****Report of the PRESIDENTIAL COMMISSIONon the Space Shuttle Challenger Accident****
**** ****
****Chapter VI: An Accident Rooted inHistory.****
**** ****

**Early Design**

[**120**] The Space Shuttle'sSolid Rocket Booster problem began with the faulty design of itsjoint and increased as both NASA and contractor management firstfailed to recognize it as a problem, then failed to fix it andfinally treated it as an acceptable flight risk.

Morton Thiokol, Inc., the contractor, did notaccept the implication of tests early in the program that the designhad a serious and unanticipated flaw.**1** NASA did not accept the judgment of its engineers thatthe design was unacceptable, and as the joint problems grew in numberand severity NASA minimized them in management briefings and reports.**2** Thiokol's stated position was that "the condition isnot desirable but is acceptable."**3**

Neither Thiokol nor NASA expected the rubberO-rings sealing the joints to be touched by hot gases of motorignition, much less to be partially burned. However, as tests andthen flights confirmed damage to the sealing rings, the reaction byboth NASA and Thiokol was to increase the amount of damage considered"acceptable." At no time did management either recommend a redesignof the joint or call for the Shuttle's grounding until the problemwas solved.

Thiokol was selected to receive the NASAcontract to design and build the Solid Rocket Boosters on November20, 1973.**4**The booster was the largest SolidRocket Motor ever produced in the United States; it was also thefirst solid motor program managed by NASA's Marshall Space FlightCenter in Huntsville, Alabama.

Costs were the primary concern of NASA'sselection board, particularly those incurred early in theprogram.

Thiokol's three competitors were Aerojet SolidPropulsion Co., Lockheed Propulsion Co. and United Technologies. TheSource Evaluation Board on the proposals rated Thiokol fourth underthe design, development and verification factor, second under themanufacturing, refurbishment and product support factor and firstunder the management factor.**5**

Thiokol received the second highest overallMission Suitability score, tied with UnitedTechnologies.**6**

In a December 12, 1973, report, NASA selectionofficials said Thiokol's "cost advantages were substantial andconsistent throughout all areas evaluated." **7** They also singled out Thiokol's joint design forspecial mention.

"The Thiokol motor case joints utilized dualO-rings and test ports between seals, enabling a simple leak checkwithout pressurizing the entire motor," the officials' report said."This innovative design feature increased reliability and decreasedoperations at the launch site, indicating good attention to low cost(design, development, testing and engineering) and production."**8**

"We noted that the [NASA Source Selection]board's analysis of cost factors indicated that Thiokol could do amore economical job than any of the other proposers in both thedevelopment and the production phases of the program; and that,accordingly, the cost per flight to be expected from a Thiokol-builtmotor would be the lowest," the officials said. "We, therefore,concluded that any selection other than Thiokol would give rise to anadditional cost of appreciable size." **9**

The Selection officials said they "found noother [**121**] factors bearing upon the selection that ranked inweight with the foregoing."

Cost consideration overrode any other-objections, they decided. We concluded that the main criticisms ofthe Thiokol proposal in the Mission Suitability evaluation weretechnical in nature, were readily correctable, and the costs tocorrect did not negate the sizable Thiokol cost advantage," theselection officials concluded.

The cost-plus-award-fee contract, estimated tobe worth $800 million, was awarded to Thiokol.

The design of the Shuttle Solid Rocket Boosterwas primarily based on the Air Force's Titan III solid rocket, one ofthe most reliable ever produced. Thiokol hoped to reduce new designproblems, speed up the development program and cut costs by borrowingfrom the Titan design. In Thiokol's Solid Rocket Motor proposal, therocket fuel is contained in four- forged steel cases which arestacked one on top of the other. The casings were connected by acircumferential tang and clevis, as were theTitans.**10**

Despite their many similarities, the ThiokolSolid Rocket Booster and the Titan motors had some significant designdifferences. For example, the joints of the Titan were designed sothat the insulation of one case fits tightly against the insulationof the adjacent case to form a more gastight fit than the Thiokoldesign. One O-ring bore seal was used in each Titan joint to stop anyhot gas pressure that might pass by the insulation overlap,**11** but in the Titan design the O-ring was able but notintended to take the brunt of the combustion pressure. In contrast,the Thiokol O-rings were designed to take the brunt of the combustionpressure, with no other gas barriers present except an insulatingputty. Also, the Solid Rocket Motor joint had two O-rings, the secondto provide a backup in case the primary seal failed.

Asbestos-filled putty was used in the SolidRocket Motor to pack the space between the two case segments toprevent O-ring damage from the heat of combustion gases.**12** Thiokol believed the putty was plastic, so when actedon by the combustion pressure at the motor's ignition the putty flowtowards the O-ring would compress the air in the gap between theputty and the primary O-ring.**13** The compressed air, in turn, would.....

** **

********
********
**Figure 1.**Comparison of Original Design to Design Used.****
** **

[**122**] ...cause theprimary O-ring to extrude into the gap between the clevis and thetang, behind the primary O-ring groove, thereby sealing the opening.If the primary O-ring did not seal, the intent was that the secondarywould pressurize and seal the joint by extruding into the gap behindits groove.**14**

Another difference in the Solid Rocket Motorand the Titan was that the tang portion of the Thiokol joint waslonger in order to accommodate two O-rings instead of one. It wasmore susceptible to bending under combustion pressure than the Titanjoint, as post-design tests and later flight experiencedemonstrated.**15**

The initial Thiokol design proposal waschanged before the production motors were manufactured. Originally,the joint seal design incorporated both a face seal and a boreseal.**16** (Figure1.) However, the motor that waseventually used had double bore O-rings. The original bore seal/faceseal design was chosen because it was anticipated that it "provides[better] redundance over a double bore ring seal since each iscontrolled by different manufacturing tolerances, and each respondsdifferently during joint assembly. " **17** Because the early design incorporated tolerancessimilar to the Titan and it also incorporated a face seal, Thiokolbelieved it possessed "complete, redundant seal capability."**18**

Nevertheless, as the Solid Rocket Motorprogram progressed, Thiokol-with NASA's concurrence-dropped theface/bore seal design for one using a double bore seal (Figure 1). NASA engineers at Marshall said the original designwould have required tapered pins to maintain necessary tolerances andassure enough"squeeze" on the face-sealingO-ring.**19** However, design analysis determined that motorignition would create tension loads on the joint sufficient to causethe tapered pins to pop out. Solving that would have meant designingsome type of pin-retainers. Moreover, the rocket assembly was mucheasier with the dual bore seals. Because inspections and tests had tobe conducted on the Solid Rocket Motor stack, horizontal assembly wasrequired. Thiokol engineer, Howard McIntosh, described this in aCommission interview on April 2, 1986:

> 
"We were concerned very much aboutthe horizontal assembly that we had to do to do the static tests. TheTitan had always been assembled vertically, and so there had neverbeen a larger rocket motor to our knowledge that was assembled(horizontally)".**20**

Because of the extremely tight tolerances inthe joints caused by horizontal assembly, McIntosh noted, "We . . .put the bore seals in there, and we opened the tolerance in the gapsslightly to accommodate that."**21** To tighten the joint's fit and to increase the squeezein the O-rings to compensate for the larger tolerances, Thiokolsubsequently put thin metal shims between the outer walls of the tangand clevis.

Another significant feature of the Thiokoldesign was a vent, or port, on the side of the motor case used afterassembly to check the sealing of the O-rings. As will be noted later,this leak check eventually became a significant aspect of the O-ringerosion phenomenon.**22**

The manufacture of the O-rings themselvesconstituted another difference between the Titan and the ThiokolSolid Rocket Motor. While both O-rings were Viton rubber, the TitanO-rings were molded in one piece. The Solid Rocket Motor O-rings weremade from sections of rubber O-ring material glued together. Thespecifications allowed five such joints, a number chosen arbitrarily,and the vendor routinely made repairs of voids and inclusions aftergetting the material supplies. Only surface inspections wereperformed by Thiokol and by the manufacturer.

Finally, unlike the Titan, the Thiokol SolidRocket Motor was designed for multiple firings. To reduce programcosts, each Thiokol motor case for the Shuttle was to be recoveredafter flight and reused up to 20 times.**23**

** **

**Early Tests**

Thiokol began testing the Solid Rocket Motorin the mid-1970's. One of the early important tests was a 1977"hydroburst test."**24**

Its purpose was to test the strength of thesteel cases by simulating a motor firing. The case was pressurizedwith water to about one and one-half times the pressure of an ignitedmotor (about 1,500 pounds per square inch) to make certain the casehad adequate structural margin.**25** Also, to measure the pressure between the O-rings,engineers attached instruments to the leak test port at a segmentjoint. Although the test was successful in that it demonstrated thecase met strength requirements, test measurements showed that,contrary to design expectations, the joint [**123**] tang and insideclevis bent away from each other instead of toward each other and bydoing so reduced-instead of increased-pressure on the 0-ring in themilliseconds after ignition.**26** This phenomenon was called "joint rotation."Testifying before the Commission, Arnold Thompson, Thiokol'ssupervisor of structures, said,

> 
"We discovered that the joint wasopening rather than closing as our original analysis had indicated,and in fact it was quite a bit. I think it was up to 52onethousandths of an inch at that time, to the primaryO-ring."**27**

Thiokol reported these initial test findingsto the NASA program office at Marshall. Thiokol engineers did notbelieve the test results really proved that "joint rotation" wouldcause significant problems,**28** and scheduledno additional tests for the specific purpose of confirming ordisproving the joint gap behavior.

