uncorroborated since the U.S. figures are also suspect. If a plane was badly damaged but managed to land, the USAF did not count as a loss even if it was too damaged to fly again. During the Vietnam war, the Soviet Union delivered 95 S-75 systems and 7,658 missiles to the Vietnamese. 6,806 missiles were launched or removed by outdating. According to the Vietnamese, the S-75 shot down 1,046 aircraft, or 31% of all downed US aircraft. By comparison, air-defense guns brought down 60% and 9% were shot down by MiG fighters. The higher rate of anti-aircraft artillery is partially caused by the fact gun units received data from the S-75 radar stations that significantly improved their effectiveness. Replacement systems Soviet Air Defence Forces started to replace the S-75 with the vastly superior S-300 system in the 1980s. The S-75 remains in widespread service throughout the world, with some level of operational ability in 35 countries. In the 2000s, Vietnam and Egypt are tied for the largest deployments at 280 missiles each, while North Korea has 270. The Chinese also deploy the HQ-2, an upgrade of the S-75, in relatively large numbers. Description Soviet doctrinal organization The Soviet Union used a fairly standard organizational structure for S-75 units. Other countries that have employed the S-75 may have modified this structure. Typically, the S-75 is organized into a regimental structure with three subordinate battalions. The regimental headquarters will control the early-warning radars and coordinate battalion actions. The battalions will contain several batteries with their associated acquisition and targeting radars. Site layout Each battalion will typically have six, semi-fixed, single-rail launchers for their V-750 missiles positioned approximately apart from each other in a hexagonal "flower" pattern, with radars and guidance systems placed in the center. This unique "flower" shape led to the sites being easily recognizable in reconnaissance photos. Typically another six missiles are stored on tractor-trailers near the center of the site. Missile The V-750 is a two-stage missile consisting of a solid-fuel booster and a storable liquid-fuel upper stage, which burns AK-20 (based on red fuming nitric acid) as the oxidizer and TG-02 (toxic mixture of 50-52% triethylamine and 48-50% isomeric xylidine) as the fuel. The booster fires for about 4–5 seconds and the main engine for about 22 seconds, by which time the missile is traveling at about Mach 3. The booster mounts four large, cropped-delta wing fins that