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NATIONAL RESEARCH COUNCIL
COMMISSION ON ENGINEERING AND TECHNICAL SYSTEMS
2101 Constitution Avenue Washington, D. C. 20418
COMMITTEE ON NASA SCIENTIFIC
AND TECHNOLOGICAL PROGRAM REVIEWS
Panel on Redesign of Space Shuttle
Solid Rocket Booster
The Honorable James C. Fletcher
Administrator
National Aeronautics & Space Administration
400 Maryland Avenue, S.W., Room 7137
Washington, DC 20546
Dear Jim:
January 15, 1987
I am pleased to submit herewith the third interim report
of the National Research Council's Panel for the Technical
Evaluation of the Redesign of the Space Shuttle Solid Rocket
Booster. The redesign effort is currently in transition from
a period devoted to creating the so-called baseline design
using analytical techniques to one concentrating on testing
the concepts and hardware. The purpose of this letter is to
critique the program at this stage.
Background
Since early October, the Panel has met formally three
times. In addition, groups of our members have participated
in a number of technical meetings with personnel from Marshall
Space Flight Center and Morton Thiokol, Inc. Members have
visited Latrobe Steel Company and the Ladish Company to review
capabilities for producing steel cases for the motor and the
U.S. Air Force Space Command to discuss methods of nondestruc-
tive evaluation of insulation bonds, as well as other aspects
of the redesign that might benefit from the Titan SRM program.
We have also kept up to date on the progress of the contrac-
tors who are developing concepts for a Block II motor.
Current Status
NASA's goal is to return a safe and reliable Shuttle to
service at the earliest possible date. To meet the goal, the
NASA team has chosen a baseline design and is in the process
15
The National Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering
to serve government and other organizahons
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Letter to the Honorable James C. Fletcher
—2—
of organizing and implementing a testing program to verify
that the design meets the requirements of the program. The
redesign baseline is aimed at correcting the causes that
contributed to the failure as well as other elements of the
original design that showed signs of trouble in flight or that
do not meet the new design criteria. Most decisions have been
made as to which features need complete redesign, which need
modification, and which can remain the same as before. Only
those changes considered mandatory for the next flight are
being made.
Only if the verification program goes very well can NASA's
schedule be met; care must be exercised in the ground-test
program to assure that the design is adequately demonstrated.
Morton Thiokol is well started on this program and should be
strongly supported in its pursuit.
As discussed in an earlier report of our Panel, however,
the test program is success-oriented. By this, we mean that
the schedule for the program reflects an assumption that each
test will produce results that are expected and understood;
there is little room in the schedule for modifying the design
if this does not prove to be the case. Preliminary tests have
already produced results that were not anticipated. For exam-
ple, O-ring materials with better low temperature resilience
were found to deteriorate when tested after extended exposure
to the rust-inhibiting grease, with the result that the base-
line design now uses the original fluoropolymer O-rings with
heaters to maintain an appropriate temperature.
It is prudent, therefore, to plan for contingencies,
particularly for those elements of the design for which
substantial uncertainties remain. If an aspect of the design
is shown not to meet safety objectives, an alternative should
be available to carry the program forward. In some areas the
program has incorporated alternatives to the baseline design.
Nonetheless, we are concerned that valuable time would be lost
should it become necessary to turn to one or more of the
alternatives as a consequence of something learned from the
test program. Further, now that the NASA/Thiokol redesign
team is nearing the completion of the selection of the base-
line design, it should be able to increase its efforts on
preparing for contingencies. We recommend, therefore, that
NASA strengthen its contingency plans for incorporating
alternatives into the program.
Aspects of the design that warrant more contingency
planning include the case field joint, the case-to-nozzle
joint, and configurations of the insulation at both the case
and case-to-nozzle joints. Materials for which contingencies
16
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Letter to the Honorable James C. Fletcher
should be in the program include the ablative materials in the
nozzle, O-rings, and the rust inhibitor. Options currently in
the program are limited. For example:
o The Panel shill believes that contingency planning
should include the design, fabrication, and testing
of a case field joint that closes under pressuriza-
tion. We understand that such a design can be
obtained without changing the configuration of the
existing forgings, but the first step in this process
is to order the appropriate forgings, which has not
been done.
