On the heels of a successful launch simulation, engineers at the Kennedy Space Center are working to close out assembly of the redesigned boosters that will be used for the first post-Challenger mission in late August.

NASA managers originally expected the boosters to be ready Wednesday for attachment of the shuttle Discovery's giant external fuel tank, but last-minute work remained to be completed late Tuesday and the fuel tank-booster "mating" was postponed 24 hours.In another major milestone, launch teams at the Kennedy Space Center and mission control personnel at the Johnson Space Center in Houston conducted an elaborate practice countdown and make-believe blastoff Tuesday to test new countdown procedures designed to improve decision-making procedures.

"All in all, it was an excellent exercise," said astronaut Robert Crippen, chairman of the new NASA mission management team that has final authority for clearing shuttles for launch.

Lee Briscoe, a flight director at the Johnson Space Center, said, "We don't get to do a simulation like this very often. It was a good idea to get everybody together on the (ommunications) loop."

The simulation took place as engineers worked to put the finishing touches on Discovery's boosters in the Vehicle Assembly Building at the Florida shuttleport.

NASA's official target launch date for Discovery - and it is no more than a target - is Aug. 22. But the actual date will depend on when Discovery is rolled to the launch pad, along with continued progress to complete pre-launch processing.

NASA's internal launch assessment date has fluctuated between Sept. 1 and Aug. 29 over the past few weeks. As of late last week it was back to Aug. 29, two days ahead of the previous internal assessment.

Discovery itself is tentatively scheduled to be moved from its hangar to the nearby VAB late next week for attachment to the external tank. If all goes well, the assembled vehicle will be hauled to the launch pad around June 25 for blastoff.

The goal of Tuesday's "super sim" was to practice using new countdown procedures and generally to give launch teams a chance to get their feet wet before an actual blastoff.

The super sim countdown began at 6:30 a.m. EDT with blastoff of the make-believe shuttle targeted for 9:50 a.m. But the mock launch was delayed until 10:45 a.m. by a series of built-in problems and malfunctions, and there was a main engine failure shortly after launch that required an emergency return to the Kennedy Space Center.