Another interesting fact is that the J58 was basically a constant speed engine. Idle was 4000 rpm and full throttle was 7400. If engine rpm kept increasing with throttle like most jet engines, by the time you hit mach 3 the engine is over speeding and burning tons of fuel...it just can't sustain that. Once engines hit max rpm, the throttle only controlled the afterburner and ejector nozzle. This is the reason the SR-71 had trouble pushing through the mach 1 sound barrier....it could do it but it burned lots of fuel and took awhile. At mach .95 the pilots would put the SR-71 in a slight dive and let gravity help push it past mach 1. This maneuver was called the dipsey doodle. At mach 1.6 the spike begins to retract and the inlet starts to take over as the main source of thrust. Kelly Johnson used to say at mach 3 the J58 was little more than a flow inducer to the rest of the system.
Great information! I am a collage student recently working on different inlet types of supersonic aircrafts. Are there any source you would recommend for me? Thanks in advance.
@Berker - Sorry for the late response. There are different types of supersonic inlets so you have to study them all. They all have to take supersonic air and slow it to subsonic so the jet engine can ingest it. The big difference is most inlets are external compression meaning they are designed to keep the shockwave outside of the inlet. This system is very stable but tends to limit speed to Mach 2. Beyond Mach 2 they burn too much fuel to be efficient. Only the SR-71 had the spike retract to bring the shockwaves inside the inlet. This system is inherently unstable and would sometimes “ unstart. The upside is it could efficiently accelerate past Mach 2.
The reason the SR-71 had trouble pushing through the Mach 1 sound barrier was the same as other jets, excessive drag in the transonic speed region, 0.95 Mach to 1.15 Mach. Rather than expend excessive amounts of fuel, being gobbled up by the afterburners, the pilots instituted a 3,000 ft. per minute descent from 33,000 ft. to transcend this region as quickly as possible. Once established at 450 KEAS (Knots Equivalent Airspeed) and supersonic flow was established in the inlet, the spike would begin its slow progression into the inlet at 1.60 Mach, 1 5/8 in. per 0.10 Mach increase, based on the computer inlet's schedule, which varied from aircraft to aircraft and inlet to inlet on the same aircraft. And they figured all this out, only using slide rules in the '60's!?
I never worked on or flew the SR-71 but I have a pretty good understanding of the engine system. I was a jet engine mechanic for 10 years and have 2 SR-71 pilot friends so if there are any questions that need answered I will do my best to get you the answers. I hung out at the Pima Air and Space Museum recently and explained to the dosent's there how this engine system works. The way I explain how the inlet produces most of the thrust at mach 3.2 is ; If you blow up a balloon and pinch off the nozzle, you now have equal pressure pushing on all sides of the balloon called static pressure. The balloon will not move forward or backward because the pressure is equal in both areas. Begin opening the nozzle and letting air out. This releases the backward pushing pressure at the nozzle and converts it to thrust....but the forward pushing pressure on the front of the balloon is still there driving the balloon forward. This is without any combustion. The forward edge of the balloon is the back side of the inlet spike and the balloon nozzle is the air entering the J58. The inlet is a working part just like the engine. This is why the inlet is known to "unstart".....if it can be unstarted it first has to be started. Once the inlet is started it begins to drive the plane forward. My profile picture has Lt Col Robert G Sowers ( first SR-71 instructor pilot and B-58 Bendix trophy winner) , myself, and Maj Gen Pat Halloran ( U2 and SR-71 pilot) taken at the Air Force Academy 2016.
Hello, I'm studying the J58, but there's a data that I can't find out. I've seen that, at high Mach numbers, some of air incoming in the inlet doesn't enter the compressor because of the cambered position of the IGV, and the bleeded air flows around the engine to cool it down. Am I right? If yes, do you have an idea of how much air is bleeded? Thank you.
@ David Galbersanini - Actually no that’s not correct. The air flowing around the engine for cooling comes from the shock trap bleed at the inlet and from the aft bypass doors when they are open. The aft bypass is controlled by the pilot but is scheduled closed at Mach 3.
@David Galbersanini - If you look up compressor bleed and IGV shift schedule which is in the SR-71 flight manual it shows a diagram.The IGV works like this. There are the 6 bypass tubes and also a external or “starter “ bleed . At engine start the IGV is cambered and 4th stage bleed are open. The starter bleed is also open to lessen the load on the back of the compressor. When you advance the throttle past idle the external bleeds close and just after the IGV and 4th stage bleed shifts to axial and closed............
