Forum: Technology

Raptor_One wrote:

If you want to poke holes in it by saying the J-58 really wasn't like a turbofan because it didn't have a "fan" (which is just a larger diameter compressor), go right ahead. We can agree to disagree. Fundamentally, it became like a variable bypass AB turbofan above Mach 1.8 or so. Not exactly like one, but in essence.

No, and I'll agree with sferrin on this one.

The J58 only "bled" in 6 specific spots on the compressor case through tubes. The primary function for the "bleed" was increased compressor stall margin.

Compressor bleed was calculated by the MFC (main fuel control) as a function of CIT (compressor inlet temperature) and engine RPM, and was not constant! The schedule was related to the IGV (inlet guide vane) position and both were varied by actuators using fuel pressure supplied by the MFC.

The J58 is a "turbojet" engine did not have a constant BPR! Rolling Eyes

Bleed air was simply an anti-stall feature akin to the RCVV, VSV, CIVV, or AJ ratio of the F110/F100 engines. It did benifit the afterburner with cool/fresh air, but that was not it primary function.

Turbofans "bypass" air constantly regardless of any operating condition, that is what makes them turbofans...

Ref: http://www.sr-71.org/blackbird/manual/1/1-19.php
Ref: http://www.sr-71.org/blackbird/manual/1/1-20.php

OR

Ref: http://www.enginehistory.org/Convention ... lsion2.pdf

Guess working on actual engines wins out here? Poke

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Raptor_One wrote:

Sferrin,

So the J-58 is closer to a turbojet at Mach 3+ than a turbofan? Just answer that question for me so I know exactly where you stand. Wink

I'll take this one Sferrin...

Depends on where you are on the following chart. There are two different engine bleeeds, one internal, one external. External bleed dumps the air overboard, so it's not even close to a fan, more like the old turbojet "bleed valves."

Depending on your engine RPM and CIT (mostly MACH dependant) the compressor needed to "dump" high pressure air so the blades didn't stall. (Increasing the compressor's stall margin) All that hot-dense air at MACH 3 would have choked that single spool compressor!

Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

Okay... so what you're trying to say is that the bypassed airflow from the 4th stage compressor above Mach 1.8-2.0 (depending on conditions) is not more like an AB turbofan than an AB turbojet? Please tell me why. And don't tell me that the six tubes/ducts didn't bypass air around the remaining sections of the compressor (after the 4th stage), the combustor, the turbine, and into the nozzle section? This doesn't remind you of a mixed flow AB turbofan? It reminds you of a plain-old AB turbojet? Okay... just answer one way or another. No fancy explanations because I asked a simple question. Which does the J-58 remind you more of above Mach 1.8-2.0. A mixed flow AB turbofan or an AB turbojet? If you refuse to answer that simple question, I will simply take it as an indication that you don't wish to cry uncle. That's fine, but I will continue to pose that simple question if you continue to avoid answering it. Smile

Raptor_One wrote:

Sferrin,

So the J-58 is closer to a turbojet at Mach 3+ than a turbofan? Just answer that question for me so I know exactly where you stand. Wink

As it has no fan it is a turbojet at all speeds. Hell, my Eclipse acts like a Ferrari, until you turn the engine on. That doesn't make it a Ferrari.

Raptor_One wrote:

Okay... so what you're trying to say is that the bypassed airflow from the 4th stage compressor above Mach 1.8-2.0 (depending on conditions) is not more like an AB turbofan than an AB turbojet? Please tell me why.

Correct, it is NOT more like an AB turbofan. Why? - A true turbofan constantly bleeds air around the core, regardless of flight condition. The J58 turbojet 'unloaded' or 'bleeds' air from the compressor as an operating feature to keep the engine stable. The bleed air is not part of the engine's CYCLE, but IS part of it's SCHEDULE.

Raptor_One wrote:

And don't tell me that the six tubes/ducts didn't bypass air around the remaining sections of the compressor (after the 4th stage), the combustor, the turbine, and into the nozzle section? This doesn't remind you of a mixed flow AB turbofan? It reminds you of a plain-old AB turbojet? Okay... just answer one way or another.

