Without setting a new launch date, shuttle program managers agreed Jan. 6 that ongoing troubleshooting of shuttle Discovery’s external tank (ET) cracks will not be complete in time for an early February launch.

The 11-day assembly mission to the International Space Station has been on hold since a Nov. 5 scrub because of eight small cracks found in five of the ET’s 108 aluminum-lithium support stringers.

The decision deferred efforts to prepare for a launch between Feb. 3 and 10. The next window opens Feb. 27 and extends through March 6, though earlier opportunities could open with changes in international cargo missions currently planned for March.

A launch date decision is expected Jan. 13, NASA shuttle program spokesman Kyle Herring says.

During the Jan. 6 session, managers also deferred a decision on whether to proceed with additional radius block modifications to Discovery’s ET. The mods strengthen the upper tips of the 21-ft.-long stringers to prevent additional cracking. Managers are evaluating whether to place radius blocks on all 108 of the support beams on the stringer section.

That decision would come Jan. 10, following a report on the progress of a limited radius block that will place the crack protection on 34 stringers distributed on either side of the two ET thrust panels. The thrust panels shoulder most of the structural forces accompanying the launch of the 4.5-million-lb. shuttle.

Meanwhile, engineers continue to search for the root cause of the cracks. Managers believe that material defects or a slight misassembly of the stringer section could have been exacerbated by the flow of chilled oxygen and hydrogen propellants into the ET during fueling.

The initial cracks on two adjoining stringers were discovered following a Nov. 5 launch scrub caused by an unrelated hydrogen leak.

The mission is to be Discovery’s last. The shuttle’s six astronauts will equip the station with a new equipment module and an external storage platform for spare parts.