Sanuj Raj
The Startup
Published in
10 min readJul 29, 2020



The SR-71 was one of the greatest achievements of the 20th century in the field of aviation. SR-71 was a product of the legendary aircraft engineer Kelly Johnson. It was built in the Skunk Works facility. SR-71 was a high altitude plane, it could fly at an altitude of about 26 km. So high that the pilots could see the curvature of the planet and the inky blackness of the sky(space) from their cockpit.

SR-71 stands for Strategic Reconnaissance-71.

It had set some records that stand unbroken to this day. It had an iconic design which was a result of thousands of hours spent in the wind tunnel perfecting the minutest of details. It was primarily used for the surveillance of enemy territory from a very high altitude. It had a camera system that was so high resolution that it could easily identify car number plates from 80,000 feet above the ground.
It was named blackbird because of the special black paint that was applied to the plane and its inability to appear on the radar.

It was the result of the cold war between the USA and the USSR. Many secrets activities were going on in USSR which the Americans wanted to know, for which they needed surveillance planes to capture the area from above. As a result, they made the U-2 which was neither fast nor stealthy, unlike SR-71. The only thing it could do is to fly at a higher altitude. The USA thought that Soviet radar couldn't detect the U-2 at such high altitudes but they were wrong. The Soviets had tracked the U-2 since day one and it was only a matter of time before they would be able to shoot one down. It turned out, simply flying high wasn't enough.

The Behemoth SR-71 Itself (Top Down View)

You might ask, a plane so advance would be having some high-tech technique to defend itself but it was the other way around, the plane was so fast that ‘changing flight path and just simply speed up’ was the primary defense system of the SR-71. Even though missiles had a higher top speed it couldn’t achieve the altitude and maneuverability of the SR-71. It was very good at escaping, out of the 32 aircraft ever built not a single one was lost to an enemy missile rather 12 were destroyed in accidents. One literally disintegrated around its pilots.

Tires were filled with 400 psi which made it as hard as a rock filled with nitrogen.

TITANIUM: The Material that Made it Happen

Very little was known about the working aspects of the material before the SR-71. The engineers were among the very first people in history to make commercially viable real use of the material despite it being the 9th most abundant material on Earth at a percent weight of 0.56%. Almost 93% of the SR-71's structure was made of titanium. In the 1940s the first reliable process to produce a chemically pure form of titanium was developed called Kroll Process. This made the SR-71 possible.
You will be amazed to know that most of the titanium ore came from USSR through ghost companies, had they known what it is going to be used for they would not have supplied it to the USA.

The primary titanium alloy used in the SR-71 was named B-120VCA. It comprised of 13% Vanadium, 11% Chromium, and 3% Aluminium. Both Cr and Al form thermally stable oxide layers on the outer skin of the metal which raises the maximum operating temperature of the metal. Whereas, the V acts as a material with higher tensile strength and better affordability, with the ability to heat treat to higher strength.

The SR-71’s speed was not limited by the power of its engines instead it was limited by the heat its structure could withstand.

Heat Map of SR-71 (Temperatures During High Mach Number)

What makes titanium special is not its tensile strength, weight or high-temperature performance, but a combination of all these properties that made it more favorable than steel or aluminum. Also, titanium has a higher strength-to-weight ratio (specific strength) than its equivalent counterparts. But nowadays very little titanium is used in fighter planes, aluminum is the primary material used as titanium is very expensive because of its refinement process.

Material Selection Diagram for the Aerospace Industry

In the first place, during manufacturing and machining of titanium, engineers were having many problems regarding the material, but thankfully they documented each and every part of the process which helped to eliminate many difficulties. Some of the difficulties that they faced were:-

  • They discovered that their Cadmium plated tools were leaving trace on bolts which would cause galvanic corrosion and ultimately cause the bolts to fail. This led to all cadmium tools to be removed from the workshop.
  • Titanium required more pressure to deform than steel or aluminum. At the time the best presses could only produce about 20% of the pressure required to do the job. Because of these inadequacies in the forming capabilities, the final forging dimensions were nowhere near the design dimensions and much of the forming process had to be completed through machining which resulted in about 90% of the material going to waste. Amazingly, by the end of the program, the engineers had found enough improvements to save $19 Million on the manufacturing processes.

Here’s a youtube video that will tell you more about the deforming pressure of titanium:

See the Video to Know Who Wins
  • The extremely low thermal conductivity made the machining very difficult. Machining materials produce a lot of heat that can damage the tools and cause unfavorable material properties in the aluminum, like hardening which means the metal under the fresh-cut, is now harder than and therefore even more damaging to the tools.
    This is minimized with coolant but titanium’s low conductivity doesn’t allow it. To deal with lower machining speeds had to be used along with a high volume of coolant which is indeed expensive. The slower speeds made the overall process incredibly slow.

Interestingly, Titanium’s maximum operating temperature is less a function of loss in strength but a function of oxiadation. Meaning, at higher temperatures oxidation will cause more damage and help dangerous cracks to form on the surface.


The SR-71 uses a hybrid engine that had a combination of turbojet and ramjet. At lower speeds, it behaved as a turbojet engine and higher Mach speeds it acted as a ramjet engine. It was the first of its kind and as a result, it was given its own name which was Turboramjet (Air Turborocket). The plane had a Pratt and Whitney J58 engine which was first developed in 1958.

