The Insane Engineering of the SR-71 Blackbird | What Makes the SR-71 so Special?

SR-71 Blackbird

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The SR-71 Blackbird was an engineering marvel

Capable of flying 26 kilometres above the surface of the planet and dissipating heat generated from aerodynamic friction. Its engines were able to function from 0 all the way up to mach 3.2, and it never lost an aircraft to enemy fire.

The Pratt and Whitney J58 turbojet engine could only provide 17.6% of the thrust required for Mach 3.2 flight, so the SR-71 used a ramjet to achieve those speeds. A ramjet uses ram pressure to operate, and slows down the supersonic airflow before entering the combustion chamber.

No moving parts, ram jet engines can fly at much higher speeds than conventional jet engines, but they need forward movement to start.

The inlet spike adjusts the airflow entering the engine and keeps the shockwave in its ideal position. It moves backwards by 41 millimetres for every 0.1 increase in mach number.

The inlet spike contains perforations that allow air to flow from the outside in, but as the plane speeds up, the airflow reverses to allow high energy fast moving air to enter.

Around the J58 engine there is a bypass area that takes air from the inlet and cools the engine, improving engine efficiency and allowing the plane to fly faster.

Air got into the bypass area in a number of ways, including the cowl bleed, suck-in doors, and forward bypass doors. The forward bypass doors were used to control the pressure level in the inlet at the optimum level, and to maintain the position of the normal shockwave.

There were 6 bypass ducts that took air from the compressor and dumped it directly into the afterburner, which transformed the engine from a turbojet into a ramjet. The afterburners are really inefficient, but they are the only feasible way to generate thrust.

The military doesn't care about fuel efficiency, but the heavier and bigger the plane gets, the more fuel it uses. The engineers managed to fill the plane up with an astounding amount of fuel.

The SR-71 used a total wet wing fuel tank system, which meant that the fuel was contained by the skin of the plane itself. The sealant applied to every gap the fuel could possibly come out of gradually deteriorated over time, allowing fuel to leak out.

The SR-71 was mostly fuel, and had a range of 5,200 km. Its range varied greatly depending on the outside temperature, and it burned nearly 13 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.

The US was not going to be landing at their target to hand over a top secret plane to the enemy, but with aerial refueling the plane could stay in the air for hours.

The SR-71's top speed was limited by overheating. A specially formulated fuel called JP7 was used, which had very low volatility with a high flash point, and was used to cool critical components like the engine oil, hydraulic systems and control electronics.

The fuel was so stable that the pilots needed to manage the Triethylborane carefully when slowing down for refuelling or managing unstarts.

The SR-71 was painted black to reflect heat and prevent the plane from overheating. This was because a good heat absorber is also an equally effective heat emitter, and the black paint helped the plane radiate heat away from the plane.

Aluminum is typically the material aircraft engineers turn to, but titanium has a much higher specific strength and is easier to machinable. Titanium is also incredibly strong and can resist temperatures up to 600 degrees celcius, reducing the thermal stresses in the aircraft.

Engineers have made huge strides in material science, and the SR-71's successor the SR-72 will take advantage of new high performance composites to reach speeds up to Mach 6.