Orion spacecraft back shell

The heat shield on NASA’s Orion spacecraft gets all the glory when it comes to protecting the spacecraft from the intense temperature of reentry. Although the blunt, ablative shield will see the highest temperatures – up to 4,000 degrees Fahrenheit on its first flight this December – the rest of the spacecraft is hardly left in the cold.

Engineers and technicians at NASA’s Kennedy Space Center have finished installing the cone-shaped back shell of Orion’s crew module – the protective cover on the sides that make up Orion’s upside down cone shape. It’s made up of 970 black tiles that should look very familiar – the same tiles protected the belly of the space shuttles as they returned from space.

But the space shuttles traveled at 17,000 miles per hour, while Orion will be coming in at 20,000 miles per hour on this first flight test. The faster a spacecraft travels through Earth’s atmosphere, the more heat it generates. So even though the hottest the space shuttle tiles got was about 2,300 degrees Fahrenheit, the Orion back shell could get up to 3,150 degrees, despite being in a cooler area of the vehicle.

And heat isn’t the only concern. While in space, Orion will be vulnerable to the regular onslaught of micrometeoroid orbital debris. Although micrometeoroid orbital debris is too tiny to track, and therefore avoid, it can do immense damage to a spacecraft – for instance, it could punch through a back shell tile. Below the tiles, the vehicle’s structure doesn’t often get hotter than about 300 degrees Fahrenheit, but if debris breeched the tile, the heat surrounding the vehicle during reentry could creep into the hole it created, possibly damaging the vehicle.

Debris damage can be repaired in space with techniques pioneered after the space shuttle Columbia accident. A good deal of information was gathered then on what amount of damage warranted a repair. But the heating environment Orion will experience is different than the shuttle’s was, and the old models don’t apply.

Engineers will begin verifying new models when Orion returns from its first flight test this December. Before installing the back shell, engineers purposely drilled long, skinny holes into two tiles to mimic damage from a micrometeoroid hit. Each 1 inch wide, one of the holes is 1.4 inches deep and the other is 1 inch deep. The two tiles with these mock micrometeoroid hits are 1.47 inches thick and are located on the opposite side of the back shell from Orion’s windows and reaction control system jets.

“We want to know how much of the hot gas gets into the bottom of those cavities,” said Joseph Olejniczak, manager of Orion aerosciences. “We have models that estimate how hot it will get to make sure it’s safe to fly, but with the data we’ll gather from these tiles actually coming back through Earth’s atmosphere, we’ll make new models with higher accuracy.”

A better understanding of the heating environment for damage on Orion’s heat shield will inform future decisions about what kind of damage may require a repair in space.