O Currently, there is no real alternative in the
program for the baseline design of the case-to-nozzle
joint, which incorporates a large number of new bolls
and bold holes, hence complex stress patterns and
potential leak paths. We consider the lack of an
alternative to be serious since the joint is critical
for safety, few tests of the final configuration are
planned, and they occur late in the test program. We
recommend that alternatives be established and
l
special emphasis be placed on early, meaningful tests
of the design.
a
The baseline design of insulation at the field case
joint incorporates an adhesive bond between opposite
sides of the gap. Two variations of this unvented
design are being studied, called the J-seal and
U-seal. The backup design, a vented configuration,
will be evaluated in subscale tests and a ful1-
duration, fu11-scale test firing, but it is not being
carried forward on a schedule that would permit its
inclusion in the program without a delay. The base-
line design of the case-to-nozzle joint also incorpo-
rates an adhesive bond. NASA should anticipate that
the details of the insulation design at both joints,
whether unvented or vented, will continue to change
and we recommend that the Agency provide for addi-
tional insulation tooling to enhance the flexibility
of the program.
Significant changes in estimates of loads in the SRB and
both of its interfaces with the External Tank have recently
been reported to us. In light of these changes and the crack
in an aft skirt during a recent structural test, we recommend
that stress analyses of the booster and its parts be carefully
reviewed and, as appropriate, calculated again using the best
data and most up-to-date models.
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Letter to the Honorable James C. Fletcher
The Panel has additional technical comments and recommen-
dations on the current design and testing program, which are
presented later in this report.
Perspective on the Future
.,,
i .
Not all the changes that the engineering team would like
to make in the solid rocket motor will be made before the next
launch. By directive from NASA headquarters, the redesign
effort is focussing on identifying those changes essential for
adequate safety or reliability for the next flight. The
purpose is to avoid a dilution of effort on the mandatory
changes, which could delay the Shuttle's return to service.
We concur with this approach in principle.
Beyond the changes deemed mandatory for the next flight,
however, additional changes may also be needed further to
enhance its safety and improve its reliability, performance,
and cost effectiveness. The solid rocket booster, no less
than other technological products, should continually be
improved by changes driven by the results of tests and data
gathered during use. It is our impression that the ongoing
technological program that supported improvements in the
reliability of the Shuttle solid rocket motor before the
accident was limited compared with that for improving
performance.
We recommend, therefore, that NASA establish and maintain
a directed program for continued evaluation and improvement in
reliability after flights are resumed, including all of the
following: use of flight instrumentation for diagnostics
throughout the flight program; continuing evaluation of flight
results and recovered hardware; continuing ground testing to
understand and, as necessary, improve the hardware promptly;
and preplanned block changes to the flight hardware to provide
opportunities for incorporating desired improvements.
In the long term completely new designs, possibly radi-
ca1ly different from the current one, may be preferred. The
nation's manned space flight program cannot afford to continue
to rely on older technology indefinitely into the future.
Consequently, we believe that NASA should vigorously pursue a
program to develop the next and future generations of motors
for use in the space transportation system. The studies of
alternative concepts for the SRM that NASA commissioned in
1986 is a commendable step in this direction.
The program outlined above--including near-term changes to
meet immediate goals, intermediate-term changes for product
improvement, and long-term changes to incorporate substan-
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Letter to the Honorable James C. Fletcher
—5—
tially improved technology--can only be accomplished with the
support of research and development. Not all of the required
R&D can be accomplished within the current recovery program,
so it may be necessary for NASA to include appropriate activ-
ities elsewhere within its programs.
Additional Conclusions and Recommendations on Technical Issues
The Case Joint. We are concerned that the baseline design may
not readily allow for reworking or inspection of the hidden
surfaces of the capture feature. We conclude that this
problem requires special attention since it could be suffi-
ciently serious to prohibit reuse of the case segments if the
finishes and absence of corrosion cannot be satisfactorily
verified.
