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The Mig 25 Foxbat was largely used for surveillance and interceptor roles. For these two purposes, the plane served well.

The engine required replacement only if the aircraft was tested to its limits: + mach 3.2 speeds.

The rivets, like Imnidiot mentioned, were in non-aerodynamic areas, and therefore economically better designed than ones wasting labour-expensive, aerodynamic rivets all over the aircraft.

Western intelligence and the MiG-25:

MiG-25RBSh with markings of 2nd Sqn/47th GvORAP (Guards independent recce Regiment)

Inaccurate intelligence analysis caused the West initially to believe the MiG-25 was an agile air-combat fighter rather than an interceptor. In response, the United States started a new program which resulted in the McDonnell Douglas F-15 Eagle. NATO obtained a better understanding of the MiG-25’s capabilities on 6 September 1976, when a Soviet Air Defence Forces pilot, Lt. Viktor Belenko, defected, landing his MiG-25P at Hakodate Airport in Japan.

The pilot overshot the runway on landing, damaging the landing gear and making the MiG-25 un-airworthy. It was carefully dismantled and analyzed by the Foreign Technology Division (now the National Air and Space Intelligence Center) of the United States Air Force, at the Wright-Patterson Air Force Base. After 67 days, the aircraft was returned to the Soviets in pieces. The analysis, based on technical manuals and ground tests of engines and avionics, revealed unusual technical information:

* Belenko’s particular aircraft was brand new, representing the very latest Soviet technology.

* The aircraft was assembled very quickly, and was essentially built around its massive Tumansky R-15(B) turbojets.

* Welding was done by hand. Rivets with non-flush heads were used in areas that would not cause adverse aerodynamic drag.

* The aircraft was built of a nickel alloy and not titanium as was assumed (though some titanium was used in heat-critical areas). The steel construction contributed to the craft’s 64,000 lb (29,000 kg) unarmed weight.

* Maximum acceleration (g-load) rating was just 2.2 g (21.6 m/s²) with full fuel tanks, with an absolute limit of 4.5 g (44.1 m/s²). One MiG-25 withstood an inadvertent 11.5 g (112.8 m/s²) pull during low-altitude dogfight training, but the resulting deformation damaged the airframe beyond repair.

* Combat radius was 186 miles (299 km), and maximum range on internal fuel (at subsonic speeds) was only 744 miles (1,197 km) at low altitude (< 1000 meter).

* The airspeed indicator was redlined at Mach 2.8, with typical intercept speeds near Mach 2.5 in order to extend the service life of the engines. A MiG-25 was tracked flying over Sinai at Mach 3.2 in the early 1970s, but the flight led to the destruction of its engines.

* The majority of the on-board avionics were based on vacuum-tube technology, not solid-state electronics. Although they represented aging technology, vacuum tubes were more tolerant of temperature extremes, thereby removing the need for providing complex environmental controls inside the avionics bays. In addition, the vacuum tubes were easy to replace in remote northern airfields where sophisticated transistor parts might not have been readily available. With the use of vacuum tubes, the MiG-25P’s original Smerch-A (Tornado, NATO reporting name “Foxfire”) radar had enormous power – about 600 kilowatts. As with most Soviet aircraft, the MiG-25 was designed to be as rugged as possible. The use of vacuum tubes also makes the aircraft’s systems resistant to an electromagnetic pulse, for example after a nuclear blast.

CITATION: Broad, William J. “Nuclear Pulse: Awakening to the Chaos Factor” and others.

Science, Volume 212, 29 May 1981, pp. 1009–1012.

http://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-25#Western_intelligence_and_the_MiG-25