How all U.S. Air Force pilots since 1968 have met their Mach.
- By Peter Garrison
- Air & Space magazine, September 2005
Northrop Grumman Corporation (NASM SI NEG. #00079050)
(Page 2 of 5)
One day a couple of engineers from General Electric’s newly formed Small Aircraft Engine Department—in those days engineers were also salesmen—turned up at Northrop with a tiny engine they had brought along as baggage on an airline flight from the East Coast. They claimed it would develop 2,500 pounds of thrust. “What scale is this model?” the Northrop people asked. “This isn’t a model,” replied the GE men. “This is the engine!”
Northrop’s vice president for engineering, Edgar Schmued, saw in the tiny engines the possibility of reversing what he believed to be a pernicious trend toward ever larger, heavier, and costlier fighters. Engine weight was critical because, by a rule of thumb, each extra pound of engine weight would require six additional pounds of airframe. The J85—the brainchild of Ed Woll, one of GE’s most gifted and influential engineers—was 22 inches in diameter and weighed less than 600 pounds, and in the form in which it would be used in the T-38, it developed 3,850 pounds of static thrust. Its thrust-to-weight ratio was superior to that of any other jet engine of its time. It sounded the death knell of the Fang project, from the ashes of which the N-156 arose.
In the initial stages of the N-156 project, Schmued favored a layout in which the engines were mounted on the wings, a location that he thought would make them easily accessible for maintenance. His chief engineer, Welko Gasich, disagreed: Wing-mounted engines produced excessive drag and aerodynamic disturbances. In Gasich’s opinion, the engines had to be within the fuselage. Schmued resisted for a long time but eventually capitulated, and a buried-engine mockup was built. Designer Lee Begin laid down the mold lines—the outer shape of the airplane. It was a thrilling shape, different from that of any other airplane of its era. The wings were vestigial, the vertical fin huge. The immensely long nose, exotically contoured and tapering practically to a point, seemed so far from the wing as almost to belong to another airplane. The swellings and hollows of its mid and aft fuselage—scooped out at the waist in accordance with aeronautical engineer Richard Whitcomb’s transonic area rule and expanding thereafter to envelop the engines—gave the fuselage a reclining-nude quality.
Northrop’s competitor for the supersonic trainer contract was the F-100F, a two-seat variant of North American’s F-100 Super Sabre. At the time, Northrop was not a supplier particularly in favor with the Air Force; North American was. Among Northrop’s pitches was one that is so obvious today that it is hard to believe that it was new to procurement offices in 1956. It was the idea of “life cycle costs,” which Gasich had brought with him from his previous employer, the RAND Corporation. While North American argued that the initial acquisition of the F-100F would be cheaper because the production line and the infrastructure for supporting the airframe and its engine were already in place, Northrop countered that in the long run, its new trainer would be cheaper because it used far less fuel and incurred lower maintenance costs. With two engines, it would also be safer; Northrop believed that it could bring the serious-mishap rate, then around 25 per 100,000 flight hours for the F-100, down to the range of seven to 10. The Air Force was persuaded. On June 15, 1956, a letter of intent was signed for what was now called the YT-38 Talon.
Several features changed during development. One was the arrangement of the vertical fin. A T-tail arrangement, like that of the F-104, with the stabilizer set atop the fin, had been considered and discarded early on, but the fin was originally designed with moderate sweep, like the wing. Flutter analysis indicated that the swept version might present difficulties. The crux of the matter was the stiffness of the aft fuselage, large portions of which had to be removable for engine access. In some jets, removing an engine required first removing the vertical fin, but it was a difficult design problem to make the fin easily detachable and yet sufficiently strong and stiff. Engineer Julius Villepique proposed what proved to be a key innovation. He fixed the fin to the keel structure that ran between the engines, and made the horizontal stabilizer, rather than the fin, the removable component, along with the “boat tail”—the entire aft shell of the fuselage surrounding the engines. The process of getting at the engines was extremely simple. Apart from the fasteners holding the fuselage shell together, the only parts that had to be disconnected were two push rods that connected the pilot’s control stick to the valve controlling the horizontal stabilizer’s hydraulic actuators. Hydraulic lines—there were only two—mated automatically, with internal check valves preventing loss of fluid.
The engines hung from rails on either side of the central keel. To avoid having to break and reconnect multiple hydraulic lines, designers mounted the hydraulic pump and other accessory drives on the fuselage, joined to the engine by a short driveshaft. Thanks to the rail support and the fuselage-mounted gearbox, once the boat tail had been detached, an engine could be removed and replaced in an hour.
Weight control was always a key factor in the trainer’s design. The N-156 was intended to weigh 10,900 pounds full up—a fantastically low figure when you consider that the fighters of the time generally weighed between 30,000 and 45,000 pounds. Eventually it would grow by about a ton—but even then it was a model of weight control. Its systems were simple. The hydraulically powered flight controls lacked manual reversion, and if both engines failed, the pilots’ only recourse was to eject. There was no fuel in the wings, no provision for external stores, and only basic systems for navigation and communication. Test pilot Lew Nelson took the prototype on its uneventful first flight on April 10, 1959.
“Science is done by single individuals, but engineering is done by a team, and I had a great team,” recalls Welko Gasich today. In his voice you can hear his affection for the airplane and its creators, and for a halcyon period—“the height of the great 1950s and ’60s screwdriver technology,” one pilot called it—during which every choice his team made turned out to be the right one.