A new Space Shuttle external tank (ET) will be used for the first time in December 1997, fitted to the STS88/ Endeavour, the first assembly mission to the International Space Station (ISS). The tank will be 3,400kg lighter than the original ET, adding an equivalent weight to the amount of payload which can be carried into orbit. The new ET is made of aluminium lithium, which is lighter and stronger than the metal alloy used in the previous tank.

The payload-capability increase is critical because the decision to alter the orbital inclination of the ISS from 28degrees to 51degrees, to accommodate its Russian partners, means that the Space Shuttle will be constrained by a lower payload capacity. NASA gave the go-ahead to the lightweight ET in 1994, awarding a $172 million contract to Lockheed Martin. The new ET will enable the Shuttle to increase its payload performance to other orbits as well, if required.

NASA and Lockheed Martin have completed tests at the space agency's Marshall Space Flight Center, in Huntsville, Alabama, to demonstrate the capability of the new ET to withstand loads greater than flight-certification requirements. The tank replaces aluminium alloy with aluminium-lithium alloy, and has changed to a waffle-like design. The alloy is 30% stronger and 5% less dense than the present material.

The test was a full-scale, but shortened, version of the ET's new aluminium-lithium hydrogen tank with the liquid-oxygen tank dome. It will later be subjected to the first-ever destructive test on a tank. It was exposed to structural verification tests simulating the loads and pressures it is put under at lift-off, as well as when it is unpressurised on the launch pad and when the solid-rocket boosters separate from the Shuttle during the ascent.

The current ET weighs about 760,000kg at lift-off, with a load of 626,000kg of liquid oxygen and 104,000kg of liquid hydrogen. This compares with the total Shuttle lift-off weight of about 2,000,000kg, with the orbiter weighing around 250,000kg and the solid-rocket boosters weighing about 1,100,000kg. The weights of the orbiter, ET and solid-rocket boosters vary significantly from mission to mission.


Payload problem

The Shuttle's payload-carrying capacity has always been a problem for the programme, whether NASA cares to admit it or not. The original payload capacity to a 28 degree orbit was to have been 29,500kg, but was never achieved.

In 1993, the design of the Shuttle was frozen after a thorough review of the effect of lift-off-dynamics forces on the system. Following that, many components were required to be made at double their original tolerance. This had increased the "stack" weight by about 11,000kg over the life of the programme (Flight International, 24-30 January, 1996).

The theoretical maximum payload capacity was reduced to 24,400kg. This is almost impossible to achieve operationally, since an extra crewman and the addition of the remote manipulator-system arm, for example, adds 675kg to the weight.

In addition to the 3,400kg saving from the new ET, NASA is trying to save another 2,500kg by using lightweight seats, reducing contingency reaction-control-system propellant. Other plans include firing the orbital-manoeuvring system engines for a brief moment, just as the solid-rocket boosters are petering out, to "buy" 115kg.

With such savings, the Atlantis, for example, will be able to carry a theoretical 21,320kg into a 51 degree orbit and 27,220kg into a 28 degree orbit. These figures, however, do not relate to those actually carried on operational missions. For example, the heaviest payload carried to a 51 degree orbit since the resumption of flights in September 1988 after the Challenger accident was 12,190kg, launched by the STS71, the first Shuttle Mir Mission (SMM), in June 1995. The SMMs STS74, 76 and 79 carried an average payload of only 7,250kg. Even with the lightweight ET and other savings, the equivalent STS71 payload has been increased to 18,100kg.

Source: Flight International