yee@trident.arc.nasa.gov (Peter E. Yee) (07/03/90)
Mark Hess
Headquarters, Washington, D.C. July 2, 1990
(Phone: 202/453-4164)
LH2 LEAK INVESTIGATION
History
During the launch attempt for the STS-35 mission on May 29,
1990, a hydrogen leak, in excess of limits established to
maintain safe operating conditions, was detected by onboard
hazardous gas detection systems. Leakage was detected both in
the aft compartment and external to the liquid hydrogen external
tank/orbiter umbilical assembly.
A subsequent tanking test, that incorporated special ground
instrumentation, further isolated the leak to the free space
between the two halves of the umbilical assembly. The umbilical
provides capability to load propellant into the external tank and
transfer propellant from the external tank to the Space Shuttle
main engines during launch. The umbilical disconnect assembly is
the separation point between the orbiter and the external tank
after main engine cutoff.
The design of the umbilical disconnect has remained the same
throughout the Shuttle flight program except that a safety
modification to incorporate a valve latch, which precludes
inadvertent closure, was authorized after the Challenger
accident. Data from the tanking test determined that the design
changes incorporated by this modification did not contribute to
the leakage.
Following rollback and orbiter demate, the LH2 External Tank
(ET) side of the umbilical was removed and tested at Rockwell
International, Downey, Calif. The testing was performed under
precisely controlled liquid hydrogen test conditions. No leaks
were detected.
On June 29, 1990, NASA conducted a modified propellant
loading test of the STS-38 Space Shuttle vehicle to ensure the
safety and integrity of the orbiter/ET umbilical. The test
revealed a hydrogen leak. The results indicate the leak is in
the vicinity of the umbilical mating plates. It appears to be
primarily from the 17-inch line but possibly with a contribution
from the 4-inch line. The leak is flow rate and temperature
dependent. It is not as high as STS-35 but it exhibits many of
the same characteristics.
Current Status
An extensive investigation is being performed to isolate the
source of the leakage observed on both the STS-35 and STS-38
vehicles.
Columbia is in the Orbiter Processing Facility (OPF). The
orbiter side of the LH2 umbilical from Columbia was shipped to
Rockwell-Downey on Saturday and is being installed in a test
stand which can test the orbiter and ET umbilicals in a mated
configuration. Engineers expect to begin cryogenic hydrogen
testing of the umbilical on Thursday to further isolate the
source of the leak.
In the interest of safety, all potential leak sources,
including the very low probability of a parent metal flaw, are
being investigated. A detailed investigation of all aspects of
the STS-35 and STS-38 component history, including acceptance
test procedure requirements and data, and design changes being
performed.
Processing of the STS-38 vehicle has been suspended.
Engineers are continuing troubleshooting efforts on the Shuttle
vehicle at Launch Pad 39-A. Today, the external tank will be
pressurized and leak checks conducted in the area of the orbiter-
to-external tank umbilical. Another special tanking test is
being considered using special leak detection sensors to help
pinpoint the leak's location.
Program Team
Leonard Nicholson, Deputy Director, Space Shuttle Program,
will lead the NASA/industry team charged with analyzing the cause
of the leak and determining corrective actions.
Under Nicholson, four work teams have been formed:
- Design and Analysis Team - to assess the flight hardware
and ground support equipment hardware designs, fabrication and
test programs, and assess ground processing procedures to ensure
compliance with design intent.
- Hardware Processing Team - to review all Kennedy Space
Center procedures associated with the ET and orbiter processing,
including "as run" data, problem reports, processes, procedures
and personnel certification.
- Data Analysis Team - to analyze data from the tanking
tests and applicable launches to identify trends, define
additional tests and instrumentation needed to understand and
isolate the source of the leak and review all tests currently
planned for completeness and appropriateness.
- Fault Tree/Test Requirements Team - to develop and provide
to the other teams a fault tree identifying failure scenarios,
and identify additional tests and data requirements.
An independent team, headed by Wayne Littles, Deputy
Director of Marshall Space Flight Center, Huntsville, Ala., also
has been formed with senior NASA and contractor representatives
who are experts in liquid hydrogen technology. While they will
work independently of the investigation teams, they will report
to Nicholson, and support the team, as appropriate.
Acceptance Test Procedure (ATP)
The ET/Orbiter umbilical is fully developed and qualified
flight hardware that has met all of the functional requirements
stipulated by specification.
An acceptance test procedure, which is a screen to verify
the manufacturing process for each component delivered, has been
developed from performance requirements. The ATP stipulates the
use of liquid nitrogen as an acceptable substitute for liquid
hydrogen, which is extremely dangerous and volatile to handle.
The disconnect component specification allows a maximum
hydrogen leakage of 200 standard cubic inches per minute (scim)
for the mated disconnect assembly at cryogenic temperature (150
scim for the ET and 50 scim for the orbiter). The ATP has a more
stringent limit on the ET disconnect, which is 50 scims. This
allowable leakage rate was established to compensate for the
temperature and media differences between liquid nitrogen and
liquid hydrogen. The orbiter disconnect underwent ATP testing
with liquid hydrogen because there are a limited number of
production units.
Future Steps
Until the leak investigation is completed, Shuttle flights
have been suspended. Returning the Shuttle fleet to flight
status is the highest priority in the Space Flight Office, and
every available resource within the Shuttle program is being
brought to bear on solving this problem. NASA is confident that
the source of the leak can be isolated quickly and the problem
fixed with minimum disruption to the Shuttle flight program.
Independent of these events, a component redesign to replace
the current umbilical disconnect with a new design has been
underway and is well into the preliminary design phase. The new
disconnect incorporates significant safety improvements,
including redundant seals at all locations.