system plus a terrain contour-matching guidance system which uses radar and images stored in the memory of an onboard computer to find its target. This allows the missile to guide itself to the target with a high degree of accuracy. The original Kh-55 had a drop-down engine; the Kh-65SE had a fixed external turbojet engine, while the Kh-SD had its engine inside the body of the missile. Production versions in 2013 were equipped with the increased-power 450 kgf Russian-made NPO Saturn TRDD-50A engine. In the nuclear role, Kh-55 carries a warhead designed TK66, with a warhead weight of . The mass-size simulator for the warhead is designated KTS-120-12. Operational history The original Kh-55 entered service on 31 December 1983. The Kh-55SM followed in 1987. The conventionally armed Kh-55SE was flight tested on 13 January 2000, and first used in exercises over the Black Sea 17–22 April 2000. The Kh-555 is thought to have entered service in 2004, the first pictures of the Kh-101 appeared in 2007. The Kh-55 can be carried by the Tu-95MS (NATO "Bear-H") and Tu-142M (NATO "Bear-F"), and the Kh-55SM is carried by the Tupolev Tu-160 "White Swan" (NATO "Blackjack"). Sixteen Kh-55's can be carried by the Tu-95MS16 variant, ten on underwing hardpoints and six on an MKU-5-6 rotary launcher. The missile was also tested on the Tu-22M (NATO "Backfire") bombers. The Kh-SD tactical version was to have been carried by the Tu-95MS (fourteen missiles) and the Tu-22M (eight missiles). The Kh-101 is expected to be carried by the Tu-160 (twelve missiles), Tu-95MS16 (eight missiles), Tu-22M3 (four missiles) and Su-34 (two missiles). The end of the Cold War left Ukraine with 1,612 Kh-55s, part of the armament of the 19 Tu-160s of the 184th Heavy Bomber Regiment at Pryluky and the 25 Tu-95MSs of the 182nd Heavy Bomber Regiment at Uzin-Shepelovka. It was reported that Ukraine demanded US$3 billion for the return of the planes and their missiles to Russia. In October 1999, a compromise was reached that saw Russia pay US$285 million for eight Tu-160 and three Tu-95MS bombers and 575 Kh-55 cruise missiles, while the rest were meant to be destroyed under U.S.-led Nunn–Lugar Cooperative Threat Reduction programme. However, in March 2005 Ukraine's prosecutor-general Sviatoslav Piskun said that in 2001, twelve Kh-55s had been exported to Iran in a deal said to be worth US$49.5 million, and an additional six Kh-55s were exported to China.