it's hard to explain the engineering
marvel that is the sr-71 blackbird a
long-range plane capable of flying 26
kilometers above the surface of the
planet so high that the pilots could see
the curvature of 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 zero all the way up to
mach 3.2 dealing with the myriad of
problems like cooling fuel efficiency
and supersonic shockwaves interfering
with airflow a plane so advanced that
when it detected a surface-to-air
missile its 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 in list yes this
plane was powered by the pratt & whitney
j58 turbojet engine forget this off the
shelf
these engines could only provide
seventeen point six percent of the
thrust required for mach 3.2 flies a
speed 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 be first 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
this needs forward movement in order to
achieve the correct compression of air
in the combustion chamber so there are
either dropped from a conventional plane
have a secondary propulsion system or
are a hybrid of a conventional jet
engine and a ramjet
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 this is
capable of moving forward and back by
0.66 meters this adjusts the inlet and
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 shock wave as
it minimizes the energy lost due to drag
as air flows over the shock wave the
inlet spike stays in the forward
position until Mach 1.6 after this point
it begins to move backwards by 41
millimetres for every 0.1 increase in
Mach number keeping the shock wave in
its ideal position the inlet spike
contains perforations which connect to
the outside of the nacelle through ducts
initially the airflow 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 clinks to the
surface of objects by bleeding this
layer of slow-moving air off 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 blade
located here which again helped minimize
boundary layer growth there were suckin
doors located here which only opened
from Mach zero 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 engine 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 shock wave if this was mismanaged
the engine would lose control of the
normal shock wave and may even spit it
out of the intake resulting in a sudden
power loss called an on stairs which
would cause the plane to violently yaw
in the direction of the faulty engine if
this happened the forward bypass doors
opened fully and the spike would move to
the forward position to reduce back
pressure and get the shockwave back into
its normal position
besides this bypass area that took air
from the inlet and dumped it into the
ejector there were also six 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 waste they basically just
inject fuel into the exhaust of the jet
engine and ignited 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 is that the engineers could
have eked out more thrust from this
engine to increase the top speed even
more ram jets can go up to Mach 5 so why
did they stop at three point two 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 west wing fuel tank
system which meant that the fuel was not
contained within a separate 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 field 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 the fuel to leak out because of
this the sr-71 had to regularly go into
maintenance and have sealant reapplied
but it usually came back still leaking
just not quite as much the number of man
hours required to reduce it to zero was
simply too great to fit it between
flights so they just had an allowable
fuel leak limit which looked like this
this plane like a rocket was mostly fuel
it's dry waste depending on sensor
payload was between 25 and 27 tons it's
wet waste was between 61 and 63 tons
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 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 temperature
was higher a fully loaded sr-71 could
expect to burn nearly 13 metric tons of
fuel accelerating from Mach 1.25 at
30,000 feet to mach 3 at 70,000 feet if
the outside temperature was 10 degrees
Celsius above standard that is 36% of
its fuel capacity if it was 10 degrees
below standard the fuel burn nearly half
to 7.2 tons and of course 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
kilometers a
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 refuelling the
plane could stay in the air more or less
indefinitely provided there was no
mechanical issues really the range was
entirely determined by the pilots the
longest operational sortie occurred in
1987 when the u.s. flew the sr-71 from
Okinawa to observe developments in the
iran-iraq war this mission lasted 11
point 2 hours and likely required at
least five aerial refuelling x'
along the way so if it wasn't the fuel
or engines that limited the sr-71 stop
speed what did at Mach 3 point to 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
this was a specially formulated fuel
called jp-7 which has very low
volatility and a high flash point this
was partially needed because the fuel
leaked on the runway and they needed a
fuel that wouldn't ignite or easily
evaporate and make the ground crew ill
but mostly they needed a fuel that
wouldn't vaporize in the tanks and cause
fuel feed and pressurization problems
the jp-7 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 shots of triethyl boring a fuel
that spontaneously ignites in the
presence of oxygen to start the
combustion cycle and afterburners
the plane usually only carried about 16
shots of this so the pilots needed to
manage them carefully particularly when
slowing down for refueling and managing
on starts 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-71 s
predecessors were unpainted which saved
waste and the areas exposed to 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 its
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 kirchoff's
rule of radiation which tells us that a
good heat absorber like a black object
is also an equally effective heat and
Mitter so the black paint helped the
sr-71 radiate heat away from the plane
as it allowed the plane to radiate more
heat than a gained from the radiation
from the Sun these efforts helped to
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 its speed limited by heat to
a much lower Mach 2 aluminium is cheap
has a great strength or a ratio and is
easily machinable titanium the material
that made up 93% of the sr-71 has only
one of these properties its strength to
weight ratio otherwise known as specific
strength is fantastic but titanium is
incredibly expensive despite it being
the seventh 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 resists heat from
breaking them apart titanium alloys can
resist temperatures up to 600 degrees
Celsius before their atoms begin to
diffuse and slide over each other
significantly allowing it to retain much
of its strength even at 300 degrees it
has also very low thermal expansion so
that expansion and contraction we
mentioned earlier is minimized reducing
the thermal stresses in the aircraft but
titanium has its 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 compsat 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-71
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 its 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 curiosity stream 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 molds 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
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