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@@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?

3.4 M.

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."

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.)

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.

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 .

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!?

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.