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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. F1 etcher
Administrator
National Aeronautics & Space Administration
400 Maryland Avenue, S.W., Room 7137
Washington, DC 20546
Dear Jim:
August 1, 1986
Based on an initial appraisal of the objectives and focus
of the program to redesign the SRB, I am pleased to provide
this interim report containing observations, conclusions, and
recommendations of the NRC Panel for the Technical Evaluation
of the Redesign of the Space Shuttle Solid Rocket Booster.
Background
Beginning with its formation in early June 1986, the Panel
has had introductory briefings by personnel of NASA Head-
quarters, Marshall Space Flight Center (MSFC), and Morton
Thiokol, Inc., on the current design and manufacture of the
solid rocket motor (SRM), the mission 51-L accident, and
preliminary plans for the redesign. We have had more detailed
briefings from and discussions with personnel of Rockwell
International, Thioko1, and NASA/MSFC on the structural loads
and dynamics of the total assembler shuttle and the resultant
loads on the SRM, as well as briefings from personnel of
United Technologies Chemical Systems Division, Aerojet Stra-
tegic Propulsion Company, and Hercules Aerospace Company on
alternative solid rocket designs and alternative joint and
seal designs. We have toured the Thiokol manufacturing and
test facilities and engaged in many detailed technical dis-
cussions with Thiokol personnel on design and testing of
proposed future SRB designs. We have alto been informed by
Air Force personnel about the results of their investigation
of the recent accident involving the Titan solid rocket motor.
1
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
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Letter to the Honorable James C. Fletcher
Focus of the SRB Redesign Program
—2—
In addressing its charge to provide a technical evaluation
of the redesign effort, the Panel is aware of four interde-
pendent factors that influence the program. They are safety,
time, costs, and performance. Above all, safety is the prime
consideration. The critical national need for the launch
capability of the Shuttle makes time a close second. Conse-
quently, we recognize that NASA is initially concentrating on
those redesign approaches that will provide the required
margins of safety with maximum use of existing facilities,
hardware, and components. In addition to engineering design,
the Panel is also reviewing the program for testing and veri-
fication of safe performance of new designs, selection and
specification of materials, and quality assurance or control.
In parallel with this initial approach, the Panel intends
to address alternative designs for the SRM to raise the levee
of not only safety and economy, but alto performance.
Design Specifications and Requirements
We understand that the specifications and requirements for
redesign of the SRM are being completely reexamined. We have
noted differences between NASA/MSFC and Thiokol personnel on
this subject, particularly on the application of past specifi-
cations and requirements for the redesign. Great care and
prompt action should be taken to eliminate these differences
and misunderstandings. We conclude that the redesign process
cannot proceed successfully unless and until all parties agree
to and understand the meaning of specifications and require-
ments placed on the performance of the new SRB.
Joints between Motor Segments
While the Panel agrees with the Presidential Commission
that faulty design of the pressure seal, compounded by the
cold temperature at launch and dynamic joint rotation, is the
most probable cause of a blow-by of the two O-ring seals in
the aft field joint of one SRB, the ensuing events which
resulted ultimately in the loss of the shuttle (almost 60
seconds later) are much less clear. It has been hypothesized
that sealing was reestablished by a plug of material which was
dislodged later, possibly during conditions of high dynamic
pressure including wind shear, leading to melting of the case
metal in and adj acent to the j oint by hot gas . The details,
however, remain uncertain and this uncertainty is very
important f or the redes ign .
2
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Letter to the Honorable James C. Fletcher
—3—
All failure modes must be examined, and all potential
weaknesses in the design of the motor should be eliminated or
reduced to the extent possible. For example, the probability
of debonding of insulation from the metal case should be
minimized as should the probability of the introduction of
moisture into the case where corrosion of metal parts or
degradation of performance of the elastomeric parts can
occur. We recommend that NASA consider using existing SRM
cases to perform tests on the joint to investigate the mode of
failure of mission 51-L to assure that our understanding of
the weaknesses in the original design is correct. The tests
should be designed to investigate alternative modes of failure
and to duplicate the failure of the 51-L joint.
Based on its work to date, the Panel recommends that each
of the following be addressed in the new design: joint rota-
tion from all sources; O-ring materials; O-ring and groove
design; verification of O-ring seating; insulation at the
joint, considering the potential for gas flow in the gaps;
asymmetries in gas circulation in the bore, gaps between seg-
ments, and nozzle from various causes; the bond between the
metal case and insulation, and the role and properties of
putty or alternative means for protecting the O-rings from
exposure to hot gases. The previous design employing two
pressure-activated O-rings was adopted in response to the
requirement that the seal be both redundant and verifiable.
