The shuttle Discovery's crew strapped in for a dress-rehearsal countdown Friday, a milestone on the road to launch around Aug. 24 on a space station resupply mission. Engineers, meanwhile, are continuing a down-to-the-wire assessment of critical insulation on Discovery's external tank in the wake of foam losses during the shuttle Endeavour's launch last month.

Credit: NASA-KSC

Foam losses in three areas were noted during Endeavour's climb to space, most of it falling from the central intertank region of the shuttle's external tank. A small bit of debris also separated from the left-hand bipod mount, where one of two struts holds the nose of the shuttle to the tank, and another piece of insulation fell from a so-called ice-frost ramp on the upper liquid oxygen section of the tank.

Engineers are still not sure what caused the intertank foam to separate. But nearly 170 "plug pull" tests were conducted on Discovery's tank - ET-132 - to assess the adhesion of the insulation to the metal structure of the intertank. In every case, the adhesion was normal.

Engineers also believe they understand how the bipod foam came off, the presumed result of voids in the foam insulation around a wiring bundle. But the ice-frost ramp foam loss from a bracket holding an oxygen pressurization line is a somewhat different issue. Endeavour's flight was the second in a row to suffer foam loss from the IFR in question, indicating a potential problem in the way the foam is applied.

The insulation on the pressurization line fitting is applied by injecting foam into a sort of mold. After the foam hardens and the mold is removed, technicians manually shape the foam into the aerodynamic ice-frost ramps visible on the side of the tank.

But the current technique can result in voids in the foam depending on how the mold injection process is carried out. If voids are present, heating from atmospheric friction during the shuttle's climb out of the dense lower atmosphere can cause air trapped in such voids to expand, popping off the overlying foam. Depending on when it is released, such foam can pose an impact hazard to the shuttle's fragile heat shield.

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In Discovery's case, engineers X-rayed the IFR in question and saw no evidence of any voids. But engineers at the Marshall Space Flight Center in Huntsville, Ala., are looking into possible remedies for upcoming tanks, including a manual application technique using a different type of foam that might reduce the odds of voids in future flights.

At the same time, shuttle Program Manager John Shannon asked engineers at the Kennedy Space Center to look into how such work could be carried out for Discovery's tank if engineers and managers ultimately conclude a fix of some sort is required.

The IFR cannot be accessed at the launch pad and any such repairs to ET-132 would require a rollback to the Vehicle Assembly Building, delaying launch until mid October at the earliest to avoid conflicts with upcoming Japanese and Russian space station missions.

As of this writing, no final decisions have been made and engineers at the Kennedy Space Center are continuing to process Discovery for launch around Aug. 24 or 25.

But the issue will be reviewed during a shuttle program flight readiness review on Aug. 12, followed by an executive-level review Aug. 18.

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