View source for RP-21 Sapfir

[[Image:RP-21MA Hu Szolnok 1.jpg|thumb|300px|right|Front view of RP-21MA radar]]
The '''RP-21 ''Sapfir''''' ([[NATO reporting name|NATO codename]]: ''Spin Scan)'' was a [[radar]] system developed by the [[Soviet Union]] for use in the updated [[MiG-21|MiG-21PF]] fighter, and used in later versions of the MiG-21.

The early MiG-21 fighter variants, MiG-21F and MiG-21F-13, were clear-weather daylight-only fighters. Although they were in use with the [[Soviet Air Force]] Frontal Aviation (VVS FA), due to their limitations, they were unsuited for the interception tasks of the [[Soviet Air Defence Forces]] (PVO). The [[Mikoyan]] [[OKB]] started development of a more sophisticated interceptor, based on the MiG-21F-13 in the late 1950s. The MiG-21P and MiG-21PF were the first MiG-21s to be equipped with a real radar that would enable them to search, track and intercept targets by night and in foul weather: the RP-21 ''Sapfir'' ('Sapphire') radar, which was given the [[NATO reporting name|NATO codename]] of "Spin Scan-A."

The RP-21 ''Sapfir'' replaced the MiG-21F-13's SRD-5M ''Kvantum'' ('Quantum') ranging- and gun-radar. RP-21 featured a mechanically steered and gyroscopic stabilised radar dish, and had a [[thermionic valve]] (vacuum tube) circuit. In theory it was able to detect fighter-sized targets from a range of 20&nbsp;km, and lock on to them at a range of 10&nbsp;km, though in practice this range reduced to 13&nbsp;km and 7&nbsp;km respectively. The associated weapon was the [[Vympel K-13]] infrared guided air-to-air missile, also known as [[Vympel K-13|R-3S]], object 310 or AA-2 "Atoll-A" by NATO, already in use with the older SRD-5M radar. Later on, newer versions of the aircraft (namely, MiG-21PFS) used the further upgraded RP-21M radar (NATO codename "Spin Scan-B"), which allowed the [[semi-active radar homing]] (SARH) missile, the new [[Vympel K-13|R-3R]] (NATO codename AA-2 "Atoll-B"). 

Though it was a great improvement over the older SRD-5M, the new intercepting capabilities were still limited, mostly due to an inherent design flaw of the MiG-21: the nose inlet limited the size of the small radar cone, so the radar field was limited to 20° vertically and 60° horizontally. Even in the latest Chinese [[Chengdu J-7|F-7MG]] copies of the MiG-21 (equipped with more advanced Israeli and Italian radars {{citation-needed|date=May 2015}}) this is still said to be a significant limiting factor for radar use. Also, because of the vacuum tube electronics, the radar system (like most other early generation radars) had a low [[mean time between failures]] (MTBF). The radar lacked [[look-down/shoot-down]] capabilities (though this was not at all a common feature of radar systems at the time), meaning that it couldn't intercept targets that were flying under the MiG-21, because the radar system was unable to filter out "[[Clutter (radar)|ground clutter]]" return. Another disadvantage of the new radar (and other avionics) was the weight increase in the MiG-21 PF, which reduced the baseline MiG-21's otherwise impressive flight characteristics. 

[[Image:RP-21MA Hu Szolnok 2.jpg|thumb|300px|right|Rear view of RP-21MA]]
The radar was simple to use and repair, and, because of vacuum tube technology, was able to withstand a powerful [[electro-magnetic pulse]] (EMP), a side-effect from nuclear explosion, radiation harmless to humans but very damaging to modern [[Solid state (electronics)|solid state]] (silicon transistor) electronics. Its capabilities, as limited as they may seem, were offset by the standard [[Protivo-Vozdushnaya Oborona|PVO]] doctrine: pilots were tied to a [[ground-controlled interception]] (GCI) system, which, through ground-based radars and data links, provided interceptors more extensive and more precise information. 

Over the years, the West became more acquainted the RP-21 Sapfir during the [[Vietnam War]] in [[Indochina]], and during the [[Six-Day War|Six-Day]] and the [[Yom Kippur War]]s in the Middle East. Its combat record is not impressive, mostly due to the primitive and overestimated early generation R-3R radar guided missiles. Also, the radar was rather quickly compromised because of defections, and its weaknesses were well known to the West, giving way for the creation of effective [[electronic countermeasures]] (ECM). Western pilots learned to distinguish the RP-21's rhythmic three tone pulse. 

Still, the RP-21 and its developments, like the [[RP-22]] radar (NATO codename "Jay Bird") of the later MiG-21bis series, had a long career and the latter was even used for export purposes in downgraded [[MiG-23|MiG-23S]] and MiG-23MS interceptors.

==Sources==
*[[:fr:MiG-21]] {{Vague|date=March 2011}}
*{{usurped|1=[https://web.archive.org/web/20060813130005/http://www.vectorsite.net/avmig21_1.html#m3 Article title]}}
*Various written sources regarding radar technology, e.g., combat aircraft {{Vague|date=February 2009}}

==External links==
*[http://www.phazotron.com/en/index.html Phazotron - NIIR Corporation, the developer of the RP-21 Sapfir radar] {{Webarchive|url=https://web.archive.org/web/20070321192415/http://www.phazotron.com/en/index.html |date=2007-03-21 }}
*[http://www.missiles.ru/_foto/MuzeyFAZOTRON/faz12.jpg RP-21 Sapfir radar] at the museum of the [[Phazotron]] company
*[https://www.cia.gov/readingroom/docs/CIA-RDP82-00038R001800190001-3.pdf R-2L RADAR MANUAL ENGLISH TRANSLATION]
*[https://www.cia.gov/readingroom/docs/CIA-RDP82-00038R001800180001-4.pdf R-2L RADAR SOVIET MANUAL]
*[https://archive.org/details/RP-21_Sapfir_radar_technical_manual RP-21MA Service manuals]

[[Category:Aircraft radars]]
[[Category:Soviet military radars]]
[[Category:Phazotron products]]