Pratt & Whitney J58 vs General-Electric J93

[OldBUFFkeeper]

The "Monster that ate Pratt & Whitney"

The beginning of your text makes me want to scratch my old, grey, head in wonder, as I have heard it before, and many times. Given that the J-58 and the J-93 were roughly the size, to hear that the J-58 was an 80%(!!) "scale model" of the J-91, originally offered as a competitor to........... the J-93, as a B-70 powerplant, one must wonder just how much of a monster that engine (the J-91) must have been, and what was P&W thinking when they offered it as a J-93 alternative? Perhaps, there was a FOUR(!!)-engined version of the Valkyrie that no one, (certainly not I), has never seen! But then again, when one hears the thrust figures for the J-91, (approx. 41,500lbs w/AB) they're not that much greater than the most powerful versions of either the J58 or J-93, both of which "clocked in" at around 33,500/35,000lbs. To add insult to injury, the weight, around four and a half TONS(!!!), gave a T/W ratio that, even in the '60's, was almost pitiful. Anyone have any further specs on the "P&W MONSTER(!!!)", or, even better yet, know eher we can find pics?


OldBuffkeeper

[lomapaseo]

OldBUFF
Ther are an awful lot of paper engines that never found a home. It's pretty hard to make comparisons between engines.

Along the way the paper engine actually morphs into development hardware with many chaqnges depending on the whims of the guy with the pocketbook (mostly Uncle Sam). After the entrance into the commercial jet age a designer might even produce drawings that mimic actual for sale hardware for commercial service, hence some of these engines that get the JT prefixes in the P&W world.

The development hardware lots of times starts with some modules (compressor, burner, Turbine, afterburner) that are tested separately against possible combinations that might sell. Thus a collection of like modules might end up with an afterburner or not depending on the mission it might fit. The hardware still in the P&W museum in East Hartford only mimics a few of these combinations. Many other combinations have already been sold for scrap.

I've heard stories of stuff 50-60 years ago of giant burners that were 2-3 times the diameter of the high turbine. I never saw any signs of these on an aircraft.

[OldBUFFkeeper]

Thanks Jane, good work

Great info there, and now recall that the first two stages of the, by then defunct, J-91 became the fan for the turbofan version of the J-57/JT-3, i.e. the TF-33/JT-3D. Since the inlet diameter of the that engine was around 53 inches, your figure for the J-91 seems borne out.
Still would like to find some pics of the J-91, and am also still wondering how/why P&W managed to use the same designation, JT9, for what now seem to be TWO COMPLETELY SEPARATE(!!!) gas generators, since there is no way the core for the JT-9D turbofan could have been 50+ inches in diameter at the first stage after the fan? BTW, does anyone out there know where one can find cutaway views of any of these powerplants?

[OldBUFFkeeper]

To Any and All

Upon measuring off a photograph of an operational version of the JT-11D-20, it became excrutiatingly self-evident that the total cross sectioal area, and, by definition, delivery capability, of the six bypass tubes coming off the compressor just after the fourth stage, was substantially less than the similarly tasked area available with a fully annular bypass duct in a classic turbofan. Which leads to my question: does anyone know what the claimed "bypass ratio" of the JT-11"D" actually was?
When looking at the respective bypass ducts of F-100s, F-110s, and F-135s, measuring the available areas there, dividing by six, then PI, extracting the square root, and multiplying by two, i.e. the reverse figuring for the area of a circle, I came up with tube diameters more than TWICE(!!!) the diameter of those on the J-58, thus exposing the JT-11D as not much more than a "leaky" turbojet, and leaving me to wonder just how useful such an arrangement could ultimately be above mach-3 when ANY(!!!) rotating machinery in the way of ramjet/afterburner airflow is about as desirable as a drag chute?

[lomapaseo]

Upon measuring off a photograph of an operational version of the JT-11D-20, it became excrutiatingly self-evident that the total cross sectioal area, and, by definition, delivery capability, of the six bypass tubes coming off the compressor just after the fourth stage, was substantially less than the similarly tasked area available with a fully annular bypass duct in a classic turbofan. Which leads to my question: does anyone know what the claimed "bypass ratio" of the JT-11"D" actually was?
When looking at the respective bypass ducts of F-100s, F-110s, and F-135s, measuring the available areas there, dividing by six, then PI, extracting the square root, and multiplying by two, i.e. the reverse figuring for the area of a circle, I came up with tube diameters more than TWICE(!!!) the diameter of those on the J-58, thus exposing the JT-11D as not much more than a "leaky" turbojet, and leaving me to wonder just how useful such an arrangement could ultimately be above mach-3 when ANY(!!!) rotating machinery in the way of ramjet/afterburner airflow is about as desirable as a drag chute?

