NASA found the cracks when they performed x-ray analysis of the brackets on the external tank of Discovery, which is scheduled to lift off next.

Four of the five brackets contained cracks in the foam covers. (The shuttle is connected to the ET at one forward attachment point and two rear (aft) points. In the rear section, brackets contain umbilical cords that carry gases, fluids, electrical power, and electrical signals, between the ET and the orbiter.)

The foam covers are made of layers of lower-density insulating foam (on top) and higher-density heat-resistant cork foam (on the bottom, and next to the ET surface). The cork is called Super Lightweight Ablative (SLA) foam core. The cracks were found in the cork layer. Such cracks are thought to increase the chances that the lighter top layer of insulating foam would tear off during the launch of the shuttle.

The function of the insulating foam is to prevent ice from accumulating on the external surface of the ET when its extremely cold liquid hydrogen fuel and liquid oxygen oxidzer is pumped into its tanks.

Foam from one of these support brackets, which holds a liquid oxygen feed line between Endeavour and the ET, broke off during the liftoff of the STS-118 mission and slammed into the underside of the space shuttle. The resulting hole caused great concern for the safety of the astronauts.

In the end, intense computer modeling and thermal analysis by NASA engineers deemed the hole to be small enough so that the intense heat of the Earth’s atmosphere during re-entry would not place the astronauts in harm’s way.

Consequently, the cracked foam covers will be replaced on the ET for the upcoming space shuttle mission for Discovery. The ET for Discovery will use foam covers that are made of foam only, removing the cork from the mix.

NASA tests have shown that the underlying heat-resistant cork foam is not necessary.

As of August 25, 2007, no delays are expected to the launch of the October 23rd launch of Discovery and STS-120 because of the estimated nine-day repair to the foam covers. However, NASA thinks that the December 6, 2007 launch of Atlantis and STS-122 may be delayed because the extra work to Discovery will push back the time needed to prepare Atlantis for lift off. However, they are unsure with the schedule at this time.

NASA Shuttle Program Manager N. Wayne Hale, Jr. said, “The point is, we will take the amount of time that we need to to get this repair done properly. We will not rush and if we happen to fall a day or two after the 23rd, that is not a huge impact to our schedule. The schedules for the following flights obviously are more fluid and we'll be reviewing those as the work goes forward."

In the future, titanium brackets will be used for the external tanks. Such a metal does not require foam to be applied on the brackets. However, these new brackets will not be installed until the STS-124 mission, now scheduled for April 2008.

The updated external tank used for the next generation Orion capsule (Project Constellation) will not need to use any insulating foam. The current shuttle external tank will be used as the foundation for the next generation external tank of the second stage of the Ares I rocket (for the manned Orion capsule) and for the first stage of the Ares V rocket (for the unmanned cargo vehicle).

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NASA  Science  Space Shuttle  Spacecraft  William Atkins 

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