HOW FAST IS IN­VIS­I­BLE?

iD magazine - - Current Events -

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.

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