Turbojet Axial Rotor Thrust - SR-71

[megan]

A reading of "SR-71 Revealed" by Richard Graham, pilot and the 9th Strategic Reconnaissance Wing Commander, he says, "at cruise, the rotor of the engine actually has a small negative thrust load on the engine".

I'm trying to understand the import of his statement.

In normal operational circumstances of a turbojet I'd expect a fairly large axial drag component on the rotor (negative thrust if you like, since the turbine is driving the compressor). Without identifying the engine or thrust capability, a NASA report cites a axial load of 3,000lb on a medium size engine. The engine at SR-71 cruise provides 17% of the thrust, the rest being 54% from the inlet and 29% from the ejector..

Is this merely indicative that most of the compression is coming from the inlet rather than the compressor?

I've seen statement that the axial loading of a turbojet rotor can reverse direction depending on circumstance, such as RPM. Anyone with insight?

[ACMS]

Not an expert but I remember reading that the Engine actually changed it’s cycle above Mach 2 ( ? ) and became a Ram Jet.

thats all I know!!

[Uplinker]

Not an expert either, and would have to read my copy of Skunk Works again to be sure, but I understood the same as you, ACMS:

At Mach speeds, the engine uses the supersonic shock wave - modified by those huge conically shaped inlet flow modifiers which moved in and out over a distance of 9 feet to present the correct airflow to the engine throughout the operating speed range. This compressed the inlet air and the airflow then bypassed the core engine and effectively became a ramjet.

The core engine would have to remain running obviously, to provide electrical and hydraulic power, and the power to force fuel into the ram jet at a sufficient rate, but by partially blocking the path, it would presumably reduce the efficiency compared to a pure ‘empty tube’ ramjet?

[Concours77]

Not an expert either, and would have to read my copy of Skunk Works again to be sure, but I understood the same as you, ACMS:

At Mach speeds, the engine uses the supersonic shock wave - modified by those huge conically shaped inlet flow modifiers which moved in and out over a distance of 9 feet to present the correct airflow to the engine throughout the operating speed range. This compressed the inlet air and the airflow then bypassed the core engine and effectively became a ramjet.

The core engine would have to remain running obviously, to provide electrical and hydraulic power, and the power to force fuel into the ram jet at a sufficient rate, but by partially blocking the path, it would presumably reduce the efficiency compared to a pure ‘empty tube’ ramjet?




”partially blocking the path....” was the inadvertent establishment of a “choke” at the compressor face that kept the engine from reaching its potential.... A “choke” is basically a dense ball of inlet air that lingers in the wrong place.

The design embellishment that resolved the “choke” was the fit of six bypass tubes that diverted some of the inlet Air from the compressor face to the afterburner (ejector). Simples, got any ethyl borate?

[pattern_is_full]

We need to check our terminology. "Rotor thrust" usually means the backwards force on the rotor or turbine discs from the expanding hot flow out of the combustion chamber, which attempts to blow the rotors out the back of the engine. Which is counteracted by 1) the connection of the rotor(s) to the turbine shaft(s), and the connection of the shaft(s) to the engine by thrust bearings, which not only allow the shaft to spin freely, but also have to carry the rotor thrust forces, in order to hold the engine together.

That is true in any axial-flow turbine engine - the turbine blades are "an obstruction" in the path of the thrust airflow. A necessary one, since the thrust they steal from the overall combustion output keeps the whole thing spinning.

In the case of the high-mach semi-ramjet hybrid engines of the SR-71 or Concorde or similar, above some speed the entire turbojet is an "obstruction" that steals a bit of available thrust - it just happens to be an unavoidable obstruction, unless one wants to install extra engines to separately power low-speed and high-speed flight. (cf: Airbus Ultra-Rapid Vehicle - pure ramjets on the wings, plus retractable fanjets and a rocket engine, for subsonic flight, and (briefly) for acceleration to ramjet speed).

If Graham had said "at cruise, the rotor of the engine still has a small negative thrust load on the engine," it might have been clearer.

That is to say, the combustion outflow pushes backwards on the rotor, which pushes/pulls backwards on the turbine shaft, which pushes backwards on the thrust bearings, which pushes backwards on the whole engine, which pushes backwards on the engine mounts, which push backwards on the airframe, and thus ever-so-slightly reduces net forward thrust for flight.

If we assume a 30000 ft/lb engine loses 3000 lbs to backwards force/thrust on the rotor (10%) and the SR-71 gets 17% of its thrust at cruise from the turbojet, then it loses less total thrust to the obstruction of the rotor - 1.7% (possibly a bit more, since as noted above, the whole turbojet unavoidably "gets in the way" of the ramjet function). A price to be paid to keep the whole inlet/nacelle/turbojet production-of-thrust system "bootstrapped" and operating, in a single (but complex) unit.

[Concours77]

Hey. “Semi-ramjet” can we expand on this? To me, this is not a “semi” or hybrid. The thrust that bypasses the case is no different from that of a high bypass turbofan. Where precisely is the ramjet function taking place?

[Concours77]

At a certain velocity, airflow can be isolated, compressed (heated) and fuel can be introduced, creating a robust expansion we call thrust? In a ramjet, the requirement for intake is through a tube. “Blocking its passage” is what the “choke” function does, there need be no mechanical parts to compress the air, but a tube, or constrictor, directs the combusted gas path “out the back”.

Where in the Blackbird is fuel introduced to a compressed (inlet) gas path, to provide thrust? Certainly in the turbo jet, but the Inlet flow?

Saying the J58 contributes 17 percent to total thrust is misleading. Shut off the fuel supply, what happens to the “Ramjet”?

Excess pressure in the compressor inlet is directed back to the ejector. It serves to cool the afterburner, relieve the compressor of high pressure stalled airflow, and contributes some small amount of thrust as it burns with AB fuel. You could claim the six tubes are the duct, the burner is the choke, and the fuel is the energy, and say that “herein lies the partial ramjet”. But the six tubes were a solution to low power in the turbojet that prevented Mach Two, let alone 3,

The lack of power in the ductless version was caused by the excess pressure in the Compressor inlet. That excess pressure is an “incipient ramjet”. But we won’t be injecting fuel into it will we?

