In early 1953 the MiG-17F day fighter entered production. The "F" indicated it was fitted with the VK-1F engine with an afterburner by modifying the rear fuselage with a new convergent-divergent nozzle and fuel system. The afterburner doubled the rate of climb and greatly improved vertical maneuvers. But while the plane was not designed to be supersonic, skilled pilots could just dash to supersonic speed in a shallow dive, although the aircraft would often pitch up just short of Mach 1. This became the most popular variant of the MiG-17. The next mass-produced variant, MiG-17PF ("Fresco D") incorporated a more powerful Izumrud RP-2 radar, though they were still dependent on Ground Control Interception to find and be directed to targets. In 1956 a small series (47 aircraft) was converted to the MiG-17PM standard (also known as PFU) with four first-generation Kaliningrad K-5 (NATO reporting name AA-1 'Alkali') air-to-air missiles. A small series of MiG-17R reconnaissance aircraft were built with VK-1F engine (after first being tested with the VK-5F engine).