Agree,

but how else would you get the vehicle up to that speed to begin with where the compresor becomes useless?

It's not the relative efficiency of the parts that's important, it's the mission objective being met

[Jane-DoH]

OldBUFFkeeper

The beginning of your text makes me want to scratch my old, grey, head in wonder, as I have heard it before, and many times. Given that the J-58 and the J-93 were roughly the size, to hear that the J-58 was an 80%(!!) "scale model" of the J-91, originally offered as a competitor to........... the J-93, as a B-70 powerplant, one must wonder just how much of a monster that engine (the J-91) must have been, and what was P&W thinking when they offered it as a J-93 alternative?

The J91 had a pressure-ratio of 7:1, and the J58 had a pressure-ratio (IIRC) of 8.8:1 so the J58 would get more thrust for it's size. I'm not sure if the J91 had a variable IGV or the airflow-controlled nozzle, but the J58 had both of these. The former may have been to keep the turbine inlet temperature in line at high mach-numbers; the latter, might have been to squeeze extra AB performance out of the engine.

I'd like to make note that I'm not sure if 80% scale meant 80% the weight, 80% the diameter, 80% the mass-flow.

Perhaps, there was a FOUR(!!)-engined version of the Valkyrie that no one, (certainly not I), has never seen! But then again, when one hears the thrust figures for the J-91, (approx. 41,500lbs w/AB) they're not that much greater than the most powerful versions of either the J58 or J-93, both of which "clocked in" at around 33,500/35,000lbs.

That's because of the lower pressure ratio. I actually didn't know the thrust was that high actually.

To add insult to injury, the weight, around four and a half TONS(!!!)

The J91 weighed 9,000 pounds?

Great info there, and now recall that the first two stages of the, by then defunct, J-91 became the fan for the turbofan version of the J-57/JT-3, i.e. the TF-33/JT-3D. Since the inlet diameter of the that engine was around 53 inches, your figure for the J-91 seems borne out.

I'm not sure if the two compressor stages were exactly the same or simply derived from the J91. Still there's a connection between the two.

am also still wondering how/why P&W managed to use the same designation, JT9, for what now seem to be TWO COMPLETELY SEPARATE(!!!) gas generators, since there is no way the core for the JT-9D turbofan could have been 50+ inches in diameter at the first stage after the fan?

Manufacturers have their own proclivities when it comes to designating their products. For example the JT3C was a J57 variant that powered the 707 and DC-8. When they made a turbofan; they called it the JT3D. After that point, virtually every turbofan they developed had a -D after it.

Upon measuring off a photograph of an operational version of the JT-11D-20, it became excrutiatingly self-evident that the total cross sectioal area, and, by definition, delivery capability, of the six bypass tubes coming off the compressor just after the fourth stage, was substantially less than the similarly tasked area available with a fully annular bypass duct in a classic turbofan.

Because it wasn't designed to use the fan to produce thrust like a propeller -- it was meant to dump excessive air around the engine as a way of lowering the pressure-ratio at high-mach as a means of lowering the turbine-inlet temperature.

The airflow was fed into the afterburner as a way of making use of that compressed air. It increased the pressure of the airflow in the afterburner inlet and also increased the air-percentages (60-65% of the engine exhaust is air -- the bypass air is 100% oxygen)

Which leads to my question: does anyone know what the claimed "bypass ratio" of the JT-11"D" actually was?

Allegedly 65% of the airflow was routed around the engine, and fed into the afterburner. So the bypass was 2.86:1

When looking at the respective bypass ducts of F-100s, F-110s, and F-135s, measuring the available areas there, dividing by six, then PI, extracting the square root, and multiplying by two, i.e. the reverse figuring for the area of a circle, I came up with tube diameters more than TWICE(!!!) the diameter of those on the J-58, thus exposing the JT-11D as not much more than a "leaky" turbojet, and leaving me to wonder just how useful such an arrangement could ultimately be above mach-3 when ANY(!!!) rotating machinery in the way of ramjet/afterburner airflow is about as desirable as a drag chute?

