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Basic History State of PVO before MiG-25 During the middle part of the 1950s, the PVO (Soviet air defense forces) were poorly equipped to deal with future threats. The majority of its interceptors consisted of aircraft such as the MiG-15, MiG-17, MiG-19, and Yak-25. These aircraft possessed sufficient performance to take on American bombers such as the B-29 and B-50, and were at least marginally capable of intercepting jets like the B-47 or B-52. However, they were horribly inadequate against coming bombers, such as the B-58 Hustler (which first flew in 1956). The arrival of Mach 2 capable interceptors such as the Su-9, and later the MiG-21 in the latter portion of the 1950s evened the playing field somewhat. However, these interceptors suffered from severe deficiencies. The Su-9 (and to a lesser extent the MiG-21) had a cripplingly short range, a major shortcoming when defending a country as large as the Soviet Union. The Yak-27, a development of the Yak-25, had better range than either of the deltas, but was inferior in terms of speed and altitude. In addition to various performance issues, the PVO’s interceptors were also handicapped by poor armaments. The Kaliningrad K-5 (NATO AA-1) radar guided missile was equipped by most PVO interceptors following its introduction in 1955. Though decent for its time, it was limited by its beam riding guidance and short range. The infrared guided K-13 (AA-2) was introduced in 1960 after being hurried copied from a captured AIM-9B. Like the K-5, the K-13 suffered from poor range, and was also limited to being fired at the rear of a target. These systems would clearly not be sufficient to reliably destroy the B-58, let alone the Mach 3 capable XB-70 (which began development in the late 1950s). The introduction of surface to air missiles such as the S-25 and S-75 had some promise (as demonstrated by the destruction of a U-2 reconnaissance aircraft by an S-75 on May 1 1960 at over 18,000 meters). However, these missiles required large fixed launch sites, leaving them incapable of covering large swaths of territory. In an environment where a single bomber was capable of destroying a whole city, the situation was clearly unacceptable. It was clear that a new interceptor was needed to equip the PVO. Numerous solutions were tried. A modified variant of the MiG-19, the MiG-19SU was tested. This aircraft was fitted with liquid rocket engines in addition to the two RD-9s, giving (briefly) improved speed and altitude capability. This allowed flights to over 20,000 meters , however, the aircraft suffered severe controllability issues. In any case, it was clearly a temporarily solution. Another program was the development of various ‘heavy interceptors’, capable of engaging NATO bombers at extreme range, well away from populated areas or strategic targets. These included the abortive La-250, and more successful Tu-128 (developed from the Tu-98 bomber prototype). Though the Tu-128 entered service (indeed, it continued on into the early 1990s), it did not arrive until the middle of the 1960s. Even then, it was a large and ponderous aircraft, capable only of transonic speeds and completely lacking maneuverability. It could not serve as the PVO’s primary interceptor. An effort with more potential was the Sukhoi T-37. This aircraft was broadly similar in planform to the Su-9 (being a tailed delta) but was much larger. Powered by an R-15-300 engine, it was planned to reach speeds near 3,000 km/h at altitudes approaching 25,000 meters. Interestingly, it was also to have been fitted with equipment enabling it to be flown automatically under ground control (the US had the SemiAutomatic Ground Environment, a similar system capable of directing F-106s). However, it was plagued by numerous issues, and was scrapped in 1960 before it ever flew. Requirements The requirements for the MiG-25 were greatly determined by its potential adversaries. The main threat was the B-58 Hustler, capable of Mach 2 at altitudes approaching 18,000 meters. In 1960, the Soviets became aware of yet another threat; a Mach 3 successor to the U-2 was under development (this was the A-12/SR-71, though the Soviets did not know this initially). As a result, it was decided that the new interceptor should be capable of flight at up to Mach 3, and at altitudes in excess of 20,000 meters. Coincidentally, the VVS (Soviet Air Force, separate from air defense forces) was looking for a new high speed reconnaissance aircraft during the late 1950s. At the time, most of the VVS reconnaissance force consisted of variants of the Il-28 or Yak-25. The survivability of these aircraft against new NATO fighters such as the F-104 or Lightning was marginal at best. The Yak-25RV was in development, and promised comparable performance