[219] The following examples of Operational Maintenance Requirements and Specifications Document violations were noted during the Commission's inquiry:1
1. The Operational Maintenance Requirements and Specifications Document indicated that the External Tank liquid hydrogen and liquid oxygen ullage pressure control and redundancy verification using simulated transducers was a requirement for this processing. However, the entire sequence was marked "not performed" in the documentation, indicating that it had not been completed. Missing any of these steps has implications for safety of flight.
2. The three requirements that verify the main engine pneumatic isolation valve actuation were not met as specifically called for in the Operational Maintenance Requirements and Specifications Document. The intent of the requirement was met.
3. One requirement (main engine pneumatic isolation check valve individual follow-through test) was not met in the Operations & Maintenance Instructions. The main engine flight readiness tests gave assurance that at least one of two check valves per system was working.
4. A main engine pneumatic regulator functional test, which checks the redundancy of individual regulators, was not verified under flow conditions.
5. The results of helium pneumatic low pressure system decay check (with closing solenoids energized) exceeded the allowable limit. The decay rate was recorded as 0.98 pounds per square inch per minute; however, a recalculation of the data revealed that the decay rate was actually 1.4 pounds per square inch per minute. The calculated allowable decay rate was 1.35 pounds per square inch per minute maximum.
6. The leak check steps for test port Number 4, after installation of the plug, were inadvertently omitted from the Operations & Maintenance Instructions.
7. Main engine protective covers were not installed at times required. A revision to the requirement is needed.
8. Several requirements cannot be satisfied during a 24-hour launch scrub turnaround due to lack of access. A revision to the requirement is needed.
9. The humidity indicator inspection requirement was not met because the engines were not in the controlled environment with a trickle purge on. The requirement needs to be updated.
Representative samples were taken from the Orbiter processing paper. Of 121 Operations & Maintenance Instructions reviewed, 47 percent had paper errors. Incomplete, incorrect or missing data recording points were found in about 13 percent of the cases and 32 percent had Quality Control buy-off stamps missing.
Also reviewed were 479 Work Authorization Documents in the Interim Problem Report, Problem Report and Test Preparation Sheet categories. Of those documents, 70 percent had [220] anomalies, including inaccurate/inadequate level of detail (36 percent), missing stamps (24 percent), correct signatures not obtained (29 percent), and inaccurately detailed summary for closure or deferral (20 percent).
In addition to normal processing, there were 22 Modification Change Requests applicable to flight 51-L. Those requests generated 51 Work Authorization Documents, all of which were reviewed as part of the post-accident study of flight 51-L processing. Although not accident related, 96 percent of the Work Authorization Documents were found to have errors of an administrative or format nature. Those examples led to the conclusion that there was a pervasive lack of discipline and lack of proper training with respect to how Work Authorization Documents are written and implemented. 2
The same lack of completeness and accuracy was discovered in review of nearly all types of paperwork in the processing system. The amount of flawed paper work-approximately 50 percent-is unacceptable. There are several contributing factors, among them signature requirements that are lengthy and require people to travel long distances to accomplish, excessively long times required to close out paper, as compared with doing the actual work; lack of understanding of the paper system; a complicated tiered control and status trail for Quality Assurance personnel; and the fact that no single organization has the responsibility for final review for closure. Basically, the system is not simplified for the originator, performer, or verifier. Therefore, it is not a useful tool, which would be the only reason for its existence. Rather, it is an impediment to good work and good records.3
The work control documentation system is cumbersome and difficult to use. Consequently, the work force does not try very hard to use it. The result is that the real-time execution of tasks and their subsequent traceability suffer. The system needs to be simplified so that it becomes "user friendly." Once it is, the work force should be trained to use it and management should place proper emphasis on rigorous observance of the documentation requirements.