** **

**Design Objections**

Reaction from Marshall to the early SolidRocket Motor test results was rapid and totally opposite ofThiokol's. In a September 2, 1977 memorandum, Glenn Eudy, Marshall'sChief Engineer of the Solid Rocket Motor Division, informed AlexMcCool, Director of the Structures and Propulsion Laboratory, thatthe assembly of a developmental motor provided early indications thatthe Thiokol design:

> 
"Allowed O-ring clearance.... Somepeople believe this design deficiency must be corrected by somemethod such as shimming and perhaps design modification to the casejoint for hardware which has not been final machined.... I personallybelieve that our first choice should be to correct the design in away that eliminates the possibility of O-ring clearance.... Sincethis is a very critical SRM issue, it is requested that theassignment results be compiled in such a manner as to permit reviewat the S&E Director's level as well as project manager."

After seeing the data from the September 1977hydroburst test, Marshall engineer Leon Ray submitted a reportentitled "Solid Rocket Motor Joint Leakage Study" dated October 21,1977. It characterizes "no change" in the Thiokol design as"unacceptable"-"tang can move outboard and cause excessive jointclearance resulting in seal leakage. Eccentric tang/clevis interfacecan cause O-ring extrusion when case is pressurized." Ray recommendeda "redesign of the tang and reduce tolerance on the clevis" as the"best option for a long-term fix." **29**

After Ray's 1977 report, John Q. Miller, chiefof the Solid Rocket Motor branch at Marshall, signed and sent amemorandum on January 9, 1978 to his superior, Glenn Eudy, describingthe problems evident in the Solid Rocket Motor joint seal. "We see novalid reason for not designing to accepted standards," the memo said,and it emphasized that proper sealing of the joint by use of shims tocreate necessary O-ring pressure was "mandatory to prevent hot gasleaks and resulting catastrophic failure."**30**

One year later, not having received a responseto his 1978 memo, Miller signed and forwarded a second memostrenuously objecting to Thiokol's Solid Rocket Motor joint sealdesign. This memo, dated January 19, 1979, opened with: "We find theThiokol position regarding design adequacy of the clevis joint to becompletely unacceptable...." **31** The memorandum made three principal objections toThiokol's joint design. The first was the "large sealing surface gapcreated by extensive tang/clevis relative movement." The memo saidthis movement, the so-called"joint rotation," caused the primaryO-ring to extrude into the gap, "forcing the seal to function in away which violates industry and government O-ring applicationpractices." **32** Moreover, joint rotation allowed the secondary O-ringto "become completely disengaged from its sealing surface on thetang." Finally, the memorandum noted that although Thiokol's contractrequired all high pressure case seals to be verifiable, "the clevisjoint secondary O-ring seal has been verified by tests to beunsatisfactory."**33** A copy of the second memorandum was sent to GeorgeHardy, then Solid Rocket Booster project manager at Marshall. Thiokolapparently did not receive copies of either Miller memorandum, and noreply from Eudy to Miller has been found.

The Commission has learned that Leon Rayactually authored the Miller memos to Eudy, although Miller signedthem and concurred in the objections raised.**34** During February, 1979, Ray also reported on a visit hemade to two O-ring manufacturers-the Precision Rubber ProductsCorporation at Lebanon Tennessee, and the Parker Seal Co. atLexington, Kentucky.**35** Eudy [**124**] accompanied Ray onthe Precision visit. The purpose of the trips was to give themanufacturers the data on the O-ring experiences at Thiokol and to"seek opinions regarding potential risks involved," Ray wrote in aFebruary 9, 1979, memo describing the visit. Officials at Precisiondid "voice concern for the design, stating that the Solid RocketMotor O-ring extrusion gap was larger than that covered by theirexperience," Ray reported. "Their first thought was that the O-ringwas being asked to perform beyond its intended design and that adifferent type of seal should be considered," Rayadded.**36**

During the Commission hearing on May 2, 1986,Ray was asked why the 1978 and 1979 memoranda were written:

> 
** **

**Mr. Ray**: Thereason they were written was as a result of test data that we had,and I have to go back to, I guess, a little bit further back in timethan these memos. When the joint was first designed, the analysisproduced by Thiokol says the joint would close, the extrusion gapwould actually close.

We had quite a debate about that until we dida test on the first couple of segments that we received from themanufacturer, which in fact showed that the joint did open. Later onwe did some tests with the structural test article, and this ismentioned in the memo as STA-1 [Structural Test Article].

At that time, we really nailed it down. We gotsome very accurate numbers on joint rotation, and we know for a factthat during these tests that, just what the memo says, the jointrotated. The primary O-ring was extruded up into the joint. Thesecondary O-ring did in fact detach from theseat.**37**

 

No records show Thiokol was informed of thevisits, and the O-ring design was not changed.

Thiokol's phase 1 certification review onMarch 23, 1979, mentioned leak check failures, and forces during casejoint assembly that resulted in clevis O-ring grooves not conformingwith tang sealing surfaces. However, this was not listed as a problemor a failure.**38**

** **

**Verification and CertificationCommittee**

While Ray was warning of problems with jointrotation, static motor tests in July 1978 and April 1980 again weredemonstrating that inner tang/clevis relative movement was greaterthan originally predicted.**39** Thiokol continued to question the validity of thesejoint rotation measurements and their effect on the availability ofthe secondary O-ring.

In 1980, NASA empanelled a Space ShuttleVerification/Certification Committee to study the flight worthinessof the entire Shuttle system. A subdivision of that group, thePropulsion Committee, met with NASA Solid Rocket Motor programpersonnel and raised several concerns about the jointdesign.**40** The Committee pointed out that the booster's leak testpressurized the primary O-ring in the wrong direction so that themotor ignition would have to move the ring across its groove beforeit sealed. The Committee added that the effect of the insulationputty was not certain. Redundancy of the O-rings was also listed as averification concern. The same report, however, said "the Committeeunderstands from a telecon that the primary purpose of the secondO-ring is to test the primary and that redundancy is not arequirement." George Hardy testified that the Committee's statementconflicted with his understanding:

> 
"The discussion there or thereference there to a telecon-and I don't know who that was with-thatimplies there was no intent for the joint to be redundant is totallyforeign to me. I don't know where they would have gotten thatinformation because that was the design requirement for the joint."**41**

In May 1980, the Verification/CertificationCommittee recommended that NASA conduct full-scale tests to verifythe field joint integrity, including firing motors at a mean bulkpropellant temperature range of 40-90 degrees Fahrenheit. The panelalso asked NASA to:

> 
"Perform case burst test with oneO-ring removed. During the burst test for final verification of themotor case safety factor, one of the two O-rings failed by extrusionand leaked. The analysis used for additional verification did notinclude further gap openings caused by joint deflection atpressurization or any deflections caused by bending loads. The panelconsiders the above to be inadequate to provide operational programreliability, and marginal to provide adequate [**125**] safety factorconfidence on [Shuttle flight] one." **42**

The NASA program response to these issues wasincluded in the final Committee report in September 1980. It saidthat the original hydroburst tests and the lightweight case tests,being conducted at the time, satisfied the intent of the Committee'srecommendations. Moreover, the response stated: "NASA specialistshave reviewed the field joint design, updated with larger O-rings andthicker shims and found the safety factors to be adequate for thecurrent design. Re-analysis of the joint with larger O-rings andthicker shims is being accomplished as part of the lightweight caseprogram.... The joint has been sufficiently verified with the testingaccomplished to date (joint lab tests, structural test article, andseven static firings and the two case configuration burst tests) andcurrently scheduled for lightweight caseprogram."**43**

** **

**Criticality Classification andChanges**

The Solid Rocket Motor certification wasdeemed satisfactory by the Propulsion Committee of theVerification/Certification Group on September 15, 1980. Shortlythereafter, on November 24, 1980, the Solid Rocket Booster joint wasclassified on the Solid Rocket Booster Critical Items List ascriticality category 1 R. NASA defines "Criticality 1R" as anysubsystem of the Shuttle that contains "redundant hardware, totalelement failure of which could cause loss of life orvehicle."**44** The use of "R", representing redundancy, meant thatNASA believed the secondary O-ring would pressurize and seal if theprimary O-ring did not. Nonetheless, the 1980 Critical Items List(CIL) states:

> 
"Redundancy of the secondary fieldjoint seal cannot be verified after motor case pressure reachesapproximately 40 percent of maximum expected operating pressure. Itis known that joint rotation occurring at this pressure level with aresulting enlarged extrusion gap causes the secondary O-ring to losecompression as a seal. It is not known if the secondary O-ring wouldsuccessfully reseal if the primary O-ring should fail after motorcase pressure reaches or exceeds 40 percent of maximum expectedoperating pressure."

