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OCR for page 25
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
June 22, 1987
The Honorable James C. Fletcher
Administrator
National Aeronautics and Space Administration
400 Maryland Avenue, S.W., Room 7137
Washington, DC 20546
Dear Jim:
~ am pleased to submit herewith the fourth interim report
of the National Research Council's Panel for the Technical
Evaluation of the Space Shuttle Solid Rocket Booster. The
baseline design is essentially complete with only a few
choices remaining to be made. The program of testing full
scale implementation of the design is about to begin. This
report addresses several of the more important questions
remaining about both the design and the verification program.
Background
Since submitting our third report, the Panel has met
formally four times. In addition, groups of our members have
participated in four technical meetings with personnel from
the Marshall Space Flight Center, Johnson Space Center,
Langley Research Center, Morton Thiokol, Inc., and several of
its subcontractors. Members have visited several subcon-
tractors to review capabilities to cut and finish case hard-
ware. The cooperation that we have received from personnel
from NASA and Morton Thioko~ has been outstanding. The
members of the redesign team have shared their technical
insights and information about the program graciously and
effectively.
Current Status
The direction of the redesign program has not changed
significantly in the last several months. The focus of
attention has been on refinements to the baseline design and
preparation for testing the new design to verify that it meets
specifications.
25
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
—2—
Schedule . The realities of this complex redesign, develop-
ment, manufacturing, testing, and verification program,
however, have forced substantial delays in the test program.
For example, the f irst test of the new case field j oint
hardware in a short-duration burn, original ly scheduled for
January, now will not take place until ache end of July. We
understand that the need] to prepare new mold tool ing for the
insulation of the modified J-seal design contributed in part
to your decision to change the launch schedule from February
to June 19 8 8 . The target date f or del ivering the f irst f ~ ight
set to Kennedy Space (enter has changed by about two months,
to the middle of December 1987, shill Leaving considerable
pressure on the booster testing program.
Contingencies. More attention is now being paid to contin-
gencies, but we remain concerned that alternatives for certain
critical aspects of the design are not being pursued with
sufficient vigor to minimize delays in the program should it
become necessary to turn to alternatives. We understand the
view of the program office that the delivery schedule will
only be met by concentrating intensively on the baseline
design--the principal need for this concentration being the
shortage of manpower rather than budget. However, as we have
stated before, paying greater attention to the development of
alternatives could, on balance, reduce the risks to the
schedule.
The types and levels of risks to schedule and performance
that are acceptable can only be decided by NASA management.
We understand that you have reviewed the program's plans for
pursuing design alternatives in critical areas in light of the
schedule for delivery and launch and have decided that the
risks to the program are acceptable.
The Development and Verification Program
The firing of the full-scaJe, full-duration Engineering
Test Motor (ETM-1A) on May 27th raised the curtain on the most
important part of the development and verification program.
Although the ETM-1A could not incorporate much of the new
hardware, the firing has given useful engineering information
and exercised both the test facility and personnel. The
essential features of the two most critical aspects of the new
design, the case field joint and case-to-nozzle joint, will
first be tested in the fulI-scaJe, fu11-duration firing of
Development Motor No. 8, currently scheduler for August 23rd.
26
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Letter to the Honorable James C. Fletcher
—3—
Reliability and Margins of Safety. Contemplation of the next
year of scheduled full-scale, fu11-duration firings of demon-
stration and qualification motors and the tests of full-scale
hardware for short duration, reminds us once again of the
unique feature of the solid propellant rocket: the flight
article itself cannot be test-fired before launch. To compen-
sate for this characteristic, the test program must establish
reliability and margins of safety in special ways. Further-
more, extraordinary care must be taken to assure that the
materials and processes used to fabricate and assemble both
test and flight articles are tightly controlled so that all
are, as nearly as practical, identical.
It is clear that conducting a statistically meaningful
number of full-scale, full-duration ground tests of the rede-
signed motor to establish reliability and margins of safety is
not feasible. In circumstances such as these, several prac-
tices can be undertaken to increase confidence in the design.
One is to make detailed predictions of performance before
major tests to demonstrate understanding by comparing the
predictions with the results of equally detailed measurements.
The redesign team intends to make explicit predictions in its
test plans in the future and we will follow progress in this
regard closely.
Testing with Defects. A complementary method is to test the
adequacy of margins of safety by introducing intentional flaws
or defects into test articles. The redesign program relies on
testing with inflicted flaws to verify the design where the
flaws are of types not likely to be detectable by available
methods of inspection. Tests with inflicted damage currently
must demonstrate not only that the primary and redundant
secondary seals are not violated under these conditions but
also that they do not show signs of thermal distress.
Since all types and sizes of flaws cannot be tested, a key
issue in this regard is what constitutes the most severe flaw
that has a reasonable probability of occurring. Testing with
flaws that are not likely to occur in practice may yield mean-
ing~ess or misleading results. Subscale tests have shown that
a small direct path between mated parts of the insulation can
result in a gas jet that impinges on an O-ring. Major damage
can result from such a flaw. Not clear is whether these
"engineered" flaws are representative of the most serious
defects that might be expected to result during manufacturing
and assembly. We conclude that determination of the "worst
credible" flaw is essential for the success of the certifica-
tion program.
