The fundamental purpose behind the MiG-31 Firefox program was to develop an aircraft capable of intercepting anything the West currently had, which at that time were Lockheed’s SR-71 Blackbird, U-2B, TR-1, and the D-21 drone. The cold war was still at its peak and the need to “keep the other guy honest” led the United States to begin overflights of Russia with spyplanes. Thus, the Mach 3.5 Lockheed D-21 drone that launched from a “mothership” SR-71 was the only real concern Russia had in 1982. The Americans already had several successful launches and recoveries of the unmanned aircraft above 100,000 feet, Mikoyan-Gurevich needed to make this their primary target. Much of what they had learned from the MiG-25 Foxbat programme was applied to create one of the most advanced aircraft to ever take to the skies. The Firefox was at the forefront of aviation technology and the United States recognized this.
The MiG-31 budget kept getting larger and the hopes of building more than two prototypes quickly began to diminish. The titanium manufacturing process alone was already far beyond the initial budget for the program, but officials pushed forward regardless. They were not to be outdone by anyone, and they most certainly would not allow the American’s to overfly their aispace unchallenged any longer. Firefox was designed to be the ultimate high-speed, high-altitude interceptor."
The Firefox used a pair of Tumansky RJ-15BD-600 high-bypass afterburning turbojets capable of producing 50,000 lbs. thrust each. These were heavily modified, uprated turbojets based upon the engines from the MiG-25 Foxbat programme. Russian engineers took what they learned from high-mach turbojet engine technology and applied acquired US engine manufacturing technology to the Tumansky 600 series engines, built within the same footprint of the current R-15BD-300 design. The introduction of high-bypass air intake systems helped produce the most powerful conventional turbojet engine of its day, surpassing even the US built 32,000lb P&W J58, used in the much revered SR-71 Blackbird.
In addition to these massive engines, the Firefox had six Soyuz/Komarov solid rocket boosters, using a solid-propellant with a proprietary ignition/shutoff system. These rockets could be used to augment the main engines, providing an additional 15,900 pounds of thrust. These were normally used during take-off under full load or high-speed dash acceleration. In some rare cases, test pilots were known to engage these rockets at extreme altitudes where thin air produced flame-outs on the main engines. The first flying prototype was taken to 131,079 feet setting a new world record, breaking the previous record of 123,492 ft. held by a Ye-266M.
The compressor blade components were manufactured of pure titanium, a first for Russian aircraft manufacturing. Fuel was cooled via the centerline ventral air intake and then pumped through a complex series of conduits around the giant Tumansky engines to keep them cool, as well as throughout much of the airframe. Much of this technology and ideology was gleaned from information obtained by Russian agents on the production of the SR-71 Blackbird, which used similar cooling principles.
By alternating wastegate control between the dorsal and ventral air intakes using ramps, thereby more accurately controlling the engine-breathing, the RJ-15BD-600 could achieve incredible thrust to weight ratio and excellent high-altitude air-breathing qualities. Achieving Mach 6 was possible, although this was considered ‘maximum’ speed which was ineffecient to maintain for any period of time due to the MiG’s massive fuel consumption. Cruise speeds were more in the range of Mach 3.8 to 4.8, and operational altitudes under normal conditions were considered to be in the range of 95,000 to 105,000 feet."