Correct; AB Turbojet. (Quit reading if you don't want to know why) Look at large P&W turbojets of the time. (J-75 & J57 below.) You will see "bleed valves" (inside the red circles) located near the center of the high-speed compressor. They were used to increase stall margin on high-performance turbojets during that time. The J58 used multiple (6ea) valves and "duct" some of this air back into the engine at specific points to gain some cooling effect.

As you know most engines have other air that runs through the bearing cavities, cases, and around/through blades as cooling air. The bleed air of the J58 is akin to this cooling flow, not actual propulsion. I doubt the velocity of the air had much propulsive effort to give the engine. After all, the pressure has been dumped from the compressor, partially dumped overboard, changed direction 90 degrees twice; none of which helps flow or velocity. (Turbofans on the other hand exhaust their flow directly aft to add to the propulsive effort of the engine.)

Guess if you had ever been hit with "bleed air" coming from an operating engine you'd know what i was talking about... Yes I was once an FNG too... Embarassed

Raptor_One wrote:

No fancy explanations because I asked a simple question.

AB Turbojet. (Don't read the rest of the post if you don't want the fancy explanation. Maybe too late, but it is here for the benefit of others who want to understand.)

Raptor_One wrote:

Which does the J-58 remind you more of above Mach 1.8-2.0. A mixed flow AB turbofan or an AB turbojet?

AB Turbojet that is using a keen "feature" to keep the compressor operating at high MACH (inlet temp) when others would stall or melt.

Bang Head

You're trying to force a square peg into a round hole. Just because one engine type has some similarities of another does not change it's basic design. Without the KNOWING how the J58 worked, and actually having your hands on one, don't make assumptions. Even you have said - "The devil is usually in the details and anecdotal evidence generally doesn't get into the details." I've offered the details you asked for, then you say you don't want fancy answers; what next? Shrug

Raptor_One wrote:

If you refuse to answer that simple question, I will simply take it as an indication that you don't wish to cry uncle. That's fine, but I will continue to pose that simple question if you continue to avoid answering it. Smile

Uncle?

Engine guys don't cry uncle, and we don't avoid things.. Cool

.
Keep 'em burnin' Thumb

Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

That_Engine_Guy wrote:

As you know most engines have other air that runs through the bearing cavities, cases, and around/through blades as cooling air. The bleed air of the J58 is akin to this cooling flow, not actual propulsion. I doubt the velocity of the air had much propulsive effort to give the engine. After all, the pressure has been dumped from the compressor, partially dumped overboard, changed direction 90 degrees twice; none of which helps flow or velocity. (Turbofans on the other hand exhaust their flow directly aft to add to the propulsive effort of the engine.)

You're just wrong. That statement above is just 100% wrong. This shows you don't know much about the J-58 or you don't understand what you read about it. Calling those 6 bypass ducts bleed air valves is the most ridiculous thing I've heard in this thread so far. Sorry, but this is where I have to draw the line. You're posting falsehoods... 100%. Why don't you read the SR-71 manual because you obviously don't know what you're talking about. You're contradicting what's said in the manual as well as general public information regarding the J-58 engine's operation. Why you're doing this, I don't know. Perhaps you don't like losing debates and will make things up like you did above to try to place yourself on the winning side. I think your above post is disingenuous to be honest with you and I don't think you'd even buy half the things you're trying sell in it. Bleed air valves... ha...hahaha...hahahahaha.... no.

Raptor_One wrote:

That_Engine_Guy wrote:

As you know most engines have other air that runs through the bearing cavities, cases, and around/through blades as cooling air. The bleed air of the J58 is akin to this cooling flow, not actual propulsion. I doubt the velocity of the air had much propulsive effort to give the engine. After all, the pressure has been dumped from the compressor, partially dumped overboard, changed direction 90 degrees twice; none of which helps flow or velocity. (Turbofans on the other hand exhaust their flow directly aft to add to the propulsive effort of the engine.)

