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OCR for page 7
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:
October 10, 1986
I am pleased to submit herewith the second interim report
of the National Research Council Panel for the Technical
Evaluation of the Redesign of the Space Shuttle Solid Rocket
Booster. The purpose of this letter is to provide an
independent technical evaluation of the progress of the
redesign program thus far.
Background
During August and September, the Panel formally met twice
with personnel of NASA Headquarters, Marshall Space Flight
Center, and Morton Thiokol, Inc. The Panel also conducted a
visit to the Kennedy Space Flight Center, where we inspected
recovered parts of Mission 51-L, were briefed on techniques
for recovering spent solid rocket boosters (SRB) from the
ocean, and witnessed a demonstration of the preparation of
booster segments for mating. Members of the Panel visited the
Rocket Propulsion Laboratory at Edwards Air Force Base to
inspect the dormant vertical test stands there, and some
attended the meeting of the Program Requirements Review
Board. Members also revisited Morton Thiokol for more
thorough exchanges with the company's engineering personnel.
Perspective on the Redesign Program
The redesign effort has progressed greatly in these two
months, principally refining in detail the earlier redesign
ideas for the case field joint, igniter, and nozzle joints, as
well as for changes of the nozzle linings. The focus of the
effort is still on designs that will permit the use of booster
rocket case forgings ordered previously. If this approach is
successful, i.e., if the test program succeeds and the level
of safety is judged acceptable, the Shuttle flight program can
resume at the earliest time. The Panel concludes that the
chances of success of this approach are sufficiently good that
it should be pursued.
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 organizations
OCR for page 8
Letter to the Honorable James C. Fletcher
The New Baseline Design
—2—
The baseline design of the new case field joint incorpo-
rates a capture feature that engages the inside leg of the
clevis to restrict relative motion between the tang and
clevis. We concur that the design will significantly con-
strain the motion.
There are however a number of practical aspects of the
design that are uncertain and of concern. The increased
rigidity of the joint will cause additional bending stresses
to be carried by the case membrane on either side of the
joint; the additional stresses need to be fully understood.
The capture feature may make mating and demating the segments
more difficult than in the past. The multiplicity of addi-
tional seals in the new design (an adhesive bond in the
insulation, the O-ring on the capture feature, and the
meta1-to-metal interference fit of the capture feature) will
make it difficult to qualify and verify by test the primary
and secondary pressure seals, which by definition are the two
O-rings between the clevis leg and tang. While the bond
between facing insulators is not intended to qualify as a
pressure seal, it will normally prevent exposure and evalua-
tion of the metal joint during qualification tests (as the
putty did in previous tests and flights). The new design
imposes more stringent dimensional tolerances and the effects
of reuse in meeting the tolerances remain to be determined.
These and other uncertainties must be resolved through a
vigorous test and engineering program.
Regarding design of the insulation in the gap between case
segments, the Panel notes that the baseline bonded insulator
designs, while advantageous for protecting the metal joint and
insulator-to-case bond, may also pose probe ems with assembly
and demating. Alternatively, an unvented joint may be
obtained without using an adhesive by employing unbended insu-
Jating materials to fill the gap between facing inhibitors.
The inserts would have to be compressed during assembly and be
sufficiently resilient to accommodate the expansion of the gap
that occurs at ignition.
The program has also considered and will test a vented
design employing a labyrinthine gap between ablative surfaces
that causes combustion gases to travel a longer distance
before reaching the joint. This design as presented may pre-
vent hot combustion gases from impinging on the pressure seal
components. Alternative approaches to a vented design may
also be practical, including those intended to assure that any
gases reaching the joint have cooled to acceptable tempera-
tures. Such designs, which have not been considered in the
program, could, for example, employ porous ablative materials
8
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Letter to the Honorable James C. Fletcher
—3—
in the gaps between segments to act as thermal barriers. The
structure could incorporate a volume or reservoir of rela-
tively cool, stagnant combustion gases between the mated
insulating segments upstream of and in series with the first
seal along a potential leak path. This volume could be fi1 leaf
with a porous ablative material that would physically inhibit
gas exchange and absorb the sensible heat that enters the
reservoir. This approach could also, of course, be used with
an unvented insulator design adding further to its overall
reliability. In either case , such a reservoir should be
designed to dampen acoustic oscil~ ations in the gap.
Pending the results of full scale tests, the Panel regards
both vented and unvented insulator designs as potentially
satisfactory. Therefore, the Panel recommends that both
vented and unvented designs be pursued in parallel in the
development effort. This recommendation is based on serious
concern about the validity and scope of the testing of the
baseline (bonded) design.
The Panel urges that a final decision on the design of the
insulation between segments be delayed until:
a) the performance of both unvented and vented designs
has been evaluated in full diameter, full duration tests; and
b) the feasibility of assembling and disassembling the
unvented design is evaluated.