.......at that point the exhaust nozzle closes down a bit so the inlet will reach military operating temperature. The IGV switches back to cambered automatically when it reaches a compressor inlet tem (CIT) of 85 - 115 C which occurs about Mach 1.7, roughly the same time as the spike begins to retract and the bypass bleeds open.
Hello, I am currently studying Turbo Ramjet engine for my semester project. Was really fascinated by the intelligent techniques employed by engineers back then. My goal is to design a Turboramjet engine... And I wanted to learn more about it. Can someone please suggest some good source for the same. I have seen almost all the videos on TH-cam... still want some more insights.
Many people have a hard time understanding this system so they simplify it and say it turns into a ramjet at high speeds. While it does indeed possess ramjet qualities it was not the designed intent. The engine compressor cannot handle the amount of air entering the inlet at Mach 3 so it is rerouted around the engine and into the turbine. This gives some additional thrust but also helps cool the turbine. The secret is the inlet which is shown to produce 54% of the thrust at Mach 3. The inlet cannot operate on its own (no free lunch) so it also needs the afterburner and nozzle to work which produce 29%. Together they produce about 80% of the thrust but the inlet is the key and produces more than half of the thrust all by itself. Many people argue that the inlet cannot produce thrust but there is such a thing as ram thrust. It can also said the inlet produces forward momentum if that makes it easier to understand.
The engine compressor face at 3.2 Mach actually has a slight negative effect on the aircraft's forward thrust. Rather than classifying the inlet/J-58 system as a turboramjet, it's more like a ram-pressure recovery turbojet, much like a balloon, as the exiting air drives the balloon forward. In this example the balloon remains constantly inflated, while the front of the balloon corresponds to the inlet spike and the back of the balloon corresponds to the afterburner. There's 100,000 cubic feet of air, entering the inlet each second. At 80,000 feet, where the outside air pressure is 0.4 psi and the inlet duct pressure is 18 psi, that's a lot of forward thrust vector. Imagine a balloon, with a constant air pressure of 18 psi, zipping through the atmosphere, where the pressure is only 0.4 psi, and you'll understand the dynamics.
@@Bluelevitron I think the problem with the balloon analogy is that it neglects the speed difference. The air coming in to the inlet is slowed down and consequently compressed, which creates a pressure difference that serves to push the plane forward. Upon exit however, it needs to speed back up again which would have the opposite effect as you can't get something for nothing. If I understand correctly, the engine and afterburner work to increase velocity such that there is no great pressure drop to match cruise speed? In this very simplified explanation the inlet provides pressure and combustion provides the velocity, both of which are important for thrust. Is this reasonably close to the reality or am I completely wrong?
@@sammo303 At Mach 3.2 the 0.4 psi air at -65 degrees F. enters the inlet and, in milliseconds, is compressed to 18 psi and 800 degrees F. by the captured oblique shock wave, located just inside the inlet lip. This instantaneous compression of this air creates a huge amount of energy, of which 88 % is recovered, when this air, not needed by the engine, is routed around the engine through shock tubes and the aft bypass doors into the afterburner, providing additional thrust, while cooling the engine with 800 degree F. air, i.e., ram recovery. The six by-pass tubes, which usually open simultaneously with IGV shift, are only routing the air from the fourth stage of the compressor into the front part of the afterburner, also providing additional thrust and cooling. With apologies to Lardawg 67, the inlet guide vanes are in the axial position for takeoff and intermediate supersonic cruise, switching to the cambered position around Mach 1.7-2.3, as a function of CIT between 85 to 115 degrees C. So, energy is created at the front end of the inlet and recovered at the back end. We're not "getting something from nothing." The engine and afterburner still have to get the aircraft's speed up to Mach 1.6, before the shock wave off the spike is captured and the inlet "started."