Flight 51-L Booster Processing
With Shuttle mission STS-6 in April 1983, NASA introduced the "lightweight" version of the Solid Rocket Booster, about 4,000 pounds lighter than its 185,000-pound (empty weight) predecessors. The weight reduction was achieved by shaving the thickness of each steel casing by two to four hundredths of an inch. On flight 51-L, all but the forward segments of the two boosters had lightweight casings.
There are 11 separate case components in each Solid Rocket Booster. Only two of the 22 components in the 51-L stack were new. The remaining 20 components had been used a combined total of 29 times previously, in ground tests and in flight.
The new components were the right forward center tang and the left forward dome. The right forward segment (Number 085) had been part of the flight 51-C (January 24-27, 1985) left forward field joint that had experienced O-ring erosion and deposited soot behind the primary O-ring. None of the other 51-L case segments had experienced O-ring problems on previous use.
Segment L-60, the right aft center tang component, had been flown on 41-D (August 30-September 5, 1984) as the left forward center tang component. Segment L-06, the right aft clevis component, had been flown on 51-C as the left aft clevis member. Segment L-06 had undergone another burn in addition to 51-C; it had been used as part of the left aft segment in a static test firing.4
The first of the eight motor segments for flight 51-L arrived by rail at Kennedy Space Center on October 11, 1985. The last reached Kennedy on November 4. The segments for 51-L were designated booster integration set BI026.
Grain inspection and offloading began on October 24. Stacking preliminaries for the left booster got under way on October 28 with the mating of the aft segment to the skirt that surrounds the nozzle. The stacking of the right booster began on December 4. During the stacking operation, which involves assembling the components of the Solid Rocket Booster one atop the other on the Mobile Launch Platform (MLP), a number of minor deviations and a few unusual situations were experienced. They were carefully reviewed by the NASA report team and by the Commission. With one possible exception, explained below, these incidents did not have significant impact on the performance of the Solid Rocket Boosters.
Before stacking of the right hand booster, measurements of the right aft center tang and the right aft clevis diameters indicated a potential for....
....stacking interference. Taken across the 0-180 degree axis, the tang diameter measurement exceeded the corresponding clevis dimension by +.512 inch. The maximum allowable tang to clevis difference is +.250 inch.
Normal Operations and Maintenance Instructions procedures were followed for bringing the out-of-round segment into allowable tolerances. While the right aft center segment was hanging from four points on a lifting beam, the first step was to adjust the lifting beam to create a two-point lift across the 90-270 degree axis. The weight of the segment itself would decrease the tang diameter across the 0-180 degree axis. This process reduced the excess measurement to + .334 inch, but it was still outside the allowable tolerance.
The next step in the procedure was to install the circumferential alignment tool. It was installed across the 16-196 degree axis and maximum allowable pressure of 1,200 pounds per square inch gauge was applied to the tool. This produced a further improvement, but again fell short of the measurement requirements. Additional deflection was obtained by turning the hex nut on the alignment tool. This caused the hydraulic pressure on the tool to increase to 1,300-1,500 pounds per square inch gauge, which exceeded the limit on the tool. The procedure produced a force of 3,254-3,766 pounds on the....
[222] Table 1
Right Aft Center Segment Tang to Aft Segment Clevis Diameter
Measurement Differentials Taken on December 7, 1985
(Positive is Tang Larger)
.
.
NOTE: Measurements to nearest .001
inch are approximate
....segment case, which was within manufacturer specifications. Although this procedure was at that time authorized by the Operations and Maintenance Instruction, it has since been deleted because the application of increased pressure on the alignment tool risks damage to the tool.
Following all of these procedures, measurement of the tang showed the differential between the tang and clevis along the 0-180 degree axis to be + .138 inch, which was considered suitable for mate. The right aft center segment was hoisted from the transfer aisle and lowered into position above the aft segment in the Vehicle Assembly Building high bay. The alignment tool was removed and final tang measurements showed a differential of + .216 inch, indicating mating was possible. Installation of both O-rings and successful stacking of the segments then took place without incident. No further problems were identified during engagement of the two segments. Table 1 shows the measurements taken at various stages of the entire procedure.5
The several sets of tang/clevis diametric measurements referred to in the foregoing discussion, and presented in Table 1, were reported by the stacking crews at Kennedy.
