No other aircraft to date has become a legend as the McDonnell Douglas F-4 Phantom II. This icon of a jet first flew in 1958 and went on to become the platform of the U.S. Air Force and Navy during the Vietnam War. Though it is a big jet, the mighty Phantom was no slouch and could reach Mach 2.23, earning its reputation for power and versatility. However, an ambitious plan now emerged to convert the F-4 into a Mach 3+ fighter and reconnaissance plane: the so-called F-4X.
The F-4 Phantom II, universally known by the U.S. Air Force aircrew as the “Rhino,” was equipped with state-of-the-art AN/APQ-120 radar, the best General Electric J79 engines, and an internal M61 Vulcan cannon. This, combined with everything else, made this Phantom variant the most capable. Alongside many other air forces, the state of Israel operated the F-4E and RF-4E and employed both in support of many operations.
In the late 1960s and early 1970s, Israel was increasingly being faced with a SAM threat supplied by the Soviets, just over the border in Egypt. What the IAF wanted was a high-speed reconnaissance vehicle that could keep tabs on these SAM sites without having to overfly hostile airspace. The RF-4E, although useful, had to fly into contested areas to obtain images of sufficient quality for the photo-interpreters, thus making it vulnerable to SAMs.
In 1971, the Soviet Union started deploying MiG-25R Foxbat-B reconnaissance aircraft to Egypt. Those jets could easily shun Israeli fighters, flying at Mach 2.83, and even at Mach 3.2 if necessary. Time-pressed the IAF to find a countermeasure against that threat.
General Dynamics made an offer for a radical modernization of the F-4E, which would make it faster by using pre-compressor cooling (PCC) with water injections. This was to be achieved by injecting water into the inlets, where the engine would cool the air, thereby increasing its density, and in turn, the engine’s thrust. The modified F-4, subsequently dubbed the F-4X, was to be outfitted with large conformal water tanks and inlets for the reconfigured PCC system. The aircraft would theoretically be capable of sustained speeds over Mach 3.
Although the concept was the most promising, the United States Air Force canceled it; that probably had to do with the then-upcoming high-performance F-15A. There were also qualms from the U.S. State Department about exporting technology so advanced that it could affect the strategic balance and even find its way into Soviet hands.
This restriction was the one that General Dynamics tried to get around by developing an F-4X-based reconnaissance aircraft, the RF-4X, that carried the advanced HIAC-1 LOROP camera. It was this work that caught the attention of Israel and resulted in Israel loaning an F-4E to General Dynamics for development work. This project was also terminated primarily due to technical difficulties and a lack of adequate funding.
Despite the ability to be the fastest fighter jet ever created, the F-4X Phantom never even made it past the mock-up stage. The program was vast in its scope, encompassing the taking of the F-4 to a Mach 3-plus reconnaissance aircraft; however, that is all this proves to be, very much a “what could have been” in the history of aviation.