NASA
is teaming up technology developed for the space shuttle and
designs used for the Apollo Program to produce elements of
the next spacecraft destined to deliver astronauts to the
moon.
An early sign of that combination has made its way to NASA's
Kennedy Space Center in the form of a prototype heat shield,
which is the same size and dimensions of the one planned to
protect the Orion spacecraft as it enters Earth's atmosphere
on the way back from the International Space Station or the
moon.
The arrival of the heat shield stirred up excitement from
workers on the Constellation Program as they were able to
see for the first time one of the first pieces of Orion's
full-scale test hardware.
"When (it) got here at the end of November, it was very
exciting because it is the first piece of hardware," said
Joy Huff, a NASA shuttle orbiter thermal protection system
engineer who is spearheading Kennedy's work on the Orion
heat shield. "Not flight hardware, but it is flight-type
material. And just to see the full size, it really gives you
a scale of the size of it."
At five meters in diameter, the heat shield is the
largest one of its kind ever built. The prototype was
built largely just to prove it could be done, Huff said.
Also known as a manufacturing demonstration unit, or MDU,
the prototype was also created by the need to develop heat
shield evaluation, inspection and handling procedures, said
Jim Reuther, project manager of the Crew Exploration Vehicle
thermal protection system at NASA's Ames Research Center.
Although parts of Orion's thermal protection system, which
serves as a barrier against the heat upon re-entry into
Earth, will use shuttle tile materials, the base of the heat
shield endures the most heat and will burn away or "ablate"
as it descends through the atmosphere at more than 25,000
mph.
The use of ablative materials mirrors that of the Apollo
Program's approach, in which the entire entry capsule was
covered with an ablator, Reuther said. The Orion heat shield
also uses techniques perfected for the shuttle's thermal
protection system, particularly the bonding method used to
attach the segments of ablative material to the base heat
shield. But since the area to be protected is much smaller
than that of the shuttle, and because the base of Orion's
heat shield will not be reused, its design is simpler in
respect to the number of parts and reusability.
The
prototype heat shield is made of the leading candidate
material called PICA, which stands for phenolic impregnated
carbon ablator material. The PICA material was previously
used for the base heat shield of Stardust, a small robotic
spacecraft that successfully completed its mission of
obtaining comet samples and returned to Earth in January
2006.
Because Stardust was less than 3 feet in diameter, it was
possible to cast its heat shield in a single piece as
opposed to the many pieces needed to make Orion's heat
shield. At 16.5 feet in diameter, Orion's heat shield
will require up to 200 pieces of PICA blocks.
"The actual final number of PICA blocks will be determined
by both manufacturing and thermal-mechanical design
constraints,"Reuther said. "However, when compared to the
roughly 24,000 tiles used on the shuttle, the final number
of blocks will be very manageable."
The blocks on the heat shield share the same delicate
characteristics as the shuttle's tiles. Designers also plan
to include gap fillers between blocks, just as with the
shuttle.
NASA chose an ablative heat shield that slowly burns off
because it can handle higher temperatures than the shuttle's
reusable tiles. A spacecraft returning from a lunar
mission is expected to encounter temperatures as high as
5,000 degrees Fahrenheit during re-entry into Earth’s
atmosphere, compared to about 2,300 degrees for a space
shuttle re-entry. Because of this, the Orion heat shield
can only be used once, Huff said.
The prototype heat shield rests in Hangar N at Cape
Canaveral Air Force Station where will it undergo several
months of nondestructive evaluation testing, or NDE, that
mainly includes laser scans and X-rays. The tests will be
used to reveal flaws purposely built into the heat shield.
"We want to get it into the X-ray facility to use X-rays to
look for these known flaws," Huff said. "That's part of the
NDE task to come up with standards, so when you get a flight
unit, you know what you're flying."
But before any NDE testing can be performed, the team at
Kennedy has to learn the best way to move and handle the
heat shield. Because of the size of the prototype, they also
will have to test and develop new handling standards that
will be applied to the actual flight heat shields.
Backup procedures call for using a crane to handle the
prototype, but Huff said she hopes to see an adaptor made
that will allow a forklift to be used instead. The forklift
would make it easier to handle the prototype inside
buildings and as it undergoes testing procedures.
"The materials (for the adaptor) have been ordered, (so we)
should be fabricating (it) within the next few weeks, get
that built and then we'll put the MDU on the adaptor and
start NDE testing," Huff said.
She's looking forward to a special milestone to take place
by late summer: turning all handling and NDE testing results
over to Lockheed Martin.
And when that happens, the Constellation Program's mission
of putting man on the moon and beyond will be one big step
closer.
From NASA