The “Ramjet” reputation emanates from the patent language. One is given free rein in patent language, it need not be proven. Acquiring a patent involves proving something is “novel”. Novelty is the only requirement. Nothing covered by patent needs to work, or even make sense. The protection covers the “uniqueness” of the invention, not its functionality.

Puffery. Hyperbole.

Whatever its called, the Pratt J58 is an elegant, no amazing propulsion system.

[Goldenrivett]

The design embellishment that resolved the “choke” was the fit of six bypass tubes that diverted some of the inlet Air from the compressor face to the afterburner (ejector).
According to https://www.youtube.com/watch?v=F3ao5SCedIk , the bypass tubes take air from the 4th stage of the compressor.

[Concours77]

According to https://www.youtube.com/watch?v=F3ao5SCedIk , the bypass tubes take air from the 4th stage of the compressor.

Mybad. I write from memory and probably shouldn't. The video calls the removal of high pressure airflow from the compressor a “partial ramjet.” How can we conflate a reduction in pressure with the introduction of a “partial ramjet.”?

The cold inlet air reintroduced to the ejector is a “ram effect”. Not a Ramjet.

I have two turbochargers on me Corvette. Both have turbine wheels. I think I’ll start referring to my engine as a “partial turbine”.

[VinRouge]

Mybad. I write from memory and probably shouldn't. The video calls the removal of high pressure airflow from the compressor a “partial ramjet.” How can we conflate a reduction in pressure with the introduction of a “partial ramjet.”?

The cold inlet air reintroduced to the ejector is a “ram effect”. Not a Ramjet.

I have two turbochargers on me Corvette. Both have turbine wheels. I think I’ll start referring to my engine as a “partial turbine”




I believe turbine inlet temperature at higher Mach became an issue for the turbomachinery section. The inlet was exceptionally efficient at compression using reflected shocks, with the net effect that Mach 3+ air was slowed to low subsonic, but at a much higher pressure and temperature. Passing this air through the turbine would have melted it I believe.

Wasnt normal Fuel either; the fuel they used had a much higher flash point and was ignited hypergolically. I believe there was a small tank of the lighter fuel which meant they could only attempt a certain number of relights during the flight.

Cracking book is Sled Driver.

Pretty good video explaining here:

https://www.youtube.com/watch?v=aqSSRKl1Rew

[wiedehopf]

The compressor does less compression than the ram effect, so it's only fair to call it a partial ramjet, no?

[Concours77]

The compressor does less compression than the ram effect, so it's only fair to call it a partial ramjet, no?

Wait. The airflow that bypasses the case is at higher pressure than the mechanical compressor? The airflow from the compressor into the ducts is passive, what is the pressure of the six tubes or the cold flow out side the Case? Greater than the mechanical result? Either way, if the compression is mechanical, and not passive, it fails the Ramjet qualifier by definition.

A Ramjet does not use mechanical means to heat the airflow, nor does it need independent ignition. You’re saying that the airflow in the space between the case and the nacelle is at higher pressure than that entering the combustion chamber? The flow outside the case provides its thrust as a fraction of the total ejector production?

methinks we need further definition of terms,

Eg. Isn’t hypergolic the ignition of two separate materials on contact with each Other?

the “other fuel” was Tri ethyl borane, I don’t know if it could be used in the air, it is extremely unstable.

if inlet air would have melted the machinery, why not the exhaust nozzle?

thanks for responding!

[VinRouge]

if inlet air would have melted the machinery, why not the exhaust nozzle?

thanks for responding!






Because the issue is a combination of centrifugal loading in rotating turbomachinery (typically 2/3 of the stress in the blade) and the effect of the temperature. Failure mode of a cooked blade is typically plastic stretching due to the centrifugal load, making contact with the case.

I think the propulsion system was termed turboramjet.

Sorry, my bad, not hypergolic.

From Wikipedia:

Triethylborane was used to ignite the JP-7 (https://en.wikipedia.org/wiki/JP-7) fuel in the Pratt & Whitney J58 (https://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58) turbojet (https://en.wikipedia.org/wiki/Turbojet)/ramjet (https://en.wikipedia.org/wiki/Ramjet) engines powering the Lockheed SR-71 (https://en.wikipedia.org/wiki/Lockheed_SR-71),[2] (https://en.wikipedia.org/wiki/Triethylborane#cite_note-2) and its predecessor A-12 OXCART (https://en.wikipedia.org/wiki/A-12_OXCART). Triethylborane is suitable for this because of its pyrophoric (https://en.wikipedia.org/wiki/Pyrophoric) properties, especially the fact that it burns with very high temperature. It was chosen as an ignition method for reliability reasons, and in the case of the Blackbird, because the JP-7 (https://en.wikipedia.org/wiki/JP-7) fuel has very low volatility and is difficult to ignite. Conventional ignition plugs posed a high risk of malfunction. It was used to start each engine and to ignite the afterburners (https://en.wikipedia.org/wiki/Afterburner).[3] (https://en.wikipedia.org/wiki/Triethylborane#cite_note-3)

[wiedehopf]

First the bypass air is compressed by the shockwaves and shape of the inlet, then it is compressed by the first 4 stages, then it bypasses the rest of the turbine via tubes and mixed with fuel in the afterburner to add thrust. (timecode 1:30 in the linked video)

We are not talking the other bypass air that bypasses the afterburner as well.

[Concours77]

First the bypass air is compressed by the shockwaves and shape of the inlet, then it is compressed by the first 4 stages, then it bypasses the rest of the turbine via tubes and mixed with fuel in the afterburner to add thrust. (timecode 1:30 in the linked video)

We are not talking the other bypass air that bypasses the afterburner as well.

The purpose of the six ducts was not to create thrust, or to provide a designed for “ramjet” effect.

It was a repair. The engine could not accelerate beyond M 1.7. It was discovered that the compressor section was getting clogged with very dense inlet gas at this speed. The ducts were added to relieve this “choke”, And allow the motor to achieve its full potential. Unplanned benefits included a cooling effect for the AB, and some added thrust.

My understanding.