The idea evidently was to basically operate as a turbojet (which you need for takeoff and landing), and progressively relieve the excess pressure more and more as you went faster and faster and feed it into the afterburner which doesn't have the same limits as the turbo-compressor (the core of the engine) -- plus I'm not 100% sure of this, even without turbine temperature limits, there might be a point of diminishing returns as to your ability to compress air and get propulsive efficiency out of it (without cooling the air, which is used with the SABRE and Scimitar engine designs used for SKYLON and the Reaction Engines LAPCAT A2)

Regardless there's only a certain degree of efficiency you need to accomplish a task -- the rest is gravy.

[OldBUFFkeeper]

to JaneDoH and Iompaseo

Brilliant work, from both of you! I'm impressed! HOWEVER(!!!), given the actually inability of a one-inch hose to deliver the same flow, all other factors being equal, of a two-incher, I must continue to scratch my head as to the actual bypass ratio of the JT-11"D", especially as the cited figure of "2.86(!!!??) to one" not only exceeds that of the very much below mach-3 F-101 on the B-1, but also, (again) considering that pressures at that longitudinal point, just aft of the fourth stage, would be, more or less, uniform from hub to casing, the combined amount of area of the six tubes just doesn't provide that much relief as to provide the bypass ratio you cite. Maybe I'm missing something.

Also, as to the common use of a designation, "JT-3" remained that "D", or no "D". Why not the same for the "JT-("?


OldBUFFKkeeper

[lomapaseo]

Also, as to the common use of a designation, "JT-3" remained that "D", or no "D". Why not the same for the "JT-("?

Come again

[OldBUFFkeeper]

Iampaseo

The point I was trying to make, in my clumsy way, was that if only for some semblance of corporate continuity, designations have tended to be contiguous, with "JT", i.e. "jet turbine" being the same for P&W until a few decades ago when they went to a new system based, among other things, on the thrust of the engine. Until then, the sequential arrangement was quite clear, i.e. "JT-3" (J-57), followed by "JT-4"(J-75), etc. Of course, even this can still be confusing when one notes that the next developement, the "JT-8", has a LOWER(!!) military designation "J-52" than either the J-57, and J-75, both of which it is clearly a technological successor. In any case, as if that wasn't enough, I'm having a serious problem with the allegation, offered in several sites, that BOTH(!!!) the J-52, and the J-58 were scaled down descendants of the massive J-91.

As I've noted before, externally, the basic J-58 looks roughly like a J-75, minus the N-2 casing, as well as the distinctive "bump" in the diffuser casing of the J-57. By contrast, the J-52 (i.e. JT-8) looks like nothing more than a "miniature J-75 (JT-4)", at least as it appeared to me in the "dash-4" for each engine, as I gleaned when thumbing through each. Not insignificantly, the site claiming common ancestry with the J-91 then shows a cross section, allegedly of the JT-8 (J-52), that looks nothing at all like what I saw in the tech manuals. Specifically, the JT-3 through JT-8 had an N-1 in which the hub was parallel to the axis and the casing tapered inward, moving rearwardly, and an N-2, in which the casing was of a constant diameter while the hub increased as one moved towards the final stages. In the view provided at this one site, the arrangement was REVERSED(!!), giving an external profile unlike anything I'd even seen from P&W, and roughly resembling a RR Avon, but with the compressor divided into two separate sections.

Having been "professionally aquainted" with the JT-8 from both extremes, beginning with its first employment as the J-52, still too "hot" and unreliable to be used on a manned aircraft, instead driving the GAM-77 Hound Dog missile, of which the B-52 I crewed carried two, and its "fully tamed" ultimate application as the JT-8D turbofan, three of which, drove the B-727 airliner, countless numbers of which, I serviced, while employed at United Air Lines, after leaving the service, not to mention watchng the RM-8 afterburning version of the turbofan, drive the Saab J-37, a spectacular demonstrator at air shows of the real meaning of "specific excess power", I know what one looks like, and now stand even more confused about the P&W lineage than before. HELP!!!

P.S. Have also learned of a planned genuine turbofan version of the J-58 that was the first proposal for propulsion of the L-2000, along with other claims that the core of the actual JTF-17 was, in fact, nothing but a modified J-58.

