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The solution to Israel’s speed issues came in the form of a proposal from General Dynamics to convert/modify existing F-4Es to a different engineering standard. The overall aerodynamics of the jet would largely remain the same, save for the inclusion of two large conformal tanks on both sides of the fuselage just above the engines, carrying around 2500 gallons of water (9600 liters for our friends who use the metric system). Now, you’re thinking- wait a second… did I just read that right? What the heck are they going to do with 5000 gallons of water on a fighter jet? Water injection. The boffins at General Dynamics figured that the secret to lifting the Phantom’s speed was pre-compressor cooling (PCC for short), where water would be injected into the air rushing through the engine inlets on its way to the combustion cores, reducing the temperature of the air passing in. By cooling down the air, the mass and density would theoretically be increased, giving the F-4 a major improvement on its thrust output, especially at higher altitudes. The corporate suits at General Electric, the company that designed and built the Phantom’s J79 engines, weren’t thrilled with this suggestion but nevertheless assisted nominally with the research into the concept. This wasn’t anything especially new- General Dynamics had previously attempted something similar with the Convair F-106 Delta Dart, though the work that was put into modifying the F-106 never amounted to anything substantial. Testing done at the Arnold Engineering Development Center, operated by the USAF, found that with pre-compressor cooling, engines could be run with the afterburner engaged for incredible periods of time (e.g. they managed to keep a J75 going with the afterburner lit for 40 hours). PCC had also been used by McDonnell Douglas when the F-4 was first built, just to help break and set a few speed records with the then-new jet, though the system installed in the early F-4 was very basic and barebones, compared to what General Dynamics wanted to do with the F-4X. The new PCC setup for the F-4X would “mist” the water into the engine, thus preventing moisture buildup inside the engine. To make the most of PCC, the air intake inlets were redesigned with larger scoops and a redefined shape, improving the airflow moving into the compressor chambers. To help manage the airflow as efficiently as possible, controllable intake ramps were installed as well as vortex generators for both of the J79s installed. What was the final result of all of this? An F-4 Phantom II that could fly at more than three times the speed of sound for sustained dashes, and would be able to cruise at above Mach 2.4 during missions. The fastest fighter jet ever made (that we know of). The US Air Force quickly gave up on the concept, floated to them first by General Dynamics before Israel got wind of it; likely due to the coming of the high-performance McDonnell Douglas F-15A. While Air Force brass were unwilling to demonstrate any semblance of interest in such an idea because of the impending air superiority fighter acquisition, the State Department had a different worry in mind. At that point in history, the SR-71, flown solely by the United States, gave the US an unparalleled and untouchable intelligence-gathering capability. Allowing foreign customers, even ones with closely-held allegiances to the US, the ability to posses and operate an aircraft with such strengths as the F-4X would be a less than optimal situation. Soviet human intelligence (HUMINT) agents would potentially garner information on the jet, or heaven forbid, actually take an F-4X for analysis, giving the USSR the opportunity to build and field a counter-aircraft that could take out the SR-71 and severely hinder the USAF/CIA’s intel program. The government immediately banned the export of the jet. Working quickly, General Dynamics removed the F-4X’s weapon systems and hardpoints, disarming the jet and circumventing the ban. Instead of flying as a fighter, the aircraft would be equipped with the previously-mentioned HIAC-1 LOROP camera in the nose. Dubbed the RF-4X, it wasn’t the fighter that Israel wanted, but it still fit their reconnaissance purposes well. After securing permission to shop the RF-4X to Israel in 1974, the, the Israeli Air Force loaned General Dynamics an F-4E (serial no. 69-7576) to work with as a mockup, and later, an RF-4X prototype/testbed. Physical work commenced on the RF-4X in November, soon after the aircraft’s delivery, and carried on into the next year. Cardboard and papier-mache was used to simulate the new intake/inlet architecture, as well as the large PCC blister tanks on the sides of the fuselage. The nose of the Rhino was also taken apart and modified to house the HIAC-1. The IAF hounded General Dynamics to build the PCC system as soon as possible, since their need was urgent. However, engineers discovered that they needed far more time than what Israel had to offer. After the USAF withdrew interest from the RF-4X, the program was cut. There was no way Israel could fund such a project on its own. The fastest Phantom in history was dead, having never flown or even proceeded past the mockup stage. Phantom phanatics, I hope you had a box of tissues handy while reading the above. http://tacairnet.com/2015/06/18/redeveloping-the-f-4-phantom-ii-into-a-mach-3-fighterspy-plane/