When asked about the text of the 1980Criticality 1R classification, Arnold Aldrich, NASA Manager of theNational Space Transportation System, said,

> 
"The way that . . . language[reads], I would call it [criticality] 1."**45**

Notwithstanding this apparent contradiction inthe classification 1R and the questionable status of the secondarydescribed in the text of the CIL, the joint carried a 1 Rclassification from November 1980 through the flight of STS-5(November 1982).

The Space Shuttle first flew on April 12-14,1981. After the second flight, STS-2, in November 1981, inspectionrevealed the first in-flight erosion of the primaryO-ring.**46** It occurred in the right Solid Rocket Booster's aftfield joint and was caused by hot motor gases.**47** The damage to the ring proved to be the worst everfound on a primary O-ring in a field joint on any recovered SolidRocket Booster.**48** Post-flight examination found an erosion depth of .053inches on the primary O-ring; nonetheless, the anomaly was notreported in the Level I Flight Readiness Review for STS-3 held onMarch 9, 1982. Furthermore, in 1982 the STS-2 O-ring erosion was notreported on the Marshall problem assessment system and given atracking number as were other flight anomalies.**49**

In mid- 1982, two significant developmentstook place. Because Thiokol believed blow holes in the insulatingputty were a cause of the erosion on STS-2, **50** they began tests of the method of putty layup and theeffect of the assembly of the rocket stages on the integrity of theputty. The manufacturer of the original putty, Fuller-O'Brien,discontinued the product and a new putty, from the Randolph ProductsCompany, was tested and selected in May 1982.**51** The new Randolph putty was eventually substituted forthe old putty in the summer of 1983, for the STS-8 Solid Rocket Motorflow.**52**

A second major event regarding the joint sealoccurred in the summer of 1982. As noted before, in 1977-78, Leon Rayhad concluded that joint rotation caused the loss of the secondaryO-ring as a backup seal. Because of May 1982 high pressure O-ringtests and tests of the new lightweight motor case, Marshallmanagement [**126**] finally accepted the conclusion that the secondaryO-ring was no longer functional after the joints rotated when theSolid Rocket Motor reached 40 percent of its maximum expectedoperating pressure. It obviously followed that the dual O-rings werenot a completely redundant system, so the Criticality 1R had to bechanged to Criticality 1.**53** This was done at Marshall on December 17, 1982. Therevised Critical Items List read (See pages 157 and 158):

> 
"Criticality Category 1.

"Failure Mode and Causes: Leakage at caseassembly joints due to redundant O-ring seal failures or primary sealand leak check port O-ring failure.

*"Note. Leakage of the primary O-ring sealis classified as a single-failure point due to possibility of loss ofsealing at the secondary O-ring because of joint rotation after motorpressurization.*

"Failure Effect Summary: Actual Loss-*Loss of mission, vehicle and crew dueto metal erosion, burn through, and probable case burst resulting infire and deflagration. .*

"Rationale for Retention:

"The Solid Rocket Motor case joint design iscommon in the lightweight and regular weight cases having identicaldimensions. The joint concept is basically the same as the singleO-ring joint successfully employed on the Titan III Solid RocketMotor.... On the Shuttle Solid Rocket Motor, the secondary O-ring wasdesigned to provide redundancy and to permit a leak check, ensuringproper installation of the O-rings. Full redundancy exists at themoment of initial pressurization. However, test data shows that aphenomenon called joint rotation occurs as the pressure rises,opening up the O-ring extrusion gap and permitting the energized ringto protrude into the gap. This condition has been shown by test to bewell within that required for safe primary O-ring sealing. This gapmay, however, in some cases, increase sufficiently to cause theunenergized secondary O-ring to lose compression, raising question asto its ability to energize and seal if called upon to do so byprimary seal failure. Since, under this latter condition only thesingle O-ring is sealing, a rationale for retention is provided forthe simplex mode where only one O-ring is acting" [emphasis added] .**54**

The retention rationale for the "simplex" orsingle O-ring seal was written on December 1, 1982, by HowardMcIntosh, a Thiokol engineer.**55** This document gave the justification for flight withthe single functional O-ring. It reported that tests showed theThiokol design should be retained, citing the Titan history, the leakand hydroburst tests, and static motor firings as justification.However, it also contained the following rationale which appeared toconflict with the Criticality 1 classification that the secondaryO-ring was not redundant:

> 
"Initial information generated ina lightweight cylinder-to-cylinder proof test shows a total movementof only .030 inch at pounds per square inch, gauge pressure in thecenter joint. This . . . indicates that the tang-to-clevis movementwill not unseat the secondary O-ring at operatingpressures."**56**

Testimony in hearings and statements given inCommission interviews support the view that NASA management andThiokol still considered the joint to be a redundant seal even afterthe change from Criticality 1R to 1. For example, McIntosh'sinterview states:

> 
** **

**Question**:[After the Criticality I classification], what did you think it wouldtake to make [the joint seal] 1R?

**Mr. McIntosh**: I thought it was already 1R. I thought that afterthose tests that would have been enough to do it.

**Question**:Well, you knew it was 1 but you were hoping for 1R?

**Mr McIntosh**:Yeah, I was hoping for 1R, and I thought this test data would do it,but it didn't.**57**

 

At the time (in 1982-83), the redundancy ofthe secondary O-ring was analyzed in terms of joint or hardwaregeometry, with no consideration being given to the resiliency of thering as affected by temperatures.**58** Moreover, Marshall engineers like Ray and Millerdisagreed with Thiokol's calculations on the measurement of jointopening.**59** That engineering debate eventually went to a "referee"for testing which was not concluded until after the 51-Laccident.

[**127**] Notwithstanding theview of some of Marshall engineers that the secondary ring was notredundant, even at the time of the Criticality revision, MarshallSolid Rocket Motor program management appeared to believe the sealwas redundant in all but exceptional cases. Dr. Judson Lovingood toldthe Commission:

> 
" . . . [T]here are two conditionsyou have to have before you don't have redundancy. One of them iswhat I call a spatial condition which says that the dimensionaltolerances have to be such that you get a bad stackup, you don't haveproper squeeze, etc. On the O-ring so that when you get jointrotation, you will lift the metal surfaces off the O-ring. All right,that's the one condition, and that is a worst case conditioninvolving dimensional tolerances.

"The other condition is a temporal conditionwhich says that you have to be past a point of joint rotation, and ofcourse, that relates back to what I just said.

"So first of all, if you don't have this badstackup, then you have full redundancy. Now, secondly, if you do havethe bad stackup, you had redundancy during the ignition transient upto the 170 millisecond point, whatever it is, but that is the way Iunderstand the [Critical Items List]."**60**

George Hardy and Lawrence Mulloy sharedLovingood's view that the secondary seal was redundant in all butsituations of worst case tolerances.**61** However, there is no mention of this caveat in theCritical Items List itself, nor does it appear in the subsequent"waiver" of the Criticality 1 status granted by NASA Levels I and IIin March, 1983.**62** This waiver was approved to avoid the obligationsimposed on the Shuttle Program by Paragraph 2.8 of the Space ShuttleProgram Requirements Document, Level I, dated June 30, 1977. Thatparagraph states:

> 
"The redundancy requirements forall flight vehicle subsystems (except primary structure, thermalprotection system, and pressure vessels) shall be established on anindividual subsystems basis, but shall not be less than fail-safe.'Fail-safe' is defined as the ability to sustain a failure and retainthe capability to successfully terminate the mission. Redundantsystems shall be designed so that their operational status can beverified during ground turnaround and to the maximum extent possiblewhile in flight." **63**

Glynn Lunney, the former manager of the STSProgram (Level II at JSC) described the Criticality 1 change andresulting waiver to the Commission on May 2:

** **

> 
**Mr. Lunney**: Well, the approval of the waiver in March of 83, atthe time I was involved in that. I was operating on the assumptionthat there really would be redundancy most of the time except whenthe secondary O-ring had a set of dimensional tolerances add up, andin that extreme case there would not be a secondary seal.

So I was dealing with what I thought was acase where there were two seals unless the dimensional toleranceswere such that there might only be one seal in certain cases.

**Chairman Rogers**: Now, to me, if you will excuse the expression, thatsounds almost contradictory, what you just said. What you first saidwas you came to the conclusion that you could only rely on theprimary seal and therefore you removed the R.

**Mr. Lunney**:Yes, sir.

**Chairman Rogers**: And now you're saying, if I understand it, thatexperience showed that there was redundancy after all.

**Mr. Lunney**:No, I don't know of any experience showing that. What I'm saying isthat the removal of the R is an indicator that under allcircumstances we did not have redundancy. There were a certain numberof cases under which we would not have redundancy of the secondaryO-ring.

Recognizing that, even though there were a lotof cases where we expected we would have redundancy we changed thecriticality designation.

