A make-believe space shuttle blasted off Tuesday after a mock countdown that served as a major test for the post-Challenger decision-making process.

In full-scale dress rehearsal for the next shuttle mission, scheduled for launch in late August, the so-called "super sim," the most extensive launch simulation conducted since the Challenger disaster, began in earnest at 6:30 a.m. with the start of a mock countdown.Following a series of make-believe problems that delayed the countdown - including an apparently unexpected problem at T-minus 31 seconds - the imaginary shuttle blasted off at 10:45 a.m., 55 minutes late.

The mock launch at the Kennedy Space Center was a key milestone on the road to resuming U.S. manned space flights. The test wound up around 11:19 a.m. after a simulated emergency landing at the Kennedy Space Center.

Three astronauts in a shuttle simulator in Houston played the role of the flight crew, but the exercise focussed on the performance of the entire launch team at the Kennedy Space Center as well as mission control personnel at the Johnson Space Center in Houston.

A major goal of the simulation was to exercise NASA's "mission management team," a group of top agency managers under astronaut Robert Crippen charged with making the final launch decision.

The presidential commission that investigated the Challenger disaster charged NASA's decision to launch the doomed shuttle was "flawed." The mission management team addresses such concerns by improving communications and clearly defining the chain of command.

When Discovery takes off for real, it will have been 2 1/2 years since the launch of Challenger in January 1986, and NASA managers want to make sure new procedures and lines of communication will work as advertised.

Throughout the countdown, simulated problems were thrown at the launch team to determine how they would respond. Most were relatively minor but one make-believe glitch, a leak past a fuel valve in the shuttle's No. 2 main engine, would have forced a launch delay had the countdown been for real.

Under new launch rules designed to improve safety, the mission management team will be responsible for making a launch recommendation to Crippen during a final 10-minute hold in the countdown at the T-minus nine-minute mark.

Crippen, in turn, has the ultimate responsibility for allowing a launch to proceed.