airframe life and provide a higher safety margin. One of Tupolev's web site pages states that "TU-144 and TU-160 aircraft operation has demonstrated expediency of limitation of cruise supersonic speed of M=2.0 to provide structure service life and to limit cruising altitude". Materials The aircraft was designed for a 30,000-hour service life over 15 years. Airframe heating and the high temperature properties of the primary structural materials, which were aluminium alloys, set the maximum speed at Mach 2.2. 15% by weight was titanium and 23% non-metallic materials. Titanium or stainless steel were used for the leading edges, elevons, rudder and the rear fuselage engine-exhaust heat shield. Tu-144DA A project study, assigned the number Tu-144DA, increased the wing area and the take-off weight, and replaced the engines with the RD-36-61 which had 5% more thrust. The Tu-144DA increased fuel capacity from 98,000 kg to 125,000 kg with a higher maximum certified take-off weight (MCTOW) of 235,000 kg. and range up to 7,500 km. Variants Tu-144 – (izdeliye 044 – article 044) The sole prototype Tu-144 aircraft Tu-144S – (izdeliye 004 – article 004) Six redesigned production aircraft powered by Kuznetsov NK-144A engines in widely spaced nacelles, and redesigned undercarriage Tu-144D – (izdeliye 004D – article 004D)(D-Dahl'neye – long-range) Production Tu-144 aircraft powered by Koliesov RD36-51 non-afterburning engines. One aircraft converted from Tu-144 СССР-77105(c/n10031) and five production aircraft (СССР-77111 [c/n10062] to СССР-77115 [c/n 10091]) plus one (СССР-77116) uncompleted Tu-144DA – Projected improved version of the Tu-144D with greater fuel capacity and therefore longer range increased up to Tu-144LL – One Tu-144D aircraft (СССР-77114 [c/n10082]) converted to a flying laboratory with four Kuznetsov NK-321 afterburning turbofan engines and re-registered RA-77114. The first flight took place on 29 November 1996 with the 27th and last flight on 14 April 1999 Proposed military versions Early configurations of the Tu-144 were based on the unbuilt Tupolev Tu-135 bomber, retaining the latter aircraft's canard layout, wings and nacelles. Deriving from the Tu-135 bomber, early Tupolev's design for supersonic passenger airplane was code-named Tu-135P before acquiring the Tu-144 project code. Over the course of the Tu-144 project, the Tupolev bureau created designs of a number of military versions of Tu-144 but none were ever built. In the early 1970s, Tupolev was developing the Tu-144R intended to carry and air-launch up to three solid-fueled ICBMs. The launch was to be performed from within Soviet air space, with the aircraft