I heard that the inlet temp restriction was very conservative due to the engineers knowing that hot shot pilots would push the limits... I believe I read that in Ben Rich's book . Best explanation of the inlet spikes i've heard yet, thank you :-)
I worked for P&W Florida in the 80's and the J58 was very dear to our hearts. The question always came up: how fast could the SR-71 fly? There is a simple calculation to determine a Mach that it cannot fly above and that is when the spike shock cone impinges on the cowl lip edge. This is a simply geometric property and it corresponds to around 3.6 to 3.7 Mach. While the speed is still classified, those dimensions tell the story. An interesting anecdote about what you can discern from pictures: an extremely talented RC airplane builder made the first mold for the F22. Those bodies were worth many thousands of dollars. He scaled all the dimensions from every photograph he could get of the airplane. His brother told me that all the angles on that plane were the same value (something in the 40 degree range ) which fouled up radar reflections (this was all worked out in some 1930's Russian mathematician's discertation.) I mentioned this at a get-together with my P&W friends when I visited FL and there was a hush at the table. Turns out that angle was classified.
My understanding is that the oblique shock wave off the front of the aircraft at 3.6 to 3.7 Mach will now be entering the inlets, causing all kinds of havoc with the spike's oblique and reflected shock waves in the inlet, creating an immediate "unstart."
@@Bluelevitron That's definitely true and a good point. But it begs the question how they designed "safety" margins between the 2 types of unstarts: nose or spike. WARNING: Ramblings of the uninformed follow: LOL I would guess that since the spike was specifically designed to create an inlet shock wave, it would have been the primary = especially since it produces an axi-symmetric shock cone. The nose shock would be highly asymmetric at the inlet No idea if the asymmetric shock would be more dangerous but it surely would be more difficult to analyze since everything was done with a slide rule. My wild ass guess is that the spike was the limiting geometry not totally unlike how a canard stalls thus preventing the main wing from stalling. Another factor would be if the spike is continually adjusting it's position while in flight. At a given throttle setting, air speed is always changing and angle of attack also with climb rate. My guess is such a control system may have been beyond the capability back then, and these variations were built into safety margins. Since the nose is dramatically farther from the inlet, such changes in the nose mach cone would be greater at the inlet. So there would be an even greater safety margin for it. But when trying to outrun a missile and death, who knows how far the pilot could bust those margins. I think there was a spec in the public information that if pressed into that last danger region regarding speed, the engines had to be overhauled - or something like that (I apologize, my memory is garbage these days.)
The inlet guide vanes, IGV's, are in the axial position (parallel to the air flow) for take-off and subsonic speeds and shift to the cambered position around Mach 2, similar to shifting a car into 5th gear for cruise. The IGV shift immediately increased air pressure in the duct, which caused the forward bypass doors to open excessively, around 25 %, increasing drag as the slower exiting air hit the supersonic airstream. The pilot would then open the aft bypass doors manually, which closed down the forward bypass doors, as the excess air was now rerouted around the engine and into the after burner section for increased thrust.
Bill have you seen the video of Richard Graham explaining the cockpit controls. The throttles were interesting. You can push the throttles forward until it hits a stop which is military power. You then have to lift the throttles up and over the stop which lights the afterburners. After that you can continue pushing the throttles forward for more power. I didn’t know that’s how it worked so it kinda filled in the blanks for me.
The only slight error that I saw in the video is when you mention the heat limitation of the J58. The heat limitation is the air coming into the compressor, not the turbine. Most of the air at that speed is bypassed to the afterburner and throttle increase only adds more fuel to the afterburner.
I believe this is incorrect I don’t claim to understand supersonic flow but this seems to violate the laws of physics if true this is the “perpetual motion machine” I believe the intake compresses the air and then fuel is burned in the afterburner section heating the air and expanding/accelerating it out the nozzle and that’s how this works as using the turbo machinery to compress the air wouldn’t be possible due to the high temperatures involved and running that heated air from the combustion cans into a turbine would melt any known material axial flow compressors don’t function well at high altitudes if designed to function at low altitudes so they used the shockwave to compress the air at speed bypassing the comperssor
If we place 5 cone structures we can create 10 reflection of oblique shocks and for this 10 when we use different combustion chambers to each pair of shock then we reach mach12 for sure🙂
I think Richard Graham said he took an SR to 3.5 mach ..one time , it's on youtube ...people forget that at mach 3.2 , the SR's skin temperature is almost 1000 degrees .....and the Russians were going to shoot-down a SR with the MIG 25 ( made out of aluminum ), yeah, right ...! The SR-71 is the only air-breathing vehicle that gets better mileage the faster it goes ..!
It's hard to comprehend that at 80,000 the outside air temp is approx. -60 degrees F and within the short distance of the spike inlet to engine compressor it's heated to 800 degrees F.
