"The cost of a scrub is approximately one million dollars," said spokesman Allard Beutel at NASA's Kennedy Space Center here.

Along with the cost of filling, draining and then refilling the external tanks so many times with specialized liquid hydrogen and oxygen fuel, the cost is also boosted by overtime pay for NASA employees and other workers at the space centers here.

The overall 4.5-million-dollar cost provided by information from NASA is "marginal" in Nasa's overall operating budget, added Beutel. The agency says the space shuttle Endeavour alone, built to replace the shuttle Challenger, cost some 1.7 billion dollars.

The weather has been blamed for scuttling chances to launch Endeavour in recent days, but NASA has promised a sixth attempt late Wednesday to reach the International Space Station mission.

The shuttle is now set to lift off at 6:03 pm (2203 GMT) Wednesday from NASA's Kennedy Space Center at Cape Canaveral, Florida, with the US space agency saying there is only a 40 percent chance of unfavorable weather conditions this time.

A launch was also being considered for Thursday, the last possible date before interfering with the July 24 lift-off of the Russian cargo craft Progress to the ISS, launch integration manager Mike Moses told reporters.

Although Russian space officials have accepted the Thursday launch option, Moses noted that it would force NASA to abort the fifth spacewalk planned for Endeavour's mission to the ISS.

If the shuttle does not take off on Wednesday or Thursday, the next launch window would begin on July 26.

Forecasters said nearby thunderstorms, forced NASA to scrub Monday's launch attempt.

"Again, the vehicle and our team were ready, but the weather has just bitten us again with the lightning within 20 nautical miles" (37 kilometers), said launch director Pete Nickolenko shortly after the latest cancellation, just minutes prior to the scheduled launch.

Lift-off has been cancelled three times since Saturday because of the weather and two earlier attempts were aborted after potentially hazardous fuel leaks were discovered, apparently caused by a misaligned plate linking a hydrogen gas vent line with the external fuel tank.

"Technically, we've been really clean the last two days with our vehicle," Moses said of Endeavour's launch attempts on Sunday and Monday. "It's just been the weather scenario that got us."

NASA conducted repair work that they hoped would pave the way for the launch of the shuttle, scheduled to rendezvous with the ISS to complete the assembly of the Japanese Kibo laboratory.

Endeavour's crew of six Americans and one Canadian are scheduled to install a platform on the ISS for astronauts to conduct experiments in the vacuum of space, 350 kilometers (220 miles) above Earth's surface.

The 48-hour delay following the latest scuttled attempt awarded engineers an opportunity to replace the covers made from Tyvek -- a high-density synthetic material -- that protect the shuttle's nose thrusters.

One of the covers had come loose, which could have allowed rain to penetrate the thruster nozzle. The rain would have frozen when the shuttle was in orbit and could have had an impact on maneuvers, such as docking Endeavour.

In the summer, Florida weather is often unstable in the afternoon, with violent storms and heavy rains that can prevent launches from taking place.

Weather proved a thorn in NASA's side on its previous shuttle mission, in May, when Atlantis's return to earth was postponed by three days as stormy conditions forced the shuttle to touch down at its alternative landing spot in California.

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