The redesign program focuses on joint designs that employ a
double clevis arrangement to reduce rotation and either two or
three O-ring seals, among other things. It is not cd ear to
the Panel that two pressure-activated O-rings always provide
redundancy.
In these early stages of the redesign program, it is
important to maintain consideration of a diversity of designs,
including both modifications of the proposed double-clevis
joint, and simpler alternatives such as reinforcing the case
walls to reduce rotation. An array of alternatives should be
investigated and carried through the redesign process as far
as is practical.
Additional discussions of joint redesign are scheduled for
an early August meeting of the Panel.
The Nozzle
The nozzle of the SRM has experienced serious problems in
the nozzle-to-case joint and the ablating material in the
nozzl e itself. Work to correct these probe ems has been
underway for several years but much more needs to be done.
3
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Letter to the Honorable James C. Fletcher
—4—
Alternative designs in which joint rotation tends to close
gaps between parts rather than to open the gaps (as in
previous shuttle SRM designs) appear to be inherently safer.
We recommend that NASA include such designs in the redesign
program.
Materials
Materials have always posed serious challenges in solid
propulsion rocket design and new problems are emerging.
Clearly the putty used heretofore in the SRB is one such
material. The expected and actual performance of the putty
seem quite unclear.
. . .
require Its use.
Designs are being considered that do not
Results of the investigation of the Titan accident
suggests a new materials problem, a potentially erosive
interaction between molten alumina and the steel case, that
accentuates the need to avoid exposing steel casing to prope1-
lant gases. We recommend that this phenomenon be researched
vigorously because of its potential importance for the safe
design of future solid rocket motors. We also recommend that
NASA review its requirements and specifications for assurance
of the adequacy of bonds in light of this recent development.
There is also a great dependence on bonded materials
without techniques for assured inspection of bonds. Progress
is currently being made in non-destructive inspection tech-
niques, but they sti11 leave great room for improvement. We
recommend that NASA carefully monitor development of tech-
niques for non-destructive testing of bonds being carried out
by the Air Force and apply the techniques, if appropriate.
It may also be anticipated that asbestos, currently
incorporated into the inhibitor material for the SRM, may not
be avail able in the future. We recommend that NASA consider
developing and qualifying asbestos-free insulating material s
for the SRM within the current redesign effort to avoid a
subsequent requal i f ication when asbestos is no longer
available .
Quality control procedures for materials suppl fed by
vendors appear to have depended largely on the ability of
product specifications to define adequately and compl etely the
material to be supplied, backed up by the qualif ication of
vendors, at least for critical items. This procedure is not
adequate for many materials, polymers in particular being
notorious for their inability to be completely described by
product speci f ication. The system should be strengthened for
4
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Letter to the Honorable James C. Fletcher
—5—
most components by: qualifying vendors' vendors (and in a few
cases, such as the O-rings, vendors' vendors' vendors) and
imposing a process "freeze" on the part of vendors (and
vendors' vendors, etc.) with requalification required for any
process change. We recommend that NASA review the experience
of the military with control and surveillance of materials for
its solid rocket motors and where applicable adapt that
experience to the SRM program.
Testing
The fact that a solid fuel rocket can never truly be
tested except in flight places unusual emphasis on adequate
subsca~e and full scale testing with appropriate simulations
and statistical analysis. It is our first impression that the
proposed test program at best only meets a minimal require-
ment; it is clearly success oriented. The Panel needs more
information on the detailed tests to be conducted before we
can reach an opinion about the feasibility of meeting the
current test schedule, but it appears clear that the planned
sample size may not be adequate for testing all specified
requirements. An opportunity exists, however, to reduce some
limit testing through design choices, for example using
heaters at the joints would obviate the need to test joints at
cold temperatures.
The special issue of vertical vs. horizontal testing is
sti11 open and will be further discussed when the Panel meets
in early August. Dynamic performance of the shuttle system
and its effects on the SRB need both testing and analysis.
The critical issue is the extent to which the actual dynamic
conditions of launch and flight can be simulated in a vertical
test stand. A corollary issue is whether vertical testing
provides enough additional data--or indeed as much useful
data--as to be worth the additional cost. For equivalent
expenditures and time, a larger number of horizontal tests can
be performed.
Sincere]
my ,
H. Guyford Stever
Chairman
cc: Adm. Richard H. Truly
Panel Members
5
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Representative terms from entire chapter:
honorable james