2:00 the inlet doesn't produce thrust, it just compresses the air (ram effect) and doing so produces drag (no free lunch). There is a very good animation how J58 engine and its inlet work in SR-71: th-cam.com/video/F3ao5SCedIk/w-d-xo.html
It's a nice video but I believe it is incorrect that at 3.2 Mach, the Oblique shock meets the inlet cowl lip. That would mean at M3.3, the oblique shock would be inside the inlet.
Inlets do generally produce drag, but that’s because they push the air in front of them...especially when they are approaching Mach 1. They are called external compression inlets. The SR-71 spike begins to retract about Mach 1.6 which begins to bring the shockwave into the inlet rather than push it externally. This removes much of the drag and gives the power to overcome what’s left. This is the reason the throttles had to be retracted the faster the SR-71 went.
I apologize for my uneducated ignorance and i have trouble understanding how the inlets produce thrust without combustion. If air is compressed it is more dense and can burn more fuel but without combustion no thrust can occour. ( in my erroneous thinking) Thanks for yr patience with me!
@@BilGriffith Thank U sir for the reply .The balloon has potential energy stored as air pressure transformed by the "nozzle" in to thrush. But kenetic energy is limited to air in the ballon. I am missing something core to understanding this but i can't figure it out.
... and this is NOT an "Oswatitsch-type" inlet (f.e. , as in MiG-21, although it looks similar, if not the same) - but completely unique construction of cone inlet, in order to be Jet & Ram-Jet inlet, depending upon regime of flight, allegedly designed by Benjamin Robert Rich; (With increase of the speed, in Oswatitsch-type inlet - the cone is moving FORWARD, in A-12/SR-71, with increase of the speed, the cone is moving BACKWARD)🤓
Another interesting fact is that the J58 was basically a constant speed engine. Idle was 4000 rpm and full throttle was 7400. If engine rpm kept increasing with throttle like most jet engines, by the time you hit mach 3 the engine is over speeding and burning tons of fuel...it just can't sustain that. Once engines hit max rpm, the throttle only controlled the afterburner and ejector nozzle. This is the reason the SR-71 had trouble pushing through the mach 1 sound barrier....it could do it but it burned lots of fuel and took awhile. At mach .95 the pilots would put the SR-71 in a slight dive and let gravity help push it past mach 1. This maneuver was called the dipsey doodle. At mach 1.6 the spike begins to retract and the inlet starts to take over as the main source of thrust. Kelly Johnson used to say at mach 3 the J58 was little more than a flow inducer to the rest of the system.
Great information! I am a collage student recently working on different inlet types of supersonic aircrafts. Are there any source you would recommend for me? Thanks in advance.
VERY interesting fact about mach 1!!!
@Berker - Sorry for the late response. There are different types of supersonic inlets so you have to study them all. They all have to take supersonic air and slow it to subsonic so the jet engine can ingest it. The big difference is most inlets are external compression meaning they are designed to keep the shockwave outside of the inlet. This system is very stable but tends to limit speed to Mach 2. Beyond Mach 2 they burn too much fuel to be efficient. Only the SR-71 had the spike retract to bring the shockwaves inside the inlet. This system is inherently unstable and would sometimes “ unstart. The upside is it could efficiently accelerate past Mach 2.
The reason the SR-71 had trouble pushing through the Mach 1 sound barrier was the same as other jets, excessive drag in the transonic speed region, 0.95 Mach to 1.15 Mach. Rather than expend excessive amounts of fuel, being gobbled up by the afterburners, the pilots instituted a 3,000 ft. per minute descent from 33,000 ft. to transcend this region as quickly as possible. Once established at 450 KEAS (Knots Equivalent Airspeed) and supersonic flow was established in the inlet, the spike would begin its slow progression into the inlet at 1.60 Mach, 1 5/8 in. per 0.10 Mach increase, based on the computer inlet's schedule, which varied from aircraft to aircraft and inlet to inlet on the same aircraft. And they figured all this out, only using slide rules in the '60's!?
I never worked on or flew the SR-71 but I have a pretty good understanding of the engine system. I was a jet engine mechanic for 10 years and have 2 SR-71 pilot friends so if there are any questions that need answered I will do my best to get you the answers. I hung out at the Pima Air and Space Museum recently and explained to the dosent's there how this engine system works.
