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  • 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

  • film makers available on CuriosityStream right now, and a subscription costs just 11.99 for

  • an entire year. With that subscription, you will also get free access to Nebula, the streaming

  • service created by me and my fellow YouTube creators. Here you can watch my Logistics

  • of D-Day series that details the planning and technology that made D-Day possible. The

  • first episode is available for free on this channel, if you would like a taste of what

  • you are signing up for.

  • By signing up to CuriosityStream and Nebula you will be helping not just me, but the entire

  • educational community, as we work together to build a place where we can create unique

  • experimental series, like Tom Scott's new game show, Money.

  • As always, thanks for watching and thank you to all my Patreon supporters. If you would

  • like to see more from me the links to my instagram, twitter, subreddit and discord server are

  • below.

It’s hard to explain the engineering marvel that is the SR-71 Blackbird. A long range

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SR-71 黑鸟的疯狂工程(The Insane Engineering of the SR-71 Blackbird)

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    joey joey 發佈於 2021 年 06 月 09 日
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