27
OCR for page 28
Letter to the Honorable James C. Fletcher
Another possible cause of thermal distress is circumferen-
tial flow around the periphery of the joint. The JES test
series, which has provided much understanding, has a burn
duration of less than one second. There is concern that this
duration may not be sufficiently long to reveal other possible
modes of failure.
Case-to-Nozzle Joint. The area of our greatest concern
regarding testing with inflicted flaws, however, is the
case-to-nozzle joint. Defects in the form of holes in the
adhesive may be introduced as the nozzle housing slides past
the case dome during assembly. Since the integrity of neither
the adhesive nor the wiper O-ring will be assured by test
after assembly, neither can be considered seals. Thus, the
performance of the nozzle must be tested with defects in
both. In our opinion, the probability is high that the
primary O-ring will experience thermal distress when tested
with a flaw that extends directly through both the adhesive
and wiper O-ring. Therefore, Me conclude that the current
baseline design of the case-to-nozzle joint is likely to fail
to meet the requirement prohibiting the appearance of thermal
distress when tested in the presence of this flaw.
Consequently, we recommend that the redesign team address
this problem with urgency. The worst credible flaw should be
determined systematically using assembly demonstration tests
and supporting analyses. Potentially modest modifications of
the design which might preclude O-ring damage with such flaws
should be reviewed. Aggressive pursuit of an alternative
design, including acquisition of hardware and testing, also
appears warranted to minimize consequences for the schedule if
the current design or modifications of it cannot meet require-
ments.
Verification of Pressure Seals. Another matter of concern is
the development of satisfactory procedures for verifying the
O-ring pressure seals after assembly. The maximum allowable
leak rate past pressure seals must be below the rate that
would result in thermal distress, including erosion, of the
O-rings but not so low that it cannot reliably be detected by
the adopted method. The redesign team currently favors a
maximum leak rate of 0.01 standard cubic centimeters per
second. The current leak check protocol includes establishing
the test pressure in a volume adjacent to the O-ring and
watching for a decay of 2 psi within 10 minutes. Laboratory
tests conducted by the redesign team indicate that thick
coatings of the grease can interfere with the detection of
leaks of this size. However, large amounts of grease are also
28
OCR for page 29
Letter to the Honorable James C. Fletcher
.
—5—
thought to protect against leaks during firing. Removing as
much grease as possible may assist in the detection of smaller
leaks but may make the design less forgiving in operation.
Testing at high pressure may also reveal small leaks, but
the procedures are sensitive to very small changes in tem-
perature. Under current leak check protocols, changes in
temperature of the order of a few tenths of a degree Kelvin at
a test pressure of 1,000 psi can falsely either indicate a
leak or mask one. Also, the higher the test pressure the
greater the potential for damaging the insulation, insulation
bond lines, or O-rings.
Work is underway to develop the most appropriate leak
check procedures in light of the several trade-offs involved.
The additional data from this program should be available in
the next month. Further analysis and testing appears to be in
order to determine if a higher maximum leak rate, hence lower
test pressure, might be acceptable. Also, in some cases it
might be appropriate to measure leak rates using alternative
approaches, such as by measuring the buildup of test or tracer
gases downstream of a seal rather than by measuring the decay
of the test pressure upstream.
Nozzle Ablatives
The redesign of the internal parts of the nozzle incorpo-
rates the carbon-phenolic materials used in the previous
design, with changes in the orientation of the carbon fibers
in the matrix and improved specification and qualify control
of materials. The changes are intended to provide reduced,
more consistent erosion and to eliminate fracture problems.
Early evaluation of some features of the new design (fiber
angle in two nozzle parts) has been provided by three full-
sca~e static tests (two in the filament-wound motor program
and the ETM-1A). Because the performance of ablative mate-
rials is relatively unpredictable, a number of tests are
needed to establish the adequacy of the design, materials,
workmanship, and production. Thus far, we are encouraged by
the work on redesign of the ablative parts of the nozzle.
Nonetheless, as many tests of these parts should be scheduled
as practical. Furthermore, the performance of the ablative
parts in particular, should be closely monitored and evaluated
by inspection of recovered flight motors on a continuing basis
beyond the verification phase of the program.
29
OCR for page 30
Letter to the Honorable James C. Fletcher
Materials and Process Control
—6—
As we previously observed, a successful solid rocket motor
program depends on careful control of the materials and proc-
esses used I manufacture both the test and flight articles.
The manufacture of nonmetallic parts under carefully defined
specifications is a continuing concern.
O-r; He M~teri al . The effort of the past year to find an
O-ring material with better low temperature resilience while
meeting all other performance requirements has not been suc-
cessful. Therefore, we agree that the fluoroelastomer used
previously is an appropriate choice at this time since it will
be used in combination with the modified metal parts and
heaters to provide temperature control and hence satisfactory
. . .
resl fence.