to the U-2 (it did not fully deliver), but this was an incremental step at best. A wholly new aircraft with exceptional performance was needed. By 1960, it was realized that the new recon aircraft had broadly similar requirements to the new PVO interceptor program (by then in development for over a year), so the programs were merged. The ability to carry photorecon equipment was added as a requirement to the interceptor program. Development The Ye-150/152 can be considered the direct ancestor of the MiG-25. Developed in the late 1950s and first flying in 1960, the Ye-150 was broadly similar in appearance to the MiG-21, but was significantly larger and was capable of reaching much higher speeds. The Ye-152 was a further development; it was fitted with R-15-300 engines, the same R-15s which would be fitted to the MiG-25. Like the Ye-150, it represented a massive performance leap over the MiG-21, but it was very much a testbed. The R-15s were supremely unreliable (early models had an average engine lifetime of roughly 20 hours), and the weapons system was tempermental. There was also the Ye-152A, which was fitted with two R-11 turbojets (the same engine as the MiG-21) instead of a single R-15. Though these aircraft did not enter production, they provided the Mikoyan-Gurevich design bureau with valuable data on high speed flight and the R-15 engine. There is some uncertainty about the initial genesis of the MiG-25 program. There are rumors that Mikoyan instructed chief designed Seletisky to develop an interceptor similar to the North American A-5 Vigilante (which had first flow in late 1958) powered by two R-15-300s. Other sources state that work began before the first flight of the A-5. While there are obvious similarities in layout between the A-5 and MiG-25, there are also massive differences. At most, the MiG-25 was somewhat inspired by the A-5, and is very much not a ‘clone’ or reverse engineered copy. (Later proposed Vigilante derivatives such as the NR-349 or rumored J58 powered version would have been much more analogous to the MiG-25). The new aircraft was assigned the designation Ye-155 by Mikoyan (the prefix Ye for yedinitsa denoting a prototype or testbed aircraft). It was decided very early to use the R-15-300 engine. However, there remained uncertainty as to the layout of the engines. Though a conventional side by side layout was ultimately chosen, a vertically stacked layout (such as on the English Electric Lightning) was also considered, as was a staggered engine arrangement (as on the I-320 prototype). Unlike previous Mikoyan-Gurevich designs, it was decided not to utilize a nose mounted intake, but instead place the intakes under the wings. This allowed for a smaller fuselage (in both length and cross section). The underwing intake placement worked well with the shoulder mounted wing placement, which was needed to allow for the carriage of very large AAMs. This wing layout also allowed the wings to be constructed in one piece, simplifying constructing and improving structural rigidity. Numerous planforms were proposed for the wing before the design of the prototype was finalized. At one point variable geometry designs were considered (this would have significantly predated the introduction of the Su-17, though variable sweep designs had been studied at least since 1945). The variable sweep design was seriously considered enough that a model of a recon MiG-25 with variable geometry was made, it somewhat resembles an F-14. One of the benefits of the variable geometry design would have been improved takeoff and landing performance, however, it was decided that this was not worth the added complexity and weight associated with such a design. Following numerous wind tunnel tests, a trapezoidal wing of low aspect ratio was chosen. As with many Soviet designs in the 1950s and 1960s, lift jets were considered for use to improve takeoff and landing distance. The proposed STOL design would have used two small RD36-35 lift engines, mounted in a slightly staggered arrangement in the fuselage, with intakes along the back of the aircraft. While the lift jets would likely have improved takeoff and landing distances greatly, they also decreased the internal volume available for fuel. This was a serious drawback, especially for the reconnaissance version. As a result, the lift jets were discarded, with the STOL design not progressing beyond the model stage. As the MiG-25’s design grew closer to being finalized, materials selection became a serious issue. The leading edges of the wings, inlets, and nose of the aircraft would experience extremely high temperatures at Mach 3, well above the melting point of conventional aluminum alloys commonly used in aircraft construction. Alternative materials had to be found, and many were considered. At