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  • If you've ever tried to understand the SR-71's engines, chances are

  • you've come across these diagrams from the SR-71's flight manual.

  • Let's face it though: they're not as clear as they could be. So let's clean it

  • up and simplify.

  • There we go! The whole diagram is the complete engine nacelle

  • made up of the airflow inlet, the Pratt & Whitney J58 engine, the convergent

  • divergent ejector,

  • and the airplane body. At speeds below Mach 2, the J 58 acts like any other after

  • burning turbojet engine.

  • Air flows into the nacelle through the inlet, where it's allowed to diffuse behind

  • the supersonic shockwave before moving into the multi-stage axial compressor

  • which looks like this, but bigger. Here, the air is compressed before heading into the

  • burner where fuel is added for combustion.

  • The heated exhaust turns the turbine and provides the engine's forward thrust as

  • its accelerated to high speeds by the ejector.

  • The turbine turns the compressor and keeps the engine cycle going.

  • Just after the turbine

  • is the afterburner, where more fuel is added to the exhaust in order to get as

  • much of the oxygen

  • out of the air as possible. While afterburners allow for powerful bursts

  • bursts of acceleration,

  • they're really inefficient, costing huge amount of fuel for the increased force.

  • What makes the J58 engine so different than all other turbojets are these 6 bypass tubes

  • which you don't find on these diagrams. The tubes open when the plane is flying at

  • speeds greater than Mach 2.2,

  • moving compressed air from the fourth stage the compressor directly into the

  • afterburner

  • This allows the engine to act more like a ramjet, which allows the SR-71's

  • afterburner to operate at a much higher fuel efficiency,

  • the forward motion aircraft handling most to the air compression

  • at a ratio of about 39:1 (38.8:1), with the four turbines ages adding an

  • additional compression about 1.6:1.

  • The combined action of the turbine and ramjet compression makes the J 58 a very

  • unique type of engine:

  • a turboramjet; and allows the plane to cruise at speeds that would make a

  • normal turbojet melt.

  • The Blackbird's inlet design is as important to allowing the J58 to do its thing as

  • the engine itself so let's see how it works.

  • In the middle of the inlet is this symmetrical spike, called the inlet spike,

  • or centerbody,

  • and behind it is the diffuser, where compressed air spreads out before

  • entering the engine.

  • At supersonic speeds the inlet spike takes the pressure the leading

  • supersonic shockwave

  • off of the engine so that the engine gets the best airflow. Inside the inlet a

  • second shockwave is formed called the normal, where the air coming into the

  • nacelle transitions from low-pressure, supersonic speeds, to high-pressure,

  • subsonic speeds.

  • Where the normal ends up inside the inlet depends on the speed at which the

  • aircraft is moving and the shape of the inlet

  • and inlet spike. When the aircraft hit's Mach 1.6,

  • the normal ends up in the best place inside the inlet for pressure recovery,

  • Which is the percentage of the pressure caused by the plane's supersonic flight

  • forward that gets translated into usable pressure inside the diffuser for the engine.

  • This ratio is a very high ninety percent, for the SR-71 when flying at Mach 3.2.

  • So to keep the normal in the optimal position for pressure recovery,

  • the spike retracts 1.6 inches for each point-one increase in Mach number above

  • Mach 1.6.

  • This changes the relative geometry of the inlet, keeping the normal

  • at the optimal position. When the plane reaches its cruising speed of Mach 3.2,

  • the external shock wave is positioned directly at the inlet's lip,

  • called the cowl, and the inlet spike has retracted 26 inches.

  • It's at the speed that the J 58 turboramjet has its maximum fuel efficiency,

  • with the pressure recovery at the inlet doing most to the air compression work

  • for the afterburner.

  • Okay!

  • Now that we've seen how the inlet, engine, and ejector all work together, let's look at

  • the other details found inside the engine nacelle. Positioned inside the cowl

  • is a cowl bleed,

  • that captures some of the incoming air and passes it through a ring of circular

  • openings called shock traps, that drop the air speed to subsonic

  • and guide it through the cell body around the engine for cooling.

  • The air is drawn out of the nacelle by the fast-moving exhaust flowing through the

  • ejector.

  • At speeds below Mach 0.5, not enough air is coming through the inlet for cooling,

  • so more air comes in through suck-in doors that are position midway

  • along the nacelle. These close at Mach 0.5, which the plane only hits just

  • after takeoff,

  • and just before landing. Before the ejector are set up tertiary doors that

  • also open at low speeds

  • to prevent the ejector from creating places a drag caused by not enough air

  • and exhaust flowing

  • through it. These close at Mach 1.2 and stay closed for most of the Blackbird's flight,

  • opening only for takeoff, landing, and refueling.

  • Both the suck-in doors and the tertiary doors allow the powerful J58 to operate

  • at speeds much slower than its high cruise speed.

  • The engines are so big that each needs two muscle car motors on the ground to start it,

  • and rev it up to a self-supporting speed. But, I digress.

  • The last three details found in the nacelle are required for keeping the

  • normal shockwave in place in the inlet.

  • The first is the centerbody bleed, which connects agrill on the outside the

  • nacelle to set of slits on the spike through the hollow centerbody middle.

  • At low speeds, the centerbody bleed allows the engine to pull additional air

  • into the inlet, while at higher speeds the bleed wicks away the boundary layer

  • a layer of low-pressure turbulent air that normally sticks to the spike and reduces

  • pressure recovery.

  • Inside the inlet is a series of slots that run between the shock tubes that lead

  • directly out of the plane.

  • These are the forward bypass doors, which allow an analog computer to lower the

  • pressure inside the diffuser by sending some of it outside the aircraft.

  • But if the pilot wants to reduce drag during acceleration, or provide

  • additional cooling to the engine, he or she can open the aft bypass doors,

  • which route the additional pressure through the nacelle and out the ejector.

  • And that's it! That's how the J58 turboramjet inside the Blackbird engine nacelle, works!

  • Now when you look at these diagrams been on scene so complicated after all!

  • Don't forget to like this video and subscribe to Tech Laboratories for more

  • mind-blowing videos on science and technology!

  • I'm Tech Adams saying Keep Thinking and thanks for watching!

If you've ever tried to understand the SR-71's engines, chances are

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B2 中上級

マイティJ58 - SR-71の秘密の動力源 (The Mighty J58 - The SR-71's Secret Powerhouse)

  • 90 6
    Shelby Lai に公開 2021 年 01 月 14 日
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