The way I explain how the inlet produces most of the thrust at mach 3.2 is ; If you blow up a balloon and pinch off the nozzle, you now have equal pressure pushing on all sides of the balloon called static pressure. The balloon will not move forward or backward because the pressure is equal in both areas. Begin opening the nozzle and letting air out. This releases the backward pushing pressure at the nozzle and converts it to thrust....but the forward pushing pressure on the front of the balloon is still there driving the balloon forward. This is without any combustion. The forward edge of the balloon is the back side of the inlet spike and the balloon nozzle is the air entering the J58. The inlet is a working part just like the engine. This is why the inlet is known to "unstart".....if it can be unstarted it first has to be started. Once the inlet is started it begins to drive the plane forward.
My profile picture has Lt Col Robert G Sowers ( first SR-71 instructor pilot and B-58 Bendix trophy winner) , myself, and Maj Gen Pat Halloran ( U2 and SR-71 pilot) taken at the Air Force Academy 2016.
Hello, I'm studying the J58, but there's a data that I can't find out. I've seen that, at high Mach numbers, some of air incoming in the inlet doesn't enter the compressor because of the cambered position of the IGV, and the bleeded air flows around the engine to cool it down. Am I right? If yes, do you have an idea of how much air is bleeded? Thank you.
@ David Galbersanini - Actually no that’s not correct. The air flowing around the engine for cooling comes from the shock trap bleed at the inlet and from the aft bypass doors when they are open. The aft bypass is controlled by the pilot but is scheduled closed at Mach 3.
@David Galbersanini - If you look up compressor bleed and IGV shift schedule which is in the SR-71 flight manual it shows a diagram.The IGV works like this. There are the 6 bypass tubes and also a external or “starter “ bleed . At engine start the IGV is cambered and 4th stage bleed are open. The starter bleed is also open to lessen the load on the back of the compressor. When you advance the throttle past idle the external bleeds close and just after the IGV and 4th stage bleed shifts to axial and closed............
.......at that point the exhaust nozzle closes down a bit so the inlet will reach military operating temperature. The IGV switches back to cambered automatically when it reaches a compressor inlet tem (CIT) of 85 - 115 C which occurs about Mach 1.7, roughly the same time as the spike begins to retract and the bypass bleeds open.
Hello, I am currently studying Turbo Ramjet engine for my semester project. Was really fascinated by the intelligent techniques employed by engineers back then. My goal is to design a Turboramjet engine... And I wanted to learn more about it. Can someone please suggest some good source for the same.
I have seen almost all the videos on TH-cam... still want some more insights.
Many people have a hard time understanding this system so they simplify it and say it turns into a ramjet at high speeds. While it does indeed possess ramjet qualities it was not the designed intent. The engine compressor cannot handle the amount of air entering the inlet at Mach 3 so it is rerouted around the engine and into the turbine. This gives some additional thrust but also helps cool the turbine. The secret is the inlet which is shown to produce 54% of the thrust at Mach 3. The inlet cannot operate on its own (no free lunch) so it also needs the afterburner and nozzle to work which produce 29%. Together they produce about 80% of the thrust but the inlet is the key and produces more than half of the thrust all by itself. Many people argue that the inlet cannot produce thrust but there is such a thing as ram thrust. It can also said the inlet produces forward momentum if that makes it easier to understand.
Sorry I meant the bypass tubes are rerouted around the engine and into the afterburner and helps cool the afterburner not the turbine. My bad.
Lardawg67 brilliant explanation!
The engine compressor face at 3.2 Mach actually has a slight negative effect on the aircraft's forward thrust. Rather than classifying the inlet/J-58 system as a turboramjet, it's more like a ram-pressure recovery turbojet, much like a balloon, as the exiting air drives the balloon forward. In this example the balloon remains constantly inflated, while the front of the balloon corresponds to the inlet spike and the back of the balloon corresponds to the afterburner. There's 100,000 cubic feet of air, entering the inlet each second. At 80,000 feet, where the outside air pressure is 0.4 psi and the inlet duct pressure is 18 psi, that's a lot of forward thrust vector. Imagine a balloon, with a constant air pressure of 18 psi, zipping through the atmosphere, where the pressure is only 0.4 psi, and you'll understand the dynamics.
@@Bluelevitron I think the problem with the balloon analogy is that it neglects the speed difference. The air coming in to the inlet is slowed down and consequently compressed, which creates a pressure difference that serves to push the plane forward. Upon exit however, it needs to speed back up again which would have the opposite effect as you can't get something for nothing. If I understand correctly, the engine and afterburner work to increase velocity such that there is no great pressure drop to match cruise speed? In this very simplified explanation the inlet provides pressure and combustion provides the velocity, both of which are important for thrust. Is this reasonably close to the reality or am I completely wrong?
