Forum: Technology

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.

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?

Hardy, har, har. No fan, no "turbofan". Wow... that's deep. Tell me, what's the difference between a mixed flow AB turbofan's fan stages and compressor stages? The fan is bigger and passes a portion of the air it compresses around the engine core. Could you make a turbofan engine by simply bypassing some of the flow from the 2nd, 3rd, 4th, or whatever stage of the compressor around the rest of the core via dedicated bypass tubing? Umm... yeah! Same concept. The only difference lies in what's compressing the cold flow and how a portion of that cold flow then gets bypassed around the (remaining) compressor stages, combustor, and turbine. Seriously, now who's behaving like a little child? I am well aware of the obvious differences between how the J-58 operates above Mach 1.8-2.0 and how a mixed flow, AB turbofan operates throughout its entire envelope. I've already pointed them out several times and I've just had to do so again for your benefit it seems.

Based on the way you argue, I could take the same childish tact and say that the J-58 is not a turbojet because turbojets don't employ six huge bypass ducts to bypass some portion of the cold flow around the latter compressor stages, combustor, and turbine above a certain Mach number. By the strictest of definitions, pure turbojets do not employ any form of bypass flow the way turbofans do. I find it completely disingenuous for you to still be trying to rip down my argument based purely on semantics. I believe it was brought up in this thread already that low bypass, mixed flow turbofans are sometimes described as leaky turbojets and that Rolls Royce even referred to the turbofan as a bypass turbojet or some such.

Hear yee, hear yee. Sferrin has now declared the definition of a turbojet as a jet that has no "fan". It doesn't matter if the jet bypasses some of its compressed cold flow around the remaining core sections. If this bypass flow doesn't get compressed by a "fan" then we call this jet a turbojet. Why? Because like Sferrin says... no fan, no "turbofan". Gosh... what if the gas turbine propulsion system known as the "turbofan" instead became know as "bypassjet" for whatever arbitrary reason. Then you'd probably be on my side, right? You'd have to say something clever like, "If a jet employs bypass flow, it's a bypassjet." And what if the "fan" sections ended up being referred to as the bypass compressor (as opposed to core compressor). What if the term "fan" just never stuck. I wonder what clever little comments based solely on arbitrary naming conventions you'd have then.

Go ahead... keep telling yourself the J-58 is a turbojet above Mach 1.8-2.0. Juuuuuuust a turbojet. Nothing out of the ordinary about that turbojet... nope... nothing at all.

AFAIK the F-136 DOES use Variable cycle technology, the F-120 used a hybrid tandem fan arrangement mounted on the low pressure shaft and used bypass ducts and doors to alter the bypass ratio right through the flight envelope. Exactly how the F-136 does it they are keeping close to their chests. It isn`t the same engine as the F-120 but uses a lot of the ideas/ technology from the F-120 which after all, was a GE motor.
There are quite a few methods to achieve variable cycle, which leads me onto this....

Check out the ADVENT propulsion effort led by RR USA.

http://www.insideindianabusiness.com/ne ... p?ID=24985

The F120 DEFINITELY doesn't use a variable cycle.

This is a quote from Aviation news in the UK...

"General Electric’s design was the GE37, a variable-cycle turbojet/turbofan with a dual-spool, vaneless, counter-rotating turbine"

This was the YF-120 engine, GE called it the GE-37 in house.
In my book the YF-120 was indeed a variable cycle engine...I can`t find anything that contradicts this, please enlighten me otherwise.

Below is another page with the same info`, check out the sources at the bottom of the page, they look credible enough. The GE-RR F-136 seems a direct derivative of the YF-120 the more I read. Look at paragraph 41.
http://home.att.net/~jbaugher4/f22_1.html

The J-58 has also been described as a "Variable cycle engine".

The J-58 has also been described as a "Variable cycle engine".

Sferrin,

Obviously I'm not that smart. At the beginning of this thread I thought the SR-71's J-58 engine core was completely bypassed at high Mach. I thought the SR-71's propulsion system was basically that of ramjet at such high speeds. I have no problem being wrong, admitting I was misinformed, etc. You'll have a difficult time bruising my ego. Feel free to continue with your snappy comebacks though.

Now would you agree with "Variable Cycle TURBOJET"

(Being that the engine starts as a turbojet and uses the bleed to increase stall-margin, cool, and increase thrust?)

Another interesting thing I've found reading about the J58 is that it was developed at almost the exact same time as P&W's TF33 engine.

The TF33 (JT3D) is a turbofan version of the J57 turbojet. The first two compressor stages of the canceled J91/JT9 replaced the first three compressor stages of the J57. The TF-33 actually flew first in 1958 about 4 years prior to the J58.

"5 October 1962: A-12 flies with J75 (in left nacelle) and J58 (in right nacelle) engines."

I'd venture to say - At that time, turbojets had more promise than the turbofans when it came to high MACH propulsion, as the engine chosen to developed for the A-12 was a turbojet that didn't even have the bleed bypass. Which consequently would not have done the job but was modified versus being abandoned in favor of other engines, namely a turbofan based engine.

The J58 did not incorporate the "Recovery Bleed Air" feature as originally designed for the NAVY, but was added later for Lockheed's project according to Dr. Abernethy. He said, "I quickly understood that the J58 would never get to Mach 3 as designed. At about Mach 2.5 the exhaust pressure was equal to the inlet pressure, the compressor was deep in surge, and there was no cool airflow for the afterburner liner that would therefore melt. This is not good."

Now the quote below confirms my statement about the J58's "bleed valves" (That someone laughed at me about... )

Dr. Abernethy - "One of the problems was that if you open the bleed valves at high Mach number there would be a hiccup in the airflow which could unstart the inlet and possible destroy the aircraft. My solution was to open the valves at a lower Mach number, around 2.0 where there would be no bleed flow and no hiccup."

Also note there is no flow through these bleed valves even at MACH 2. Between MACH 2-3 the pressure inside that compressor must have went insane!?

In August 2004, Allison Advanced Development Co. (AADC) shared with Lockheed Martin a USD175 million five-year contract for the Rattlers (revolutionary approach to time-critical long-range strike) hypersonic unmanned vehicle. Part of the National Aerospace Initiative, and administered by the Office of Naval Research, the Rattlers mission is long-range strike. Its YJ102R turbojet-based combined-cycle engine is being designed as "an inexpensive propulsion system for Mach-4+, with advanced turbine cooling and no afterburner, yet with cruise specific thrust six times that of the Pratt & Whitney J58". If the eventual J102R should find a manned application, it will feature in Jane's Aero-Engines. In any event, its technology can hardly fail to read across to the propulsion of future Mach-4+ manned aircraft. In July 2005 AADC was renamed Rolls-Royce North American Technologies Inc.