[wiedehopf]

Oh well one would then need to quantify how much air is going through the tubes versus through the engine i guess.
Also it doesn't matter if they created that almostramjet on purpose or during the design process for other reasons.

Anyway googling the numbers you mentioned for thrust sources, this seems like quite a lot of reasonable points:

https://www.reddit.com/r/SR71/comments/52kghn/qan_argument_about_the_sr71_propulsion_system/

[Concours77]

Oh well one would then need to quantify how much air is going through the tubes versus through the engine i guess.
Also it doesn't matter if they created that almostramjet on purpose or during the design process for other reasons.

Anyway googling the numbers you mentioned for thrust sources, this seems like quite a lot of reasonable points:

https://www.reddit.com/r/SR71/comments/52kghn/qan_argument_about_the_sr71_propulsion_system/

i have a list of the design requirements for the bleed air ducts. I’ll post it when I find it, the first requirement is “to expand the range of Compressor pressures to widen the stall limits.” Makes sense, else why evacuate (decrease) the compressor pressure area only to reintroduce it into the gas path? Intuitively bleed air is a negative connotation. For me.

[Turbine D]

Try reviewing the thread "Blackbird's thrust question". It contains a wealth of information regarding the J58 engine and its design features... :ok:

[wiedehopf]

Getting all the extra air through the other compression stages, combustor and turbine was impossible also because of thermal constraints. So as you don't need as much compression for the turbomachinery to produce thrust with the already compressed inlet air you use the "extra" air you have to produce extra thrust via the afterburner.

Bleed air is just power you extract from the engine, it's not normally used for thrust. But in the SR-71 it is used for thrust production via the afterburner so why should it be negative.

Another mental image would be to see it more as an afterburning turbofan that uses the bypass air at low speeds to keep the turbine itself going (the bypass ducts can be closed can't they?)
First 4 stages are the "fan" part and produce air for the core and the bypass.

Anyway i feel the arguments in the reddit link are quite a bit more refined compared to mine.

[lomapaseo]

OP

I've seen statement that the axial loading of a turbojet rotor can reverse direction depending on circumstance, such as RPM. Anyone with insight?

Yes, it's true for the operative word "can" . It helps to control load conditions on the bearings to ensure they remain positively loaded in certain flight and thrust conditions.

However that's not to say what is being done in the SR71 in your topic so the answers above may help.

[megan]

Unless we can get some traction on my OP I'll delete the thread. The link provided by Turbine D at #18 provides all the information that Concours77 is derailing this thread with.

[lomapaseo]

Unless we can get some traction on my OP I'll delete the thread. The link provided by Turbine D at #18 provides all the information that Concours77 is derailing this thread with.

It isn't nice to the readers or other posters for the OP to delete their thread in a technical forum. It's like my Prof tearing up my exam book as not worth anything since it will never make an "A" grade.

[Pugilistic Animus]

I have the POH for the SR71 and though it mentions spike position it doesn't say anything about this subject..My feeling is unless a very old or dead P&W engineer or a Habu who has flown it, comments, the fairest answer is that nobody knows

[wiedehopf]

Is this merely indicative that most of the compression is coming from the inlet rather than the compressor?

That question is exactly what is discussed in the reddit link i posted. So it was perfectly on topic.

Regarding the thrust load i get the impression we need to clarify what the rotor is, i would assume it's the complete rotating spool of the engine yes?

In normal operational circumstances of a turbojet I'd expect a fairly large axial drag component on the rotor (negative thrust if you like, since the turbine is driving the compressor)

Now we are discussing the direction/magnitude of the load on the thrust bearing of the spool of a turbojet.
That's equal to the combined aerodynamic axial load of compressor and turbine.
Turbine will be pulled backwards and the compressor will be pulled forwards.

Now why would you expect a large axial drag component?
Check the picture on this page: http://aeromodelbasic.********.com/2012/05/thrust-distribution-distribution-of.html
Edit: why would this forum censor ********? anyway via link shortener http://goo.gl/RbZFpY

Now if you shove air down the compressor as in the SR71 the forward thrust load on the compressor reduces so that the forces on the spool eventually sum up in the other direction.
The overall forces as described in the picture still need to sum up as a force thats forward for thrust to be produced. But there are forward components on other parts of the engine that take care of that.

If any turbojet is at idle or while the aircraft is at high speed i would also expect the thrust load on the rotor to become negative.
This could still happen while the engine as a whole is producing thrust as the combustion chamber for example is being pushed forwards while the rotor is being pushed backwards a bit.

Anyhow clarify what exactly it is you are asking or what you are not clear about and i'm sure there will be some more answers.

[Concours77]

Notwithstanding OP’s query, there is a wealth of discussion on the recommended thread (thanks TurbineD)

In my opinion, it answers point proffered in this thread, from lomapaseo and wiedehopf Both, and I conclude without doubt that the engine/airframe in question here, the SR-71, is a Ramjet powered aircraft. In fact, it is by definition powered by partial contributions of three distinct power plants: Turbojet, Ramjet, and “transitional”. (Hybrid).

To realize the nature of this airplane is to have to confess to awe. Nothing short of pure aeronautical genius......

concours

[glad rag]

I am sure the Olympus engines on Concorde achieved a similar "thrust for very little" at cruise...

[lomapaseo]

I am sure the Olympus engines on Concorde achieved a similar "thrust for very little" at cruise...

Well that opens up a more involved discussion. I suspect that with the much lower Mach and the technology of the day, that the Concorde had a much simpler inlet and less opportunity to recover thrust.

[pattern_is_full]

Lower Mach and mission requirements and so on surely helped, but I thought Concorde's Olympi (or Olympuses or whatever) were ranked as the most efficient engines ever at pressure recovery - something like 94%. And the inlet design slightly beat the SR-71 by producing 85% of the thrust (vs. 83% for the Blackbird.)

If I have my facts approximately right, SR-71 could sustain Mach 3 for 90 minutes with afterburners and occasionally supercruise at Mach 1.5 without AB, Concorde could sustain Mach 2.02 in supercruise (no afterburner/reheat thrust) for around 180 minutes (London-Bridgetown, and occasionally, if the winds/weather and weight were right, Caracas-Paris).