first, titanium seemed a logical choice. It has excellent thermal properties, is quite strong for its weight, and the Soviet Union possessed large reserves of the material. However, the Soviet aircraft industry had little experience working with titanium, which was notoriously temperamental (for instance, when manufacturing the SR-71, Lockheed engineers were forced to use special tools, as normal tools contained cadmium, which made the titanium brittle). In particular, the automated welding methods commonly used in Soviet aircraft manufacturing plants would not be suitable for titanium. As a result, it was decided to use steel alloys for the majority of the MiG-25s structure (titanium was used in some areas, but at much lower quantities). Initially, there were some doubts as to whether certain components (such as the integrated fuel tanks) could be made strong enough without seriously increasing weight, or absorbing the cyclical flight loads. Tests showed that this was not the case, and the MiG-25 ended up being built using large quantities. In addition to the issues with materials selection for the aircraft structure, the thermal loads associated with high speed flight caused trouble in other areas. Normal lubricants and hydraulic fluid would break down at the temperatures the MiG-25 would experience, and normal canopy glass would melt. In many ways, this mirrors the problems faced by the Skunk Works team designing the A-12 at around the same time. Throughout 1962 and 1963, the design of the MiG-25 was further refined. In 1963, construction began on the first prototype, Ye-155R1. Its completion took most of the year, and it was not rolled out from the Zenit Machinery Works (the common name for the MiG bureau’s experimental aircraft factory) until December. Despite this, Ye-155R-1, a prototype of the reconnaissance variant, was missing much of its operational equipment. Still, it had more than enough to validate the basic flight characteristics of the airframe. In addition to lacking various pieces of equipment, Ye-155R-1 differed from production MiG-25s in several ways. For one, the aircraft had a pair of 600 liter fuel tanks mounted at the wingtips. These both increased fuel capacity, and prevented flutter. Ventral fins were attached to the fuel tanks, to improve lateral stability at high speeds. Both of these features would be absent from production MiG-25s. Ye-155R-1 also had provisions for the fitment of canards to the sides of the forward portion of the air intakes; these would have been used for pitch control at high Mach numbers, but they were never installed. The first prototype suffered from numerous issues. Among the most serious of these was roll control issues in the transonic regime, in some cases severe enough to render the aircraft uncontrollable. The wingtip fuel tanks caused vibration as fuel was depleted and sloshed about. The intakes were inefficient at high Mach numbers, and the aircraft’s static margin decreased as well. Finally, Ye-155R-1 was overweight, causing its range to fall short of the target. The second prototype, Ye-155R-2, was also a prototype of the recon MiG-25. This aircraft was broadly similar to the previous prototype, though it did incorporate some refinements. There was one immediately obvious difference; the wingtip fuel tanks were deleted. This would be the last prototype built at the Zenit plant; the factory was tasked with producing the first MiG-23 prototypes, and had no more room for further Ye-155s. As a result, it was decided to build prototypes from Ye-155R-3 onward at Mikoyan’s Gorkii plant, the same as would produce the production aircraft. This resulted in a delay in producing Ye-155R-3 as the factory was retooled, but it would ultimately allow production to commence quicker. Ye-155R-3 was a milestone in and of itself; it was fitted with a full suite of photorecon equipment, and would be used for testing various camera arrangements (among other things). Belenko Defection Later Variants Post-Soviet service Variants Prototypes Ye-155 models Operational MiG-25P/PD/PDS Export MiG-25P MiG-25RB MiG-25R MiG-25BM MiG-25PU Other Ye-266M Ye-155MP Buran testbeds / training aircraft Various other concepts and variants Production In Red Air Force Service Other Operators Combat Performance Egypt vs. Israel Iraq MiG-25 vs. SR-71 Structure Materials Engines Electronics Radar Early versions vs. Later Recon variants - cameras Armaments Air to Air MiG-25P MiG-25PD Ground MiG-25RB MiG-25BM Other Systems of Note Citations http://www.kamov.net/russian-aircraft/mig-19su/ http://www.kamov.net/russian-aircraft/sukhoi-t-37/ MiG-25 'Foxbat', MiG-31 'Foxhound': Russia's defensive front line By: Gordon, Yefim. Aerofax 1997 Skunk Works: a personal memoir of my years at Lockheed By: Rich, Ben R., and Leo Janos. Little, Brown 1994