Turbojet Axial Rotor Thrust - SR-71
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Turbojet Axial Rotor Thrust - SR-71
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'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?
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?
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?
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?
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?
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?
Last edited by Concours77; 8th Sep 2018 at 14:48.
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.
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.
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?
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.
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.
Last edited by Concours77; 8th Sep 2018 at 19:33.
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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).
Sorry
According to https://www.youtube.com/watch?v=F3ao5SCedIk , the bypass tubes take air from the 4th stage of the compressor.
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”.
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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”
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:
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!
Last edited by Concours77; 8th Sep 2018 at 20:18.
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I think the propulsion system was termed turboramjet.
Sorry, my bad, not hypergolic.
From Wikipedia:
Triethylborane was used to ignite the JP-7 fuel in the Pratt & Whitney J58 turbojet/ramjet engines powering the Lockheed SR-71,[2] and its predecessor A-12 OXCART. Triethylborane is suitable for this because of its 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 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.[3]
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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.
We are not talking the other bypass air that bypasses the afterburner as well.
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.
We are not talking the other bypass air that bypasses the afterburner as well.
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.
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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/commen...ulsion_system/
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/commen...ulsion_system/
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/commen...ulsion_system/
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/commen...ulsion_system/
Last edited by Concours77; 8th Sep 2018 at 22:40.
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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.
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.
OP
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.
I've seen statement that the axial loading of a turbojet rotor can reverse direction depending on circumstance, such as RPM. Anyone with insight?
However that's not to say what is being done in the SR71 in your topic so the answers above may help.