The final launch of Space Shuttle Discovery will take place no earlier than 3 December.

Weather permitting, the launch - originally set for September but postponed to 1 November because the payload was not ready and then to 30 November after a hydrogen leak was discovered while filling the external fuel tank - is now expected at 02:52 Eastern Standard Time.

The current launch window will be open until 5 December.

The countdown-stopping leak was at the ground umbilical carrier plate, an attachment point between the external tank and a 178mm (7in) pipe that carries gaseous hydrogen safely away from Discovery to the flare stack, where it is burned off.

Two previous Shuttle missions in 2009 - Discovery's STS-119 mission and Endeavour's STS-127 - had similar problems, NASA says.

While emptying the fuel tank, engineers also discovered a foam crack on the external fuel tank's liquid oxygen intertank flange and cracks in the 6.4m (21ft) support beams called stringers. NASA says technicians, using a known repair technique, "installed new sections of metal, called 'doublers', because they are twice as thick as the original stringer metal, to replace the two cracked stringers on Space Shuttle Discovery's external fuel tank".

Such cracks became more common after 1998, when super-lightweight tanks were being built with a more brittle aluminium-lithium alloy. Since then, 29 cracks in stringers making up the ribbed intertank section separating the liquid hydrogen and oxygen tanks have been found and repaired in 18 external tanks, including Discovery's and a tank scheduled for use on Atlantis's final flight in summer 2011.

In addition to the stringer repairs, NASA replaced the hydrogen vent line quick-disconnect fitting after tracing the leak to a misalignment issue. A suspect circuit breaker believed to be responsible for slight electrical anomalies was also replaced.

The US space agency's engineers were still reviewing the completed repairs as most of the USA prepared to take time off for the Thanksgiving holiday 25 November.

Before Discovery can be cleared for launch, engineers must be confident that any new cracks that form in the tank stringers after the Shuttle is refuelled would not cause a loss of protective foam when the Shuttle climbs out of the dense lower atmosphere.

After 26 years of service, Discovery has faced a string of problems trying to get off the ground for its final flight. The original September launch was initially pushed back when payloads to be delivered to the International Space Station were not ready on time.

Then the planned 1 November take-off was repeatedly scrubbed, first to repair leaking helium and nitrogen gas lines on the Shuttle's orbital manoeuvring system pod, which took longer than expected. Electrical problems discovered during a routine engine power-up and check-out and weather added further delays.

The mission - the 133rd for the Shuttle programme and a record 38 for Discovery - will deliver a pressurised logistics module called Leonardo to the 10-year-old ISS.

Leonardo, built by the Italian space agency, will be permanently attached to the space station and used to transfer cargo to and from the station. The Shuttle will also carry Robonaut 2, a 136kg (300lb) humanoid robot developed by NASA, making its first trip into space for testing.

Source: Flight International