Editor's Note...
This story has been updated to provide additional information about the so-called "tin whiskering" phenomenon that may be present in an ascent thrust vector control unit. This update also corrects an editing oversight (ATVC units control main engine, not aerosurface, movement).

Initial wind tunnel tests indicate recent modifications to the foam insulation on the shuttle's external tank may not be as easily analyzed as initially hoped, sources say. While additional testing may resolve the matter, showing the removal of wind deflectors called PAL ramps from the tank will not compromise safety, other ongoing technical issues, including a new concern about possible circuit board problems, threaten the July 1 target date for the next shuttle mission.

Shuttle program managers, meanwhile, are studying proposed launch dates for subsequent missions in the wake of a March 15 decision to delay Discovery's launch on the second post-Columbia mission from May 10 to July 1. Managers plan to meet Thursday to review the following proposed "no-earlier-than" launch targets:

• STS-121/ULF1.1 (Discovery): 07/01/06 (3:52 p.m. EDT; launch window closes July 19)
• STS-300* (Atlantis; emergency rescue flight if needed): 08/04/06
• STS-115/12A (Atlantis): 08/28/06
• STS-301* (Discovery; emergency rescue flight if needed): 11/11/06
• STS-116/12A.1 (Discovery): 12/14/06
• STS-117/13A (Atlantis): 03/22/07
• STS-118/13A.1 (Endeavour): 06/14/07

* NASA is processing shuttles in parallel to permit emergency rescue flights in case of major problems during Discovery's mission or that of Atlantis in August. In both cases, the crew of a damaged shuttle could move aboard the international space station to await rescue. If the next two shuttle flights go smoothly, however, NASA will stand down from emergency flight processing.

The schedule hinges on Discovery's processing flow and while the recent delay to July 1 gives engineers some much needed time to fix known problems and resolve open issues, it's not yet clear whether it will be enough. Here is the schedule as it currently stands (readers are advised to take these dates with a grain of salt; they are targets only and almost certainly will change as processing continues):

• 05/12: Discovery is moved from its hangar to the Vehicle Assembly Building
• 05/19: Rollout to launch pad
• 06/06: Discovery's payload is taken to the launch pad
• 06/06: Debris verification review
• 06/13: Program design certification review
• 06/13: Dress-rehearsal countdown and crew training exercise
• 06/16: Countdown test ends
• 06/22: Flight readiness review
• 07/01: STS-121 launch*
• 07/13: STS-121 landing

* A summary flight plan is posted here.

Discovery's launch delay from May 10 to July 1 was ordered by program manager Wayne Hale to give engineers time to replace suspect engine cutoff sensors in the hydrogen section of Discovery's external fuel tank. One of the four hydrogen ECO sensors in the tank showed a slight shift in electrical resistance that indicated a possible failure mode. Multiple failures in flight could lead to a premature engine shutdown or fool the ship's flight computers into running the engines long enough to suck the tank dry. Both scenarios are potentially catastrophic.

The ECO sensor swap out requires engineers to remove foam insulation at the base of the tank and to open a large manhole cover to permit workers to climb inside. Once the sensors are replaced, the tank must be closed up, refoamed and retested.

That three-week process provides an umbrella of sorts for engineers working other issues that also threatened the original May 10 target date.

Ongoing wind tunnel tests using scale models to assess the acoustic and aerodynamic forces acting on the tank during launch still represent the long pole in the tent.

After a large piece of foam insulation fell away from the hydrogen protuberance air-load - PAL - ramp on Discovery during launch last year, NASA managers decided to simply remove the wind deflector from future tanks. The ramps were in place to smooth the flow of air across an external cable tray and two pressurization lines as the shuttle rockets through the region of maximum aerodynamic pressure during the climb out of the dense lower atmosphere.

Managers decided to remove the ramps based on preliminary results from sophisticated computer models and on the assumption that wind tunnel testing would confirm the pressurization lines and cable tray were strong enough to withstand whatever buffeting they might experience in the absence of the PAL ramps.

NASA has not revealed what the wind tunnel testing has shown to date, but sources say the initial results were not as clear cut as engineers had hoped. The ambiguous results, however, could be the result of scaling issues - problems making sure data collected with sub-scale mockups correctly reflects full-scale reality - and a NASA spokeswoman cautioned that testing and analysis are far from complete.

The worst-case scenario would be unambiguous data showing the tank does not have the desired safety margin without the PAL ramps, a result that would derail plans to launch Discovery this summer and raise questions about the future of the program. The best-case scenario would be confirmation that the tank and its external fittings were conservatively designed, as many hope, and more than tough enough to withstand the expected environment.

As of this writing, it's not clear how this issue might play out.

On another, more positive front, engineers now believe they have a good understanding of the behavior of foam insulation around brackets running up the side of the tank that hold the pressurization lines in place. The so-called ice/frost ramp foam, in place to prevent ice from forming on the brackets prior to launch, was once partially protected from aerodynamic buffeting by the now removed PAL ramps. Engineers believe they have a handle on what sort of changes are needed to minimize foam loss from the ice/frost ramps.

Another tank issue is the replacement of a gaseous oxygen vent valve that helps maintain the proper pressure. During initial work to replace the valve, a light fixture contacted foam insulation in the area, but a NASA spokesman said the incident caused only minor, easily-repaired damage.

Of more pressing concern, engineers have discovered a potential electrical issue, this one involving the ascent thrust vector control - ATVC - system used to move the shuttle's main engine nozzles during launch.

During ground tests, sources said, an ATVC control box suffered a malfunction. The failure may have been due to a phenomenon known as "whiskering," in which certain metals - like the tin used in circuit card guides - develop crystalline extrusions that pose a short-circuit threat should they break off and fall across unprotected circuit components. In this case, the concern focuses on tin whiskers possibly growing from circuit board guides found in ATVC boxes and others that share a common chassis design.

This may be the result of a change in the way Honeywell built the boxes in question or, more worrisome, an age-related or stress-related phenomenon, officials say. It's not yet known how extensive the whiskering might be or what impact it might have on Discovery's launch. But the ATVC system in question is critical for flight safety and if hardware must be replaced aboard Discovery, the July launch target could be in jeopardy.

In other developments, two of Discovery's cockpit windows have been replaced because they were not tested properly and engineers are now discussing whether to replace two others.

At the same time, engineers studying the results of a new analytical technique for assessing the structural "loads" on the external tank have been surprised by results suggesting brackets near the base of the tank might not have the expected safety margin.

More tests are planned and engineers stress the picture almost certainly will change as additional data come in. Even so, some have suggested modifying the flight profile for Discovery's next mission to lower aerodynamic stress on the tank as the shuttle climbs out of the lower atmosphere.

A so-called "low-Q" flight profile was developed to reduce loads on the cockpit windows. The windows in question are being replaced, but the modified ascent profile could be used, some have suggested, to lower stress on the tank and its fittings if necessary.

At this point, such talk is just that - talk - and the issue has not been formally discussed by program managers.

One other issue appears to have been resolved. Engineers now believe a small fragment of metallic contamination seen in a mesh filter just above the point where a liquid oxygen propellant line attaches to one of Discovery's main engines does not pose an ignition threat. But the debris cannot be removed without breaking into the main propulsion system, a complex process that engineers want to avoid unless absolutely necessary. As such, it appears NASA managers will agree to simply leave the contamination in place for Discovery's launching.

In addition, work to repair the shuttle's robot arm, dinged in a hangar incident earlier this month, is ahead of schedule. The arm should be re-installed next month with no impact on the July launch target.