You're just wrong. That statement above is just 100% wrong. This shows you don't know much about the J-58 or you don't understand what you read about it. Calling those 6 bypass ducts bleed air valves is the most ridiculous thing I've heard in this thread so far. Sorry, but this is where I have to draw the line. You're posting falsehoods...

Guess you need to learn how to read as the graphic he posted clearly says "bleed air." You need to get over yourself as you're starting to sound like a spoiled child. Suck it up and admit you're wrong.

Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

A spoiled child? Fine. So be it. That has no bearing on the discussion. If the both of you want to insist that the SR-71's J-58 is not effectively a turbofan engine (minus the dedicated fan... the first 4 stages of the compressor operate as the fan above Mach 1.8-2.0), then you will have to explain exactly why you think it's more like a turbojet. Remember, those 6 bypass ducts are not for bleed air. How can you say they're for bleed air when it's right in the SR-71 manual that they're NOT for bleed air! LOL!

Raptor_One wrote:

If the both of you want to insist that the SR-71's J-58 is not effectively a turbofan engine then you will have to explain exactly why you think it's more like a turbojet.

Again; a true turbofan constantly bleeds air around the core, regardless of flight condition. The J58 turbojet is 'unloaded' or 'bleeds' air from the compressor as an operating feature to keep the engine stable. The bleed air is not part of the engine's CYCLE, but IS part of it's SCHEDULE. The compressor bleed is primarily used to improve compressor stall margin.

Raptor_One wrote:

How can you say they're for bleed air when it's right in the SR-71 manual that they're NOT for bleed air! LOL!

Where does it say that? The diagram I am looking at here (below) calls them "Bleed Bypass Tubes (6)".... Index number 9 to be specific.

The J58 is a turbojet when it starts up, and it is a turbojet when it shuts down. What it does in between is a function of it's operation, not it's cycle.

SR-71 Manual wrote:

"Thrust is supplied by two Pratt & Whitney JT-11D-20 bleed bypass turbojet engines with afterburners."

"The internal bypass bleed control and actuation system consists of four two-position actuators to move the bleed valves and a pilot valve, within the main fuel control to establish the pressure to the actuators. The pilot valve controls the bleed valve position in response to a mechanical signal from the main fuel control. Bleed valve position is scheduled within the main fuel control as a function of engine speed and CIT. The external bleed control and actuation system is similar to the internal bleed system, except that three actuators are used."

Did I miss something when I retyped the entire section concerning "Engine External and Internal Bleeds" from the SR-71 Flight Manual?

Raptor_One wrote:

You're just wrong. That statement above is just 100% wrong... ...Perhaps you don't like losing debates and will make things up like you did above to try to place yourself on the winning side. I think your above post is disingenuous to be honest with you and I don't think you'd even buy half the things you're trying sell in it. Bleed air valves... ha...hahaha...hahahahaha.... no.

Sorry can't hold my tongue anymore, I appoligize to everyone else in advance and ask forgiveness...

I stop using "generalizations" and give the "details" Raptor_One says are required for a "scientific debate"; he completely disregards them and now throws a little temper tantrum? I play by his rules now and he doesn't agree with the facts so he calls me a lier and laughs at me? In my book this is a big insulted to me and my experience.

Raptor_One the way you act I find it hard to believe you have anything to do with aircraft propulsion, or real aircraft engines. Your attitude toward differing opinions is unforgivable even a debate setting such as this. I doubt you would last long on a flight-line or in backshop acting like this.

I'm done with your pompous, self righteous attitude Raptor_One ; get over yourself sir. The rest of us are trying to exchange ideas and information here, not be bashed or attacked on a personal level due to a differing points of view.
.

Wow, a very good debate in parts guys. Laughing

I have to say something about "The jet engine" by RR. I too have this book amongst others, R-one is correct that it isn`t a scientific reference but a detailed, well illustrated intro`/overview on jet propulsion. It isn`t really suitable for undergrads but is required reading on some lower level courses to serve as a basic introduction to the technology and concepts. It`s weakness, as I see it, is it`s very RR centric view by concentrating on the RR 3 spool design cores and doesn`t explore other manufacturers engines but it is a RR book so hey!
As an amateur I also found "Jet Engines" by Klaus Hunecke a good basic intro` before wading into the RR tome...RR did indeed call turbofans "leaky turbojets" but that was before they settled on the term "turbofan".