@@sammo303 At Mach 3.2 the 0.4 psi air at -65 degrees F. enters the inlet and, in milliseconds, is compressed to 18 psi and 800 degrees F. by the captured oblique shock wave, located just inside the inlet lip. This instantaneous compression of this air creates a huge amount of energy, of which 88 % is recovered, when this air, not needed by the engine, is routed around the engine through shock tubes and the aft bypass doors into the afterburner, providing additional thrust, while cooling the engine with 800 degree F. air, i.e., ram recovery. The six by-pass tubes, which usually open simultaneously with IGV shift, are only routing the air from the fourth stage of the compressor into the front part of the afterburner, also providing additional thrust and cooling. With apologies to Lardawg 67, the inlet guide vanes are in the axial position for takeoff and intermediate supersonic cruise, switching to the cambered position around Mach 1.7-2.3, as a function of CIT between 85 to 115 degrees C. So, energy is created at the front end of the inlet and recovered at the back end. We're not "getting something from nothing." The engine and afterburner still have to get the aircraft's speed up to Mach 1.6, before the shock wave off the spike is captured and the inlet "started."
I learn a lot. As a aeronautics student this explanation is important to me. Thank you sir
I heard that the inlet temp restriction was very conservative due to the engineers knowing that hot shot pilots would push the limits... I believe I read that in Ben Rich's book . Best explanation of the inlet spikes i've heard yet, thank you :-)
I worked for P&W Florida in the 80's and the J58 was very dear to our hearts. The question always came up: how fast could the SR-71 fly? There is a simple calculation to determine a Mach that it cannot fly above and that is when the spike shock cone impinges on the cowl lip edge. This is a simply geometric property and it corresponds to around 3.6 to 3.7 Mach. While the speed is still classified, those dimensions tell the story.
An interesting anecdote about what you can discern from pictures: an extremely talented RC airplane builder made the first mold for the F22. Those bodies were worth many thousands of dollars. He scaled all the dimensions from every photograph he could get of the airplane. His brother told me that all the angles on that plane were the same value (something in the 40 degree range ) which fouled up radar reflections (this was all worked out in some 1930's Russian mathematician's discertation.) I mentioned this at a get-together with my P&W friends when I visited FL and there was a hush at the table. Turns out that angle was classified.
My understanding is that the oblique shock wave off the front of the aircraft at 3.6 to 3.7 Mach will now be entering the inlets, causing all kinds of havoc with the spike's oblique and reflected shock waves in the inlet, creating an immediate "unstart."
Iam a nautical graduate i had exclusive skills on propulsion how i can i get a sit in p&w
By recognition by one of SR-71 pilots, airplane is tested up to 3.4 M, which was its upper "officially allowed" top speed. 🤓
@@Bluelevitron That's definitely true and a good point. But it begs the question how they designed "safety" margins between the 2 types of unstarts: nose or spike.
WARNING: Ramblings of the uninformed follow: LOL
I would guess that since the spike was specifically designed to create an inlet shock wave, it would have been the primary = especially since it produces an axi-symmetric shock cone.
The nose shock would be highly asymmetric at the inlet No idea if the asymmetric shock would be more dangerous but it surely would be more difficult to analyze since everything was done with a slide rule. My wild ass guess is that the spike was the limiting geometry not totally unlike how a canard stalls thus preventing the main wing from stalling.
Another factor would be if the spike is continually adjusting it's position while in flight. At a given throttle setting, air speed is always changing and angle of attack also with climb rate. My guess is such a control system may have been beyond the capability back then, and these variations were built into safety margins.
Since the nose is dramatically farther from the inlet, such changes in the nose mach cone would be greater at the inlet. So there would be an even greater safety margin for it.
But when trying to outrun a missile and death, who knows how far the pilot could bust those margins.
I think there was a spec in the public information that if pressed into that last danger region regarding speed, the engines had to be overhauled - or something like that (I apologize, my memory is garbage these days.)