The Blackbird was actually the "older technology" (first flight 1964, A-12 1962) although the core strategy (no pun intended) was the same, just different engineering tactics (ramps vs. spikes, etc.), and more or less concurrent with Concorde. And both brilliant.

Save Concorde Group » Engines (http://www.saveconcordegroup.co.uk/engines/)

[megan]

Originally Posted by Concours77Ad hominem is an admission of ignorance.

No ad hominem involved, merely a statement of fact. All your are posts, except for the one above, have absolutely nothing to do with the OP, and direction was given you to where you could find information to educate yourself. Your, Now comes the controversy. The inlet is claimed to provide fifty four percent of the thrust.

Where does it provide this thrust? Not at the inlet. Not in the engine, not in the exhaust.

In the Ejector. Total thrust is measured at the rim of the fully open Ejector.demonstrates a need for education and understanding. You're not Lyman rebirthed? I've been a student of the -71 for some years, and have communicated with the J58 designer about certain aspects, but unfortunately he is no longer available for my OP question to be put. It isn't nice to the readers or other posters for the OP to delete their thread in a technical forum. It's like my Prof tearing up my exam book as not worth anything since it will never make an "A" grade lomapaseo, at my school if your paper didn't answer the question you received 0%, I realise schools these days believe everyone deserves a medal, any indication of failure having dire consequences on ego. As for deleting the thread, one poster is hijacking it with superfluous information that can be found on https://www.pprune.org/tech-log/506122-blackbird-s-thrust-question-3.html and not addressing the OP. Waste of bandwidth.

[lomapaseo]

at my school if your paper didn't answer the question you received 0%, I realise schools these days believe everyone deserves a medal, any indication of failure having dire consequences on ego. As for deleting the thread, one poster is hijacking it with superfluous information that can be found on Blackbird's thrust question (https://www.pprune.org/tech-log/506122-blackbird-s-thrust-question-3.html)and not addressing the OP. Waste of bandwidth.

There are a few places on PPRUNE including this one where it's hard to waste bandwidth even with a deviated technical direction in some eyes. Did you ever notice that very few threads get closed here? They just continue as long as there is readership and no playing the man instead of the ball.

If the OP were to judged on-topic responses then I suspect that there is an agenda rather than a question of learning something of benefit for the rest of us.

[megan]

The only agenda is to get an answer to the question posed. All else covered by others on this thread not answering the question is more than adequately covered on https://www.pprune.org/tech-log/506122-blackbird-s-thrust-question-6.html I suggest you post there if you wish a discussion, this thread is about a very specific aspect, axial-rotor-thrust. Sorry.

[Concours77]

That question is exactly what is discussed in the reddit link i posted. So it was perfectly on topic.

Regarding the thrust load i get the impression we need to clarify what the rotor is, i would assume it's the complete rotating spool of the engine yes?



Now we are discussing the direction/magnitude of the load on the thrust bearing of the spool of a turbojet.
That's equal to the combined aerodynamic axial load of compressor and turbine.
Turbine will be pulled backwards and the compressor will be pulled forwards.

Now why would you expect a large axial drag component?
Check the picture on this page: http://aeromodelbasic.********.com/2012/05/thrust-distribution-distribution-of.html
Edit: why would this forum censor ********? anyway via link shortener model aircraft: Thrust distribution DISTRIBUTION OF THE THRUST FORCES (http://goo.gl/RbZFpY)

Now if you shove air down the compressor as in the SR71 the forward thrust load on the compressor reduces so that the forces on the spool eventually sum up in the other direction.
The overall forces as described in the picture still need to sum up as a force thats forward for thrust to be produced. But there are forward components on other parts of the engine that take care of that.

If any turbojet is at idle or while the aircraft is at high speed i would also expect the thrust load on the rotor to become negative.
This could still happen while the engine as a whole is producing thrust as the combustion chamber for example is being pushed forwards while the rotor is being pushed backwards a bit.

Anyhow clarify what exactly it is you are asking or what you are not clear about and i'm sure there will be some more answers.

So. At high speed, the engine thrust causes the engine to want to move forward in its mounts, the rotor has (net) forward thrust.

At a certain Airspeed, the Pressure on the Compressor end of the rotor is rearward, and of equal value to the thrust moment forward.

It is at this condition that the rotor “floats” in its thrust bearings, net thrust (on the rotor) is neutral?

Increase the Airspeed more, and the inlet pressure becomes dominant. The net thrust on the rotor is rearward (aftward).

Is that it?

Question. At the face of the compressor, is there a point where the inlet air is at such pressure that the compressor is driven by the inlet, and morphs into a turbine? If so, do we expect there to be a neutral point where the compressor blades can accurately be described as stators? That would be a complicated answer due to increasing angle of attack of the blades in sequence?

Where is the inlet drag, (pressure) expressed as thrust? It is dominant, per megan’s OP, fifty four percent of total. At high cruise, most of it is dumped into the Ejector, to be heated, expanded and ejected as thrust?

Here to learn.

concours

apropos not much: “took my girl for a ride in the Skylane. She pointed to.an instrument on the dash. “What’s that? Everything looks so complicated.” The pilot in me wanted to Say ‘Chronometer’. I had to say instead, ‘’honey, that’s a clock...’

“If this description of "thrust" is mostly true for structural calculations, I find it strange it was mentioned to pilots (or the general public for that matter) that have little interest in this. This issue actually seems to fool pilots, and some engineers alike, if you'd take a look at this quote from Design and Development of the Blackbird: Challenges and Lessons Learned (http://enu.kz/repository/2009/AIAA-2009-1522.pdf) (page 25)”

The inlet produces no thrust until the gas contents escape.

[CliveL]

@ Concours

This might help you understand what forces act where

https://cimg1.ibsrv.net/gimg/www.gmforum.com-vbulletin/661x529/z184_zpsed5e397c_186cf00d536a4100de67a876d86f928e09e0502d.jp g

@ Megan

Given the typical distribution of internal forces in a simple single spool jet, it is difficult to see how the remark you are questioning could refer to anything other than the complete compressor/turbine spool
Check your PMs

https://cimg5.ibsrv.net/gimg/www.gmforum.com-vbulletin/2000x1070/thrust_distribution_ff3d10e55c67ca48ad4111792f87be5fa10831d8 .jpg

[Concours77]

An honor to be addressed personally by you sir. Thank you

I accept: The engine is an integrated system. Only with caution should we simplify by “dividing” the whole into “zones.”

