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RedditEarly experimental airplanes were often paired with heroes like Chuck Yeager and Scott Crossfield, but that pattern was broken after the X-6, a huge Convair B-36 bomber that was modified for nuclear propulsion research and meant to be flown by a crew of five. The X-plane that followed, the X-7, was its opposite in every way: tiny and very fast instead of gigantic and lumbering. And it carried nary a human.
Lockheed built it and surprised the establishment by choosing an unconventional way to attain speeds well above Mach 1: Instead of wing sweep, the company used a unique wing with a razor-thin airfoil. The trapezoidal shapes of the stubby wings and tail would show up later in the company’s XF-104 fighter prototype, but the X-7’s job was not warfare but pure research—in this case, into high-speed ramjets. It was a minimalistic vehicle: just a long, slender fuselage ending in a sharp spike. After its flight was over, the vehicle descended under a parachute nose-first, and the spike would dig into the earth. Like a lawn dart, it could be pulled out and re-used. A test ramjet was slung beneath the aircraft in a configuration that would have made conventional landing gear impractical. The combination looked like a balsa glider carrying some weird round phone booth.
To test the ramjets, Lockheed had to boost the X-7 to the high speeds that such engines need for igniting. The company had a good off-the-shelf solid rocket booster, the XC202-C3, which the Allegany Ballistics Laboratory in Pinto, West Virginia, had developed in wartime secrecy under the oversight of George Washington University of Washington, D.C. With its 100,000-plus pounds of push, the rocket’s task was to get moving through the air fast enough so that the ramjet’s carefully shaped intake would compress the incoming flow the way the rotating compressor does on a turbojet. Having no turbomachinery, ramjets work aerodynamically: Their intakes convert the inlet air’s energy from velocity to pressure, after which fuel can be added to the air flow and ignited. Most ramjets have internal surfaces to help maintain the flame once the engine starts. In some respects, ramjets can be thought of as rockets that use air for their oxidizer.
Because they were so simple and cheap, ramjets found a ready market powering expendable target drones. But they were also loud fuel hogs that only now are making a minor comeback as a way of extending the range of missiles.
The X-7 once held the speed record for an air-breathing aircraft—Mach 4.31 (almost 2,900 mph)—but NASA’s more recent scramjet experiments easily topped that mark.
The National Aeronautic Association is giving UAVs their due, by recording their achievements in a category of their own in the sporting code that governs record attempts. In the NAA’s archives, for example, you’ll find that a Northrop Grumman RQ-4A Global Hawk flew 8,214.44 miles from Edwards Air Force Base in California to Edinburgh Royal Australian Air Force base on April 23, 2001.
I enjoyed the article on the X-7. As a new engineer at Lockeheed Missiles Van Nuys facility in 1956, I was assigned to work with Senior dynamicist Dale Branchflower on a problem vexing the early X-7B vehicles, viz, they were disintegrating shortly after launch by the powerful solid propellant booster. A rear looking camera was mounted on the nose of the following shot and it showed the wings in a diverging oscillation, after a coupe of cycles of which, the wings departed the fuselage and the vehicle augered in. Branchflower assesed it as aileron divergence, as a result of torsional vibration on the X-7's trapezoidal planform wing. We modeled the wing and analyzed it via a "Stodola" method and the assessment was verified. Later, I was given a new desktop computer (burroughs E-101) to evaluate and used it to program Branchflower's equations and run several test modes for analysis. A report was written on it (which I just found by GOOGLING). I belive the problem was fixed by adding weights to the wing spar at the critical points to damp out the oscillations. But, I was transferred to the new Sunnyvale, CA facility and the Polaris program and never thought of it again until reading this issue of Air Space Smithsonian. Keep up the fine magazine. I'm a charter subscriber and read every issue through. Don Marioni Port Townsend, WA
Posted by Don Marioni on July 21,2008 | 01:40PM