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!

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