October 2005 
Volume 04, Issue 6 
Integrated Defense Systems

Analyze this

Analyze this

Boeing provides data, work to NASA for safe Space Shuttle re-entry


Performing a first-of-its-kind repair is a challenge in itself. But try assessing the need for repairs and developing repair techniques while the vehicle in question circles the earth at 17,500 mph (28,200 kilometers per hour) and an altitude of 225 miles (362 kilometers).

Boeing played a critical role in doing just that by helping NASA resolve several on-orbit anomalies during the most recent Space Shuttle mission, STS-114.

Boeing engineers fill console positions in the NASA Mission Evaluation Room at Johnson Space Center in Houston. The MER team, which includes representatives from NASA and industry, including Boeing, examines all data during a flight and makes recommendations on corrective actions for any observed problems.

Following a review of camera imagery from inspection of the Orbiter Thermal Protection System (TPS) with the Orbiter Boom Sensor System, engineers spotted two protruding "gap fillers" between tiles on Space Shuttle Discovery's belly.

During re-entry, the shuttle depends on the smooth flow of air over the underside tiles to maintain vehicle temperatures within safety limits. Anything that disrupts this smooth flow can create significantly higher heating. That in turn could cause downstream temperatures to exceed material capabilities, which could result in the loss of the vehicle and crew. Because of these concerns, STS-114 astronaut Steve Robinson performed the first "in-flight TPS repair" in the Shuttle Program, removing the protruding gap fillers during the crew's Aug. 3 spacewalk.

Boeing brought together a large team to work on these issues. "The feeling of ownership was huge. We had around-the-clock staffing in every key discipline," said John Dunn, a Boeing project manager for the TPS group.

About gap fillers and thermal blankets

Boeing played a leading role in analyzing damage to the Space Shuttle Discovery's Thermal Protection System during the STS-114 mission. An integrated team across the Space Shuttle program, International Space Station program and Boeing engineers in Philadelphia helped analyze tile and wing leading edge Reinforced Carbon-Carbon materials, while also providing recommendations to remove two gap fillers and leave a torn thermal blanket in place during reentry.

Here's some background on the gap fillers and thermal blankets.

Gap fillers: Ceramic coated cloth spacers that are bonded with room-temperature-vulcanizing adhesive between tiles to dampen the ascent vibrations from a shuttle launch, keeping the tiles from banging into each other. Additionally, if the gap between adjacent tiles is too wide or the step between tiles is too large, there could be extra heating in that area, so the gap filler helps to reduce any heating effect between tiles.

There are thousands of gap fillers on each orbiter. On STS-114, two gap fillers had come loose and were protruding about one inch above the surface between two tiles. One was located in the center of the belly about three feet aft of the port-side nose-landing-gear door, while another was located a little further aft and outboard on the starboard side.

Advanced Flexible Reusable Surface Insulation blankets: The thermal blankets are bonded directly to the orbiter by Room-Temperature Vulcanizing adhesive. The torn blanket on STS-114 was a closeout blanket at a transition below the port side Commander's window. Advanced Flexible Reusable Surface Insulation blankets are quilted, flexible insulation blankets that consist of a fibrous ceramic insulation with a densified quartz fabric cover. Advanced Flexible Reusable Surface Insulation can withstand temperatures up to 1,500 degrees Fahrenheit (816 C).

—Ed Memi

The level of TPS analysis on STS-114 was orders of magnitude more complex than on any previous mission. To get to that level, Boeing spent the last 2 1/2 years building more than 20 integrated models to predict debris-impact damage to the Orbiter and assess the effect on the spacecraft's ability to safely reenter the earth's atmosphere.

Since last October, engineers have run simulations to validate their tools for use in determining if the Orbiter is safe for re-entry. When the gap filler protrusion and other small TPS damage areas were identified, Boeing engineers ran their models; this time, they had detailed imagery to help analyze these problems and use as a reference in comparisons against analysis results. 

Engineers looked at 32 tile damage sites on the bottom of the Orbiter. "Most of them were cleared as they were within allowable limits," said Hussein El-Lessy, the Boeing critical debris process lead engineer.

The team narrowed their investigation to four damage sites that needed a closer look with the laser imagery sensors on the Orbiter Boom Sensor System, El-Lessy said. Besides the Boeing analysis, NASA also formed a team of independent experts to review the results.

"They evaluated our tool capability and test database, and determined the level of uncertainty was such that they could not guarantee a safe re-entry either, forwarding the same recommendation that we had to remove the gap fillers," said Brian Anderson, Boeing Aero-Heating Debris lead engineer. Boeing also helped develop the plan to remove them. 

Besides the gap fillers, a torn insulation blanket also had to be evaluated. After extensive analysis and execution of three wind tunnel tests at NASA's Ames Research Center in California, mission managers concurred with the engineering team's recommendation that Discovery could safely return to Earth with the blanket left in place. Arriving at that decision took a lot of hard work by Boeing engineers, including extensive thermal and structural analysis and NASA-led wind-tunnel tests to see if any parts could shed during re-entry and possibly damage the Orbiter.

As NASA looks toward STS-121, Boeing engineers will be busy refining their debris assessment processes, models, and engineering tools, improving the wing leading edge impact sensors and performing many other tasks to improve safety. 



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