Shuttle Landing to Test Future Heat Shield Tiles

CAPE CANAVERAL, Fla. - Shuttle Discovery will make a fiery plunge through the atmosphere this weekend, serving as a hypersonic test bed for new heat shield tiles that might be used to protect astronauts on NASA's next-generation spacecraft.

Flying Discovery upside down and tail-first, mission commander Lee Archambault will fire twin maneuvering engines Saturday, slowing the shuttle enough to drop it out of orbit and into an hour-long free fall through the atmosphere.

The ship will flip, pointing its belly in the direction of travel, and the compression of smooth-
flowing air around its leading edge will form a protective blanket around the orbiter.

Then a specially designed tile with a built-in "speed bump" will trigger turbulent airflow, increasing re-entry heating. The anticipated temperature increase is only 15 to 20 degrees, which is not enough to cause damage.

The idea is to see if the candidate tile is safe to fly on the Orion space capsules, which will encounter temperatures of up to 3,400 degrees Fahrenheit as astronauts return from missions to the moon, Mars and other celestial destinations. The shape of the Apollo-like capsule will result in re-entry temperatures 500 degrees above those encountered by the shuttle, which is slated for retirement next year.

"We have returned to using the space shuttle as a research vehicle," said NASA shuttle program manager John Shannon. "We're trying to learn more and more about spaceflight and hypersonic re-entry."

Discovery and seven astronauts are scheduled to land at Kennedy Space Center at 1:38 p.m. Saturday - about five minutes earlier than previously calculated. The deorbit burn is scheduled for 12:36 p.m. that day.

Returning shuttles normally experience this disruption of supersonic airflow when the velocity of the orbiter drops to around Mach 8 or Mach 10.

The switch from smooth to turbulent airflow is a phenomenon known as "boundary layer transition," and any bulge or protrusion on shuttle heat shield tiles could trigger it. NASA engineers call this "tripping the boundary layer."

If it occurs early in re-entry - when the velocity is higher - the tiles are exposed to greater temperatures for longer periods of time. Resulting damage could breach the orbiter's heat shield, endangering the spaceship and the astronauts aboard it.

The presence of two protruding pieces of fabric - so-called "gap fillers" between tiles on the belly of the shuttle - prompted NASA to order an emergency spacewalk during the agency's first post-Columbia test flight in 2005.

Veteran U.S. astronaut Stephen Robinson and Japanese crewmate Soichi Noguchi were sent to retrieve the gap fillers before mission managers cleared the shuttle for re-entry.

The special tile on Discovery is an improved version of original shuttle tiles. It was installed on the underside of the orbiter's left wing. Its built-in "speed bump" is 4 inches long and sticks out about a quarter of an inch. Temperature sensors were installed on that tile and nine others downstream of it.

NASA computer models indicate that the "speed bump" will trip the boundary layer as the shuttle slows to Mach 12 or Mach 14.

The transition from smooth to turbulent airflow is expected to occur as Discovery soars over the Gulf of Mexico on its way to KSC. A Navy NP-3D Orion aircraft will fly below the shuttle and use an infrared camera to monitor heating on the underside of the orbiter.

The imagery and data from the temperature sensors will give engineers a better understanding of the phenomenon, predicative models will be improved, and NASA engineers will determine whether the upgraded tile - or a version of it - might be used on Orion spacecraft.

"There are a lot of positives to this small test," Shannon said. "It's a great test to understand in a controlled manner the flight environment for re-entry."

SPACE.com is providing continuous coverage of Discovery?s STS-119 mission  with reporter Clara Moskowitz and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed.

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