The Super Lightweight Tank used 2195 Aluminium-Lithium alloy as the main structural material in place of the 2219 aluminium alloy of the original design. This saved 3,500 kg in empty mass, increasing shuttle payload by the same amount. This change was made in anticipation of Shuttle-Mir and Shuttle-ISS flights to high inclination 51.6 degree orbits. The tank was first used on STS-91 in June 1998.