The inlet guide vanes, IGV's, are in the axial position (parallel to the air flow) for take-off and subsonic speeds and shift to the cambered position around Mach 2, similar to shifting a car into 5th gear for cruise. The IGV shift immediately increased air pressure in the duct, which caused the forward bypass doors to open excessively, around 25 %, increasing drag as the slower exiting air hit the supersonic airstream. The pilot would then open the aft bypass doors manually, which closed down the forward bypass doors, as the excess air was now rerouted around the engine and into the after burner section for increased thrust.
Really enjoyed that explanation but makes me wonder what we could achieve by taking those lessons we learnt then ... Further today!
Bill have you seen the video of Richard Graham explaining the cockpit controls. The throttles were interesting. You can push the throttles forward until it hits a stop which is military power. You then have to lift the throttles up and over the stop which lights the afterburners. After that you can continue pushing the throttles forward for more power. I didn’t know that’s how it worked so it kinda filled in the blanks for me.
Nicely done.
This was very informative and well done.
How they controll the inlet spike / Center body according to shock wave ???
The only slight error that I saw in the video is when you mention the heat limitation of the J58. The heat limitation is the air coming into the compressor, not the turbine. Most of the air at that speed is bypassed to the afterburner and throttle increase only adds more fuel to the afterburner.
True. You are probably right about it being for the turbine. Amazing how they figured this all out before it even flew. I like this video .
Bill: Interesting and cool video. Thanks!
I believe this is incorrect I don’t claim to understand supersonic flow but this seems to violate the laws of physics if true this is the “perpetual motion machine”
I believe the intake compresses the air and then fuel is burned in the afterburner section heating the air and expanding/accelerating it out the nozzle and that’s how this works as using the turbo machinery to compress the air wouldn’t be possible due to the high temperatures involved and running that heated air from the combustion cans into a turbine would melt any known material axial flow compressors don’t function well at high altitudes if designed to function at low altitudes so they used the shockwave to compress the air at speed bypassing the comperssor
If we place 5 cone structures we can create 10 reflection of oblique shocks and for this 10 when we use different combustion chambers to each pair of shock then we reach mach12 for sure🙂
I think Richard Graham said he took an SR to 3.5 mach ..one time , it's on youtube ...people forget that at mach 3.2 , the SR's skin temperature is almost 1000 degrees .....and the Russians were going to shoot-down a SR with the MIG 25 ( made out of aluminum ), yeah, right ...! The SR-71 is the only air-breathing vehicle that gets better mileage the faster it goes ..!
3.4 M.
It's hard to comprehend that at 80,000 the outside air temp is approx. -60 degrees F and within the short distance of the spike inlet to engine compressor it's heated to 800 degrees F.
2:00 the inlet doesn't produce thrust, it just compresses the air (ram effect) and doing so produces drag (no free lunch).
There is a very good animation how J58 engine and its inlet work in SR-71:
th-cam.com/video/F3ao5SCedIk/w-d-xo.html
It's a nice video but I believe it is incorrect that at 3.2 Mach, the Oblique shock meets the inlet cowl lip. That would mean at M3.3, the oblique shock would be inside the inlet.
Inlets do generally produce drag, but that’s because they push the air in front of them...especially when they are approaching Mach 1. They are called external compression inlets. The SR-71 spike begins to retract about Mach 1.6 which begins to bring the shockwave into the inlet rather than push it externally. This removes much of the drag and gives the power to overcome what’s left. This is the reason the throttles had to be retracted the faster the SR-71 went.
Search TH-cam - Understanding Shockwaves in aerospace applications.
I apologize for my uneducated ignorance and i have trouble understanding how the inlets produce thrust without combustion. If air is compressed it is more dense and can burn more fuel but without combustion no thrust can occour. ( in my erroneous thinking)
Thanks for yr patience with me!
@@BilGriffith
Thank U sir for the reply .The balloon has potential energy stored as air pressure transformed by the "nozzle" in to thrush. But kenetic energy is limited to air in the ballon.
I am missing something core to understanding this but i can't figure it out.
@@BilGriffith conservation of energy still applies?
Thank you
Good question.
Thanks!
... and this is NOT an "Oswatitsch-type" inlet (f.e. , as in MiG-21, although it looks similar, if not the same) - but completely unique construction of cone inlet, in order to be Jet & Ram-Jet inlet, depending upon regime of flight, allegedly designed by Benjamin Robert Rich;
(With increase of the speed, in Oswatitsch-type inlet - the cone is moving FORWARD, in A-12/SR-71, with increase of the speed, the cone is moving BACKWARD)🤓
compresiion wouldnt produce any thrust i dont think