**Chairman Rogers**: It was saying to everybody else you can't necessarilyrely on the primary seal, and if the primary seal fails, as you'vesaid here, there may be loss of vehicle, mission and crew.

**Mr. Lunney**:I would adjust that to only say you cannot rely on the secondaryO-ring [**128**] but we would expect the primary O-ring to always bethere.**64**

 

The criticality waiver was processed outsidethe formal NASA Program Requirements Control Board, however,representatives of that group "signed off" on thedocument.**65** It was forwarded to Level I and approved by AssociateAdministrator for Space Flight (Technical), L. Michael Weeks on March28, 1983. Weeks told the Commission he signed the waiver because ofthe Certification/Verification Review of the Propulsion Committee in1980. Weeks explained, "We felt at the time-all of the people in theprogram I think felt that this Solid Rocket Motor in particular orthe Solid Rocket Booster was probably one of the least worrisomethings we had in the program." **66** The waiver was signed less than one week prior to thelaunch of STS-6 on April 4. According to interviews of Arnold Aldrichand of Richard Kohrs, the latter having been involved with the waiverreview at Johnson Level II, the waiver was approved so that STS-6could fly.**67** However, Weeks denied any connection between the LevelI waiver approval and the flight of STS-6.**68**

Although some Thiokol engineers and officialsclaimed that they had no notice of the Criticality change and waiverin December, 1982 and in March, 1983, from the approval signatures(including Thiokol's Operations Manager at Marshall, Maurice Parker)and the distribution of the Criticality and Waiver documents,apparently Thiokol officials were sent copies and were involved inthe criticality reclassification. **69** Nonetheless, the Commission has also determined thatseveral documents tracking the O-ring erosion at Thiokol and Marshallrefer to the Solid Rocket Motor field joint seal as Criticality 1-R,long after the status was changed to Criticality 1.**70**

** **

**STS 41-B O-Ring Erosion**

As Figure 2shows,**71** prior to STS 41-B, the O-ring erosion/blow-by problemwas infrequent, occurring on a field joint of STS-2 (November, 1981),nozzles of STS-6 (April, 1983) and a nozzle of QM-4 (March, 1983), aqualification test motor fired by Thiokol.**72** However, when STS 41-B flew on February 3, 1984, theleft Solid Rocket Booster forward field joint and the right nozzlejoint primary O-rings both suffered erosion damage. Thiokol engineersreacted to this discovery by filing a problem report on the O-ringerosion found on STS 41 -B. Thiokol presented a series of charts tothe Marshall Solid Rocket Booster Engineering Office about the 41-BO-ring erosion. Thiokol told Marshall that recent joint rotationmeasurements in tests indicated the secondary O-ring will not unseat,providing confidence that the secondary was an adequate backup. KeithCoates described his view about Thiokol's data in a February 29, 1984memorandum to George Hardy:

> 
"We have two problems with theirrationale. The effect of 0.065 inch erosion on O-ring sealingcapability is not addressed. We have asked Thiokol to provide theirdata to justify their confidence in the degraded O-ring. The secondconcern is the amount of joint rotation. L. Ray does not agree withThiokol numbers, and he has action to discuss his concern with R.Boisjoly (Thiokol) and reach agreement.

"Thiokol definition of their plans onresolution of the problem is very weak."

The erosion problem was identified and trackedby the Marshall Problem Assessment System as Marshall Record A07934and by Thiokol as Thiokol Contractor Record DR4-5/30, "Slight charcondition on primary O-ring seal in forward field joint on SRM A57 ofSTS-11 flight, Mission 41B." **73** The Marshall Problem Assessment System Reportstates:

> 
"Remedial action-none required;problem occurred during flight. The primary O-ring seal in theforward field joint exhibited a charred area approximately 1 inchlong .03-.050 inches deep and .100 inches wide. This was discoveredduring post-flight segment disassembly at KSC."

A March 8, 1984 entry on the same reportcontinues:

> 
"Possibility exists for someO-ring erosion on future flights. Analysis indicates max erosionpossible is .090 inches according to Flight Readiness Review findingsfor STS-13. Laboratory test shows sealing integrity at 3,000 psiusing an O-ring with simulated erosion depth of .095 inchesTherefore, this is not a constraint to future launches."**74**

**** ****
********
**[**129-131**] Figure 2.**O-Ring Anomalies Compared with JointTemperature and Leak Check Pressure**.**

 

**Flight or         MotorDate(Solid Rocket         Booster)Joint/O-RingPressure         (psi)ErosionBlow-byJoint Temp         °FFieldNozzle
.

DM-1

07/18/77

-

-

NA

NA

-

-

84

DM-2

01/18/78

-

-

NA

NA

-

-

49

DM-3

10/19/78

-

-

NA

NA

-

-

61

DM-4

02/17/79

-

-

NA

NA

-

-

40

QM-1

07/13/79

-

-

NA

NA

-

-

83

QM-2

09/27/79

-

-

NA

NA

-

-

67

QM-3

02/13/80

-

-

NA

NA

-

-

45

STS-1

04/12/81

-

-

50

50

-

-

66

STS-2

11/12/81

(Right)

Aft Field/Primary

50

50

X

-

70

STS-3

03/22/81

-

-

50

50

NA

NA

80

STS-4

06/27/82

Unknown: hardware lost at         sea

50

50

NA

NA

80

DM-5

10/21/82

-

-

NA

NA

-

-

58

STS-5

11/11/82

-

-

50

50

-

-

68

QM-4

03/21/83

-

Nozzle/Primary

NA

NA

X

-

60

STS-6

04/04/83

(Right)

Nozzle/Primary

50

50

(1)

-

67

(Left)

Nozzle/Primary

50

50

(1)

-

67

STS-7

06/18/83

-

-

50

50

-

-

72

STS-8

08/30/83

-

-

100

50

-

-

73

STS-9

12/28/83

-

-

1002

100

-

-

70

STS 41-B

02/03/84

(Right)

Nozzle/Primary

200

100

X

-

57

(Left)

Forward         Field/Primary

200

100

X

-

57

STS 41-C

04/06/84

(Right)

Nozle/Primary

200

100

X

-

63

(Left)

Aft Field/Primary

200

100

(3)

-

63

(Right)

Igniter/Primary

NA

NA

-

X

63

STS 41-D

08/30/84

(Right)

Forward         Field/Primary

200

100

X

-

70

(Left)

Nozzle/Primary

200

100

X

X

70

(Right)

Igniter/Primary

NA

NA

-

X

70

STS 41-G

10/05/84

-

-

200

100

-

-

67

DM-6

10/25/84

-

Inner Gasket/Primary

NA

NA

X

X

52

STS 51-A

11/08/84

-

-

200

100

-

-

67

STS 51-C

01/24/85

(Right)

Center Field/Primary

200

100

X

X

53

(Right)

Center         Field/Secondary

200

100

(4)

-

53

(Right)

Nozzle/Primary

200

100

-

X

53

(Left)

Forward         Field/Primary

200

100

X

X

53

(Left)

Nozzle/Primary

200

100

-

X

53

STS 51-D

04/12/85

(Right)

Nozzle/Primary

200

200

X

-

67

(Right)

Igniter/Primary

NA

NA

-

X

67

(Left)

Nozzle/Primary

200

200

X

-

67

(Left)

Igniter/Primary

NA

NA

-

X

67

STS 51-B

04/29/85

(Right)

Nozzle/Primary

200

100

X

-

75

(Left)

Nozzle/Primary

200

100

X

X

75

(Left)

Nozzle/Primary

200

100

X

-

75

DM-7

05/09/85

.

Nozzle/Primary

NA

NA

X

-

61

STS 51-G

06/17/85

(Right)

Nozzle/Primary

200

200

X5

X

70

(Left)

Nozzle/Primary

200

200

X

X

70

(Left)

Igniter/Primary

NA

NA

-

X

70

STS 51-F

07/29/85

(Right)

Nozzle/Primary

200

200

(6)

-

81

STS 51-I

08/27/85

(Left)

Nozzle/Primary

200

200

X7

-

76

STS 51-J

10/03/85

-

200

200

-

-

79

STS 61-A

10/30/85

(Right)

Nozzle/Primary

200

200

X

-

75

(Left)

Aft Field/Primary

200

200

-

X

75

(Left)

Center Field/Primary

200

200

-

X

75

STS 61-B

11/26/85

(Right)

Nozzle/Primary

200

200

X

-

76

(Left)

Nozzle/Primary

200

200

X

X

76

STS 61-C

01/12/86

(Right)

Nozzle/Primary

200

200

X

-

58

(Left)

Aft Field/Primary

200

200

X

-

58

(Left)

Nozzle/Primary

200

200

-

X

58

STS 51-L

01/28/86

.

.

200

200

.

.

31
**
- Dash (-) denotes no   anomaly.    NA denotes not   applicable.

NOTE: A list of the sequence oflaunches (1-25), identified by STS mission designation, is providedon pages 4 thru 6.

