their long- and medium-range needs. The perception of a declining need for close-in "dogfighting" capabilities resulted in the original decision to not install internal cannons in the Phantom. Combat experience and E-M theory However, real-world experience in the Vietnam War revealed some shortcomings in American fighter capabilities, as early generation Soviet jet fighters proved to be more of a challenge than expected for U.S. designs. Although U.S. pilots had achieved favorable kill-to-loss ratios, combat had revealed that air-to-air missiles (AAM) of this era were significantly less reliable than anticipated. Furthermore, the rules of engagement in Vietnam precluded long-range missile attacks in most instances, as visual identification was normally required. Under these conditions, combat invariably closed to short ranges where maneuverability and short-range air-to-air weapons became critical, even for dedicated interceptors like the Convair F-102 Delta Dagger. Based on his experiences in the Korean War and as a fighter tactics instructor, in the early 1960s Colonel John Boyd and mathematician Thomas Christie developed the Energy-Maneuverability (E-M) theory of the value of aircraft specific energy maintenance as an advantage in fighter combat. Maneuverability was the means of getting “inside” an adversary’s decision-making cycle, a process Boyd called the "OODA" loop (from "Observation-Orientation-Decision-Action"). This approach emphasized an aircraft design capable of “fast transients” – quick changes in speed, altitude, and direction. A fighter that is superior in its ability to gain or lose energy while out-turning an opponent can initiate and control any engagement opportunity; a fast transient capability allows the pilot to stay inside a hard-turning opponent when on the offensive or to force an overshoot of an opponent when on the defensive. These parameters called for a small, lightweight aircraft – which would minimize drag and increase the thrust-to-weight ratio – but a larger, higher-lift wing to minimize wing loading – which tends to reduce top speed while increasing payload, and can lower range (which can be compensated for by increased fuel in the larger wing). Lightweight Fighter program The need for new air superiority fighters led the USAF to initiate two concept development studies in 1965: the Fighter Experimental (F-X) project originally envisioned a 60,000-pound (27,200 kg) class twin-engine design with a variable-geometry wing. This would be a very expensive aircraft, too expensive to consider equipping the entire Air Force with it as its standard aircraft. For this reason a less-expensive Advanced Day Fighter (ADF) was also considered, a lightweight