A phone call from a concerned
Thiokol engineer in 1997 led Dr. Bruce Steinetz and
Mr. Patrick Dunlap of the Glenn Research Center to work
on developing a flexible barrier that could withstand
the extreme temperatures generated by solid rocket motors.
The braided carbon-fiber thermal barrier they invented
has won the NASA Government Inventions and Contributions
Board Invention of the Year Award for 2004, which was
presented at a special NASA Headquarters ceremony in
Engineers at Thiokol (now ATK Thiokol) worried about
heat effects they were finding on the O-rings that sealed
the joints between nozzle sections of their reusable
solid rocket motors, or RSRMs. In its worst form and
combined with brittleness caused by cold weather at
launch, a similar problem in the motor’s field joint
led to the Challenger disaster in 1986; lesser nozzle
O-ring heat effects have been observed on flights since
then. One of the ATK Thiokol engineers remembered reading
about an award Steinetz and Sirocky (a former colleague)
had received for high-temperature seal work. He called
Steinetz and asked if the seal team could apply their
expertise to the solid rocket motor’s seal problem.
Steinetz and Sirocky's 1997 Invention of the Year award
was for a ceramic, braided rope seal used in the engines
of the F-22 and other applications that require an effective
insulator that is also flexible and resilient. But the
RSRM presented a much greater challenge. The original
ceramic seal was exposed to temperatures of around 1500°F.
In ATK Thiokol's rocket motor, the thermal barrier would
face temperatures of around 5500°F. When the engine
?red, pressure in the RSRM nozzle joints would go from
ambient atmospheric pressure of 14 lb./ square inch
to 1,000 lb./square inch in less than a second. The
thermal barriers would have to protect the booster’s
O-rings from high temperatures while allowing enough
gas through to provide the pressure needed to seat the
O-rings firmly in their sealing positions. And the thermal
barriers would have to survive for close to two-and-a-half
minutes, the length of time that the rocket fired.
The challenge was all the greater because Steinetz
and Dunlap had no money earmarked for the research.
They could borrow a little funding from other projects
that would benefit from what they learned, but they
would have to find an inexpensive way to develop and
test possible solutions.
Luckily, a standard oxy-acetylene torch generates about
5500°F of heat, so they had a ready-made, low-cost
tool for screening potential thermal barrier materials.
They first tested their ceramic braid seal. The torch
burned through the material in just over six seconds.
Next they tried a thermal barrier made of braided phenolic,
a heat-resistant plastic. It lasted approximately 40
seconds. Their third test material was an eighth-inch
diameter rope braided out of an aerospace grade carbon
fiber, a close cousin to the material used in some golf
club shafts, fishing rods, and other sports equipment.
The carbon braid was promising, lasting about two minutes
before it burned through. A quarter-inch braid lasted
more than six minutes. They had taken a major step forward
in solving the problem and communicated the results
to ATK Thiokol.
The carbon thermal barrier seal passes a torch
test with flying colors.
ATK Thiokol ran tests on the carbon-fiber thermal barriers,
first in rockets with 70 lb. of thrust (as compared
with the millions of pounds of thrust of their full-scale
rockets) and then on larger, though still scaled-down
versions of the RSRM. The thermal barriers passed with
(literally) flying colors. Tests that recorded temperatures
more than 2500°F on the upstream side of the dual
thermal barriers registered less than 200°F on
the other. The braided structure was key to solving
the problem of letting sufficient pressure through to
seat the downstream O-rings while protecting them from
heat. The new thermal barriers will be used on ATK Thiokol
RSRMs for shuttle mission STS-122.
Steinetz says that how the barrier produces so great
a temperature drop is "a little bit of a mystery." Looking
back on the discovery process, he remarks on how "uneven"
technical development is. "A lot of things lined up
perfectly, and we were able to come to a solution in
short order," he says. "Many research projects don't
go like that."
The thermal barriers have also solved a problem in
the solid rocket motors Aerojet manufactures for the
Lockheed-Martin Atlas V. The 67-foot-long Aerojet rockets
are a single piece, with no joints except where the
rocket body connects with the nozzle. In the summer
of 2002, only months before the scheduled launch of
an Atlas V to put a commercial satellite in orbit, an
Aerojet rocket suffered a dramatic failure on the test
stand due to hot gases compromising the nozzle seals
and joint. Aerojet engineers asked Steinetz and Dunlap
for help and quickly redesigned the joint using three
of the braided carbon-fiber thermal barriers. The rocket
was certified in June 2003, and the commercial launch
took place the following month. Since then, Atlas Vs
with pairs of Aerojet rockets have successfully launched
the AMC-16 satellite that provides DISH network services
and Inmarsat 4-F1, which delivers broadband communications
to 86 percent of the world.
NASA will use the Atlas V to launch the Pluto Horizons
Spacecraft in 2006 and is considering its use to launch
payloads for the International Space Station and future
Exploration Initiative missions. Steinetz and Dunlap
have turned their attention to other challenges, including
developing docking and berthing pressure seals for the
Crew Exploration Vehicle and seals designed to mate
a new Apollo-like heat shield to the vehicle structure.
Steinetz and Dunlap are pictured with their thermal
barrier and patent.