- 1  On STS-6, both nozzles had a hot gas path   detected in the putty with an indication of heat on the primary   O-ring.    2
   On STS-9, one of the right Solid Rocket   Booster field joints was pressurized at 200 psi after a   destack.
  - 3  On STS 41-C, left aft field had a hot gas   path detected in the putty with an indication of heat on the   primary O-ring.      4
     On a center field joint of STS 51-C, soot   was blown by the primary and there was a heat effect on the   secondary.
    - 5  On STS 51-G, right nozzle has erosion in   two places on the primary O-ring.        6
       On STS 51-F, right nozzle had hot gas   path detected in putty with an indication of heat on the primary   O-ring.
      - 7  On STS 51-I, left nozzle had erosion in   two places on the primary O-ring.

****

 

[**132**] This last entry isalso a summary of the briefing given by Thiokol to Lawrence Mulloyabout the 41-B erosion at the Level III Flight Readiness Review forSTS 41-C held at Marshall on March 8, 1984. At that same briefing,the Chief Engineer for United Space Boosters, George Morefield,raised prior Titan experience with O-ring problems. He explained in amemorandum to Mulloy the following day:

> 
"I alluded to the Titan III SRMhistory which is quite similar to the current STS Solid Rocket Motorexperience. Post-fire inspection of Titan Solid Rocket Motor statictest motors showed that pressurization of the single O-rings in thepressure vessel routinely occurred via a single break-down pathacross the joint putty. There was also evidence that some O-ringsnever see pressure in the Titan motor. The segment -to-segment caseinsulation design results in a compression butt joint whichapparently is often sufficient to withstand Pc, ....

"Your review showed that there was sufficientmargin of O-ring remaining to do the job. I'm sure you haveconsidered that if it does burn through, the secondary O-ring willthen be similarly pressurized through a single port. So, some concernremains.

"I recommend that you set up a panel to studythe use of putty and consider some alternatives:

"1) Is putty needed at all?

"2) If the tradition can't be broken, can theputty be applied with multiple (6 or 8) pressurization paths builtin?

"I think that the primary seal should beallowed to work in its classical design mode. Both the Titan and STSSolid Rocket Motors have been designed for this not to happen. Titanhas flown over a thousand pressure joints with no failure. My opinionis that the potential for failure of the joint is higher for the STSSolid Rocket Motor, especially when occasionally the secondary sealmay not be totally effective." **75**

When the 41-B erosion was taken to the Level IFlight Readiness Review for 41-C on March 30, 1984, it was briefed asa"technical issue". A recommendation to fly 41-C was approved byLevel I "accepting the possibility of some O-ring erosion due to thehot gas impingement." **76** The rationale for acceptance was the same as thatgiven at the Level III Flight Readiness Review and entered into theMarshall problem assessment report. An outgrowth of this review wasan April 5, 1984, directive from NASA Deputy Administrator Dr. HansMark to Lawrence Mulloy at Marshall. This "Programmatic Action Item"was signed by Weeks and asked Mulloy to conduct a "formal review of'the Solid Rocket Motor case-to-case and case-to-nozzle joint sealingprocedures to ensure satisfactory consistent closeouts."**77** This action item had been preceded by a letter writtenfrom NASA Associate Administrator for Space Flight General Abrahamsonto Marshall Center Director Lucas.**78** That letter, sent January 18, 1984, requested thatMarshall develop a plan of action to make improvement in NASA'sability to design, manufacture and fly Solid Rocket Motors.Abrahamson pointed out that NASA was flying motors where basic designand test results were not well understood. The letter addressed theoverall general Solid Rocket Motor design but did not specificallymention O-ring erosion.

After Mulloy received the April 5, 1984 STS41-C action item on the O-rings, he had Lawrence Wear for-ward aletter- to Thiokol which asked for a formal review of' the boosterfield joint and nozzle joint sealing procedures. Thiokol was toidentify the cause of the erosion, determine whether it wasacceptable. define necessary changes, and reevaluate the putty thenin use. The Wear letter also requested small motor tests reflectingjoint dynamics as well as analysis of the booster assemblyprocess.**79**

Thiokol replied to the Marshall STS 41-Caction item on May 4, 1984, with a program plan entitled "Protectionof' SRM Primary Motor Seals." The plan was prepared by Brian Russell,then Thiokol's Manager of Systems Engineering. It outlined asystematic program to isolate the 0-ring erosion and charring problemand to eliminate damage to the joint seals. **80** Proposed areas of inquiry included the leak checkpressures, assembly loads, case eccentricity and putty layup. TheThiokol response in May 1984 was merely a proposal. The actual finalresponse to the directive from Marshall was not completed until theAugust 19, 1985 briefing on the Solid Rocket Motor seal held at NASAheadquarters some 15 months later. **81**

** **

******[**133**]******
********
**Figure 3. Graphs depict flight anomalyfrequency for both field and nozzle joint of solid motors for avariety of leak check pressures.******
**** ****

**Leak Check and Putty**

In addition to the action item from NASAHeadquarters, another result of the 41-B erosion was a warningwritten by John Q. Miller, Marshall chief of the solid motor branch,to George Hardy, through Keith Coates.**82** Miller was worried about the two charred rings on 41-Band the "missing putty" found when the Solid Rocket Boosters wererecovered and disassembled. He specifically identified the putty'ssensitivity to humidity and temperature as potential sources ofproblems. "The thermal design of the [Solid Rocket Motor] jointsdepends on thermal protection of the O-ring by the [putty]," Millersaid. Failure of the putty to "provide a thermal barrier can lead toburning both O-rings and subsequent catastrophic failure." Thememorandum also said that "the O-ring leak check procedure and itspotential effect on the (putty) installation and possibledisplacement is also an urgent concern which requires expedition ofpreviously identified full scale tests."

From the beginning, Thiokol had suspected theputty was a contributing factor in O-ring erosion, even afterSTS-2.**83** In April 1983, Thiokol reported on tests conducted tostudy the behavior of the joint putty. One conclusion of the reportwas that the STS-2 erosion was probably caused by blow holes in theputty, which allowed a jet of hot gas to focus on a point on theprimary O-ring. Thiokol discovered the focused jet ate away or"impinged" on portions of the O-ring. Thiokol calculated that themaximum possible impingement erosion was .090 inch, and that lab testproved that an O-ring would seal at 3,000 psi when erosion of .095inches was simulated. This "safety margin" was the basis forapproving Shuttle flights while accepting the possibility of O-ringerosion. **84**

Shortly after Miller's routing slip to Hardyabout the "urgent concern" of the missing putty on 41-B, at Thiokol,Brian Russell authored a letter to Robert Ebeling which analyzed theerosion history and the test data. Russell's April 9, 1984 conclusionwas that the putty itself and its layup were not at fault but thatthe higher stabilization pressure adopted in leak check procedures,first implemented in one field joint on STS-9, may increase thechances of O-ring erosion. The conclusion by Miller and Russell wasthat the air pressure forced through the joint during the O-ring leakcheck was creating more putty blow holes, allowing more focused jetson the primary O-ring, thereby increasing the frequency oferosion.**85**

This hypothesis that O-ring erosion is relatedto putty blow holes is substantiated by the leak check history(Figure 3). Prior to January, 1984, and STS 41-B, when the leakcheck pressure was [**134**] 50 or 100 psi, only one field joint O-ring anomalyhad been found during the first nine flights. However, when the leakcheck stabilization pressure was officially boosted to 200 psi forSTS 41-B, over half the Shuttle missions experienced field jointO-ring blow-by or erosion of some kind. **86**

Moreover, the nozzle O-ring history ofproblems is similar. The nozzle joint leak check was changed from 50psi to 100 psi before STS-9 launched in November 1983. After thischange, the incidence of O-ring anomalies in the nozzle jointincreased from 12 percent to 56 percent of all Shuttle flights. Thenozzle pressure was increased to 200 psi for mission 51-D in April,1985, and 51-G in June, 1985, and all subsequent missions. Followingthe implementation of the 200 psi check on the nozzle, 88 percent ofall flights experienced erosion or blow-by. **87**

Both Thiokol and NASA witnesses agreed thatthey were aware that the increase in blow holes in the putty couldcontribute to O-ring erosion. The Commission testimony of May 2,1986, reads:

** **

> 
**Dr. Walker**: The analysis that some of our staff has done suggeststhat after you increase the test pressure to 200 pounds, theincidence of blow-by and erosion actually increased.

**Mr. Russell**:We realized that.

 

Lawrence Mulloy was also questioned above theblow holes in the putty:

** **

> 
**Dr. Walker**: Do you agree that the primary cause of the erosion isthe blow holes in the putty?

**Mr. Mulloy**:I believe it is. Yes.

**Dr. Walker**:And so your leak check procedure created blow holes in theputty?

**Mr. Mulloy**:That is one cause of blow holes in the putty.

**Dr. Walker**:But in other words, your leak check procedure could indeed cause whatwas your primary problem. Didn't that concern you?

