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.