The baseline design now specifies fluoropolymer O-rings
lubricated with HD-2 grease, as used in all the previous
flights, with heaters to maintain the O-rings at the tempera-
ture (75°F) required to assure a satisfactory seal. The
Panel is concerned with this development because of added
complexities and potential reliability problems associated
with the heaters. For example, overheating the joints could
adversely affect adhesive bonds, which are invariably weakened
at elevated temperatures, or the properties of components,
such as O-rings or grease, in unpredictable ways. It seems
likely that an alternative grease can be found or formulated
that will both have acceptable corrosion protective properties
and not interact adversely with the silicone or nitrile O-ring
materials. We recommend that a priority effort be aggres-
sively pursued to improve understanding of the properties of
these materials and to develop a better combination of O-rings
and grease for use as soon as possible.
The redesign team has considered two concepts for the
design of insulation between adjacent segments, vented and
unvented, and has selected the latter as the baseline
approach. In this concept, a properly functioning insulation
would shield the metal joint from combustion gases. Should a
leak occur in the interface, however, the joint has been
designed to inhibit further propagation of the high-pressure
gases through the O-ring seals. The small joint rotation
anticipated with the capture feature, interference metal fit
between the clevis leg and capture feature, and three O-rings
are configured to provide redundant protection in sealing
against various upstream leak scenarios. Because the unvented
insulation design, if successful, will shield the O-rings from
exposure to pressure on hot gases, tests with imposed flaws
are particularly important for determining the adequacy of the
design of the seals.
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Letter to the Honorable James C. Fletcher
—6—
In response to the requirement that pressure seals be
verified, the program envisions testing the O-rings after
assembly by applying pressure through ports. Applying an
inappropriate pressure could damage the insulation or bond
line between case segments. We remain concerned with this
issue and expect to pursue it further as verification
procedures are developed.
The Nozzle. The Panel currently perceives that uncertainties
in the design and hence risk to the program are now greater in
the area of the case-to-nozz~e joint and for the other aspects
of the nozzle than in the case field joints. Unlike many of
the issues associated with the field joints that can be
resolved to a reasonable level of confidence with subscale or
short duration tests, several of the critical questions on the
nozzle design can only be really answered with full scale,
full duration testing.
Mating and sealing the segments of insulation at the
case-to-nozzle joint is a very different proposition from that
at the case field joint. During assembly of the case-to-
nozzle joint, the insulation on the fixed nozzle housing must
slide for several inches over the insulation on the aft dome,
complicating design of the insulation and adhesive for an
unvented design. Here the method of sealing of the mating
segments of insulation is still open to question. We
recommend continued model~ing and subscale testing of
alternatives that insure that only cooled, low momentum gas
can reach the O-rings in the event of a leak in the seal
between mating segments of insulation at the joint.
The general requirement for pressure seals to be redundant
and verifiable is being applied in the current baseline design
to all of the internal nozzle seals. This requires the addi-
tion of a second O-ring in many cases and the addition of
ports to verify and seat the seals. The second O-rings are
intended to enhance reliability, but overall reliability may
not be increased because of the penetrations of the nozzle
added for the purpose of verifying the O-rings.
Tests of the nozzle ablative materials will not be
performed until late in the verification program. Marginal or
unsatisfactory performance in these tests could severely
affect the schedule for the next launch. There are no backups
in the program for design of the Jayup of ablative materials
in the nozzle. Given the empirical nature of the technology
and its unpredicted behavior in past flights, we recommend
that, in addition to the planned full scale, full duration
tests of the nozzle, the redesign team undertake a series of
subscale, long duration firings to evaluate the conceptual
basis and materials technology for the redesign of the
ablatives.
20
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Letter to the Honorable James C. Fletcher
—7—
The Test and Verif ication Program . As indicated in an earl ier
report, we regard the proof of the redesign to depend on the
results of the program to test it and verify that it meets the
requirements of the system. The test and verif ication program
is just getting underway and is shill evolving. Some prelimi-
nary tests have been conducted, but f irings with new case
hardware are not expected before March, 19 87 . The test
program appears to be somewhat constrained by the abil ity of
the support organizations to provide test articles,
instrumentation, etc. We intend to follow the development and
progress of the test program carefully. Meanwhile, we make
the following observations and recommendations.
