If you take the time to read the linked site (http://www.bbc.com/future/story/20160905-the-pilot-who-stole-a-secret-soviet-fighter-jet), you will discover that Western assessments of the capabilities of the MIG-25 (FOXBAT) aircraft when it was introduced to the Soviet Air Force in 1970 were very wide-of-the-mark.

Rather than being a very high-altitude, high-speed, maneuverable Cold-War technology gap threat, it was a typical Soviet brute-force platform that (1) had large wing-areas not for maneuverability at high altitude but to keep the enormous weight of the MIG-25 airborne, (2) used two jet engines developed for use in a cruise-missile that developed such high-heat that, at speeds above mach-2.8, the engines suffered significant damage (“ate” engine parts), and (3), it was made of hand-welded steel whereas the US SR-71 was made of titanium extracted from rutile ore purchased from the Soviet Union through third-country front companies.

All of these erroneous assessments were exposed on 6 Sep 1976 when a Soviet pilot (Viktor Belenko) defected: landing his MIG-25 in Japan. Technical exploitation of the aircraft showed it to be “an expensive, and cumbersome aircraft, and it wasn’t particularly effective in combat.”

So, what has all of this to do with the BOREY and YASEN Class Russian Nuclear Submarines? A lot, as already discussed in detail by numerous postings of this site.

Like the MIG-25, BOREY and YASEN Class units use existing propulsion technology: in this case, 30-year old main propulsion power trains scavenged from OSCAR and AKULA Class units. These power trains used double-reduction planetary gear systems fabricated before improved gear-cutting tolerances became available to reduce radiated noise-levels, a vulnerability the Russians addressed in BOREY and YASEN by using PG-141M dc motors for patrol operations and moderate speed transits. Cannibalized bow and stern AKULA pressure-hull sections also have been used.

As previously stated, BOREY and YASEN Class submarines appear not to be fourth-generation nuclear platforms but “Frankenstein” hulls built to replace aging SSBNs and SSNs; be generous: call them generation 3.5.