U-2 became operational in June 1956, CIA officials estimated that its life expectancy for flying safely over the Soviet Union before the Soviets developed countermeasures would be between 18 months and two years. After overflights began and the Soviets demonstrated the ability to track the U-2 and made credible attempts to intercept it, this estimate was adjusted downward; in August 1956, Richard Bissell reduced the number to six months more. In practice, this window proved slightly longer; but the general point was alarmingly demonstrated in the U-2 Crisis of 1960. A replacement for the U-2 had been under consideration even before its operational missions began. Originally these studies focused entirely on the reduction of the radar cross section (RCS), but after Franklin Rodgers introduced the idea of spoofing the blip/scan in 1957, the plans were changed to research high-speed, high-altitude designs instead. Lockheed calculated that in order to be effective against known Soviet radars, an aircraft would have to travel between Mach 2 and Mach 3 at 90,000 ft and have an RCS of about 10 square meters. This led to a number of proposals which were down-selected to the Lockheed A-12 and Convair Kingfish. It was during the development of these aircraft that problems with blip/scan avoidance became clear. It was discovered that the high-temperature exhaust of these aircraft engines reflected radar energy at certain wavelengths, and persisted in the atmosphere for some time. It would be possible for the Soviets to modify their radars to use these frequencies, and thereby track the targets indirectly but reliably. It was also realized that since blip/scan avoidance relied more heavily on a problem in Soviet displays rather than in the principles of radar, changing these displays could render the technique moot. A system that recorded the radar returns in a computer and then drew the targets on the display as an icon whose brightness was independent of the physical return (a system in which returns did not have to "add up" in order to appear on the display) eliminated the potential for operator confusion. This was particularly worrying, because the USAF was itself in the process of introducing precisely this sort of display as part of their SAGE project. Finally, the introduction of the first effective anti-aircraft missiles dramatically changed the game. Radars for plotting an air intercept were generally made as long-range as possible in order to give the operators