sure look like low pressure turbine vanes. I suppose you could put something in the back designed to "deflect radar" -- bounce the radar into the sides of the engine and shape the engine to try to trap the radar inside the engine similar to how the air intakes are shaped to also trap radar -- but let it spin around like a wind mill. Still, if you don't extract any energy from the "spinning", you are just creating blockage / drag inside the engine.

So IMO this is the rear of the low pressure turbine. And I expect shaping has been applied to the rear of the turbine, possibly to the consternation of the engine fluid dynamicists, to deflect radar into the engine sidewall, which itself has been shaped to "trap" radar reflections in the engine itself. I would also guess that some sort of ceramic coating(s), probably high in iron ferrites, have been applied to the rear of the low pressure turbine blades along with the interior of the back of the engine, to absorb the radar. The conical-shaped hub at the center of the engine is probably also shaped to reflect radar into the engine sidewall to prevent it from reflecting back out the end of the engine. Shaping the aft end of the engine to "trap" radar would be consistent with some statements from pilots and others that the F-35 possesses true all-aspect stealth.

I would not be surprised to learn that the Skunk Works guys "coached up" or otherwise assisted the P&W engineers in stealthifying their motors, as their relationship dates back to at least 1995 when LM entered into an exclusive arrangement with P&W whereby P&W would only work with LM on a shaft-driven lift fan engine.

Lastly, notably absent is a lack of afterburner flame holder structure in the aft end of the motor. I seem to recall videos of multi-stage afterburners lighting off. (I may be recalling a video sequence of an F-14 catapult launch in Topgun where you see multiple stages of the afterburner lighting. Now I cannot recall what I saw looking in the back of an F-16 at an airshow... Do F-100 / F-110 / modern 4th gen afterburning jet engines have some sort of flameholder structure in the back of the engine related to afterburner usage? If so, I would think that would be a radar reflector par excellence.

Several posts / papers have noted that the F135 implemented the afterburner in a new manner to avoid radar signature problems.
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SpudmanWP

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Unread post07 Dec 2016, 20:06

steve2267 wrote:So IMO this is the rear of the low pressure turbine.


That's what I fist thought but .... where are the AB holders??

The only decent pic of a F135 cutaway is this one which shows the burners coming after the last LP turbine. Has anyone ever made them come before the turbines before? Looking at other engine cutaways I have yet to fine ones with such large blades as the last stage as in the F135 pic above.

Image

For a comparison, here is the F100

Image

One last question... In the above pic that I posted, is the F35 engine on since there are no hold-down chains and it's not under tow?
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steve2267

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Unread post07 Dec 2016, 20:22

SpudmanWP wrote:
steve2267 wrote:So IMO this is the rear of the low pressure turbine.


That's what I fist thought but .... where are the AB holders??

The only decent pic of a F135 cutaway is this one which shows the burners coming after the last LP turbine. Has anyone ever made them come before the turbines before? Looking at other engine cutaways I have yet to fine ones with such large blades as the last stage as in the F135 pic above.

Image



I thought I had read they had somehow embedded the F-135 3-stage afterburner into the sidewall of the engine? I'll go see if I can find the quote. It may be in a PDF from a magazine or webzine article.

I have seen that F-135 cutaway image before, but never paid close attention. Based on your comment, it is very possible that what I am alleging is the low pressure turbine is not, in fact the low pressure turbine. I'm still having a tough time believing they (engine designers) would turn the engine flow through those vanes with the vanes being fixed. I would think that would be terribly inefficient, and very stressful from a materials (and weight) point of view. On the other hand, if they do spin, and they spin freely (aren't connected to anything), I would think they get up to a gawdawful high spin rate, which again introduces all sorts of stress issues, especially with such large span vanes.

The only other though that occurs to me is that if they are fixed, then perhaps the flow velocity in that part of the engine is not that high, and most flow acceleration, especially in reheat, occurs downstream of those blades / vanes?
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Unread post07 Dec 2016, 20:53

The first page of this thread has the quote about the possible/variable/3-stage AfterBurner (their guess) + PDF:

viewtopic.php?f=56&t=25691

Engine F135 & LiftFan STOVL F-35LightningII pp6ed.pdf
download/file.php?id=19133 (PDF 1.5Mb)
Powering the Lightning II
April 2012 Chris Kjelgaard

"...Another key feature of the F135 is its augmentor, or afterburner system. While available details of the augmentor are sketchy, the F135 is known to employ multi-zone (probably three-zone) fuel injection aft of the afterburner’s pilot light. These zones inject fuel independently, so that the afterburner does not act in an all-or-nothing way but instead provides a variable range of additional, smoothly transitioning wet thrust at the pilot’s command. Also, like the F119 augmentor, the F135 augmentor is stealthy: The design of the two engines’ augmentors places multi-zone fuel injection into curved vanes which eliminate conventional spray bars and flame holders and block the line of sight to the turbine when looking into the engine from behind...."