Two conspicuous aspects of the 51-L right aft field joint warrant comparison with joint history of earlier flights. Those aspects are the use of the circumferential alignment tool and the large tang-to-clevis negative diameter difference of - .393 inch along the 120-300-degree axis. However, the NASA Operations and Maintenance Instructions do not specify a limit to negative differences between tang and clevis.
The alignment tool had been used five times previously; its usage is shown in Table 2. 6
Table 2. Alignment Tool Use
History
.
Mission
.
51-B
Left Aft
None
51-F
Left Fwd
None
61-B
Left Aft
None
61-C (2 joints)
Left Aft
Erosion
Right Aft
None
Of the five field joints on which the alignment tool was used, one experienced erosion.
There were 13 Solid Rocket Booster joints on missions 51-C (January 1985) through 61-C (January 1986) that had negative differences greater than -.320 inch. Three of those joints had negative differences greater than the 51-L right aft field joint. None of those 13 earlier joints experienced O-ring damage. Table 3 indicates the joints and missions with negative differences greater than -.320 inch.7
[223] Table 3.
Negative Diameter Differences Greater Than .320 Inches for
Field Joints: STS 51-C Through 61-C
.
.
51-C
Right Fwd
51 - B
Right Aft
51 - D
Right Fwd
Left Aft
Left Fwd
51 - G
Right Aft
51 - F
Right Center
51 - I
Left Center
Right Aft
61 - B
Left Center
Right Center
61-C
Left Center
Right Center
.
* Negative diameter differences
greater than 51-L
It was found that the negative dimension differences on 51-L were not the most troublesome ever experienced and that a significant number of joints on other flights had initial negative differences in excess of the worst-case design clearance between the tang and the clevis. One significant uncertainty is the degree to which segments may tend to circularity after being mated.
The procedures used in mating the right side aft and aft center segments were carefully examined and appear normal, properly followed and executed by well-experienced personnel according to specifications.
The 51 -L joint negative diameter difference has been examined for the light it may shed on whether this discrepancy may have contributed to the fatal booster joint failure.
The large negative diameter difference indicates a potential for an interference between the tang and inner clevis leg that can lead to a flat on flat condition when the tang section is lowered into the clevis section on assembly.
Subscale test on sections of the full scale joint cross section were performed which purposely produced a flat on flat condition as these sector sections were forced together. Test results showed that metal slivers were sheared from the flats, and that these slivers could be pulled into the O-ring region during assembly.
However, a flat on flat condition probably did not exist on the STS 51-L lower joint. Past assembly practice has shown that if the difference of all diametrical readings of the mating halves is less than + .250 inches a flat on flat condition will not occur. Furthermore during the mating process the halves are brought slowly together with stacking personnel positioned around the joint. A potential for flat on flat is looked for during this critical period. It has been shown through experience that a flat on flat condition is readily apparent when viewing the mating section while the upper tang section is suspended just above the inner leg of the clevis. Thus both the physical measurements and assembly procedures make a flat on flat condition unlikely during assembly.
While the tang of the 51-L right aft center segment was burned through near the 300 degree arc point where the largest negative dimension occurred, this dimension was an assembly condition only and it is not certain that it persisted until launch. Examination of the STS 61 -E destacked segments subsequent to the 51-L accident indicated that their ovality had changed after assembly while awaiting launch.
If the very tight tang-to-clevis assembly gap did persist to time of launch, it could have resulted in near maximum compression of the O-rings. Such compression, in conjunction with cold temperatures, joint dynamics, and the variable performance of the insulating putty has been shown to have detrimental influences on the joint's ability to seal. Several joints on STS 51-L, however, may have had areas where the O-ring was at near maximum compression.
References