I believe that engine technology is so advanced now that it is possible to make turbofans perform at very high speeds but this hasn`t always been the case. For a long time the turbojet was the state of the art for high speed aircraft, things change. The newest fighters are being produced with turbofans whereas they came out of the factory with Turbojets 30 years ago. I would say yes, the turbojet is dying out as the preferred means of propulsion for fighters, the turbofan offer better economy overall and that seems to be the key parameter.

I don`t see why we can`t produce a turbofan than can touch M3 today, the Mig-31 nearly achieves this...

I will like to correct my thoughts concerning the J58 bleed air not providing "much usable thrust." I was so upset by the personal insults I've dug deeper into my books and the Internet and found a VERY revealing document about the J58.

I am not "posting falsehoods... 100%" as Raptor_One would have everyone believe. The statement in question was: "The bleed air of the J58 is akin to this cooling flow, not actual propulsion. I doubt the velocity of the air had much propulsive effort to give the engine."

I didn't say I was certain, or it was a fact; I said "doubt" Shrug

After finding "More Never Told Tales of Pratt &Whitney" By Dr. Bob Abernethy, I have very interesting technical data that details the engine's "Recover Bleed Air" feature. Very Happy

Dr. Bob Abernethy wrote:

In October 1958 the solution for all these J58 problems was clear to me. Bypass the bleed air around the compressor at high Mach number into the afterburner and it would solve the surge problem, provide cool air to afterburner and increase the mass flow and thrust significantly. Actually it converted the engine into a partial ramjet with capability above Mach 3! I called it the Recover Bleed Air engine on my patent.

Other data from the article...

Dr. Bob Abernethy wrote:

Recover Bleed Air Benefits Bleeds Open to Bleeds Closed
• Airflow Increase +22%
• Net Thrust Increase +19%
• Installed Thrust Increase +47%
• Installed Fuel Consumption -20%
• Increased Afterburning Temperature
• Reduced Blade Flutter

Those wanting to know more about the J58 MUST read this!
http://www.bobabernethy.com/pdfs/Never% ... 20P&W3.pdf

As you can see I was wrong about the benefits of the bleed air in the J58, and I apologize to all for my error. I have been hit by bleed air from other turbojet engines and it doesn't have much punch even at MIL power. What I failed to appreciate is the massive amount of pressure those 6 ducts will posses at MACH 3.

I don't lie or post "100% falsehoods", but I am human and make honest mistakes. I will admit to my mistakes and not hesitate to correct them. (I've almost finished the humble pie sitting at the desk here Embarassed

)

In some respects I can thank Raptor_One for making me mad enough to dig even deeper and find my mistake, but his attitude and tone were inappropriate for the forum.

This does not change the fact the J58 is a TURBOJET. Yes it may have some similarities to a turbofan, but don't all jet engines share similarities at most levels?

Exclamation

Thought - Didn't GE's F120 and F136 have variable cycles? Does that make them actually a turbojet at high speed? I'm sure that is a different thread...

Sorry for the confusion,
Keep 'em flyin'
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Hmm, I did think that the J-58 bleed air component would be beneficial in terms of thrust by not a small margin but was unsure..Thanks for the clear up T_E_G.

That Engine Guy wrote:

Exclamation Thought - Didn't GE's F120 and F136 have variable cycles? Does that make them actually a turbojet at high speed? I'm sure that is a different thread...

Yes, variable cycle. If that means the bypass flow is continuously adjusted and there is very little bypass air at high speeds, but not at ZERO, it is still a turbofan with a fan up front! Laughing

That_Engine_Guy wrote:

Thought - Didn't GE's F120 and F136 have variable cycles? Does that make them actually a turbojet at high speed? I'm sure that is a different thread...