Source: http://militaryrussia.ru/forum/download ... p?id=28256 (PDF 14Mb)
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Unread post07 Dec 2016, 21:02

So what we are seeing appears to the the AB Augmenters themselves.

Thanks for connecting the dots...
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steve2267

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Unread post07 Dec 2016, 21:15

SpudmanWP wrote:So what we are seeing appears to the the AB Augmenters themselves.

Thanks for connecting the dots...


Yeah, I think you are correct. "Curved vanes" in Spaz' quote would seem to be the clincher.

I still am having difficulty wrapping my brain around the flow blockage / drag that the vanes would seem to cause.
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Unread post07 Dec 2016, 21:18

We know from other material that there is a variable afterburner catapult shot to stop 'engine pop stalls' (new name for 'popcorn'?) [by ingesting catapult steam] similar to Super Hornet method devised - both all automatic during catapult.

I guess I can make a new video showing the variable launch with the added 'up the tailpipe view'? No? This is an OLD ONE.

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Unread post07 Dec 2016, 21:23

Looking at that vid (thanks again Spaz), I can see tiny static "fingers of flame" (ie dark & light bands) coming from the augmenter so it seems that it does not rotate. That being said, the curvature can be gentle so as not to impose too much restriction to the flow.

Next question.. how much torque does this flow impose on the engine?
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Unread post07 Dec 2016, 21:52

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Unread post07 Dec 2016, 22:07

one thing for certain is that it isn't an AB spray bar, the F-35 doesn't have one.
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Unread post07 Dec 2016, 22:22

If the stator is in line with the exhaust vortex it blocks line of sight with augmentation without impeding gas expansion. Maybe it's ceramic on one side with space shuttle tile material on the backside to resist infrared emissions
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Unread post07 Dec 2016, 22:25

Has an integral spraybar/radar blocker, just like the F119 on the F-22.
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Unread post07 Dec 2016, 22:40

optimist wrote:one thing for certain is that it isn't an AB spray bar, the F-35 doesn't have one.


Regarding no AB spray nozzle... I think we have come to the conclusion that the curved vanes, seen from behind the aircraft in the above video screencapture , is the engine augmenter. So it is the spraybar, if you will, it just looks like nothing we've come to expect as far as conventional afterburner plumbing.
spazsinbad wrote:
Powering the Lightning II
April 2012 Chris Kjelgaard

"Also, like the F119 augmentor, the F135 augmentor is stealthy: The design of the two engines’ augmentors places multi-zone fuel injection into curved vanes which eliminate conventional spray bars and flame holders and block the line of sight to the turbine when looking into the engine from behind...."

Source: http://militaryrussia.ru/forum/download ... p?id=28256 (PDF 14Mb)
Engine F135 & LiftFan STOVL F-35LightningII pp6ed.pdf
download/file.php?id=19133 (PDF 1.5Mb)
Take an F-16, stir in A-7, dollop of F-117, gob of F-22, dash of F/A-18, sprinkle with AV-8B, stir well + bake. Whaddya get? F-35.
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Unread post07 Dec 2016, 22:49

madrat wrote:If the stator is in line with the exhaust vortex it blocks line of sight with augmentation without impeding gas expansion. Maybe it's ceramic on one side with space shuttle tile material on the backside to resist infrared emissions


If I'm understanding you correctly, if there is significant swirl in the flow as it comes aft off the low pressure turbine (LPT), you could design a stator (i.e. a row of static vanes) to straighten out the flow, and at the same time, the stator blocks the view of the engine upstream of the stator. While they were at it, they figured out how to inject fuel into the flow using the stator vanes, thus killing a few birds with one stone.
Take an F-16, stir in A-7, dollop of F-117, gob of F-22, dash of F/A-18, sprinkle with AV-8B, stir well + bake. Whaddya get? F-35.
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Unread post08 Dec 2016, 02:22

optimist wrote:one thing for certain is that it isn't an AB spray bar, the F-35 doesn't have one.


Yes and no. From AW&ST 3/17/11:

Pratt says screech is a phenomenon caused by pressure pulsations in the afterburner at low altitude and high speed. The problem was discovered during development testing around March 2009, having previously been encountered - and solved - in the F-22's F119 engine, from which the F135 is derived.

Pratt points out that the F119 and F135 are the only production engines with stealthy augmentors. Their design eliminates conventional spray bars and flame holders and integrates multi-zone reheat fuel injection into curved vanes that block the line-of-sight to the turbine.

Building on its experience with the F119, the fix for the F135 includes "minor hardware changes to the fuel system, reduced aerodynamic leakages and upgraded software," says Pratt, adding that the modified engine "now provides full max augmented thrust throughout the flight envelope."
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