****Report of the PRESIDENTIAL COMMISSIONon the Space Shuttle Challenger Accident****
**** ****
****Appendix C: Observations Concerningthe Processing and Assembly of Flight 51-L.****

 

 

[**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....

 

**[**221**]**
****
**Drawing depicts steps in the stackingof the aft and aft center segments of the Solid Rocket Booster in theVehicle Assembly Building (VAB).**
** **

....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)

.
- Circumferential            Location 
- 4-Point Lift    0145 hrs
- Initial 2-point            Lift    0305 hrs
- Intermediate 2-Point Lift                0354 hrs
- Final 2-Point Lift    0415 hrs
- Alignment Tool Installed            16°/196°    0925 hrs
- Alignment Tool            Removed    0945 hrs
.0° + .512+ .393+ .334+ .334+ .138+ .21630°+ .158+ .295+ .315+ .315N/A + .15860°- .334 - .236- .157- .157- .079- .11890°- .728- .571- .531- .531- .295- .334120°- .669- .571- .531- .531- .374- .393150°+ .0590+ .020+ .020- .39- .059
.

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
Field JointO-Ring Damage
.

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

.
MissionDifference (Inches)Location (Degrees)
.

51-C

Right Fwd
- .360120
51 - B

Right Aft
- .36090- .372120
51 - D

Right Fwd
- .3360
Left Aft
- .324120
Left Fwd
- .372120
51 - G

Right Aft
- .354120
51 - F

Right Center
- .3850- .433*150
51 - I

Left Center
- .3350
Right Aft
- .32730
61 - B

Left Center
- .334150
Right Center
- .473*120
61-C

Left Center
- .355150- .3540
Right Center
- .394*120
.

* 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**

- 1. NASA Pre-Launch   Activities Team Report, Appendix D, pages 214 and 215.    2. Ibid, pages   179-181.
  - 3. NASA Pre-Launch   Activities Team Report, Appendix I, page.      4. Morton Thiokol Inc,   SRM Steel Case Segment Use Record, April I, 1986.
    - 5. NASA Pre-Launch   Activities Team Report, Appendix B, pages 5-90 through   5-117.        6. Ibid, page 6-9.
      - 7. Ibid, pages 6-1   through 6-3.

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