Wind tunnel tests at the U.S. Air Force's Arnold Engineering Development Center are beginning to show that potential new foam problem areas on the shuttle external tank can be corrected with minor changes and minimal risk to a July launch schedule.

The new concerns arose this month when modified tank foam components blew off an external tank mockup undergoing wind tunnel tests at AEDC. Engineers tweaked the design of the components, and later tests were favorable, with no foam loss. Liftoff of Discovery on STS-121 is planned for July 1.

Additional wind tunnel tests of that same external tank hardware over the next 2-3 weeks will determine if the modified version, of the already modified components, can be adopted for use on the next flight's tank. That, in turn, could have a domino effect on the final stacking plan for the shuttle vehicle elements for the second return-to-flight mission, following the 2003 Columbia accident.

How the modifications on the external tank, ET 119, play out are pressing factors in the schedule here to mate the tank with its solid rocket boosters on a mobile launcher platform, then add Discovery to the tank. Critical schedule milestones for those operations will begin to occur at the launch site here next week.

Test results are preliminary from AEDC, the Air Force's major aerodynamics research center near Tullahoma, Tenn. But if they can be repeated, the data have the potential of further reducing concerns about aerodynamic effects. The areas in question either have already been changed or are due for modification in the wake of the Columbia accident and other foam-debris incidents during the launch of the STS-114 Discovery mission last July.

The coming STS-121 mission, again with Discovery, must launch between July 1-19 or be slipped to a different International Space Station rendezvous window that opens Aug. 28. Although STS-114 was successful, the flight had an unacceptable loss of foam off the hydrogen protuberance air load (PAL) ramp of its external tank during ascent. The several-foot-long PAL ramp has been used on all previous tanks to divert high-speed aerodynamic flow away from gaseous oxygen and hydrogen pressurization lines and a cable tray running down the side of the tank.

After extensive computer analysis, engineers determined they could remove the PAL ramp entirely, deleting that particular foam risk.

The 154-ft. tank for the next flight is entering final processing here without the ramp, but several other points on the tank's exterior could still be modified here during the next month.

Wind tunnel information on tank aerodynamics collected at the NASA Ames and Glenn research centers, as well as at AEDC, are being used to validate the analytical data. But the AEDC runs on a full-scale section of the external tank, without the PAL ramp, is being used to check flow patterns especially on surrounding components.

The other elements are specifically the foot-long "ice/frost ramps" designed to prevent ice debris buildup on the brackets that hold the pressurization lines to the external tank. There are seven of them on the hydrogen part of the tank and two on the oxygen section. Ice buildup on those components can cause an ascent debris hazard. Engineers would like to reduce the size of the hydrogen frost ramps to lessen the chance they could fly off and become debris.