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| **NASA** Facts                         | ![](media/im                |
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| National Aeronautics and               | hei                         |
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| Space Administration                   |                             |
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| **Marshall Space Flight Center**       |                             |
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| Huntsville, Alabama 35812              |                             |
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#### FS-2003-09-108-MSFC September 2003

##### **Redesign of the External Tank\'s Bipod Fitting** 

When the Space Shuttle returns to flight, the External Tank will have a
newly redesigned bipod fitting \-- one that removes all foam from the
top of the fitting, including the foam insulation ramp, or the bipod
ramp. Space Shuttle Columbia accident investigators believe insulating
foam from Columbia's left bipod ramp broke off during liftoff of the
Shuttle\'s STS-107 science mission on Jan. 16, 2003, and struck the
Orbiter's left wing. The new design meets the recommendation of the
Columbia Accident Investigation Board to minimize potential debris by
eliminating the foam ramp and replacing it with heaters.

The new design also meets the four objectives established by the
External Tank Project Office\'s redesign team: It creates no new risks;
eliminates the possibility of a foam ramp failure; it can be certified
for flight and implemented quickly; and it can be retrofitted to the
existing fleet.![](media/image2.jpeg){width="2.651388888888889in"
height="2.276388888888889in"}

The External Tank Project Office began its redesign efforts prior to the
loss of Columbia \-- after insulating foam from the left bipod ramp area
came off during the launch of Space Shuttle Atlantis on the STS-112
mission in October 2002. During the launch of Columbia on its STS-107
mission in January 2003, a similar loss prompted NASA\'s Office of Space
Flight to mandate a redesign of the bipod ramp before the Shuttle fleet
could return to flight.

The bipod ramp was a wedge-shaped foam structure, approximately 30
inches long, 14 inches wide and 12 inches tall. The ramp was designed to
protect the bipod fitting, which is near the front of the Space Shuttle
at the point where the External Tank attaches to the Orbiter. The foam
ramp protected the fitting from aerodynamic loads during liftoff and
prevented ice from forming on the fitting when the liquid hydrogen tank
was filled prior to launch. The ramp was made during the final stages of
the tank\'s preparation by hand-spraying BX250/265 foam and carving it
into a ramp shape.

**Design Changes**

The new bipod fitting design will allow the fitting to fly bare \--minus
the insulating foam ramp \-- but still capable of withstanding
aerodynamic heating. The design consists of a bare bipod fitting with
four rod heaters mounted below the fitting to prevent ice and frost from
forming when the External Tank\'s hydrogen tank is filled. The fitting
itself is the same basic design as the one now used on the External
Tank.

The new design includes four rod heaters \-- only two heaters are
needed; the other two are redundant or back-up heaters. The heaters are
placed below the fitting in heater retaining covers made of Inconel 718,
a strong alloy composed of nickel, chromium and iron. The heaters sit on
top of a copper plate sandwiched between the fitting and the phenolic
isolator that attaches to the tank. The phenolic isolator is a thermal
pad of laminated, fiberglass-reinforced material that is made with a
phenolic resin \-- a hard, dense material made by applying heat and
pressure to layers of paper or glass cloth impregnated with synthetic
resin. The isolator separates the heater from the tank.

The heaters are cartridge-type heaters with a wire coil inserted into a
tube filled with magnesium oxide. They are 0.25 inches in diameter and 5
inches in length. Each cartridge is a 300-watt heater and operates at
120 volts AC.

Other changes include the elimination of the hole in the spindle face
that once provided clearance for the bipod spindle heater element. That
heater was removed once the four rod heaters were placed beneath the
spindle. The elimination of the spindle heater meant a smaller end cover
could be used on the fitting. Once the spindle heater was removed, the
end cover\'s only purpose was to protect the internal bearing surfaces
from dust, sand and other debris that could impact the its ability to
rotate freely.

To ensure the bare spindle end cover can withstand the aerodynamic
heating experienced during launch, the cover is now made of Inconel 718,
which can withstand temperatures of up to 2,000 degrees Fahrenheit.

The new design also requires additional cabling to operate the heating
system. It includes eight circuits -- four for each bipod -- that run
from the External Tank Ground Umbilical Carrier Plate to the heaters
under the bipod fitting.

The new design is an alternative derived from three original redesign
options proposed by the project office to the Space Shuttle Program
Requirements Change Board on May 9, 2003, and later presented by the
Columbia Accident Investigation Board to the public.

**Testing**

Testing is an important factor in any redesign or modification because
it validates the integrity of the design. Though testing cannot
duplicate actual flight, it can significantly reduce risk because it
allows for careful observation and precise control over the test
article. The new bipod fitting design has undergone wind tunnel tests,
vibration and structural tests, and thermal tests, during both its
design and implementation phases, to certify it is ready for flight.
These tests ensure the new design does not affect the current External
Tank loads and stresses.

Structural tests performed at NASA\'s Michoud Assembly Facility in New
Orleans demonstrated the load capability \-- an external force or other
action acting on a structure \-- of the redesigned fitting. Tests
demonstrated the effects of cryogenic \-- subzero \-- temperatures at
the External Tank\'s intertank/flange area and the effects of ascent
temperatures.

Thermal testing performed at Eglin Air Force Base, Fla., demonstrated
that the redesign does not permit ice or frost to form during fueling of
the hydrogen tank. Thermal tests were also used to duplicate the launch
environment, recreating the temperatures, humidity and wind velocities
experienced during liftoff.

Wind tunnel tests performed at Arnold Engineering Development Center at
Arnold Air Force Base, Tenn., demonstrated the design\'s aerodynamic
capabilities and its capacity to resist high temperatures generated
during ascent.

**Implementation of the new design**

Once testing is completed on the new bipod fitting design and the new
design has passed its final certification review, work is expected to
begin in October 2003 to retrofit the new design on the eight tanks
already completed. Once the design has been retrofitted on existing
tanks, it will be implemented on new tanks.

Lockheed Martin Space Systems will do the work at NASA\'s Michoud
Assembly Facility in New Orleans. Delivery of the retrofitted tanks to
the Kennedy Space Center in Cape Canaveral, Fla., is expected in
November 2003.