It’s hard to explain the engineering marvel that is the SR-71 Blackbird. A long range
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
is typically needed.
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
in mid flight.
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
from a turbojet into a ramjet.
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
thrust, but they are really inefficient.
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
5. So why did they stop at 3.2 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 engineering that caused it.
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
skin of the plane itself. The engineers applied sealant to every gap the fuel could possibly
come out of, but because the titanium skin of the plane expanded and contracted with
every flight it gradually deteriorated over time. Allowing fuel to leak out.
Because of this the SR-71 had to regularly go into maintenance and have sealant reapplied,
but it usually just came back still leaking, just not quite as much. The number of manhours
required to reduce it to zero was simply too great to fit between flights, so they just
had an allowable fuel leak limit, which looked like this.
This plane, like a rocket, was actually mostly fuel. It’s dry weight, depending on sensor
paid load, was between 25 tonnes and 27 tonnes. It’s wet weight was 61 to 63 tonnes. Making
it by weight 59% fuel to feed those hungry engines.
Even then, without the ability to refuel in the air this plane would have had a terrible
range for what was supposed to be a long range spy plane.
Range varied greatly. For example the engines became significantly less efficient when the
outside temperatures were higher. A fully loaded SR-71 could expect to burn nearly 13
metric tonnes of fuel accelerating from Mach 1.25 at 30,000 feet to Mach 3.0 at 70,000
feet if the outside temperature was 10 degrees celsius above standard. That’s 36% of its
fuel capacity. If it was 10 degrees below standard, the fuel burn nearly halved to 7.2
tonnes. [Page 26 of ].
And ofcourse the range was severely affected by their speed and use of the afterburner,
but on average the SR-71 had a range of about 5,200 kilometres [Page 27]. About enough for
a one way trip from New York to London. Not terribly useful, the US was not going to be
landing at their target to hand over a top secret plane to the enemy. However, with aerial
refueling the plane could stay in the air more or less indefinitely provided there were
no mechanical issues. Really the range ended up being entirely determined by the pilots.
The longest operational sortie occured in 1987 when the US flew the SR-71 from Okinawa
to observe developments in the Iran-Iraq war. This mission lasted 11.2 hours and likely
required at least 5 aerial refuelings along the way.
So, if it wasn’t the fuel or engines that limited the SR-71s top speed. What did?
At Mach 3.2 the nose of the SR-71 reached 300 degrees celsius, while the engine nacelles
could reach 306 at the front and 649 at the back. This is what truly limited the top speed
of the SR-71. Without careful material selection and design, the plane would simply overheat
Even the fuel needed to be specially formulated to get around these overheating issues. It
was a specially formulated fuel called JP7. Which had very low volatility with a high
flash point. This was needed partially because the fuel leaked on the runway and they needed
a fuel that wouldn’t ignite easily or evaporate and make the ground crew ill, but mostly they
needed a fuel that wouldn’t vaporise in the tanks and cause fuel feed and pressurisation
problems. The JP7 fuel was so stable that it actually doubled as a coolant for the entire
plane. The fuel was pumped around the airframe to cool critical components like the engine
oil, hydraulic systems and control electronics. When the fuel got too hot it was simply sent
to the engines for combustion. The fuel was so stable that the plane actually needed to
carry Triethylborane, ,a fuel that spontaneously ignites in the presence of oxygen, to start
the combustion cycle and after burners. The plane usually only carried about 16 shots
of this, so the pilots needed to manage them carefully, particularly when slowing down
for refuelling or managing unstarts.
One huge question I had about the SR-71 was why it was painted black. Airliners are all
white to reflect heat and prevent the plane from overheating. If that applies to an airliner,
why not the SR-71? The SR-71s predecessors were unpainted, which saved weight, and the
areas exposed to the highest temperatures were painted black.
Why was this? Surely black would absorb more heat? The Concorde was once painted blue for
a Pepsi ad campaign and had to lower it’s speed, as it absorbed too much heat from the
sun. However the Concorde did not fly nearly as high or as fast as the SR-71, and as the
SR-71 rose the energy it absorbed from the sun dwindled in comparison to the heat it
gained from aerodynamic friction.
For this we have to refer to something called the Kirchoff’s Rule of Radiation, which
tells us that a good heat absorber, like a black object, is also an equally effective
heat emitter. So, the black paint helped the SR-71 radiate heat away from the plane, as
it allowed the plane to radiate more heat than it gained it from radiation from the
These efforts helped keep the plane cool, but the structure of the plane still needed
to be incredibly heat stable. Aluminium is typically the material aircraft engineers
turn to. It was used for the Concorde, but as we saw it too had it’s speed limited
by heat to a much lower Mach 2. Aluminium is cheap, has a great strength to weight ratio
and is easily machinable.
Titanium, the material that made up 93% of the SR-71, has only one of these properties,
it’s strength to weight ratio, otherwise known as specific strength, is fantastic.
But, Titanium is incredibly expensive, despite it being the 7th most common metal in Earth’s
crust. The refinement process is incredibly long and requires expensive consumables. It’s
also not easily machinable as it readily reacts with air when welding or forging, becoming
For these reasons, titanium is rarely used in structural parts in aviation. However,
the real benefit of titanium is its ability to resist heat. The reasons for this are complex
that we will explore in depth in future. However, the gist is that titanium alloys have incredibly
strong bonding within its crystal lattice that resist heat from breaking them apart.
Titanium alloys can resist temperatures up to 600 degrees celcius before their atoms
begin to diffuse and slide over each other significantly. Allowing it to retain much
of it’s strength even at 300 degrees. It also has a very low thermal expansion, so
that expansion and contraction we mentioned earlier is minimised. Reducing the thermal
stresses in the aircraft.
But Titanium too has it’s limits, and for the SR-71 this was about 3.2 Mach.
Today engineers have made huge strides in material science.
The SR-71 used heat resistant composite materials as radar absorbing wedges between the structural
frame, located in these locations. The manufacturing techniques needed to make composite materials
as load bearing structures did not yet exist, but that has changed. The SR-71s successor
the SR-72, which is now in development, will take advantage of new high performance composites,
which will allow it to reach speeds up to Mach 6. Many of it’s engine components will
likely be 3D printed titanium with cooling ducts printed right into the part. It’s
range also won’t be determined by pilots, as it will be an autonomous drone.
The insane engineering that makes planes like this possible fascinates me, and I recently
watched an excellent documentary on Curiositystream that details the build process for the world’s
largest airliner, the A380. Chronicling the massive sheet metal cutting machines that
cut the aluminium skin, the vacuum moulds that form it, and the biggest oven in Britain
that locks the shape in place. This is just one step in the process, and the documentary
is nearly an hour long. This is just one of thousands of documentaries by award winning
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