Any info there, anyone?

Am at least realising substantial savings on haircuts, as ripping it out over this stuff seems to work fairly well.

[lomapaseo]

One can only imagine what it must have been like in the smoke filled rooms in the back offices of P&W back in the 50's when all this was going on.

On one side of the room you have the cigar chomping military sales guys stating what the latest paper airplanes were being drawn up in the far off skunk works, and that millions of development money would be there for the asking if P&W would offer up product designs on paper to match the paper airplanes the government had already decided to fund for study.

It wouldn't have taken much for the P&W advanced engines groups to dust off a previously desgned and protyped model, add some stages, and afterburner and then claim that they could easily extropolate a performance and thrust size increase based on old data and with a few million actually commit some componet (scale hardware) on rigs to verify the feasibility in a step funded program.

Then on the other side of the room the fleldgling commerical applications dream merchants were saying that some of the military big guys like Douglas and Boeing were thinking of entering the jet age if an engine might be avialble in 5 years or so.

Not to worry it would be relatively easy to move some accessories around take off an afterburner and offer up paper model like the JT3 series and then a b then c as the specs tried to keep pace with the same dreams in the commerial sides of Douglas, Lockheed and Boeing. Actually with enough development money it actually worked as the Collier trophy for the J57 attests.

By the time the commecrial airplane guys finally settled on a plane that matched the available thrust to weight, then the various development models finally settled on a compressor and turbine combination that would match and a salable model would actually be sold in quantity like the JT3c

This went back and forth through the 50's and other models jumped in like the J60/JT12 and the J52/JT8 J58/JT11, even though the actual sales numbers might actually have been low and the model itself stored as a development only machine for future possibilities.

So when the first concept of a commercialy viable fan came along all they did was trot out a bigger compressor blade and bypass part of the air and calim better fuel consumption and quieter noise4 signature. While at the same time changing the4 model number by one charcted from a "C" to a "D". This was such a hit with the JT3c moving to a JT3D that they figured they could do the same thing with the orphaned JT8 by adding a new compressor and a fan and since the "D" change meant oodles more performace they just decided that was te way to desgnitae any ne4w commecial engine with a fan and two separate compressor sections.

Now as for the mysterious J57, J60, J58 designation you need to ask the airforce how they came about in their smoke filled back rooms. Remember since the government was spending their money their were a lot of number combinations that existed on paper both in the government offices as well as the engine manufacturers design and component development groups..

Some of these combinations even had hardware like tiny turbines behind boron fuel tubes and gears between the turbine and the large compressors, with afterburners 6 ft in diameter. I never heard of one finally coming together as a full engine though.

One of my neighbors worked there in the test department straight out of college, but sadly he died just before this thread was started so I can't trot over there for a beer and questions anymore

[kilomikedelta]

"The airflow was fed into the afterburner as a way of making use of that compressed air. It increased the pressure of the airflow in the afterburner inlet and also increased the air-percentages (60-65% of the engine exhaust is air -- the bypass air is 100% oxygen)" Do you mean that the engine exhaust is 60-65% air and 35-40% combustion product gases and the bypass air was 100% air (21% oxygen) with no other gases? Regards, KMD

[OldBUFFkeeper]

ALL

Am still curious as to the actual efficacy of those six "tubes" and wondering if they collectively had the air delivery capacity of an "honest" classic bypass duct (eg. F-100/101/110/RM-8/M-53, F-414, et.al.), and, why, even if they somehow did, the concept was not precluded by energy loss intrinsic to the no less than FOUR(!!) right-angle turns,(i.e. out of the compressor after the fourth stage, and, by definition, essentially perpindicular to the longitudinal axis, then back parallel to the axis to head aft past the high pressure stages, combustor and turbines, back in, at the AB, and perpindicular again, with a final turn to match the prevailing gas flow), and the issue resolved with a "simple" radial extension of the low pressure stages, ala Snecma M-53?

Not to "Monday morning quarterback", but, given the sheer aerodynamic elegance of the Blackbird, the tubes seem appalingly crude.

[barit1]

Well, if the Mach in the ducts is low enough, pressure loss is minimized, but then so is the flow minimized...

[OldBUFFkeeper]

Right on, Barit!