Breaking down the dynamic system into “partial thrust values” in my opinion suggests that “Thrust” is created within that zone. Is that right? Because, the gas “load” suggests only where the “effects” of thrust occur.

i am perfectly comfortable with the diagram. Zone 1-2 “feels” the most thrust, while the zone 2-3 feels second most thrust. The structure that “experiences” the bulk of thrust I take to be the aft portion of the sloping centrebody.

The structure acting as the Newtonian wall (“reaction”) is the inner forward portion of the combustion chamber?

It is tempting to say that the loads are assigned by position and distance from exhaust. The ultimate impinged structure is the furthest from the efflux, imo. The Inlet.

Another thing. Single spool integral rotating mass, the “rotor”. Related to the original question, is it permitted to entertain this rotor as “floating” in its thrust bearings, when the axial aerodynamic loads are equal? (Compressor, Turbine).

Best to you,

John


OK, it took two hours and twenty minutes....
hallelujah...got it. Forever in your debt CliveL.

[lomapaseo]

Single spool integral rotating mass, the “rotor”. Related to the original question, is it permitted to entertain this rotor as “floating” in its thrust bearings, when the axial aerodynamic loads are equal? (Compressor, Turbine).

I hope not, Bearings floating in their races have a tendency to skid and wear like hell. Simple to just bleed some high pressure air against a rotating diaphragm like the interior of a rotor disk

[Concours77]

I hope not, Bearings floating in their races have a tendency to skid and wear like hell. Simple to just bleed some high pressure air against a rotating diaphragm like the interior of a rotor disk

Not suggesting it lingers in this abnormal. How does “slight negative thrust” appear without pulling the shaft through the transition from “positive thrust”?

This is a direct comment to his (OP) question about axial reversal?

(Btw, are these thrust bearings similar to the Trent-9 ball bearings?)
concours

[lomapaseo]

The thrust bearings are typically ball bearings. The only time I heard of conical rollers were the very old RailRoad adds of the 40's. Things go tits-up when a cage breaks and the balls migrate away from the radial load. Fortunately the shafts find a home cuddling inside some other bearing compartment until the plane lands.

It's like a bowling ball inside a washing machine. Best not to be in the same room with it.

[Concours77]

The thrust bearings are typically ball bearings. The only time I heard of conical rollers were the very old RailRoad adds of the 40's. Things go tits-up when a cage breaks and the balls migrate away from the radial load. Fortunately the shafts find a home cuddling inside some other bearing compartment until the plane lands.

It's like a bowling ball inside a washing machine. Best not to be in the same room with it.

I think there is more to the original question. Axial thrust impinging the rotating mass of a single spool turbojet is a simple question, something the OP undoubtedly has the answer to. “....I have spent years studying the SR-71, and have communications with the engine builder...”

If the purpose was to tease out some conjecture, (and I have no way of knowing...), I’ll offer some thought.

The purpose of propulsion is to meet and overcome drag, with enough left over to accelerate the airframe.

A percentage of inlet air is continuously dumped (evacuated) from the dorsal airframe bleeds. That shows the design is competent in regards to drag, thrust, and excess power.

The rotor, compressor, and turbine are functionally integrated in a solid structure. Conceivably, with sufficient drag at the inlet, this propulsive design could operate as Ramjet, prior debate notwithstanding.

If it did, it would demonstrate the reversal of axial thrust at the rotor. The inlet air would exceed in Force the effort of the turbine, and force the rotor forward, into its thrust bearings with forward force. At this point in the transition to our ramjet, the turbine and its compressor adjunct would be superfluous, even deleterious to the engine’s current “configuration”. The inlet (drag) overpowers the aft turbine, driving the compressor and reversing output of the turbine.

The inlet stators have trailing edges that articulate from “adding camber” to “free stream” at high engine output; the swirl contribution is unnecessary at high speed.

Imagine that the compressor blades and turbine could be “subtracted” from the design in transition to pure Ramjet? If not remove, then “feather” the blades.

Make no mistake, that is exactly what the six bypass tubes accomplish, only partially. Bypassing the bulk of the compressor, the diffuser, the combustion chamber, and dumping inlet air directly into the afterburner section, it is a “partial” ramjet system.

If correct, I am sure this is not new to the OP. But it is new to me, just trying to catch up to those further along the learning curve.

much respect to megan,

concours

[Turbine D]

Concours77,
You are making this discussion much more complicated than it needs to be. Pratt had a problem with the initial engine design. A solution to that problem was identified and patented. It worked. It worked because of understanding what was taking place inside the engine that had to be modified. It had nothing to do with inlet stator trailing edges or feathering blades. Follow along this summary of the problem and solution:

A turbo-ramjet engine is exactly that, a turbojet working together with a ramjet to power an aircraft to Mach 3+ that could not be achieved independently of one another. To simplify this, A ramjet generates no static thrust and needs a booster to achieve a forward velocity high enough for efficient operation of the intake system. The turbojet is the booster. Ramjets generally give little or no thrust below about half the speed of sound, and they are highly inefficient until the airspeed exceeds 1000 km/h (600 mph) due to low compression ratios. The turbojet is very efficient in this regime. Ramjets work by ingesting relatively low speed air and expelling the air at a higher speed. The difference in speed results in a forward thrust. The burning fuel creates higher pressures inside the engine, causing higher exhaust speeds. But the thrust of the engine depends entirely upon how much air flows through it. No matter how hot the burning air-fuel mixture is, and how high the pressure, if not much air flows into the front of the engine not much thrust is produced. So the trick to improving ramjet efficiency is to increase airflow through the engine. This is accomplished by the spike or obstruction called an innerbody. It is pointed on both ends and thick in the middle and fits inside the intake tube. Air passing into the tube must flow around the innerbody, and the area around it is less than the area of the intake opening. Consequently the air is compressed as it flows around and reaches a maximum pressure in the narrow throat between the innerbody and the intake tube. The same amount of air flows into the engine, but it is raised to a higher pressure. This increases the pressure that the burning gasses must push against, causing the overall pressure inside the tube to increase. Higher internal pressures mean greater amounts of air in the engine, so more fuel can be burned. The result is still higher pressures, increased exhaust gas speed, and greater thrust. But there is a problem that must be dealt within the turbojet compressor area. When the pressure becomes too high in the compressor, the rotating blades tend to flutter, may break, the compressor can stall and the high temperatures can result in mechanical failures. So the P&W designers cleverly bled off air from the compressor to lower the pressure and temperature and fed it back to the burner in the afterburner section through the magical bypass tubes, it worked extremely well.

