The “Ultra Sabre”

From 1943 through the early 1960s, America’s premier air superiority fighter company was North American Aviation.

After producing such legendary aircraft as the war-winning P-51 Mustang and the F-86 Sabre, which dominated Soviet MiGs over Korea, they also developed the first American fighter to break the sound barrier in level flight — the highly successful F-100 Super Sabre. The name “North American” had become almost synonymous with “Air Force air superiority fighter.”

North American followed the F-100 with a design for an air superiority fighter capable of close to Mach 2. Named the F-100B (since the design had not been submitted to the Air Force), it was similar to the F-100 but with thinner wings, a more area-ruled fuselage, and a variable inlet for an afterburning engine with 16,000 pounds of thrust that would hopefully give it a speed of  Mach 2 in level flight.

The design retained the rear fuselage and tail section of the F-100 and, like the F-100, had a single pilot and four M-39E 20 millimeter cannons as its fixed armament. However, it soon became clear that any new air superiority fighter would need radar, so a radar was added to the nose and the engine air intake inlet moved to below the nose, like today’s F-16.

Unfortunately for North American, aircraft types reflect national defense policy, and after the Korean War the U.S. Air Force was dominated by the bomber generals of the Strategic Air Command, which would not brook air-to-air tactical fighters. In mid-1954, as a reflection of this, the USAF issued a requirement for a tactical fighter-bomber that was capable of delivering a nuclear bomb, which meant a bomb bay, and dropped any interest in a pure air superiority fighter, no matter how fast.

North American scrambled to modify the F-100B to meet the requirement but quickly saw that the position of the air intake under the nose would interfere with releasing the nuclear bomb, especially since the bomb would have to be carried semi-submerged because there was no room for a bomb bay. As a result, the intake was relocated to above the fuselage right behind the cockpit, similar to the previously discussed German He 162 “Volksjäger.” This arrangement had never been tried before on an American aircraft, for what seem to be obvious reasons.

First, if the pilot had to eject with the engine operating, he stood an excellent chance of being sucked into the air intake. Second — and more important to a fighter pilot, who never thinks about ejecting — the intake completely destroyed any visibility to the rear. But, since the new aircraft’s role was now one of high speed, low level delivery of a nuclear weapon, this seemed to be a moot point.
In June 1954, the USAF gave North American an order for 33 Mach 2 fighter-bombers and changed the program name to YF-107, but less than a month later the Air Force changed the order to three prototypes and only nine service test aircraft.

The YF-107 first flew on Sept. 10, 1956, and achieved Mach 1 easily. But on landing, the drag chute failed and it ran off the end of the runway and into a ditch, where it sheared off its nose landing gear and the nose plowed into the dirt.
The accident showed the single advantage of the location of the high mounted air intake — the engine was not damaged from ingesting the ground and the aircraft was back flying in three days, soon exceeding Mach 2.

As the program progressed, it became clear the USAF was interested in the YF-107 only as a backup for their first choice of a nuclear fighter-bomber, the Republic YF-105, later to become F-105 “Thunderchief” but more commonly known as the famous “Thud” of  Vietnam War fame.

But the YF-105 had huge problems — early models took 150 hours of maintenance for every flying hour, it was grounded at least three times in its career, and it only flew six shows with the Air Force demonstration team, the Thunderbirds, before it was dropped because of structural failures. The F-105 program lost both of their prototypes in testing while the YF-107 program “only” lost one of its three in flight testing, and for a short time it appeared there was hope for the North American aircraft.

However, the YF-107 had problems of its own. The variable geometry air intake was totally unreliable, which limited the aircraft to 1.2 Mach, and the aircraft systems were failure prone, and in the end the Air Force settled on the F-105.
After the completion of its test program, the two YF-107 survivors were sent to the National Advisory Committee for Aeronautics. NACA flew them briefly — one four times, the other 40, mainly to evaluate a side stick that was eventually used on the F-16 — then gave up on them. These two aircraft survive in museums.
The YF-107 was the final practical foray into military aviation for the North American Aircraft Corporation, except for the XF-108 Rapier, a proposed Mach 3 interceptor that never flew.

In the end, the lasting legacy of the YF-107 is the air intake located above the cockpit and the question: “What were they thinking about when they put that on a fighter?” The answer was, “the Air Force at that time didn’t believe in fighters.”

(For questions or comments, contact Dr. Michel at marshall.michel@ramstein.af.mil)