Image: Discovery’s STS-121 reentry
To cameras tuned to the visible range of the light spectrum, Discovery’s STS-121 reentry July 17 looked like this just before its successful landing.
By Staff Writer
updated 7/25/2006 6:43:55 PM ET 2006-07-25T22:43:55

The space shuttle Discovery’s mission may be over, but NASA is still evaluating a set of infrared tools used during the flight to help predict whether an orbiter’s heat shield is clear to return astronauts back home safely.

Engineers are poring through infrared video recorded by a pair of high-flying aircraft as Discovery reentered the Earth’s atmosphere last week to make a near-perfect landingat Florida’s Kennedy Space Center to end its STS-121 mission.

The video has given NASA scientists their first complete look at how the scorching temperatures of reentry are distributed across a shuttle’s belly at supersonic speeds, something only hinted at in previous ground-based attempts.

“It’s a really quite a spectacular image,” said Bob Blanchard, a lead senior scientist at George Washington University who participated in the STS-121 and earlier shuttle heat shield studies. “It was something we kind of dreamed about when we were doing this low-altitude stuff earlier.”

A team of engineers at NASA’s Langley Research Center in Hampton, Virginia are now working to match the infrared video against data from computer models and wind tunnel tests to determine how close their predictions were to Discovery’s actual reentry.

“It’s not just about good science,” said Tom Horvath, a senior research engineer for NASA with Langley’s aerothermodynamic branch. “Being able to reduce the uncertainty in this prediction tool … that has a huge impact on whether we [see] any damage on a future flight and clear it to enter as-is or recommend a repair.”

During Discovery’s STS-114 flight in July and August of 2005, engineers found that two protruding gap fillers could have unacceptable heating results to the orbiter’s heat shield, prompting a spacewalk repair to pluck out the offending ceramic cloth bits.

Meanwhile, a separate team is going over data from an orbital test of a new digital infrared video camera used by STS-121 spacewalkers Piers Sellers and Michael Fossum. The astronauts scanned heat shield test samples, Discovery’s wing leading edge, space station radiators and themselves during the demonstration.

“It came off, I think, better than we had expected,” Michael Gazarik, spacewalk infrared camera lead at Langley for the STS-121 mission, told “The imagery looks good, the data looks good, we’ve got a lot of data.”

A gap filler’s tale
Before Discovery plunged through the Earth’s atmosphere, NASA engineers already had a target to seek out with infrared cameras during the orbiter’s reentry.

A small bit of ceramic cloth — known as a gap filler — was jutting from between two of the thousands of black, heat-resistant tiles guarding the shuttle’s belly against temperatures of up to 2,300 degrees Fahrenheit (1,260degrees Celsius) during reentry.

Wind tunnel tests and computer modeling determined that the torn, off-kilter gap filler would interrupt the aerodynamic flow aft of its location — near the orbiter’s rear — and cause hotter temperatures downwind, yet not pose a danger to the shuttle. Engineers were eager to see that heating process in action.

“This [prediction] tool that we’ve developed is based on a very limited set of flight data,” Horvath said, adding that Discovery’s belly carries only five temperature sensors — or thermocouples — in all. “We need more flight data.”

A U.S. Navy P-3 Orion aircraft watched Discovery as it passed below an altitude of about 180,000 feet (54,864meters) at speeds about Mach 13, while a high-altitude observatory (HALO) aircraft recorded the spacecraft as it slowed from Mach 9 to Mach 7 about 150,000 feet (45,720meters) above Earth. A third aircraft did not catch the spacecraft’s reentry. Mach 1 is the speed of sound and is about 760 miles (1,223kilometers) per hour at sea level.

“None of the air crews even saw the vehicle as it went past,” said Marty Ross, of the firm Aerospace Corp., who coordinated air-based reentry observations for NASA. “It was all done with the [infrared] cameras.”

Engineers are now working to develop a temperature map to match the infrared data. Meanwhile, post-landing inspections found that Discovery’s primary gap filler protrusion — there were several — was worn down to a nub and left a clear path from the apparent higher temperatures down the orbiter’s aft.

Seeing red
Discovery’s STS-121 mission also proved that spacewalkers could use a handheld infrared camera to scan vital areas of an orbiter’s heat shield — its nose cap and wing leading edges — for signs of subsurface damage that the human eye may not catch.

Covered in a composite material known as reinforced carbon carbon (RCC), those heat shield areas are subjected to the most extreme temperatures experienced by an orbiter during reentry. Astronauts currently use boom-mounted laser and visible light camerasto examine heat shield surfaces for damage in-flight, but the new tool—known as an EVA IR camera — would search for cracks or voids between layers of RCC material that could come loose during reentry.

“The operation of the camera seemed to go fine,” Gazarik said, adding that testing the camera was the tool’s sole reason for being aboard Discovery. “There’s a lot of things could have gone wrong and I think it was a very good day for us.”

NASA engineers said analysis from both infrared tests will continue over the next few weeks, with some data to be presented at a Flight Readiness Review meeting for the agency’s STS-115 mission currently set to launch aboard Atlantis no earlier than Aug. 28.

“Ultimately, we’re preparing for the next flight on the books,” Horvath said.

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