RP 21 Sapfir Spin Scan-A

The RP 21 Sapfir was a radar system, developed by the Soviet Union for use in the updated MiG-21PF fighter, and later versions of the MiG-21.

The early MiG-21 fighter variants, MiG-21 F and MiG-21F-13, were clear-weather daylight-only fighters. Although they were in use with the Red air force frontal aviation, due to their limitations they were unsuited for the interception tasks of the PVO (Homeland air defence of the USSR). The Mikoyan OKB started development of a more sophisticated interceptor, based on the MiG-21F-13 in the late 1950's. The MiG-21P and MiG-21PF were the first MiG-21's 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 codename of "Spin Scan-A".

The RP 21 Sapfir replaced the Mig-21F-13's SRD-5M Kvantum (Quantum) ranging- and gun-radar. It featured a mechanically steered and gyroscopic stabilised radar dish, and had a thermionic valve (vacuum tube) circuit. It was in theory able to detect fighter-sized targets from a range of 20km, and lock on to them at a range of 10 km, though in practice this got reduced to 13 km and 7 km respectively. The associated weapon system was the K-13 infrared guided air-to-air missile, also known as 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 Mig-21 (MiG-21 PFS) used the further upgraded RP-21M radar (NATO codename "Spin Scan-B") which provided guidance for the new R-3 radar guided AAM, called AA-2 "Atoll-B" by NATO.

Though it was a great improvement over the older SRD-5M, the new intercepting capabilities were still limited, toughh this was 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 F-7MG copies of the Mig-21 (equipped with more advanced Israeli and Italian radars) 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 MTBF (mean time between failure). 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 "ground clutter" return. Another downside 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.

In its defence, the radar was simple to use and repair and was, because of the vacuum tube technology, able to withstand high amounts of EMP radiation (Electro Magnetic Pulse, a "side effect" radiation from nuclear weapons, harmless to man but very damaging to modern solid state (silicon transistor) electronics. Also, and very important, its limited capabilities were offset by standard Soviet PVO doctrine: pilots were tied to a ground-controlled interception system, which provided, trough data links, interceptors with more extensive and more precise information from ground based radars.

Over the years, the west got to know the RP 21 Sapfir a better during the Vietnam war in Indochina and the Six-Days and Yom Kippur Wars in the Middle East. Its combat record is not impressive, mostly due to the primitive and overestimated early generation R-3 radar guided missiles. Also, the radar was rather quickly compromised because off defections, and its weaknesses were well known to the west, giving way for the creation of effective ECM (electronic countermeasures). 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 in early or for export purposes downgraded MiG-23S and MiG-23MS interceptors.

ources

*http://fr.wikipedia.org/wiki/MiG-21
*http://www.vectorsite.net
*Various written sources regarding radar technology, e.g., combat aircraft vague

External links

* [http://www.phazotron.com/en/index.html Phazotron - NIIR Corporation, the developer of the RP-21 Sapfir radar]
* [http://www.missiles.ru/_foto/MuzeyFAZOTRON/faz12.jpgPicture of RP-21 Sapfir radar diplayed in the museum of the Phazotron company]