What made the SR-71 so fast?
The Blackbird's speed came from three things working together: a hybrid engine, a variable inlet and an airframe that could survive the heat.
The J58: half turbojet, half ramjet
Each SR-71 had two Pratt & Whitney J58 engines, each rated at about 32,500 pounds of thrust with afterburner. At low speed the J58 worked like a normal afterburning turbojet. As speed climbed, six bypass tubes routed some air from the compressor around the core and straight into the afterburner. At high Mach, the engine behaved largely like a ramjet.
The spike inlets did most of the work
The cone-shaped spike at the front of each nacelle moved backward by up to 26 inches (66 cm) as the aircraft accelerated. That kept the supersonic shock wave in the right place and slowed incoming air to subsonic speed before it reached the compressor. At Mach 3.2 cruise, the inlet itself produced most of the propulsive thrust and the engine core only a small fraction. If the shock wave got pushed out of the inlet, called an "unstart," the airplane yawed violently. Later automatic inlet controls made unstarts much rarer.
Shape and fuel
The flattened "chines" along the fuselage added lift and reduced the radar signature. The aircraft burned a special low-volatility fuel, JP-7, which was hard to ignite. The engines were lit with a chemical called triethylborane (TEB), which bursts into flame on contact with air.
How hot did the SR-71 get at Mach 3?
Air friction at Mach 3 heated the skin to roughly 600°F (316°C) across most of the airframe. The hottest spots ran far hotter: Lockheed expected temperatures on the leading edges to exceed 1,000°F (540°C). Aluminum would have lost its strength, so about 93 percent of the structure was titanium alloy. Some of the raw titanium came, unwittingly, from the Soviet Union: the CIA bought it through front customers to supply Lockheed.
The heat also explains two famous quirks:
- It leaked fuel on the ground. The airframe was built to grow as it heated in flight, so sitting on the ramp the tanks dripped JP-7. The skin shrank again whenever the jet slowed down to meet its KC-135Q tankers, and the tanks leaked until it climbed back to speed.
- It was painted black. The dark, high-emissivity paint radiated heat away, lowering skin temperatures. That gave the airplane its nickname.
The pilots wore full pressure suits similar to early astronaut suits. Above about 63,000 feet, the Armstrong line, an unprotected human's blood would boil.
How high could the SR-71 fly?
The Blackbird's record is 85,068.997 feet (about 16.1 miles, 25.9 km) in sustained horizontal flight. Typical operating altitudes were 80,000 feet (24,400 m) and above. At that height the crew could see the curvature of the Earth, and the sky overhead was nearly black. Altitude and speed were its main defense: when a missile launch was detected, the standard response, as the Smithsonian puts it, was "to simply accelerate and outfly the assailant."
How long did it take the SR-71 to cross the US?
On March 6, 1990, on its delivery flight to the Smithsonian after the Air Force had retired the fleet that January, SR-71 #61-7972 flew from Los Angeles to Washington, D.C., in 64 minutes and 20 seconds. That's an average of 2,144.8 mph (3,451.8 km/h) over 2,299.7 statute miles. The same flight set the following records:
- West Coast to East Coast in 67 minutes and 54 seconds
- Kansas City to Washington in 25 minutes and 59 seconds
- St. Louis to Cincinnati, 311 miles, in 8 minutes and 32 seconds
In 1974, the same type flew from New York to London in 1 hour, 54 minutes and 56.4 seconds, averaging about 1,807 mph (2,908 km/h).
Why was the SR-71 retired?
The Air Force retired the SR-71 in 1990, citing operating costs and the rise of reconnaissance satellites. Congress funded a brief reactivation in 1995, and the last USAF operational flights ended in 1998. NASA flew two Blackbirds for high-speed research until 1999. In all, 32 SR-71s were built and 12 were lost in accidents. None was ever shot down by hostile action, though hundreds of surface-to-air missiles were fired at it, according to former SR-71 pilot Buz Carpenter.
Today surviving airframes are on display. They include #61-7972 at the Smithsonian's Steven F. Udvar-Hazy Center in Chantilly, Virginia. Lockheed Martin's Skunk Works has talked publicly about a hypersonic successor, often called the SR-72, but no such aircraft has been officially shown flying.
Frequently asked questions
Is the SR-71 still the fastest plane in the world?
It's the fastest crewed air-breathing jet. Rocket planes like the X-15 and uncrewed scramjets like NASA's X-43A (about Mach 9.6 in 2004) have flown faster, but they hold different records.
What is the SR-71's top speed in mph?
The official record is 2,193.2 mph (3,529.6 km/h), set July 28, 1976.
How fast is Mach 3 in mph?
The speed of sound depends on air temperature. At the SR-71's cruising altitudes, Mach 1 is about 660 mph (1,062 km/h), so Mach 3 is roughly 1,980 mph (3,190 km/h).
Could the SR-71 outrun a missile?
That was its standard defense. It flew so fast and so high that surface-to-air missiles generally couldn't reach and catch it in time. No SR-71 was ever lost to enemy fire.
How many SR-71s were built?
Thirty-two: 29 SR-71As, two SR-71B trainers and one SR-71C hybrid trainer.
Where can I see an SR-71?
Museums with SR-71s include the Smithsonian's Udvar-Hazy Center (Virginia), the National Museum of the US Air Force (Ohio) and the Museum of Aviation in Warner Robins (Georgia), home of the 1976 speed-record jet. The Intrepid Sea, Air & Space Museum (New York) shows an A-12 and the Museum of Flight (Seattle) an M-21, both earlier members of the Blackbird family.
