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If you've ever tried to understand the SR-71's engines, chances are
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you've come across these diagrams from the SR-71's flight manual.
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Let's face it though: they're not as clear as they could be. So let's clean it
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up and simplify.
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There we go! The whole diagram is the complete engine nacelle
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made up of the airflow inlet, the Pratt & Whitney J58 engine, the convergent
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divergent ejector,
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and the airplane body. At speeds below Mach 2, the J 58 acts like any other after
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burning turbojet engine.
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Air flows into the nacelle through the inlet, where it's allowed to diffuse behind
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the supersonic shockwave before moving into the multi-stage axial compressor
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which looks like this, but bigger. Here, the air is compressed before heading into the
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burner where fuel is added for combustion.
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The heated exhaust turns the turbine and provides the engine's forward thrust as
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its accelerated to high speeds by the ejector.
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The turbine turns the compressor and keeps the engine cycle going.
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Just after the turbine
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is the afterburner, where more fuel is added to the exhaust in order to get as
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much of the oxygen
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out of the air as possible. While afterburners allow for powerful bursts
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bursts of acceleration,
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they're really inefficient, costing huge amount of fuel for the increased force.
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What makes the J58 engine so different than all other turbojets are these 6 bypass tubes
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which you don't find on these diagrams. The tubes open when the plane is flying at
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speeds greater than Mach 2.2,
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moving compressed air from the fourth stage the compressor directly into the
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afterburner
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This allows the engine to act more like a ramjet, which allows the SR-71's
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afterburner to operate at a much higher fuel efficiency,
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the forward motion aircraft handling most to the air compression
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at a ratio of about 39:1 (38.8:1), with the four turbines ages adding an
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additional compression about 1.6:1.
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The combined action of the turbine and ramjet compression makes the J 58 a very
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unique type of engine:
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a turboramjet; and allows the plane to cruise at speeds that would make a
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normal turbojet melt.
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The Blackbird's inlet design is as important to allowing the J58 to do its thing as
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the engine itself so let's see how it works.
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In the middle of the inlet is this symmetrical spike, called the inlet spike,
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or centerbody,
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and behind it is the diffuser, where compressed air spreads out before
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entering the engine.
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At supersonic speeds the inlet spike takes the pressure the leading
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supersonic shockwave
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off of the engine so that the engine gets the best airflow. Inside the inlet a
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second shockwave is formed called the normal, where the air coming into the
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nacelle transitions from low-pressure, supersonic speeds, to high-pressure,
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subsonic speeds.
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Where the normal ends up inside the inlet depends on the speed at which the
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aircraft is moving and the shape of the inlet
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and inlet spike. When the aircraft hit's Mach 1.6,
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the normal ends up in the best place inside the inlet for pressure recovery,
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Which is the percentage of the pressure caused by the plane's supersonic flight
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forward that gets translated into usable pressure inside the diffuser for the engine.
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This ratio is a very high ninety percent, for the SR-71 when flying at Mach 3.2.
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So to keep the normal in the optimal position for pressure recovery,
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the spike retracts 1.6 inches for each point-one increase in Mach number above
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Mach 1.6.
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This changes the relative geometry of the inlet, keeping the normal
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at the optimal position. When the plane reaches its cruising speed of Mach 3.2,
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the external shock wave is positioned directly at the inlet's lip,
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called the cowl, and the inlet spike has retracted 26 inches.
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It's at the speed that the J 58 turboramjet has its maximum fuel efficiency,
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with the pressure recovery at the inlet doing most to the air compression work
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for the afterburner.
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Okay!
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Now that we've seen how the inlet, engine, and ejector all work together, let's look at
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the other details found inside the engine nacelle. Positioned inside the cowl
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is a cowl bleed,
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that captures some of the incoming air and passes it through a ring of circular
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openings called shock traps, that drop the air speed to subsonic
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and guide it through the cell body around the engine for cooling.
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The air is drawn out of the nacelle by the fast-moving exhaust flowing through the
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ejector.
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At speeds below Mach 0.5, not enough air is coming through the inlet for cooling,
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so more air comes in through suck-in doors that are position midway
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along the nacelle. These close at Mach 0.5, which the plane only hits just
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after takeoff,
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and just before landing. Before the ejector are set up tertiary doors that
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also open at low speeds
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to prevent the ejector from creating places a drag caused by not enough air
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and exhaust flowing
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through it. These close at Mach 1.2 and stay closed for most of the Blackbird's flight,
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opening only for takeoff, landing, and refueling.
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Both the suck-in doors and the tertiary doors allow the powerful J58 to operate
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at speeds much slower than its high cruise speed.
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The engines are so big that each needs two muscle car motors on the ground to start it,
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and rev it up to a self-supporting speed. But, I digress.
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The last three details found in the nacelle are required for keeping the
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normal shockwave in place in the inlet.
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The first is the centerbody bleed, which connects agrill on the outside the
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nacelle to set of slits on the spike through the hollow centerbody middle.
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At low speeds, the centerbody bleed allows the engine to pull additional air
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into the inlet, while at higher speeds the bleed wicks away the boundary layer
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a layer of low-pressure turbulent air that normally sticks to the spike and reduces
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pressure recovery.
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Inside the inlet is a series of slots that run between the shock tubes that lead
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directly out of the plane.
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These are the forward bypass doors, which allow an analog computer to lower the
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pressure inside the diffuser by sending some of it outside the aircraft.
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But if the pilot wants to reduce drag during acceleration, or provide
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additional cooling to the engine, he or she can open the aft bypass doors,
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which route the additional pressure through the nacelle and out the ejector.
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And that's it! That's how the J58 turboramjet inside the Blackbird engine nacelle, works!
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Now when you look at these diagrams been on scene so complicated after all!
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Don't forget to like this video and subscribe to Tech Laboratories for more
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mind-blowing videos on science and technology!
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I'm Tech Adams saying Keep Thinking and thanks for watching!