Sorry for the confusion,
Keep 'em flyin'
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The F120 was a variable bypass engine but not the F136. In fact the F136 doesn't have much in common with the F120 at all. Certainly not as closely related as the F119 and F135. IIRC it's a mishmash of a core based on a commercial engine with an afterburner somewhat related to the F120/F414 and a new fan. If someone has some definitive clarification on that I would appreciate.

sferrin wrote:

That_Engine_Guy wrote:

Thought - Didn't GE's F120 and F136 have variable cycles? Does that make them actually a turbojet at high speed? I'm sure that is a different thread...

Sorry for the confusion,
Keep 'em flyin'
Thumb

The F120 was a variable bypass engine but not the F136. In fact the F136 doesn't have much in common with the F120 at all. Certainly not as closely related as the F119 and F135. IIRC it's a mishmash of a core based on a commercial engine with an afterburner somewhat related to the F120/F414 and a new fan. If someone has some definitive clarification on that I would appreciate.

Really? I was under the impression that the F136 is the F120 core with a new low pressure spool and fan. Which commerical engine is it based on? The GEnx?

Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

That_Engine_Guy,

My only point was that above Mach 1.8-2.0, the SR-71's J-58 is more akin to an AB turbofan than an AB turbojet. Much earlier in this thread, Gums suggested that interested parties read over the SR-71 manual. At first I was stating (incorrectly) that the SR-71 was basically a ramjet at high Mach and only used the turbojet's core thrust to get it up to speeds. I originally thought that once the SR-71 was doing Mach 2.something it bypassed the engine core entirely. Well... I was obviously wrong and after reading the SR-71 manual and doing some additional research on the J-58 itself, I realized that the J-58 turns into something akin to a variable bypass afterburning turbofan above Mach 1.8-2.0. Just read what I read in the following two posts back on page 4:

http://www.f-16.net/index.php?name=PNph ... 762#105762
http://www.f-16.net/index.php?name=PNph ... 816#105816

I was misinformed in the initial stages of this thread about the J-58 and told as much. I proceeded to inform myself and then came back and posted my findings regarding the J-58. It happened to be significant to this discussion because some here (and I honestly forget who at this point) claimed that the SR-71 reached Mach 3+ speeds using a pure turbojet. I thought it reached Mach 3+ speeds by switching over to a form of ramjet propulsion at high Mach. I was very surprised to find out that it actually reached Mach 3+ speeds by switching from pure AB turbojet propulsion to something very similar to the modern day, "conventional" AB turbofan (mixed flow variety). In the second post I link to above, I clearly pointed out the differences between the J-58's turbofan-like operation above Mach 1.8-2.0 and conventional AB turbofan operation under all flight conditions. I even went as far as to point out the physical differences between a conventional AB turbofan and the J-58's turbofan-like design. I did this all in an effort to avoid what ended up happening.

Sorry, but I don't feel bad for getting annoyed at people's continued insistence that my very specific comparison of the J-58 to mixed flow AB turbofans was misguided, wrong, off-base, or whatever was implied. I tried extra hard to leave out gross generalizations in my comparison and pointed out key differences between the J-58 and conventional AB turbofans. All this fell on deaf ears and I was told how the J-58 was not really an AB turbofan because of x,y, and z. Points x and y were differences I purposefully mentioned in previous posts (obviously ignored) and point z (the bleed air thing) was not a valid rebuttal. I'm glad you finally figured this out for yourself, but it's not my fault you chose to prematurely insult my intelligence by suggesting I misinterpreted the true purpose of the J-58's bleed bypass ducts or whatever special name they were given.

I honestly don't have a hard time admitting I'm wrong nor does it bruise my ego to do so. The fact is that unless I'm 100% sure about my knowledge or understanding of a specific topic, I take a step back when someone contradicts me. I review the appropriate material to see if I really am mistaken and acknowledge/correct/confirm any previous errors if necessary. I even thank people for correcting me. I do not apologize for being correct, explaining in a clear and thorough fashion why I am correct, and telling others they are wrong and I am right... repeatedly if necessary. And yes, I am fully aware that this rubs people the wrong way. But hey... if you're going to imply someone has a reading comprehension issue, brace yourself for the possibility that they don't.

Forum: Technology

Is the Turbojet dead ?