The Tupolev Tu-160 (Russian: Туполев Ту-160 Белый лебедь, romanized: Belyj Lebeď, lit. 'White Swan'; NATO reporting name: Blackjack) is a supersonic, variable-sweep wing heavy strategic bomber designed by the Tupolev Design Bureau in the Soviet Union in the 1970s. It is the largest and heaviest Mach 2+ supersonic military aircraft ever built and next to the experimental XB-70 Valkyrie in overall length. As of 2021, it is the largest and heaviest combat aircraft, the fastest bomber in use and the largest and heaviest variable-sweep wing airplane ever flown. Entering service in 1987, the Tu-160 was the last strategic bomber designed for the Soviet Union. As of 2016, the Russian Air Force's Long Range Aviation branch has at least 16 aircraft in service. The Tu-160 active fleet has been undergoing upgrades to electronics systems since the early 2000s. The Tu-160M modernization programme has begun with the first updated aircraft delivered in December 2014. The Tu-160 is a variable-geometry wing aircraft. The aircraft employs a fly-by-wire control system with a blended wing profile, and full-span slats are used on the leading edges, with double-slotted flaps on the trailing edges and cruciform tail. The Tu-160 has a crew of four (pilot, co-pilot, bombardier, and defensive systems operator) in K-36LM ejection seats. The Tu-160 is powered by four Kuznetsov NK-32 afterburning turbofan engines, the most powerful ever fitted to a combat aircraft. Unlike the American B-1B Lancer, which reduced the original Mach 2+ requirement for the B-1A to achieve a smaller radar cross-section, the Tu-160 retains variable intake ramps, and is capable of reaching Mach 2.05 speed at altitude. The Tu-160 is equipped with a probe-and-drogue in-flight refueling system for extended-range missions, although it is rarely used. The Tu-160's internal fuel capacity of 130 tons gives the aircraft a roughly 15-hour flight endurance at a cruise speed of around 850 km/h (530 mph), Mach 0.77, at 9,100 m (30,000 ft). In February 2008, Tu-160 bombers and Il-78 refueling tankers practiced air refueling during air combat exercise, as well as MiG-31, A-50 and other Russian combat aircraft. Weapons are carried in two internal bays, each capable of holding 20,000 kg (44,000 lb) of free-fall weapons or a rotary launcher for nuclear missiles; additional missiles may also be carried externally.[citation needed] The aircraft's total weapons load capacity is 40,000 kg (88,000 lb). No defensive weapons are provided; the Tu-160 is the first post-World War II Soviet bomber to lack such defenses. For more details on the origins, development, operational history and 10 variants, click here.