While we continue to see progress in the definition of the
total test program, it is difficult to relate the objectives
of each of the numerous tests to specific design requirements
or to develop a sense of the coherence of the test program in
terms of its support of the development effort. Clear plans
that relate each test to specific design requirements or to
the development of specific components or subsystems would
help to assure coherence of the total effort.
For NASA to accept a design as safe and reliable, we
believe that the Agency should be convinced that it under-
stands how the design will behave throughout the range of
conditions that the SRB is intended to experience. Recog-
nizing that it may not be practical to test the design
throughout the range of conditions, NASA plans to verify some
aspects of the design by analysis rather than by test.
Verification by analysis relies on the use of analytical and
computational models, so to incorporate this technique
requires that the models be validated by experiment. We
recommend that tests within the current program also be
instrumented specifics ~ y to validate the models.
The verif ication program is aimed at determining if the
des ign meets requirements, but it should al so have the value
of enhancing conf idence that the redesign team understands how
the design works. The Presidential Commission concluded that
ne ither NASA nor its contractors understood the previous
des ign very wet ~ . To assure better understanding, we
recommend that before each maj or test in the verif ication and
qualification program the redesign team make expel icit analyti-
cal predictions of the performance of the test article; the
results of each test should be examined in light of the
predictions and any discrepancies full y explained. Criteria
for successful verif ication in each test should also be
estabI ished a priori to assure the obj activity of the test
program. These are common engineering practices and are
appl icable to the Shuttle program.
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Letter to the Honorable James C. Fletcher
—8—
The Panel expressed concern in its last report over the
limited capacity of full scale test facilities. We believe
that the addition of another full scale, full duration firing
test stand capable of simulating dynamic loads is a major step
forward in that regard. Conditions that should be simulated
on the stand include launch loads and the most severe fit ight
conditions. We have recently learned, however, that the new
facility may be limited in its capability to simulate these
loads and further that estimates of the maximum loads are
being revised downward. We are concerned both about how well
the loads are understood and the adequacy of the new facility.
We recommend that the design of the stand be carefully
reviewed to assure that the test results will be most useful.
NASA apparently has concluded that another Joint Environ-
ment S imulator (JES) test stand is not needed because addi-
tional test hardware will allow more frequent tests on the
existing stand. Each of the three JES tests conducted to date
(using case joints of the original design) yielded results
that were not predicted by prior analyses. For example, tests
have shown that the extent of joint gap opening depends on
whether pressurized gases reach the region between the clevis
leg and the tang. The JES-IB test showed that the silicone
O-rings cannot be used with the current grease. JES tests
also revealed that pressurized gases can leak past the seals
significantly before the occurrence of the peak pressure or
peak gap opening. These tests have provided extremely valu-
able data and insight into the behavior of joints. They also
supply data to improve the analytical models. We concur that
additional test hardware for use in the current JES and the
related Transient Pressure Test Article will enhance the full
scale test capability. If the rate of timely testing in these
facilities is not sufficiently enhanced by addition of test
hardware, we recommend that NASA reconsider its decision not
to build an additional stand.
At this writing, NASA has not formally approved the
firings of Qualification Motors 7 and 8 before flight. In
view of the number of features of the new design that depend
critically on full scale, full duration testing, the Panel
recommends that these be approved and incorporated into the
test program immediately. Furthermore, the Panel believes
that the dynamic tests in full scale, full duration firings
with structural deformations to simul ate launch and flight
loads must be completed before f ~ ight .
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Letter to the Honorable James C. Fletcher
Finally, we are grateful for the
received from NASA, Thiokol, and
redesign team deserves praise for
work.
On behaI f of the Panel, I remain ,
cc: Adm. Richard H. Truly
Panel Members
_9 _
cooperation that we have
other personnel. The
its dedication and hard
S incerely,
H. Guyford Stever
Chairman
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Representative terms from entire chapter:
honorable james