**Mr. Mulloy**:Yes, sir. **88**

 

Notwithstanding the knowledge that putty blowholes caused erosion and that higher pressure in the leak checkcaused more blow holes, Thiokol recommended and NASA accepted theincreased pressure to ensure that the joint actually passed theintegrity tests.**89**

The documentary evidence produced by NASA andThiokol demonstrates that Marshall was very concerned about the puttyerosion/blow hole problem after STS 41-B. In addition to JohnMiller's routing slip about putty on STS 41-B discussed above, thereis a report of a June 7, 1984, telephone conference between Messrs.Thompson, Coates and Ray (Marshall) and Messrs. Sayer, Boisjoly,Russell and Parker (Thiokol), among others.**90** Marshall told Thiokol that NASA was very concernedabout the O-ring erosion problem and that design changes werenecessary, including possible putty changes. The Thiokol engineersdiscussed Marshall's suggestions after the telephone conference, butdecided they could not agree a change was mandatory. A follow-uptelephone conference was held between Ben Powers of Marshall andLawrence Sayer of Thiokol on July 2. Powers told Saver that NASAwould not accept the removal of the putty from the joint and thateveryone expected the tests to show that gas jets would damage anO-ring. However, Powers expressly stated that Marshall would notaccept Thiokol's opinion that no further tests were necessary.

In mid-1984, the early tests after NASA'saction item for 41-C led Thiokol to the conclusion that O-ringerosion was a function of the putty blow hole size and the amount offree volume between the putty orifice and the O-ring. The damage tothe O-ring was judged to be worse when the blow hole was smaller andthe free volume was larger.**91**

While Thiokol did establish plans for puttytests to determine how it was affected by the leak check in responseto the 41-C action item, their progress in completing the tests wasslow. The action item was supposed to be completed by May 30, 1984,but as late as March 6, 1985, there are Marshall internal memos thatcomplain that Thiokol had not taken any action on Marshall's December1983 directive to provide data on putty behavior as affected by thejoint leak check stabilization pressure.**92**

** **

**STS 51-C and Cold Temperature**

On January 24, 1985, STS 51-C was launched.The temperature of the O-rings at launch was 53.... 

 

[**135**] Figure 4.

| *NASA Official* | *Position*                                                       | *Description of Awareness of O-Ring         Problems*                                                                                                                                                                                                                                                                                                  |
|-----------------|------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| .               |
| John Young      | Chief, Astronaut Office                                          | "The secret seal, which no one that         we know knew about." **93**                                                                                                                                                                                                                                                                                |
|                 |
| Milton Silveira | Chief Engineer                                                   | ". . .If I had known . . . I'm sure         in the '82 time period when we first came to that conclusion         [that the seal was not redundant], I would have insisted         that we get busy right now on a design change and also look         for any temporary fix we could do to improve the operation         of the seal. " **94** |
| .               |
| James Beggs     | (Former) NASA Administrator                                      | "I had no specific concerns with the         joint, the O-rings or the putty...." **95**                                                                                                                                                                                                                                                               |
| .               |
| Arnold Aldrich  | Manager, National Space         Transportation System          | None were aware of Thiokol's concern         about negative effect of cold temperature on O-ring         performance, nor were they informed of the same concern         raised after STS 51-C. **96**                                                                                                                                             |
| Jesse Moore     | (Former) Associate Administrator for         Space Flight      |
| Richard Smith   | Director, Kennedy Space Center                                   |
| James A. Thomas | Deputy Director, Kennedy Launch and         Landing Operations |

 

....degrees, the coldest to that date. O-ringerosion occurred in both solid boosters. The right and left nozzlejoint showed evidence of blow-by between the primary and secondaryO-rings. The primary O-ring in the left booster's forward field jointwas eroded and had blow-by, or soot behind thering.**97** The right booster's damage was in the center fieldjoint-the first time that field joint seal was damaged. Both itsprimary and secondary O-rings were affected by heat, and the primaryring also had evidence of blow-by of soot behind it. This was alsothe first flight where a secondary O-ring showed the effect ofheat.

STS 51-C was the second example of O-ringdamage in flight where there was evidence of blow-by erosion as wellas impingement erosion. As noted previously, impingement erosionoccurs where the O-ring has already sealed and a focused jet of hotgas strikes the surface of the ring and removes a portion of it.Blow-by erosion happens when the O-ring has not yet sealed the jointgap and the edge of the ring erodes as the hot gas flows aroundit.

Roger Boisjoly described the blow-by erosionseen in 51-C:

> 
"SRM 15 [STS 51-C] actuallyincreased [our] concern because that was the first time we hadactually penetrated a primary,, O-ring on a field joint with hot gas,and we had a witness of that event because the grease between theO-rings was blackened just like coal . . . and that was so much moresignificant than had ever been seen before on any blow-by on anyjoint . . . the fact was that now you introduced another phenomenon.You have impingement erosion and bypass erosion, and the O-ringmaterial gets removed from the cross section of the O-ring much, muchfaster when you have bypass erosion or blow-by." **98**

[**136**] Boisjoly also saidblow-by erosion was where the primary O-ring "at the beginning of thetransient cycle . . . is still being attacked by hot gas, and it iseroding at the same time it is trying to seal, and it is a racebetween, will it erode more than the time allowed to have it seal."He described the blow-by on 51-C as "over 100 degrees of arc, and theblow-by was absolutely jet black. It was totally intermixed in ahomogeneous mixture in the grease." When the blow-by material waschemically analyzed, Boisjoly said, "we found the products of puttyin it, we found the products of O-ring in it."**99**

On the Marshall problem assessment report thatwas started to track field joint erosion after STS 41-B, the STS 51-C O-ring anomaly was described as "O-ring burns were as bad or worsethan previously experienced . . . Design changes are pending testresults." **100** The changes being considered included modifying theO-rings and adding grease around the O-rings to fill the void left byputty blow holes.

On January 31, 1985, Marshall Solid RocketBooster Project Manager Mulloy sent an urgent message to LawrenceWear with the stated subject: "51-C O-Ring Erosion Re: 51-E FRR." Themessage ordered that the Flight Readiness Review for the upcomingflight:

> 
"Should recap all incidents ofO-ring erosion, whether nozzle or case joint, and all incidents wherethere is evidence of flow past the primary O-ring. Also, therationale used for accepting the condition on the nozzle O-ring.Also, the most probable scenario and limiting mechanism for flow pastthe primary on the 51 -C case joints. If [Thiokol] does not have allthis for today I would like to see the logic on a chart with blanks[to be filled in ] . " **101**

On February 8, 1985, Thiokol presented itsmost detailed analysis to date of the erosion problems to the SolidRocket Motor project office at Marshall for what was then calledShuttle mission 51-E, but later changed to 51-D. Thiokol included areport on damage incurred by the O-rings during flight 51-C at theleft forward and right center field joints. The right center jointhad hot gas past the primary O-ring. Thiokol said that caused aconcern that the gas seal could be lost, but its resolution was"accept risk." **102**

Thiokol presented test results showing"maximum expected erosion" and "maximum erosion experienced" for bothprimary and secondary O-rings for- the field and nozzle joints.Accepting damage to the primary O-ring was being justified, in part,based on an assumption of the secondary O-ring working even witherosion. However, the Criticality classification indicated theprimary seal was a "single point failure." During this flightreadiness assessment at Marshall, for the first time Thiokolmentioned temperature as a factor in O-ring erosion and blow-by.Thiokol said in its conclusions that "low temperature enhancedprobability of blow-by-[flight] 51 -C experienced worst casetemperature change in Florida history." Thiokol concluded that whilethe next Shuttle flight "could exhibit same behavior," nonetheless"the condition is not desirable but is acceptable."**103**

At the Level I Flight Readiness Reviewconducted on February 21, there was no detailed analysis of O-ringproblems presented or any reference made to low temperature effects.Instead, a single reference indicated the O-ring erosion and blow-byexperienced was "acceptable" because of 'limited exposure time andredundancy."

** **

**STS 51-B and the LaunchConstraint**

Joint seal problems occurred in each of thenext four Shuttle flights. Flight 51-D, launched April 12, 1985 hadnozzle O-ring erosion and blow-by on an igniter joint. STS 51-B,launched 17 days later, experienced both nozzle O-ring erosion andblow-by as did 51-G, which flew on the following June 17. STS 51-F,launched duly 29, 1985 had nozzle O-ring blow-by.**104**

In response to the apparent negative effect ofcold leading to the extensive O-ring problems on flight 51 -C inJanuary, Thiokol conducted some O-ring resiliency tests in early1985. **105** The tests were conducted to quantify the seal timingfunction of the secondary O-ring and the effect of joint rotation onits ability to back up the primary ring. The key variable wastemperature. The June 3 test report, which was described in an August9, 1985 letter from Brian Russell at Thiokol to Jim Thomas atMarshall, showed:

> 
"Bench test data indicates thatthe O-ring resiliency (its capability to follow the metal) is afunction of temperature and rate of case expansion. [Thiokol]measured the force of the O-ring against Instron platens, which[**137**]simulated the nominal squeeze on the O-ring and approximated the caseexpansion distance and rate.

