Lockheed Martin has delivered the Orion crew vehicle’s heat shield for the upcoming EM-1 test flight to the Kennedy Space Center from its manufacturing facility in Denver, Colorado. The heat shield, upgraded from data received after Exploration Flight Test -1 (EFT-1) in 2014, will help protect Orion from the harsh environment of reentry at speeds of approximately 11 kilometers per second (24,600 mph).
The heat shield: Improvements for Orion’s EM-1 flight
At 5 m (16.5 ft) in diameter, the heat shield design for Orion is the largest composite heat shield built and used for a spacecraft.
That heat shield served not only as a proof-of-concept design, verifying it could protect Orion from the extreme 2,200°C (4,000°F) temperatures generated by a reentry velocity of more than 32,187 kph (20,000 mph), but also served as an important hands-on tool for post-flight investigation and analysis.
Overall, the temperatures EFT-1’s heat shield experienced during reentry were just 89% of the total heat the EM-1 shield will have to protect Orion against during lunar trajectory reentry.
EFT-1 reentry temperatures were approximately 2,200°C (4,000°F), compared to the anticipated 2,482°C (4,500°F) temperatures Orion will experience during returns from the moon.
Likewise, the speed of EFT-1’s reentry was approximately 80% of what Orion will experience when it screams into the atmosphere at the completion of EM-1.
EFT-1’s reentry speed was 32,187 kph (20,000 mph), compared to the anticipated 39,600 kph (24,606 mph) reentry speed for EM-1.
Nonetheless, the EFT-1 mission provided valuable data to Orion’s design team and provided a type of test that could not be conducted in a laboratory.
“The heat shield we put to the test during Orion’s flight test [in December 2014] met every expectation we had and gave us a tremendous amount of data on its thermal and mechanical performance,” said Mark Kirasich, Orion Program Manager.
Overall, the heat shield performed within all of the tolerances set by pre-mission objectives and kept the interior temperature of the crew module in the mid-70s during reentry.
Following recovery of Orion in the Pacific Ocean, the heat shield was removed and shipped to the Marshall Space Flight Center in Huntsville, Alabama, for post-fight analysis.
But even before that analysis began, manufacturing teams and the Orion Project itself knew that changes were coming to the manufacturing process for the heat shield that will be used on the EM-1 flight.
According to Mr. Kirasich, “The process of building the heat shield as a single piece … gave us insight into how we could improve the way we build this essential element of the spacecraft.”
For EFT-1, Orion’s heat shield was designed as a single piece, composed of a titanium skeleton and carbon fiber skin (the base) with a honeycomb structure containing 320,000 individual cells filled by hand with an ablative material called Avcoat (top part that provides the heat resistance needed for reentry).
The Avcoat material is designed to wear away, or ablate, as Orion enters the atmosphere to provide the heat protection needed to survive reentry.
Avcoat ablative heat shield technology is a very different type of heat shield than was employed with the Space Shuttle, which used thermal blankets, silica ceramic tiles, and Reinforced Carbon-Carbon panels for protection against temperatures which ranged over the vehicle from 371°C – 1,260°C (700°F – 2,300°F).
During the EFT-1 heat shield build, after each honeycomb cell was filled by hand, the entire structure was cured in a large oven and then x-rayed before being robotically machined to meet precise thickness requirements.
It was during this manufacturing process that engineers realized that the strength of the Avcoat/honeycomb structure was below expectations.
While the heat shield worked for EFT-1, the EM-1 Orion will experience colder temperatures in space and hotter temperatures during reentry, requiring a stronger heat shield.
To this end, Lockheed Martin and Orion project managers tweaked the build design for the EM-1 heat shield, instituting a block build design of the honeycomb/Avcoat structure rather than a monolithic build.
In all, the honeycomb structure containing the Avcoat material is now composed of 180 individual blocks built in stages.
This change allows the heat shield to meet all strength requirements for EM-1 and future missions of Orion as well as shorten the manufacturing timeline by two months and provides some cost savings to the process as well.
With the new design in hand, Lockheed Martin began planning for the new heat shield in early 2015 ahead of the build of both the base and honeycomb Avcoat sections.
The build itself took about one year before the base and Avcoat blocks were shipped to the Kennedy Space Center this month.
Now at Kennedy, engineers will spend the next six months installing the Avcoat blocks, flight instrumentation, and multi-layer insulation onto the heat shield.
Once that is complete, the new, lighter-weight heat shield will be installed onto Orion’s crew module.
This will mark the completion of a major assembly milestone for Orion – which is currently listed as the most critical risk element of the EM-1 mission in terms of making the planned No Later Than December 2018 launch date.
Regardless of EM-1’s launch date, however, Lockheed Martin has already learned new lessons from this heat shield build, lessons that will be applied to the planning and build of the EM-2 mission heat shield – the mission that will carry NASA astronauts to cislunar space.
(Images: NASA; Lockheed Martin)