Tests of potential O-ring materials performed to date have
resulted in the identification of three elastomers that hold
promise for use in field joints. Unfortunately, because end
item specifications are not generally effective for control-
~ing elastomer formulations, uncertainties may arise regarding
the exact composition of the tested materials. To avoid
future problems, we strongly recommend that any candidate
materials be thoroughly defined, in cooperation with their
suppliers, in formal advance specifications. As described in
my letter of August 1st, we believe that the best approach
would include specifications, even for test materials, by
product, process, and supplier at all levels and to "freeze"
these specifications. These should supplement performance
specifications for O-ring materials.
The O-ring materials being considered retain the capacity
to follow the gap opening at low temperatures. In light of
reduced gap openings and improved cold-temperature resilience,
the use of supplementary heaters should not be necessary. It
seems prudent, however, to continue to develop the heaters as
a precaution in the event that these materials cannot be used
for some reason.
9
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Letter to the Honorable James C. Fletcher
—4—
The Panel also notes that the new O-ring materials are
intended to be used in the case factory joints, which will not
otherwise be changed. The factory joint is fundamentally
different from the field joint. We do not understand the
rationale for changing the O-ring material in the factory
joint. It is also not clear to us why the program continues
to use spliced O-rings. While an acceptable O-ring can be
made either by splicing or by one-piece precision molding,
good industrial practice avoids splices.
The Shuttle solid rocket motor (SRM) has frequently
experienced "minor" debonding between the case and insula-
tion. Reliability of case-to-insulation bonds can be improved
if procedures for removing grease from the metal can be
enhanced or substitutes found for the grease that is used as
protection against rust. We recommend first that recent
improvements in the technology for testing bonds nondestruc-
tive~y be exploited fully and second that any detectable
debonds be appropriately repaired before flight.
The Panel still has under consideration design issues in
the case-to-nozzle joint, joints within the nozzle, and the
igniter. These elements of the SRM are as important for the
success of the redesign as the case field joint. The baseline
design of the nozzle introduces many more bolt holes, leak
check ports, and pressure seals. Reliability could be reduced
by some of these changes.
The Test Program
Much progress has been made in laying out the program of
tests, which has begun. Significant results, for example from
the Joint Environment Simulation test, have already been
obtained, but there is still much to do. The success of the
redesign effort rests on the test program.
The Marshall and Johnson teams and their contractors have
briefed us on the potential role of a ful1-scale vertical test
firing, including structural dynamics, in the test program.
They have made the point that a variety of horizontal firings
and other tests can, in fact, verify the essential features of
the new design. It is further argued that a vertical firing
on a test stand will not reproduce the precise conditions
experienced during launch and flight. The design team has
recommended against incorporating a vertical firing in the
program. The Panel notes that the desirability of vertical
testing has been evaluated by NASA and concurs that horizontal
testing can be appropriate for this situation. We conclude
however that to be comprehensive, the totality of the test
program must include simulation of the dynamics of launch and
flight.
10
OCR for page 11
Letter to the Honorable James C. F1 etcher
—5—
Design of the test program to simulate these effects
should take into consideration how loads are actually trans-
ferred from the solid rocket booster (SRB) to the External
Tank through both fore and aft struts, the dynamic conditions
known as "twang", and vectoring of the nozzle under flight
conditions. Some members of the Panel believe that this is
best accomplished by constructing a new full scale horizontal
test stand both to be used as an alternate to the current
facility and to provide a facility suitable for conducting
dynamic tests of the structure, nozzle, seals, etc. The new
test stand should be constructed so that loads can be taken
out through realistic struts, a displacement to start the
twang oscillation can be imposed, and a complete simulation of
nozzle vectoring can be obtained.
We recommend the creation of an additional horizontal test
stand to accommodate and test full-scale motors both to pro-
vide the capability of performing more tests and to reduce the
risk to the program should a catastrophic failure occur during
a test. We also believe that a second Joint Environment
Simulator test stand and associated hardware is warranted and
should be established.
Regarding other aspects of the test program, the Panel
offers the following comments and recommendations:
a) The current test schedule envisions only one full
scale, full duration test of the new nozzle that
incorporates all the changes of the redesign. We
recommend additional full scale, full duration
testing to assure that the various elements of the
design, including ablative material and additional
pressure seals and leak check ports, operate as
expected over the range of conditions imposed by the
requirements. In our opinion, a single full scale,
full duration static test of the redesigned nozzle
(prior to the first flight) is simply not sufficient.
The number of tests required depends on the results
of the tests and professional judgment.
b) We endorse whole-heartedly the concept of testing
articles that have incorporated inflicted flaws.
c)
We believe the testing program should be designed to
distinguish between alternative designs regarding
performance. For example, both vented and unvented
joint insulation designs should be tested in regimes
in which the alternatives could be expected to behave
differently.
