Space Forum / Shuttle / April 2006

http://www.spaceflightnow.com/shuttle/sts121/060413update/

Tank foam lost during shuttle wind tunnel test
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: April 13, 2006

During wind tunnel tests earlier this week, NASA subjected a full-scale
mockup of a shuttle external tank section to aerodynamic forces greater
than a real tank would experience during launch. In one series of
tests, unmodified foam insulation used to prevent ice buildups around
external fittings suffered only minor damage while a redesigned
"ice/frost ramp" suffered major foam loss.

The redesign is being considered in a bid to remove as much insulation
as possible from the tank to minimize the threat of debris shedding
during launch. The old ice/frost ramps, which insulate the fittings
used to hold two pressurization lines and an electrical cable tray in
place, featured long, sloping ramps to smooth the flow of air.

But those very ramps, it was believed, were susceptible to cracks and
failure, providing a potential source of debris that could strike a
shuttle during launch. The redesigned ramps are much more blunt. But in
initial wind tunnel tests at the Arnold Engineering Development Center
in Tullahoma, Tenn., the old design fared better than the redesign.
During two sets of test runs this week, the new design suffered major
foam loss.

"That's exactly what testing is all about," said one official. "This is
where you want to find a problem, not on launch day."

More tests are planned and other design changes are under
consideration, along with the option of flying the ramps "as is." But a
decision on how to proceed is needed by the end of the month to give
engineers time to make any required changes before NASA's planned July
launch of the shuttle Discovery on the second post-Columbia mission.

In other developments, shuttle program manager Wayne Hale has told
project engineers to begin planning for a shuttle fueling test around
June 1 to verify performance of new low-level fuel sensors in the tank
scheduled for use by shuttle Discovery in July. The test has not yet
been formally approved and is opposed by some - including, sources say,
William Gerstenmaier, NASA's chief of space operations - because of
concern about putting unnecessary thermal stress on the tank's foam
insulation.

But Hale strongly favors the test in the wake of recent, unprecedented
work to replace four engine cutoff - ECO - sensors, which are part of a
critical backup system used to make sure the shuttle's main engines
shut down on time. Problems with ECO sensors before Discovery's launch
last July on the first post-Columbia mission were never fully resolved
and a tanking test would give engineers confidence the new sensors will
work properly on launch day. A tanking test also would provide an
opportunity to monitor the performance of a relief valve used to
maintain proper tank pressurization during flight. A valve in a tank
last year cycled more often than usual.

Discovery remains targeted for launch at 3:48 p.m. EDT on July 1 (a
detailed flight plan and the STS-121 Quick-Look page have been
updated).

To make the July 1 launch target, however, the shuttle team must
overcome several challenging hurdles, including completion of wind
tunnel testing and analysis to confirm recent changes to the external
tank's foam insulation are safe.

During Discovery's flight last July, a large piece of foam insulation
broke away from an air deflector called a protuberance air-load - PAL -
ramp running down the outside of the hydrogen section. The PAL ramp was
in place to smooth the flow of air across two externally mounted
pressurization lines and an electrical cable tray as the shuttle
rockets out of the dense lower atmosphere.

Shuttle managers decided late last year to simply remove the ramps
based on computer modeling that indicated the pressurization lines,
cable tray and attachment fittings - and the foam ice/frost ramp
insulation used to prevent ice formation on the brackets - were tough
enough to withstand the expected aerodynamic buffeting.

Discovery's tank, without PAL ramps, was shipped to the Kennedy Space
Center in March on the assumption wind tunnel testing would confirm the
computer modeling. Tests using a scale model of the shuttle and others
using a larger scale section of the tank have been completed at NASA's
Glenn and Ames research centers. Those tests were focused on learning
more about the acoustic environment the tank is subjected to during
launch as well as the aerodynamic "loads" to make sure the tank and its
external lines and fittings can stand up to ascent forces.

Those data have not yet been fully analyzed. Despite initial concern
about somewhat ambiguous results, engineers in recent days have
expressed optimism the end result will support the decision to remove
the PAL ramps.

NASA currently is carrying out wind tunnel tests at the Arnold
Engineering Development Center using a full-scale model of a tank
segment that can be equipped with ice/frost ramps of various shapes. A
turntable allows engineers to properly orient the mockup to ensure the
wind strikes it at the proper angle to reflect the forces acting at any
given moment during ascent. The testing is designed to collect data on
all aspects of the PAL ramp-free tank design.

The ice/frost ramps were partially shielded from aerodynamic buffeting
by the now-removed PAL ramps and questions were raised about whether
the old ramp design could stand up to launch forces. In one set of runs
at Tullahoma, the original design suffered only minor damage, sources
said. But in two sets of tests using the redesigned ice/frost ramps,
large chunks of foam blew off. It's not yet clear what caused the
failures and more tests are planned.

In the meantime, the launch team at Kennedy is pressing ahead for July.
A team from the Michoud Assembly Facility near New Orleans, where the
external tanks are built, is refoaming the bottom of the tank following
the ECO sensor swap out work. The tank is scheduled to be attached to a
pair of already assembled solid-fuel booster rockets on April 24 and if
all goes well, Discovery will be attached to the tank around May 12.
Roll out to the launch pad is targeted for May 19.

The proposed fueling test would not prevent a July 1 launch, engineers
say, because of contingency time built into the schedule following a
March 15 decision to delay launch from May 10 to July 1. But the
concern about subjecting the tank to unnecessary thermal stresses by
loading it with super-cold rocket fuel remains.

Engineers now believe the expansion and contraction of the tank during
multiple fueling cycles contributes to the formation of tiny cracks and
other defects that can lead to foam loss during flight. Opponents of
the fueling test believe the health of the ECO sensors can be confirmed
through electrical testing without the need for a fueling cycle.

This issue raises thorny questions about how NASA will conduct future
launch campaigns. In the past, the agency routinely fueled space
shuttles for flight even when forecasters predicted bad weather. In
some cases, the forecasters were wrong and the shuttle was able to fly.
But in others, they were right and NASA was forced to drain the tank
and try again another day.

If thermal stress can, in fact, lead to cracks and other issues with
the foam insulation, as now seems probable, NASA may be forced to
rethink its long-standing policy of fueling a shuttle when bad weather
is expected. And that could have a bearing on how long it takes to get
Discovery off the ground in July given the frequency of afternoon
thunderstorms along the Florida Space Coast.

Weather aside, the fragility of the foam also could force NASA managers
to consider putting limits on how many times a tank can be loaded with
liquid oxygen and hydrogen. But at this point, that is speculative and
no such discussions have been held or are currently planned.

Other issues still on the table include the possible swap out of main
engine No. 2 because of a computer controller issue; completion of a
complex debris verification review, showing the tank won't shed large
pieces of foam or allow dangerous ice buildups around external
fittings; and completion of a design review of the tank modifications
in general.

Another wild card is the potential threat posed by so-called "tin
whiskers" that can form on circuit card guide rails in various
electrical components. While the precise causes are not known, small
whisker-like extrusions can form in tin and other metals under certain
stress conditions. The concern is that a whisker could break off, fall
on a circuit board and cause a short.

Whiskers recently were found during inspection of an ascent thrust
vector control box, used to control the system that moves the shuttle's
main engines for steering. Discovery has numerous electrical boxes with
a common design that are susceptible to such whiskering. Sources say
engineers believe Discovery can safely fly as is based on redundancy in
the systems involved. But a final decision on how to proceed has not
yet been made.