 In response, the Soviet General Staff issued a requirement for a "Perspektivnyy Frontovoy Istrebitel" ("PFI", literally "Prospective Frontline Fighter", roughly "Advanced Frontline Fighter").Specifications were extremely ambitious, calling for long-range, good short-field performance (including the ability to use austere runways), excellent agility, Mach 2+ speed, and heavy armament.The aerodynamic design for the new aircraft was largely carried out by TsAGI in collaboration with the Sukhoi design bureau.