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!
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