HOW FAST IS INVISIBLE?
2,200 miles per hour: The SR-71 Blackbird set this world record 39 years ago, and to this day it remains unbroken. But how do you build a machine that appears to override the laws of physics?
Even then it sounded like an impossible task, and it is still one of the most ambitious objectives in the history of aviation. Around 50 years ago the U.S. government decided to build a plane that could fly faster and higher than any machine to come before it: an aircraft that, thanks to its extreme altitude and top speed, could not be shot down; a plane with turbines so powerful that the fuel it requires had yet to be invented. In short, a vehicle that should not physically be able to exist. Yet this very spy plane has set records that have stood for decades. In fact, the records set by the SR-71 Blackbird are unbroken even now…
WHY REINVENT THE PLANE?
In the early 1960s, the Lockheed Corporation was commissioned to build the SR-71 Blackbird. It soon became clear to the engineers of the Skunk Works, the research and design department of the company, that if the government requirements were to be met, the construction of the reconnaissance aircraft had to push the boundaries of engineering. “Absolutely everything needed to be reinvented,” recalls Kelly Johnson, the head designer at the company at the time. Reason: The faster a jet flies, the hotter its outer skin gets. This occurs due to the tremendous air resistance that arises when a jet flies at Mach 3.3 ( about 2,500 mph). When it comes to the 107-foot-long SR-71, frictional temperature is over 750°F, which is sufficient to weaken standard materials like aluminium.
The Skunk Works engineers were relying on titanium, but at that time America didn’t have any reserves of the light-yet-strong metal. So where would they get the titanium from?
The CIA found the answer to this pressing question: It set up bogus companies to buy large quantities of titanium from the Soviet Union. So, ironically, America’s archenemy made it possible for the SR-71 to be built in the fi rst place. Among the other materials used, the Blackbird’s cockpit glass was made with quartz to protect the pilot against the heat.
Also, the jet fuel mixture JP-7 was specially developed for the SR-71. All of the conventional fuels at the time had failed at Mach 3. After a five-year development phase, the first SR-71 rolled out of the hangar in 1965. For the first time the Pratt & Whitney J58 engines roared to life. During ignition, 50-foot-long green flames shot out the rear of the jet. At that point it was up to the pilot to fly the majestic Blackbird to the sky. But would the physics hold up?
CAN ANY PILOT FLY THE BLACKBIRD?
The recruitment program for the SR-71 Blackbird was the toughest in the history of the U.S. Air Force: Out of thousands of applicants, only 86 men managed to make the grade. Qualified candidates had to be able to fly at least two different combat aircraft. In addition, they must have mastered the technique of refueling in the air. Moreover, there could be no entries in their medical records, and the pilots were required to pass physical and mental aptitude tests designed by NASA.
No other pilot training curriculum required candidates to pass such complicated space tests. Only when they had satisfied these demanding requirements were they allowed to fly
the jet at Mach 3.3 at an altitude of 16 miles. “Adrenaline is pumping all the time. This is why you are totally spent after a three-and-a-half-hour flight,” says former Blackbird pilot Terry Pappas.
During the flight, the pilots have very little leeway: The SR-71’S angle of attack cannot be altered by more than three degrees while at Mach 3. Otherwise the intense forces would immediately cause the pilot to lose control and the aircraft would crash. Blackbird pilots can only orientate themselves using large landmarks like mountains or lakes. The reason: At a speed of 2,200 miles per hour, the machine is moving so fast that even a cannonball that’s fired from a supersonic plane could not catch up with it. So visibility is limited from inside this high-flyer. The Blackbird takes around 150 seconds to travel from Philadelphia to New York City. A commercial airline pilot is able to navigate using cities and rivers. But things look so tiny from an altitude of 16 miles that pilots can’t see much. “At Mach 3 and 16 miles of altitude, not only are you invisible, the ground is barely discernible,” says Pappas.
ALMOST INVISIBLE TO RADAR
SR-71 Blackbirds have been shot at 4,000 times—without ever being hit. Advantages of this reconnaissance aircraft include its top speed and the high altitude it attains—neither enemy aircraft nor surface-to- air missiles can touch it. The enormous thrust is generated by two Pratt & Whitney J58 engines. Normal jets are propelled when compressed air is ignited by the addition of fuel— producing thrust. But in the J58, six pipes lead directly from the air inlet to the afterburner—a second stage for more thrust. Here residual oxygen is ignited with fuel again, enabling the SR-71 to reach Mach 3.3 speed.
» AT MACH »3 THE GROUND IS BARELY DISCERNIBLE.
“We’ve trained with F-15 pilots of the Air Force. They know just how difficult it is to shoot us down. Their targeting apparatus could not even detect the Blackbird,” says former Blackbird pilot Brian Shul. To inhibit detection by radar, the Skunk Works engineers painted the jet black— which reduces heat radiation. The paint was also mixed with special particles that reduce the reflection of radar signals. Furthermore, the spy plane was outfitted with an ECM anti-radar jammer that confused any approaching missiles. All this makes the SR-71 Blackbird the mother of all modern stealth aircraft.
But problems piled up over time. The costs of keeping the SR-71 fleet operating amounted to $ 260 million per year. Though the titanium skin is fireproof and corrosion resistant, it does require regular maintenance. Also, the engineers had developed a plane for which there were no tools.
Before each flight, the Blackbird’s engines had to be brought to 3,200 revolutions per minute, which was accomplished with two V8 engines. Only then could the J58 engines be ignited with a chemical called TEB. At Mach 3 speed, fuel consumption was very high: The engines burned 22 tons of fuel per hour. That’s why the jet had to be refueled in the air every 90 minutes—and a new fleet of tanker aircraft had to be bought.
OUT OF 32 AIRCRAFT, 20 MADE IT BACK INTACT
Conditions were most dangerous when the Blackbird reached its top speed. “At Mach 3+, you’re trying to control a machine that’s on the brink of controllability,” explains Pappas. A cylinder mounted in front of each engine adjusted airflow and behaved differently depending on the speed. If the pilot accelerated the machine to its maximum speed, the cylinders were pushed back into the turbines by 26 inches. This would increase the flow of oxygen and make higher speeds possible. But therein lay the risk: The technology was prone to errors. Once it stopped functioning, the result was a thrust imbalance. The SR-71 Blackbird would become uncontrollable and lose altitude.
The Blackbird’s final flight took place in 1999. During its period of service, 12 out of 32 machines had crashed—but not as the result of an enemy attack. The remaining SR-71 Blackbirds are exhibits in museums now. But even today engineers will admit that the records these aircraft set will probably never be broken.