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
air flow.
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
used.
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
ideal position.
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
efficiency.
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 [2]].
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
and fail.
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
sun.
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
brittle.
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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