"At 100°F, the O-ring maintained contact.At 75°F the O-ring lost contact for 2.4 seconds. At 50°F,the O-ring did not reestablish contact in ten minutes at which timethe test was terminated." **106**

On June 25, 1985, the left nozzle joint of STS51-B (launched April 29) was disassembled and inspected after it hadbeen shipped back to Thiokol. What Thiokol found was alarming. Theprimary O-ring seal had been compromised because it eroded .171inches and it did not seal. The secondary O-ring did seal, but it haderoded .032 inches. Lawrence Mulloy described the 51-B problem asfollows:

> 
"This erosion of a secondaryO-ring was a new and significant event . . . that we certainly didnot understand. Everything up to that point had been the primaryO-ring, even though it had experienced some erosion does seal. Whatwe had evidence of was that here was a case where the primary O-ringwas violated and the secondary O-ring was eroded, and that wasconsidered to be a more serious observation than previously observed. . .**107**

"What we saw [in 51-B], it was evident thatthe primary ring never sealed at all, and we saw erosion all the wayaround that O-ring, and that is where the .171 came from, and thatwas not in the model that predicated a maximum of .090, the maximumof .090 is the maximum erosion that can occur if the primary O-ringseals.

"But in this case, the primary O-ring did notseal; therefore, you had another volume to fill, and the flow waslonger and it was blow-by and you got more erosion."**108**

Upon receiving the report of the 51-B primaryring failure, Solid Rocket Booster Project Manager Mulloy and theMarshall Problem Assessment Committee placed a "launch constraint" onthe Shuttle system. **109** A 1980 Marshall letter which references "AssigningLaunch Constraints on Open Problems Submitted to MSFC PAS" defineslaunch constraint as:

> 
"All open problems codedCriticality 1, 1R, 2, or 2R will be considered launch constraintsuntil resolved (recurrence control established and its implementationeffectivity determined) or sufficient rationale, i.e., differentconfiguration, etc., exists to conclude that this problem will notoccur- on the flight vehicle during pre-launch, launch, or flight."**110**

Lawrence Mulloy told the Commission that thelaunch constraint was "put on after we saw the secondary O-ringerosion on the [51-B] nozzle." "Based on the amount of charring," theproblem report listing the constraint said, "the erosion paths on theprimary O-ring and what is understood about the erosion phenomenon,it is believed that the primary O-ring [of the joint] never sealed."**111** The constraint applied to STS 51-F and all flightssubsequent, including STS 51-L. Although one Marshall document saysthat the constraint applied to all O-ring anomalies,**112** no similar launch constraint was noted on the MarshallProblem Assessment Report that started tracking the field jointerosion after STS 41-B. Thiokol officials who testified before theCommission all claimed they were not aware of the July 1985 launchconstraint; **113** however, Thiokol letters referenced Marshall Recordnumber A09288, the report that expressly identified the constraint.**114**

After the launch constraint was imposed,Project Manager Mulloy waived it for each Shuttle flight after July10, 1985. Mr. Mulloy and Mr. Lawrence Wear outlined the procedure inthe following manner:

** **

> 
**Chairman Rogers**: To you, what does a constraint mean, then?

**Mr. Mulloy**:A launch constraint means that we have to address the observations,sec if we have seen anything on the previous flight that changes ourprevious rationale and address that at the Flight ReadinessReview.

**Chairman Rogers**: When you say»address it," I always get confusedby the word. Do you mean think about it? Is that what youmean?

**Mr. Mulloy**:No, sir. I mean present the data as to whether or not what we haveseen in our most recent observation, which may not be the lastflight, it may be the flight before that, is within our experiencebase and whether or not the previous analysis and [**138**] tests thatpreviously concluded that was an acceptable situation is still valid,based upon later observations....

The constraint was put on after we saw thesecondary O-ring erosion on the nozzle, I believe.

**Chairman Rogers**: Who decided that?

**Mr. Mulloy**:I decided that, that that would be addressed, until that problem wasresolved, it would be considered a launch constraint, and addressedat Flight Readiness Reviews to assure that we were staying within ourtest experience base....

**Chairman Rogers**: Do you have ultimate responsibility for waiving thelaunch constraints?

**Mr. Mulloy**:Yes, sir, I have ultimate responsibility for the launch readiness ofthe Solid Rocket Boosters.

**Chairman Rogers:** So there was a launch constraint, and you waivedit.

**Mr. Mulloy**:Yes, sir-, all flights subsequent to.

**Dr. Ride**:I'm trying to understand how you deal with the launch constraint. Howimportant do you think a launch constraint is and how unusual is itin your system?

**Mr. Wear**: Ithink a launch constraint is a significant event in our system, andit is one that has to be addressed within the Flight Readiness cyclebecause I don't have the authority to not do that. .

**Dr. Ride**:Why didn't you put a launch constraint on the field joint at the sametime?

**Mr. Mulloy**:I think at that point, and I will react to that question in realtime, because I haven't really thought about it, but I think thelogic was that we had been observing the field joint, the field andnozzle joint primary O-ring erosion. This erosion of a secondaryO-ring was a new and significant event, very new and significant eventhat we certainly did not understand. Everything up to that point hadbeen that the primary O-ring, even though it had experienced someerosion, does seal. What we had evidence of was that here was a casewhere the primary O-ring was violated and the secondary O-ring waseroded, and that was considered to be a more serious observation thanpreviously observed.

**Dr. Ride**:Correct me if I am wrong, but weren't you basing most of yourdecisions on the field joint on analysis of what was the maximum,what you believed to be the maximum possible erosion, and you hadthat analysis for the field joint and for the nozzle joint. When yousaw the complete erosion of the primary O-ring on the nozzle joint,that showed you that your analysis on the nozzle joint wasn't anygood, I would think. That would indicate to you that your analysis onthe field joint wasn't very good, either, or at least should besuspect.

**Mr. Mulloy**:The conclusion, rightly or wrongly, for the cause of the secondaryO-ring erosion on the nozzle joint, it was concluded from test datawe had that 100 psi pressurization leak check, that the putty couldmask a primary O-ring that was not sealing. The conclusion was-andthat one was done at 100 psi. The conclusion was that in order to getthat type of erosion that we saw on the primary O-ring, that thatO-ring never sealed, and therefore the conclusion was that it neverwas capable of sealing. The leak check on subsequent nozzles, allsubsequent nozzles was run at 200 psi, which the test data indicatedwould always blow through the putty, and in always blowing throughthe putty we were guaranteed that we had a primary O-ring seal thatwas capable of sealing, and then we further did, and we already hadthat on the field joints at that time.**115**

 

While Mulloy and Wear both testified that theconstraint was still in effect and waived for Challenger's flight,they told the Commission that there had been two erroneous entries onthe O-ring erosion nozzle problem assessment report stating theO-ring erosion problem had been resolved or closed.**116** Thiokol had suggested this closure on December 10,1985 (at Marshall's request according to Brian Russell) but Wear andMulloy told the Commission they rejected that recommendation and theproblem was still being addressed in Flight ReadinessReviews.**117** NASA Levels I and II apparently did not realizeMarshall had assigned a launch constraint within the ProblemAssessment System.**118** This......

 

[**139**] **Figure 5. August 19,1985 HeadquartersBriefing.**

| **General Conclusions**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | **Recommendations**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
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| - All O-ring erosion has occurred            where gas paths in the vacuum putty are formed 
- Gas paths in the vacuum putty can            occur during assembly, leak check, or during motor            pressurization 
- Improved filler materials or            layup configurations which still allow a valid leak check            of the primary O-rings may reduce frequency of O-ring            erosion but will probably not eliminate it or reduce the            severity of erosion 
- Elimination of vacuum putty in a            tighter joint area will eliminate O-ring erosion if            circumferential flow is not present-if it is present,            some baffle arrangement may be required 
- Erosion in the nozzle joint is            more severe due to eccentricity; however, the secondary            seal in the nozzle will seal and will not erode            through 
- The primary O-ring in the field            joint should not erode through but if it leaks due to            erosion or lack of sealing the secondary seal may not            seal the motor 
- The igniter Gask-O-Seal design is            adequate providing proper quality inspections are made to            eliminate overfill conditions | - The lack of a good secondary seal            in the field joint is most critical and ways to reduce            joint rotation should be incorporated as soon as possible            to reduce criticality 
- The flow conditions in the joint            areas during ignition and motor operation need to be            established through cold flow modeling to eliminate            O-ring erosion 
- QM-5 static test should be used            to qualify a second source of the only flight certified            joint filler material (asbestos-filled vacuum putty) to            protect the flight program schedule 
- VLS-1 should use the only flight            certified joint filler material (Randolph asbestos-filled            vacuum putty) in all joints 
- Additional hot and cold subscale            tests need to be conducted to improve analytical modeling            of O-ring erosion problem and for establishing margins of            safety for eroded O-rings 
- Analysis of existing data            indicates that it is safe to continue flying existing            design as long as all joints are leak checked with a 200            psig stabilization pressure, are free of contamination in            the seal areas and meet O-ring squeeze            requirements 
- Efforts need to continue at an            accelerated pace to eliminate SRM seal erosion |

 

......communication failure was contrary tothe requirement, contained in the NASA Problem Reporting andCorrective Action Requirements System, that launch constraints wereto be taken to Level II.

