Name: The Insane Engineering of the SR-71 Blackbird
Uploaded: 2021-06-09T06:45:46.000Z
Duration: 18 min 55 s
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It's hard to explain the engineering marvel that is the SR-71 Blackbird. A long range

The subjunctive

plane capable of flying 26 kilometres above the surface of the planet. So high that the

pilots could see the curvature on the planet and the inky black of space from their cockpits.

It flew so fast that the engineers had to develop entirely new materials and designs

to mitigate and dissipate the heat generated from aerodynamic friction. Entirely unique

engines were needed to function from 0 all the way up to mach 3.2, dealing with a myriad

of problems like cooling, fuel efficiency and super sonic shock waves interfering with

A plane so advanced that when it detected a surface to air missile it's response was

simply to change course and speed up. Even though the missiles had a higher top speed,

they couldn't achieve the range and high altitude maneuverability the Blackbird could.

This allowed the SR-71 to run hundreds of missions through Vietnam, North Korea, and

Iraq without ever losing an aircraft to enemy fire, despite multiple attempts.

The entire plane was built around the propulsion system, which alone was a miracle of engineering

design. For one, no turbine driven jet engine can operate with supersonic flow at its inlet.

Yet, this plane was powered by the Pratt and Whitney J58 turbojet engine, but get this,

off-the-shelf these engines could only provide 17.6% of the thrust required for Mach 3.2

flight. A speed at which the SR-71 could cruise at for extended periods of time.

How on earth did it manage that? In order to achieve those kinds of speeds a ramjet

A ramjet, as you can probably guess from the name, relies on ram pressure to operate. Ram

pressure is simply the pressure that occurs as a plane rams itself through the air.

So, as the engine moves through the sky, it funnels this high pressure air inside. Before

entering the combustion chamber the supersonic airflow must first be slowed down. This basically

acts like the compressor stage of a normal jet engine, elevating the air pressure before

it enters the combustion chamber. Once the air enters the combustion chamber

it is mixed with fuel and ignited. It expands and accelerates once again out of the exit

nozzle. With no moving parts this type of engine is capable of flying at speeds far

greater than a typical turbine driven engine, but it cannot start from zero. It needs forward

movement in order to achieve the correct compression of air in the combustion chamber. So, they

are either dropped from a conventional plane, have a secondary propulsion system, or are

a hybrid of a conventional jet engine and a ram jet, which is precisely what the SR-71

The turbojet J58 engine of the SR-71 is nestled inside the nacelle here. In front and around

the J58 is a complicated system of airflow management. These control mechanisms allow

the propulsion system to transition from a primarily turbojet engine to a ramjet engine

First, the inlet spike. It is capable of moving forward and back by 0.66 metres.

This adjusts the inlet and the throat area, which controls the airflow entering the engine.

It also keeps the position of the normal shock wave at its ideal position between the inlet

throat and the compressor, this is the most efficient position for the shockwave, as it

minimizes the energy lost due to drag as air flows over the shockwave. The inlet spike

stays in the forward position until Mach 1.6. After this point it begins to move backwards

by 41 millimetres for ever 0.1 increase in mach number. Keeping the shockwave in it's

The inlet spike contains perforations which connect to the outside of the nacelle through

ducts. Initially the flow of air will come from the outside in to provide additional

airflow to the turbojet engines, but once the plane hits about Mach 0.5 this airflow

reverses. As the plane speeds up the inlet spike develops a significant boundary layer

of air. A boundary layer is a layer of very slow moving air that clings to the surface

of objects. By bleeding this layer of slow moving air off to the inlet spike it frees

up a greater area of the inlet area for high energy fast moving air, and thus improves

Around the engine there is a bypass area, which takes air from the inlet and bypasses

it around the J58 engine. This air was used to cool the J58, which again improved engine

efficiency and allowed the plane to fly faster. After the air passes the engine it rejoins

the airflow just after the engine afterburner, adding additional thrust as more oxygen becomes

available for combustion and increases the pressure through the ejector nozzle.

Air got into this bypass area in a number of ways. There was a shock trap, otherwise

known as the cowl bleed, located here, which again helped minimize boundary layer growth.

There were suck-in doors, located here, which opened only from Mach 0 to Mach 0.5, to add

additional air to the bypass for engine cooling.

Air from the aft bypass doors, located just before the J58 inlet, also fed into the bypass.

These together with the forward bypass doors, which vented to the atmosphere were used to

control the pressure level in the inlet at the optimum level. If it was getting too high

a pressure sensor would trigger the forward bypass doors to open allowing more air to

exit the inlet, while the aft bypass doors were controlled by the pilot. These doors

played a critical role in maintaining the position of the normal shockwave. If this

was mismanaged the engine would lose control of the normal shockwave and may even spit

it out of the intake. Resulting in sudden power loss, called an unstart, which would

cause the plane to violently yaw in the direction of the faulty engine. If this happened the

forward bypass doors would open fully and the spike would move to the forward to reduce

back-pressure and get the shock-wave back to its normal position.

Besides this bypass area that took air from the inlet and dumped it into the ejector,

there were also 6 bypass ducts that took air from the compressor and dumped it directly

into the afterburner. These ducts were the primary mechanism that transformed the engine

Afterburners are great, they significantly add to thrust without needing a whole lot

of additional weight. They basically just inject fuel into the exhaust of a jet engine

and ignite it with whatever oxygen is left to provide additional expansion and therefore

However, as the speed increases they are the only feasible way to generate thrust and they

do gain efficiency thanks to the forward motion providing the compression of air needed to

run them, instead of the turbine needing to be powered to turn the compressor stage.

The crazy thing about the SR-71 however, is that the engineers could have eked out more

thrust from this engine to increase the top speed even more. Ramjets can go up to Mach

Would they have run out of fuel? Fuel efficiency in terms of cost doesn't mean a whole lot

to a military plane like this. The military doesn't care about cost. But, the more fuel

you carry the heavier and bigger the plane gets, increasing the fuel it uses. There is

a break even point and the planes range will be limited, but the engineers did manage to

fill the plane up with an astounding amount of fuel with some clever engineering.

The plane was strictly a surveillance plane, so no internal volume was used for weapons,

freeing up space for fuel. You have probably heard that the SR-71 leaks fuel on the runway

because there were gaps in the fuselage, but that's a simple fact that ignores much of

The SR-71 used something called a total wet wing fuel tank system, which meant that the

fuel was not contained within a seperate fuel bladder. This was a weight saving measure,

separate metal fuel tanks would add too much weight and lighter plastic ones would melt

from the intense heat generated from the aerodynamic friction. So, the fuel was contained by the

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The Insane Engineering of the SR-71 Blackbird

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