The Real X-Jet- page 2 | Military Aviation | Air & Space Magazine
Current Issue
August 2014  magazine cover
Subscribe

Save 47% off the cover price!

(Lockheed Martin)

The Real X-Jet

From Transformers to the X-Men, the Blackbird is still Hollywood's favorite futuristic jet. Here's the real story of its birth.

airspacemag.com

(Continued from page 1)

The A-12 was at first provided with three camera systems that were leaps in photographic technology. Perkin-Elmer developed a stereo camera that had an 18-inch lens and a 5,000-foot film supply. This Type I camera produced paired photographs of a 71-mile-wide swatch of ground with a resolution of one foot. Roderick M. Scott, Perkin-Elmer’s resident genius, developed several innovations, including the use of a reflecting cube instead of a prism for the scanner and a concentric film supply and take-up system that minimized weight shift. Kodak created the Type II camera, also stereo, which covered a 60-mile-wide swath of territory with a 21-inch lens that resolved on the order of 17 inches. Hycon’s entry, the Type IV, was designed by James Baker and was an advanced version of his B camera for the U-2. It took seven frames that, when combined, covered a swath 41 miles wide with a ground resolution of eight inches. The Type IV also held the most film yet: 12,000 feet. Later on, a Texas Instruments infrared camera that could take pictures day or night was adapted to the A-12 and used when the need arose.

Knowing that the difference between aerodynamics and hydrodynamics was basically a matter of density—that the air in front of their aircraft would not only get hot but, in effect, thicken as the speed increased—Johnson and his team designed the A-12 to cut through very hot, dense air the way a vessel cuts through water. (The idea dates back as far as Leonardo da Vinci, who wrote that both birds and fish “fly” through different fluids in the same basic way.) That’s why the fuselage, a model of which was tested in a water-flow tank, was hull-shaped. And the forward part was stretched into what Johnson called a blended body. Its chines, when matched to a modified delta wing, dramatically increased lift at cruise speed and reduced drag significantly because it allowed the whole aircraft to create lift. The chines also formed a series of bays that were stuffed with intelligence-collecting sensors and other equipment, including electronic countermeasures.

The speed of the A-12 came from a pair of 20-foot-long engines, each weighing 6,500 pounds, that were every bit as innovative and challenging as the aircraft they propelled. That corncob of an engine was refined over the years until it developed more than 32,000 pounds of thrust, or about a third of the thrust ratings of today’s giant airline turbofans. Designed by Pratt & Whitney under tight security starting in late 1959 and designated the JT11D-20, it was developed for a Mach 3 Navy attack aircraft that was canceled by President Kennedy after the CIA convinced him that the need for a Mach-busting spyplane took precedence. It soon became better known by its military name: J-58-P2. Powerplant cognoscenti savor the J-58 as a true classic.

For security reasons, less than a dozen of the J-58’s designers knew what the whole engine looked like or was supposed to do, recalls senior project engineer Joseph A. Daley Jr. Those who did were confounded at every stage by the heat problem, just as the airframe designers at Lockheed were. “The engine operates in the most hostile environment any engine has ever been subjected to,” the Ae107 course book noted. “Air entering the compressor reaches 1,400° F. The turbine inlet temperature is 2,000° F. The temperature in the afterburner section reaches 3,200° F....”

The inlet was a particularly tough problem. Early testing showed that high inlet temperatures would hurt the efficiency of the nine-stage compressor, which, to use engineering jargon, would run out of surge margin—the engine would stall. The problem was solved in a way that gives engineers existential pleasure and evokes the word “elegant.” At the fourth compressor stage, air was bled off through six bypass tubes on the outside of the engine and routed back to the afterburner. This not only removed heat from the compressor, but caused the hot air blowing into the afterburner to arrive at the same speed as the air flowing into the inlet. The result of this was that at Mach 2, the inlet and afterburner were synchronized and therefore turned into a ramjet that provided about 80 percent of the J-58’s thrust.

The J-58 weighed more than three tons, mainly because most of it was made with Waspaloy, a nickel-based alloy that even at 1,400 degrees keeps its strength and resists oxidation. But it is also very heavy and hard to weld. The first thin sheets of the metal used for fabrication experiments were supplied by the Hamilton Watch Company. Titanium was used for the front compressor blades because it is much lighter than steel and stronger than aluminum. The forging of the blades was an art form in a titanium industry that in the late 1950s was in its infancy. Not only were the blades meticulously hollowed on the inside for cooling, but the metal’s grains were aligned for strength.

Pratt & Whitney executive Arnold J. Gunderson crystallizes the engineering challenge by pointing out that while the J-58 had parts that were machined to thousandths of an inch, the whole engine got so hot at cruise that it expanded two and a half inches in width and grew six inches in length. (The airframe expanded and contracted as well, which was why fuel leaked from its wings.) That problem was solved, says Gunderson, by putting the gear box that drives the generators, hydraulic pumps, and other subsystems on the bottom of the engine, where it effectively floats, and hanging the engine itself from the top of the nacelle so it could expand and contract without stressing its parts.

The heat also made the A-12 and the SR-71 trickier to land than to launch. Descending too quickly and shock-cooling the airframe and engines would cause a sudden shrinking of parts that could be more stressful than the expansion that occurred during heat-up. Pilots had to avoid cooling the engine at a rate that could cause shrinkage and make the ends of the turbine blades rub against their seals, ruining the engine.

Mach 3.2 was the A-12’s (and then the SR-71’s) design point. The airplane could hit Mach 3.6 and, in theory, Mach 4 if the air was cold enough. But, as Gunderson explains, “Kelly designed the airplane to fly at Mach 3.2 at 80,000 feet. The airplane cruises there. It is very, very comfortable there because that’s the point at which it was designed to fly, where every system is snug and tight-fitting.... All of the enhancements and refinements we did over the many, many years of the program were done to reinforce that. Moving away created more problems than it was worth. Every time we flew faster and higher, we ran into areas where we had not so much experience. Things would go wrong. You’d swallow the [aero]spikes, you’d flame out.”

Unlike their engines, 93 percent of the airplanes’ structure and skin were made of titanium. The metal’s quality varies widely, and the first batch of titanium sponge (which looks like a giant soap pad) had to be bought by the CIA from the Soviet Union and smuggled past U.S. Customs. Ironically, the Soviets had most of the world’s supply of the metal and exported a product of exceptionally high quality. The plane’s landing gear was the largest titanium forging produced in the United States and the only titanium landing gear in use in any U.S. aircraft. Forging and cutting special metals to tolerances as tight as .005 inch, then welding them together, were exotic specialties that were virtually developed from scratch.

Tags

Comment on this Story

comments powered by Disqus