You make a very good point here. But it still seems to make more sense to have the "fan" stages feed a classic circumfirential duct, allowing both the harnessing of that airflow for "propeller" thrust at low speeds, i.e. dragging a full fuel load off the runway for once, rather than just enough to allow one to clear the runway and scramble to the nearest tanker, and then allow the air exiting the fans to simply follow the "path of least resistance" in the intervening space between the low and high pressure sections, flowing into the N2 as needed and going around through the fan duct when either quantity or pressure are in excess. Also, noting the degree to which the French aero engine industry, specifically SNECMA, has benefited from liasons with BOTH(!!) GE and P&W, I'm surprised and a bit disappointed that that "river" could not flow the other way as well, a "J-58-as-scaled-up-M53" still looking extremely good as the quintessential Mach 3+ powerplant.

[barit1]

Ah yes. Some years ago I worked some preliminary design stuff on such an engine - Low-bypass fan, twin spools, variable shutters to gradually close down core airflow and divert more to the afterburner, so the machine migrated toward a ramjet system. Similar in principle to what OldBUFFkeeper describes.

Very complex, but should have performed well past Mach 3. It could even be used as a reusable orbital-insertion booster vehicle with fly-home capability. NASA might be interested if they only had some $$$.

[OldBUFFkeeper]

Barit-1

A variable-cycle turbo-ramjet, did you say, with possible TSTO capability? Then you're just the person to answer my next trivial question.

Given that aside from propelling the aircraft, another significant function of the powerplant is driving all those other devices (eg. generators, via CSDs, and hydraulic pumps), crucial for auxiliary capabilities, not to mention "small neccesities" such as fuel pumps, all of which derive their motive power from a tower shaft PTO of the turbocompressor drive shaft, just what do all you "RTA"-types have in mind for a substitute when the main powerplant has transitioned completly to ramjet/scramjet mode, or is it an unspoken consideration that the core will even then still be allowed to "windmill" in a small portion of the airflow as the qunitessential "RAT"?

[barit1]

OldBUFFkeeper, as best as I can recall, the gadget was as you described. A more-or-less conventional accessory package, lube & fuel pumps etc. The core kept operating if for no other reason than to pump fuel for the afterburner.

[OldBUFFkeeper]

barit

Ah yes, yet another "what if" project!! Have done much the same in desktop "doodles", but rather than taking the gas generator completely out of the loop, my "bypass relief doors", situated just foward of the HP compressor, opened outward, thus allowing excess air to go around and into the ramburner through what was otherwise the fan exhaust duct, while a "smidgeon" of the remainder still went through the core. My theory was that, by providing such a relief, both the quantity and the ambient pressure of the air just foward of the core would be reduced, allowing it to continue to perform some useful function, however limited, throughout the envelope, including accessory drive. That way, one can have it all, fan thrust at takeoff and climb, with gradual transition to ramburner as the mach number mandates. Again, "what if........................................."

[peter kent]

Answers to questions

Hi folks,
Sorry this is a bit cryptic but I'm scared to spend too much time typing words in case I lose the whole lot with one wrong mouse click. It's already happened.

Good news for oldBUFFkeeper,
Six tube BPR is 0.25 as they passed 20%. See below for this and much, much more

http://www.enginehistory.org/Convent...ropulsion2.pdf

Hope this new revelation on low flow reqd raises your opinion of the elegance or lack of it for the tubes.
BTW Jane-DOH had already summed them up in one line although I think the relevance of the statement was missed, ie "there's only a certain degree of efficiency you need to accomplish a task. The rest is gravy" or big waste of money actually.
The apparently crude tubes were engineeringly elegant I'm sure ie reqd min mods to min # of components They did what was reqd cheapest and lightest and durable.

Following answers come from "The Engines of Pratt & Whitney, a Technical History" by Jack Connors

There are 3 pages on J91.
J91 compressor, in order to pass req flow for M3 with smallest size, hadnew low level of hub-tip-ratio, about 1/3 re previous 1/2, and also transonic blading so no IGV reqt. JT3D fan was slightly scaled down 1st 2 stages.

Finish off with thought for the day:
overall PR of subject matter above at M3 is today attained by your big fan engines at zero flt speed, ie over 40:1

That's about it for now.
Any more P&W Qs I can look up for you in the book.

Thank you all for renewing my interest in above details.

PK