The J58 Turboramjet Engine

The whole engine is divided into 3 subcategories which are the airflow inlet (the pointed cone at the start of the engine), the Pratt & Whitney J58 engine, and the end convergent-divergent ejector.
At speeds below Mach 2, the J58 acts like any other afterburner turbojet engine. Air flows into nacelle through the inlet where it's allowed to diffuse behind the supersonic shockwave before moving into the multi-stage axial compressor. Here, the air is compressed before heading into the burner where fuel is added for combustion. The turbine rotates and provides the engine’s forward thrust. Just after the turbine is the afterburner where more fuel is added which allows for powerful bursts of accelerations, they are really inefficient costing huge amounts of fuel for the increased force. The flame that comes out of the end of the engine is green in color due to the contents of the fuel.


At supersonic speeds, the inlet spike takes the pressure of the leading supersonic shockwave off of the engine so that the engine gets the best airflow. Inside the inlet, the second shockwave is formed called the normal, where the air coming into the nacelle transitions from low-pressure supersonic speeds to high-pressure subsonic speeds.
When the aircraft hits Mach 1.6, the normal ends up in the best place inside the inlet for pressure recovery, so to keep the normal in the optimal position for pressure recovery, the inlet spike retracts 1.6 inches for every 0.1 increase in Mach number above Mach 1.6. When the plane reaches its cruising speed of Mach 3.2 the external shock wave is directly positioned at the inlet’s lip called cowl and the inlet spike retracted 26 inches (66 cm). It’s at this speed that the J58 engine has the maximin efficiency.

Fun Fact: Top speed of SR-71 is still unknown because beyond Mach 3.5 the heat generated from the frictional force on the surface of the plane would be dangerously high which they dared not to cross.

Retraction of Inlet Spie During the Flow of Air at Mach 1.6 to Mach 3.2

Fun Fact: At Mach 3.2 the J58 engines only produced 20% of the thrust.

Want to learn more about jet engines in detail:


Skunk Works was Lockheed’s Advanced Development Programs (ADP) where all the advanced and technically challenging projects are done. Over the years it has got some of the most technical and challenging tasks by the US government to develop the most advanced planes of the era. The newly developed F-35 is a product of Skunk Works. Over the years it has become a very valuable company as it has made some of the classified products for the US government.

The company first made twelve A-12 aircraft for the CIA which was a single-seater aircraft which was very similar to SR-71 with only a few modifications. Later the US airforce got interested in the A-12 and ordered two-seater versions of the same from Lockheed, it was named SR-71, which later became legendary.

It is named Skunk Works because of a nearby plastic facility that produced the foul smell as skunks are synonymous with smell. Its headquarters is situated in Burbank, California.


Advancement in spy satellites and aerial drones made the SR-71 look old and its inability to deliver surveillance data in real-time diminished some of the plane’s utility. Add to that, politics and infighting for defense budget and by the late 80s, the SR-71 days were numbered. They were officially retired in 1998, with two sent to NASA for testing, which the NASA retired in 1999 as well. Nowadays it sits in the museum as an art piece that was a pinnacle of engineering a generation ago.

As silly as it may sound but the engineers never figured out a way to completely seal the fuel tanks. Whenever it was there on the ground sitting it started to leak and the fuel would spill all over the place. There were panel gaps to accommodate the increase in size when the plane flew at supersonic speeds which increased its size by 6 inches lengthwise. They took off with half the amount of fuel so that it would have less fuel wastage. This is the reason they needed to refuel just after taking off. The more the fuel loaded at the ground the more leakage it would have.

SPECIALITY OF SR-71 (Interesting Facts & Theories)

  • The J58 engine has 6 bypass tubes which you will not find in any other turbojet. The tubes open the speed of the plane is greater than Mach 2.2, moving compressed air from the fourth stage of the compressor directly into the afterburners. This allows the engine to act more like a ramjet which allows the afterburner to operate at a much higher fuel efficiency. This adds an extra compression of +1.6:1 (Compression: Ambient).
The 6 bypass tubes on the J58 engine
  • One of the requirements to join the program was to be married. You couldn't be an SR-71 pilot if you're not married. It was done because married personnel were less likely to defect to the USSR. Single person is more likely to defect since there is nothing holding them back, especially if the co-pilot is also a signal male.
  • The pilots of the SR-71 were given astronauts suits to wear because the altitudes they would be dealing with would not be encountered with a normal helmet and suit. They were flying as high as 80,000 feet which was considered as the edge of space in those days. The loss in pressure, if it were to happen at such heights would only be handled by a spacesuit.
  • Very interesting but true story about the SR-71:
Incredible Story From a Former SR-71 Pilot
    This story is interesting in which a Cessna plane pilot asks for a speed check from the ATC (Air Traffic Controller) and the ATC responds by saying 100 knots, then after some time another Cessna plane pilot asks for a speed check, the ATC responds by saying 130 knots, then after a while a fighter pilot asks for a speed check to show that he is the speed king of this place right now and the ATC responds by saying 690 knots. Then the pilot of the SR-71 realized that the fighter plane has a multi-million dollar cockpit and he is asking for a speed check then it became clear he was trying to prove himself to be the fastest.
    In the end, the SR-71 pilot asks for a speed check and the ATC responded by saying 1900 knots shutting everyone in the ATC’s area. Mind you, there was no speed check thereafter.

Watch the video to know all information about the SR-71 from one of its pilot:-

Former SR-71 pilot Buz Carpenter Explain the Plane