11
OCR for page 12
Letter to the Honorable James C. Fletcher
—6—
d) In fight of the specification that Shuttle components
must be acceptable after five years' storage before
flight, a test and surveillance program should be
established that will ensure satisfactory performance
of components, such as O-rings, prior to use. Repre-
sentative samples of O-ring materials applicable to
each assembled SRB should be stored in a compressed
state in an appropriate fixture to simulate the
environment of the assembled O-rings. These mate-
rials should be tested for dynamic resilience as a
final verification before launch.
e) The tests should be fully instrumented so that
incipient failure modes can be identified.
We plan to conduct a continuing review of the test pro-
gram, including instrumentation needs and the simulation of
dynamic conditions of launch and flight, and the results over
the next several months. Our next interim report will address
the test plan in detail. We also will continue to review
aspects of the design including assembly and disassembly of
field joints and reuse.
Alternative Design Considerations
The alternative designs being considered by NASA meet the
constraint of using the case forgings that have already been
ordered. The alternatives employ various designs for the
insulation in the gap between segments, such as the ~abyrin-
thine vented joint or the cooler-gas vented joint. At least
one such alternative will be included in the test program; we
hope that several of the more promising candidates can be
pursued through testing.
Alternatives to the baseline designs of the metal parts of
both the case joint and the case-to-nozzle joint also exist
which have not been thoroughly explored on the grounds that
they will not be compatible with the forgings which have
already been ordered. We recognize the importance to the
nation of returning the Space Shuttle to service without
unnecessary delays. If, however, the design process were not
constrained by the forgings, we believe that more basic alter-
natives to the baseline design would probably be preferred
once thoroughly analyzed.
For example, it is possible to design both case and
case-to-nozzle joints, without capture features, that tend to
rotate closed upon pressurization or not to rotate at all.
Similarly, modifications of the simple tang-and-clevis joint,
12
OCR for page 13
Letter to the Honorable James C. Fletcher
such as by changing the dimensions of the parts, might reduce
the rotation substantially. The concept of a joint that
rotates closed or does not rotate at all might be implemented
for the case joint either with bolted flanges, as suggested by
the group at NASA/Langley Research Center, or with a pinned
tang-c~evis joint. With the pin concept, the tang-clevis
joint would have to be drawn into a smaller radius than the
case cylinder. This approach might be accommodated by the
current bi1 less. Using either technique might result in the
reduction or elimination of joint rotation by design rather
than mechanically by using a capture feature.
We strongly recommend that NASA maintain a program to
explore and develop original, possibly quite different designs
for the next generation of SRB in parallel with the current
redesign effort and for the contingency that the baseline
design may not offer sufficiently good performance and margin
of safety. In particular, an alternative SRB should take
advantage of designs that are insensitive to or will benefit
from the structural deformations that occur after ignition.
Requirements, Specifications, and Institutional Relationships
There does not appear to be a very clear separation of
organizational responsibility for defining requirements or
specifications and designing the hardware. The Project Office
at Marshall not only develops requirements and reviews and
approves designs, but also is deeply involved in the detailed
design process. Thus it can happen that the design team can
also approve waivers to the basic requirements. We conclude
that a clear separation of the responsibilities for specifying
requirements and implementing a design to meet those require-
ments would allow designers greater flexibility to consider
alternative solutions and assure a more independent evaluation
of the design in light of requirements.
Several issues were raised by Panel members regarding
specific requirements for the redesign during the Program
Requirements Review Board meeting on August 26-27. Of these,
we include the following points here:
a) We understand that the baseline design may not
satisfy all the requirements. Some requirements, in
fact, are actually design specifications. It appears
to us that NASA may be setting itself up for new
waivers as a consequence. We recommend that NASA
carefully reconsider the design requirements to
assure that unnecessary specifications are not
incorporated that will necessarily result in waivers.
13
OCR for page 14
Letter to the Honorable James C. Fletcher
—8—
by A number o f dif ferent types of safety factors are
quantitative y imposed by the requirements . However,
they are only loosely defined in terms of what
specifics y shout ~ be measured, the method of
measurement, and the details of the method of
calculating safety factors.
c) There seem to be different interpretations of terms
such as f actor o f sa f ety, redundant, verif table,
etc. We believe that it is important that concise,
quantif iable def initions be prepared and used .
Summary
In summary, we have conch uded that the chances for success
with the current approach to case f ield j oint redesign are
sufficiently good that it should be pursued. This choice is
the consequence of the understandable desire to use existing
hardware to the greatest extent possible, including new case
forgings previously ordered. The baseline case field joint
will be difficult to evaluate by test. Implementation of
alternative designs for metal joints, which may prove to have
significant advantages, would take considerably longer but
should be pursued in parallel with the current redesign
program.
The success of the redesign effort depends on the test
program. We believe that the planned test program requires
significant augmentation with additional facilities and
tests. It need not, however, include a full scale, full
duration vertical test provided the static and dynamic loads
of launch and flight, separately and in combination, are
appropriately simulated elsewhere in the program.
Sincerely,
H. Guyford Stever
Chairman
cc: Adm. Richard H. Truly
Panel Members
14
Representative terms from entire chapter:
honorable james