[Concours77]

Concours77,
You are making this discussion much more complicated than it needs to be. Pratt had a problem with the initial engine design. A solution to that problem was identified and patented. It worked. It worked because of understanding what was taking place inside the engine that had to be modified. It had nothing to do with inlet stator trailing edges or feathering blades. Follow along this summary of the problem and solution:

A turbo-ramjet engine is exactly that, a turbojet working together with a ramjet to power an aircraft to Mach 3+ that could not be achieved independently of one another. To simplify this, A ramjet generates no static thrust and needs a booster to achieve a forward velocity high enough for efficient operation of the intake system. The turbojet is the booster. Ramjets generally give little or no thrust below about half the speed of sound, and they are highly inefficient until the airspeed exceeds 1000 km/h (600 mph) due to low compression ratios. The turbojet is very efficient in this regime. Ramjets work by ingesting relatively low speed air and expelling the air at a higher speed. The difference in speed results in a forward thrust. The burning fuel creates higher pressures inside the engine, causing higher exhaust speeds. But the thrust of the engine depends entirely upon how much air flows through it. No matter how hot the burning air-fuel mixture is, and how high the pressure, if not much air flows into the front of the engine not much thrust is produced. So the trick to improving ramjet efficiency is to increase airflow through the engine. This is accomplished by the spike or obstruction called an innerbody. It is pointed on both ends and thick in the middle and fits inside the intake tube. Air passing into the tube must flow around the innerbody, and the area around it is less than the area of the intake opening. Consequently the air is compressed as it flows around and reaches a maximum pressure in the narrow throat between the innerbody and the intake tube. The same amount of air flows into the engine, but it is raised to a higher pressure. This increases the pressure that the burning gasses must push against, causing the overall pressure inside the tube to increase. Higher internal pressures mean greater amounts of air in the engine, so more fuel can be burned. The result is still higher pressures, increased exhaust gas speed, and greater thrust. But there is a problem that must be dealt within the turbojet compressor area. When the pressure becomes too high in the compressor, the rotating blades tend to flutter, may break, the compressor can stall and the high temperatures can result in mechanical failures. So the P&W designers cleverly bled off air from the compressor to lower the pressure and temperature and fed it back to the burner in the afterburner section through the magical bypass tubes, it worked extremely well.

TurbineD. With great respect, you do exactly what seems to have drawn some anger when I did it. You are collecting, arranging, and repeating patent knowledge. Nothing you say is novel. It is well done, but my purpose is not to display expertise or new data on my part.

The way I approach learning is to take data in, rearrange it, and try to improve my understanding. One effective way to store it is to write about it, and see how it looks. I am not a teacher, I suck at teaching, the best I can do is try to understand what has gone before.

I really don’t care about how I look: bright, posturing, wanna be, whatever, my purpose is to increase my knowledge. I am essentially writing papers; megan was prescient when she answered lomapaseo with a “classroom” rubric.

Selfish? I don’t think so. I am taking a risk when I post here; looking or being read as stupid takes second place to my personal increase in knowledge.

best Regards,

John

[Concours77]

By the way, something I can contribute that I believe is novel is not novel but in approach.

The improvement to the engine was to eliminate in the turbo jet that which is necessary to create a Ramjet, The “choke”
There is no Ramjet without choke, and there is no turbojet with a choke.

The displacement of the “problem” was not effective in increasing thrust overall, but in maximizing the turbojet’s performance, no? That is (was) not new.

Lockheed, functionally is a division of the government. It relies on public resources to pay for its work and profit.

The whole “Ramjet” thing was marketing. In marketing, it’s called “puffing” (no pun intended). Pilots, like myself, took it in and preferred rather to be impressed than skeptical....

[KenV]

I really don’t care about how I look: bright, posturing, wanna be, whatever, my purpose is to increase my knowledge. I am essentially writing papers; megan was prescient when she answered lomapaseo with a “classroom” rubric.
With respect, how do the following statements "increase your knowledge" and "answer your classroom rubric":
Lockheed, functionally is a division of the government. It relies on public resources to pay for its work and profit......The whole “Ramjet” thing was marketing.

The above appears to be more political statements than technical statements and have little if anything to do with "increasing your knowledge."

Or did I miss something?

[Concours77]

....essentially

[Turbine D]

megan,
A reading of "SR-71 Revealed" by Richard Graham, pilot and the 9th Strategic Reconnaissance Wing Commander, he says, "at cruise, the rotor of the engine actually has a small negative thrust load on the engine".

I'm trying to understand the import of his statement.

In normal operational circumstances of a turbojet I'd expect a fairly large axial drag component on the rotor (negative thrust if you like, since the turbine is driving the compressor). Without identifying the engine or thrust capability, a NASA report cites a axial load of 3,000lb on a medium size engine. The engine at SR-71 cruise provides 17% of the thrust, the rest being 54% from the inlet and 29% from the ejector..

Is this merely indicative that most of the compression is coming from the inlet rather than the compressor?

I've seen statement that the axial loading of a turbojet rotor can reverse direction depending on circumstance, such as RPM. Anyone with insight?
I assume the "at cruise" to be above Mach 3 in which case most of the compression is coming from the inlet. I assume that was what Graham was conveying.

Axial loading on rotors do reverse with thrust settings, acceleration (pouring on the coals) and deceleration (hitting the brakes). These changes have to be accounted for in the design of the rotor components. The rotors even move slightly in the axial direction forward and aft as a result of acceleration or deceleration, so spacing between the stationary vanes and rotors has to be calculated plus a safety margin to prevent clashing between rotors and stators in either direction.