Specifications. An accurate and thorough definition of the
fluoroelastomer material is urgently needed, however. The
specifications supplied to us to date are incomplete. Once
the final specifications are determined, it will be necessary
to review the existing inventory of O-rings to verify that
they were manufactured and tested in conformance with the
specifications. Similar comments apply to the specification
of composition, processing, performance, and acceptance of the
adhesives that are to be used in the new design.
While the adhesives and O-rings may contain proprietary
products, adequate control of materials and processes requires
their full and complete description. Only with this informa-
tion can adequate engineering control be exercised over the
manufacturing process. To maintain quality control for mate-
rials used in critical applications, the suppliers must be
restrained from making any changes to formulations or proc-
esses without engineering approval. Means are available for
sharing essential proprietary information to assure control
while maintaining confidentiality. We recommend that agree-
ments with suppliers be negotiated and implemented to permit
stringent engineering control of the materials and processes
used in the solid rocket booster consistent with the protec-
tion of proprietary interests.
The requirement for finishes of metal surfaces in contact
with O-rings poses a challenge to manufacturing techniques.
We are pleased to note both the reported improvements in
machining techniques and laboratory studies of the relation-
ships between surface finish and sealing effectiveness. Much
work remains to be done. We will continue to fo1 low progress
in these areas, including the measurement of surface finish.
30
OCR for page 31
Letter to the Honorable James C. Fletcher
Activities at Kennedy Space Center
-
/
As the design comes into final focus, our attention has
also turned to the activities at Kennedy Space Center, where
case segments are inspected, stored, and assembled into the
flight articles.
Inspection. Regarding nondestructive evaluation of the integ-
rity of case segments at KSC, previous practice has been to
inspect segments visually when they arrive. The inspections
are intended to detect flaws in bonds between the propellant
and inhibitors and between the insulation and case. The
segments are then stored in a vertical position until use.
During storage, the segments are subjected to varying condi-
tions of temperature and humidity and the case-to-insulation
bonds are subject to various stresses. Individual segments
can be stored for periods from months to years. In ballistic
missile programs, similar circumstances have lead to extensive
programs to control environmental conditions under which the
motors are stored and to determine the response of the com-
ponents to aging. We suggest that similar considerations
apply in the case of the Shuttle SRB. Further, we recommend
that motor segments be inspected again using more than simple
visual inspection when removed from Jong-term storage before
assembly.
Process Control. Environmental conditions may also affect the
.
assembly of the segments into boosters. The new design re-
quires that a critical bonding process be performed during the
assembly of field joints. While specific requirements are not
yet defined, control of temperature, humidity, and cleanliness
are essential for achieving reliable bonds. Other critical
processes, such as lubrication of the field joint O-rings, are
also performed in the Vehicle Assembly Building, emphasizing
the need for a higher level of cleanliness in the facility
than previously existed.
We recommend that the contemplated design and installation
of an environmental control system suitable for protecting
case segments during assembly be vigorously pursued. The
objective should be to insure the most complete environmental
control feasible. Requirements for enhanced cleanliness and
environmental control should be reviewed, established, and
implemented.
31
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Letter to the Honorable James C. Fletcher
Plans for Flight Evaluation and Product Improvement
- —8—
While we, like NASA, have been preoccupied primarily with
issues of redesign and testing, we continue to be concerned
with the evaluation of performance and improvement of the
product after flights resume. The 51-L accident demonstrated
the importance of conducting an on-going program designed both
to evaluate the reliability and performance of the booster and
its components in flight and to provide a basis for near-term
product improvement.
A flight evaluation plan should provide for a comprehen-
sive program of measurements in flight, inspection of
recovered motors, and expeditious assessment of results of
each flight before the next one. The verification plan
currently envisions collecting extensive performance data on
the first six flights of the new booster. But NASA must also
remain in position to identify weaknesses in the design,
manufacture, or assembly of the boosters in the long term so
that corrective actions can be taken if necessary. Collection
of performance data therefore should be continued until the
data base is sufficient to demonstrate the required reiiabil-
ity and answer outstanding questions about production control
and component functioning. Thereafter, surveillance data
should be gathered and the program should constantly be alert
to the need for taking additional flight data.
The flight evaluation program should also provide for
design, production, evaluation, and introduction of limited
modifications of the SRB in response to concerns of engineers
regarding reliability of flight performance or problems in
manufacturing, inspection, or assembly. The program should
provide for preplanned opportunities for changes in order to
introduce improvements required as a result of analysis of
flight performance data.
Finally, once again I would like to express our appreci-
ation of the willingness of the redesign team to discuss
technical details and plans with us. The team members are
dedicated professionals working hard to achieve their goals
and to keep us informed, too.
On behalf of the Panel, I remain
cc: Adm. Richard H. Truly
Panel Members
32
Sincerely,
H. Guyford Stever
Chairman
Representative terms from entire chapter:
redesign team