Thrust bearings are of a ball bearing design of which there are several types. They are designed to be contact as uniformly as possible and never free floating. Roller bearings are not used in the axial direction, but are used in some situations in a radial direction.

[Concours77]

megan,

I assume the "at cruise" to be above Mach 3 in which case most of the compression is coming from the inlet. I assume that was what Graham was conveying.

Axial loading on rotors do reverse with thrust settings, acceleration (pouring on the coals) and deceleration (hitting the brakes). These changes have to be accounted for in the design of the rotor components. The rotors even move slightly in the axial direction forward and aft as a result of acceleration or deceleration, so spacing between the stationary vanes and rotors has to be calculated plus a safety margin to prevent clashing between rotors and stators in either direction.

Thrust bearings are of a ball bearing design of which there are several types. They are designed to be contact as uniformly as possible and never free floating. Roller bearings are not used in the axial direction, but are used in some situations in a radial direction.

1. The rotors (sic) move slightly (axially) fore and aft. There is spacing between Stators and blades.

2. This movement is precipitated by acceleration and deceleration.

3. If there is a constant positive flow against the turbine, there will be no float.

4. The turbine is parasitic, removing only enough power to drive the compressor, by design.

5. If at any time the inlet “overpowers” the compressor, by eliminating the “suck”,

6. The drive is overpowered by the inlet, the parasitic turbine is “removed” from the flow now either in free stream,

7. Or in “compression’, now actually providing thrust, rather than removing it.

8. The net thrust (rotor) is forward.

9. The thrust (on the rotor) has been reversed, and the bearings must have unloaded, and are now loaded in the opposite direction.

OR,

Colonel Graham is mistaken, thinking a reduction in positive thrust amounts to a reversal into the (net) negative.

No?

Do you have an analysis of unstart? Seems to me this system is extremely sensitive to variations in thrust, and homogeneity of inlet compression?

So, in my simple mind, I want to know if Graham is talking about a “reduction” in thrust that causes “negative” only in “net”? Or, an actual reversal of vector of net rotor thrust, a reversal.

If the rotor is pushed into the aft face of the forward bearing, instead of the usual forward face of the aft bearing, as the result of a reversal, then by definition there is “float” between faces, however undesirable that may be?

Bear in mind, the rotor is expected to be independent of the gas path as a whole, by definition as to variation of gas path flow. Within the limits of the thrust bearings.

It seems to me this reversal is not and should not be “catastrophic”. It merely means that for brief periods, the function of the rotor “flips”; it still remains in the gas path, its relative thrust doesn’t effect any substantial changes in the engine’s performance. It is the inlet that is the critical player.

You claim there is movement between stators (attached to the case) and blades of the rotor.

Stopping the rotor short of clash is the job of both thrust bearings. You know there have to be at least two? Unless the design requires one bearing to protect both directions of thrust?

imo

[lomapaseo]

Cocours77

Are you postulating, stating facts or asking a question ?

Without a SR-71 J58 design manual at hand I can't state any reliable facts. I participate only from the standpoint of design theory.

[megan]

To all who have made contribution from a basis of knowledge or considered opinion, I thank you. This is my final input.

[PeterKent]

ref Megan's original question :

"at cruise, the rotor of the engine actually has a small negative thrust load on the engine" from Richard Graham's book . He was obviously repeating an explanation from someone else, ie would have been second or third hand, perhaps with a bit of miscommunication thrown in. Ended up confusing rotor thrust with engine thrust, which some people do. They say, big fan engine for example, most of the engine thrust comes from the fan but then presume it comes through the fan rotor thrust bearing. Except airfoil aerodynamic load contributions to total rotor load are not necessarily the biggest nor in the same direction. More to the point FWIHR J58 bearing skidding only occurred during windmilling . Also the follow-on supersonic cruise engine, JTF17, design had to be based heavily on previous high mach experience , ie J58, and would have had 4,000 lb forward at M3, 70,000 ft.

So, should he really have said engine load was rearwards? in which case he would have been repeating what David Campbell says in next line. He was Ben Rich's man on propulsion system and inlet patent holder.
"at M3+ if afterburner reduced to minimum engine would be dragging on engine mounts" ie t/mc was actually a drag item and needed more than min a/b to provide thrust through mounts.
The effect of a/b setting from an internal thrust imbalance pov is given by "converging engine nozzle is a drag item. Afterburning allows nozzle to be opened up which reduces rearward load and this is the origin of thrust boost, ie internal thrust out-of-balance ( = engine thrust) now has a reduction in one of the rearward contributions" RR book
Or conversely closing down nozzle, as in first line, increases nozzle drag so engine is now dragging on mounts.
We would expect a turbojet to run out of thrust when reaching a certain speed, on paper that is, "for a turbojet engine, depending on compressor pressure ratio, ram recovery and turbine inlet temperature, thrust drops to zero at M3+" NACA performance study. ie needs an afterburner

And it looks like the J58 t/mc had already ceased to be a thrust producer at cruise (not withstanding its bleed-tube-enabled increase in compressor mass flow and hence thrust) in so far as its pressure loss meant it didn't contribute to a/b nozzle pressure ratio "J58 t/mc pr 0.9 at cruise" ( P&W engine/nacelle total pressure schematic avail at enginehistory.org)

[Gauges and Dials]

The engine at SR-71 cruise provides 17% of the thrust, the rest being 54% from the inlet and 29% from the ejector.

I've read this many times, and I think I understand the way the ramjet and conventional turbojet interact, and I still don't understand the way the terms are being used. 100% of the thrust is provided by the fuel that is being burned. None of the fuel is burned in the inlet. The inlet does interesting things to the airflow but it does not create thrust for any meaningful sense of "create". Or what am I missing?

[Intruder]

The pressure recovered from the inlet, sending the air through the bypass tubes directly to the afterburner, "provides" the air for the thrust, in the same way the compressor "provides" the air for the thrust in the engine core or the fan "provides" the thrust in a fanjet.

[Gauges and Dials]

The pressure recovered from the inlet, sending the air through the bypass tubes directly to the afterburner, "provides" the air for the thrust, in the same way the compressor "provides" the air for the thrust in the engine core or the fan "provides" the thrust in a fanjet.

I understand that if you did force analysis or put load sensors on the shafts and bearings, you'd see that the fan and compressor rotors are trying to pull the airplane forward, and the turbine rotors are trying to pull it backwards. And if you did the same thing for the stationary parts of the engine, you'd see that the forward walls of he combustion chambers and the compressor stators were trying to push the airplane forward, and the aft walls of the combustion chambers and the turbine stators were trying to push the airplane backward. But the thrust all ultimately comes from the expanding fuel-air mixture being burned. The compressor consumes one form of energy -- the rotation of the shaft -- and produces another -- compression of the air. I guess similarly the inlet nozzle in the J58 consumes one form of energy -- the forward motion of the airframe through the air -- and produces another -- compression of the air. But it doesn't really "produce thrust" any more than a ram air intake "produces power".

.

[tdracer]

Think of it this way - at cruise Mach numbers, the engine itself doesn't do much - it's basically there to generate the airflow through the inlet/exhaust. Ramjets don't work at lower Mach numbers - much below ~Mach 2 there simply isn't enough ram compression - hence the need for the turbojet engine. However, the at Mach 3+, you no longer need the turbojet, and pure Ramjets/Scramjets become quite efficient above Mach 3 (really impressive Thrust Specific Fuel Consumption - TSFC). At least in theory, the SR-71 could get better cruise fuel burn if they put some doors in that shutoff the inlet to the turbojet at cruise, let all the airflow bypass the core, and use duct burning as a pure Ramjet. Given the people who designed the SR-71 were no dummies, I assume they determined that the weight/space/complexity of closing off the turbojet inlet wasn't worth the potential TSFC gain. Hence the hybrid turbojet/ramjet system.

[PeterKent]

I assume they determined that the weight/space/complexity of closing off the turbojet inlet wasn't worth the potential TSFC gain. Hence the hybrid turbojet/ramjet system.
Yes. They had an existing engine which fitted in the allowed nacelle diameter. The challenge was modifying the existing engine with least carve-up possible and still give what was required, hence no concentric bypass duct instead of 6 tubes, for example. Never mind a huge duct bypassing the whole turbomachine which would have had to pass more than five times as much air as the tubes. For insight into other paper-only options for "the problem/challenge" to get a J58 to do M3.2 , eg GE-style variable stators, see U S patent 3,344,606.

[Gauges and Dials]

Given the people who designed the SR-71 were no dummies...

Also contemplate that they accomplished a lot of that magic with not much more than slide rules, pencils, and graph paper!

[PeterKent]

I understand that if you did force analysis or put load sensors on the shafts and bearings, you'd see that the fan and compressor rotors are trying to pull the airplane forward, and the turbine rotors are trying to pull it backwards. And if you did the same thing for the stationary parts of the engine, you'd see that the forward walls of he combustion chambers and the compressor stators were trying to push the airplane forward, and the aft walls of the combustion chambers and the turbine stators were trying to push the airplane backward. But the thrust all ultimately comes from the expanding fuel-air mixture being burned. The compressor consumes one form of energy -- the rotation of the shaft -- and produces another -- compression of the air. I guess similarly the inlet nozzle in the J58 consumes one form of energy -- the forward motion of the airframe through the air -- and produces another -- compression of the air. But it doesn't really "produce thrust" any more than a ram air intake "produces power".

.
Maybe it's better not to use words like create /produce/ provide.
The inlet feels (as we would if trying to stop it moving) a thrust or drag force because air is flowing through it. It doesn't matter why the air is moving. The force is still there. Air can flow through it in different scenarios all of which use a different energy source. On the front of a running engine air flows because fuel is burned. or if flamed out because the aircraft is losing altitude. When being tested in a wind tunnel air flows because an electric motor spins.

Nothing consumes energy , it converts it to another form. For an inlet the free stream has a certain amount of energy because it's "coming at" the engine with speed. The idea is to convert speed to pressure because that's the reason for the inlet. It's a better compressor than no inlet. (The compression with no inlet "just happens" as opposed to being manipulated by careful inlet design and actually occurs when an inlet unstarts). However some proportion of the "speed" ends up as thermal energy and hence we hear "the inlet has 80% pressure recovery". It only managed to get 80% of the "speed" to supercharge the engine compressor. And when the inlet unstarts it's doing nothing special as shown by a pressure recovery of, say, 40%.

The energy conversion above is exactly what happens in the spinning compressor. Instead of being given speeding air it has to make its own, stage by stage. The rotor speeds it up and the stator converts as much as it can to pressure.

[lomapaseo]

Also contemplate that they accomplished a lot of that magic with not much more than slide rules, pencils, and graph paper!

they did have IBM punch cards and Freiden calculators that you could program a task to play pseudo music until the machine crashed. Also the early wet copiers to cut down on carbon copies and of course lots of test points from running engines. We also had "wang" calculators that I could program with punch cards to solve open 3 variables.I saw every part of this

[Intruder]

Think of it this way - at cruise Mach numbers, the engine itself doesn't do much - it's basically there to generate the airflow through the inlet/exhaust. Ramjets don't work at lower Mach numbers - much below ~Mach 2 there simply isn't enough ram compression - hence the need for the turbojet engine. However, the at Mach 3+, you no longer need the turbojet, and pure Ramjets/Scramjets become quite efficient above Mach 3 (really impressive Thrust Specific Fuel Consumption - TSFC). At least in theory, the SR-71 could get better cruise fuel burn if they put some doors in that shutoff the inlet to the turbojet at cruise, let all the airflow bypass the core, and use duct burning as a pure Ramjet. Given the people who designed the SR-71 were no dummies, I assume they determined that the weight/space/complexity of closing off the turbojet inlet wasn't worth the potential TSFC gain. Hence the hybrid turbojet/ramjet system.
Don't forget that the engine had to do more than move the airplane... It had to run air conditioning, generators, fuel pumps, and all those "minor details". If the cameras and radar didn't work, there was no reason for the airplane to fly...