** **

**Escalating Concerns**

When the burn through of the primary nozzleO-ring on the left Solid Rocket Booster of STS 51-B was discovered inUtah on dune 25, 1985, an engineer from the NASA headquarters ShuttlePropulsion Group was on the scene. Three days after the 51-Binspection, a memorandum was written to Michael Weeks, also atHeadquarters, reporting on the primary O-ring burnthrough.**119** The memo blamed the problem on the faulty 100 psi leakcheck and reminded Weeks that Thiokol had not yet responded to theO-ring erosion action item sent out after STS 41-B one yearearlier.

Engineers at Thiokol also were increasinglyconcerned about the problem. On July 22, 1985, Roger Boisjoly of thestructures section wrote a memorandum predicting NASA might give themotor contract to a competitor or there might be a flight failure ifThiokol did not come up with a timely solution. **120**

Nine days later (July 31) Boisjoly wroteanother memorandum titled "O-ring Erosion/Potential FailureCriticality" to R. K. Lund, Thiokol's Vice President ofEngineering:

> 
"The mistakenly accepted positionon the joint problem was to fly without fear of failure and to run aseries of design evaluations which would ultimately lead to asolution or at least a significant reduction of the erosion problem.This position is now changed as a result of the [51-B] nozzle jointerosion which eroded a secondary O-ring with the primary O-ring neversealing. If the same scenario should occur in a field joint (and itcould), then it is a jump ball whether as to the success or failureof the joint because the secondary O-ring cannot respond to theclevis opening rate and may not be capable of pressurization. Theresult would be a catastrophe of the highest order-loss of humanlife."

Boisjoly recommended setting up a team tosolve the O-ring problem, and concluded by stating:

> 
"It is my honest and very realfear that if we do not take immediate action to dedicate a team tosolve the problem, with the field joint having the number onepriority, then we stand in jeopardy of losing a flight along with allthe launch pad facilities." **121**

[**140**] In reply tospecific questions from Marshall on August 9, Thiokol's Brian Russellreported the test data on the dune 3 resiliency tests. As notedpreviously, he indicated O-ring resiliency was a function of thetemperature and case expansion. Also, he wrote, Thiokol had no reasonto suspect that the primary O-ring would fail after motor ignitiontransient. He said the secondary O-ring would seal within the periodafter ignition from 0 to 170 milliseconds.**122** From 170 to 330 milliseconds, the probability of thesealing of the secondary O-ring was reduced. From 330 to 600milliseconds, there was only a slight chance the secondary seal wouldhold.

On August l9, 1985, Thiokol and Marshallprogram managers briefed NASA Headquarters on erosion of the motorpressure seals.**123** The briefing paper concluded that the O-ring seal wasa critical matter, but it was safe to fly. The briefing was detailed,identifying all prior instances of field joint, nozzle joint andigniter O-ring erosion. It recommended an "accelerated pace" toeliminate seal erosion but concluded with the recommendation that "itis safe to continue flying existing design as long as all joints areleak checked with a 200 psig stabilization pressure, are free ofcontamination in the seal areas and meet O-ring squeezerequirements." The briefing conclusions and recommendations appear inFigure 5. **124**

Thiokol's Robert Lund, VicePresident-Engineering, noting that "the result of a leak at any ofthe joints would be catastrophic," announced the establishment of aThiokol O-ring task force on August 20, 1985, to "investigate theSolid Rocket Motor case and nozzle joints, both materials andconfigurations, and recommend both short-term and long-termsolutions." **125**

Two days later, A. R. Thompson, Thiokol'ssupervisor of structures design, said in a memorandum to S. R. Stein,project engineer, that the "O-ring seal problem has lately becomeacute." Thompson recommended near-term solutions of increasing thethickness of shims used at the tang and clevis mating, and increasingthe diameter of the O-ring. "Several long-term solutions look good;but, several years are required to incorporate some of them,"Thompson wrote. "The simple short-term measures should be taken toreduce flight risks." **126** During a Commission hearing, Thompson was asked aboutthe larger diameter O-ring solution:

** **

> 
**Dr. Walker**: Why didn't you go to the larger O-ring, then?

**Mr. Thompson**: One problem in going to larger O-rings is in fieldjoints-plant joints, excuse me. In the plant joints, if you put inthe 295 and you take the worst on worst, when the joint is raised toa temperature of 325 degrees during the curing of the insulation, itis an overfill condition because of the alpha problems with the case,and the rubber.

**Dr. Walker**:There is no reason why a field joint and a plant joint had to havethe same O-ring, is there?

**Mr. Thompson**: There were some that were afraid of the QC people,that were afraid of the confusion that might be developed between twonearly the same sized O-ring.**127**

 

Thiokol's revised O-ring protection plan,dated August 30, 1985, indicated that NASA and Thiokol were still notin agreement on the magnitude of the joint rotation phenomenon. Itsaid that "presently there are conflicting data from Solid RocketMotor case hydrotest and [static tests] concerning the magnitude ofcase field joint rotation under motor pressure. A referee test willbe devised, which is mutually acceptable to NASA and Thiokol, todetermine joint opening characteristics." **128**

** **

**Design Questions Resurface**

Also in late August, Thiokol submitted"Preliminary Solid Rocket Motor Nozzle/Field Joint Seal Concepts" toNASA, which were "formulated to solve the [Solid Rocket Motor]sealing problems." The document contained 43 possible design conceptsfor field joints and 20 for nozzle joints. The report said Thiokol"feels the case field joint poses the greatest potential risk in thatits secondary seal may not maintain metal contact throughout motoroperation. The nozzle joint is also of major concern because thefrequency and severity of seal damage experienced has been greaterthan any other joint."

In September 1985, Thiokol's plans called fortest-firing a static motor with various O-ring configurations. In aSeptember 10 presentation to Marshall, Thiokol discussed erosionpredictions, and evaluated primary engineering concerns includingjoint deflection and secondary O-ring resiliency. Temperature was notmentioned.**129**

[**141**] Prior to thatThiokol presentation, Marshall Science and Engineering DirectorKingsbury had informed Solid Rocket Booster Program ManagerMulloy:

> 
"I am most anxious to be briefedon plans for improving the Solid Rocket Motor O-ring seals.Specifically, I want to review plans which lead to flightqualifications and the attendant schedules. I have been apprised ofgeneral ongoing activities but these do not appear to carry thepriority which I attach to this situation. I consider the O-ring sealproblem on the Solid Rocket Motor to require priority attention ofboth Morton Thiokol/Wasatch and MSFC." **130**

Early in October, internal warnings about thelack of results from the O-ring task force came when Thiokol'smanagement got two separate memoranda complaining aboutadministrative delays and lack of cooperation. One memorandum waswritten by Roger Boisjoly on October 4, 1985, and it warned Thiokolmanagement about lack of management support of the O-ring team'sefforts.**131** He said that "even NASA perceives that the team isbeing blocked in its engineering efforts to accomplish its task. NASAis sending an engineering representative to stay with us startingOctober 14th. We feel that this is the direct result of their feelingthat we [Thiokol] are not responding quickly enough on the sealproblem."

R. V. Ebeling, manager of Thiokol's SolidRocket Motor ignition system, began his October 1, 1985, report toMcDonald with the alarming word "HELP!" Ebeling said the seal taskforce was "constantly being delayed by every possible means.""Marshall Space Flight Center," he said, "is correct in stating thatwe do not know how to run a development program." Ebelingcontinued:

> 
"The allegiance to the O-ringinvestigation task force is very limited to a group of engineersnumbering 8-10. Our assigned people in manufacturing and quality havethe desire, but are encumbered with other significant work. Others inmanufacturing, quality, procurement who are not involved directly,but whose help we need, are generating plenty of resistance. We arecreating more instructional paper than engineering data. We wish wecould get action by verbal request, but such is not the case. This isa red flag." **132**

Shuttle flight 61-A was launched October 30,1985. It experienced nozzle O-ring erosion and field joint O-ringblow-by. **133** These anomalies were not mentioned at the Level IFlight Readiness Review for flight 61-B. That flight was launched onNovember 26, 1985, and sustained nozzle O-ring erosion and blow-by.**134**

The following month (December) Thiokol'sproblem status report which tracked the field joint erosion anomalystated that the O-ring task force had made one hot gas test andpreliminary results indicated the test chamber needed to beredesigned.**135** Mr. Ebeling of Thiokol became so concerned about thegravity of the O-ring problem that he told fellow members of the sealtask force that he believed Thiokol should not ship any more motorsuntil the problem was fixed.

In testimony before the Commission, Ebelingsaid:

** **

> 
**Mr. Ebeling**: Well, I am a hydraulics engineer by profession, andO-rings and seals and hydraulics are very sacred, but for the mostpart, a hydraulics or pneumatics engineer controls the structure, thestructural design, the structural deformation to make sure that thisneat little part that is so critical is given every thing it needs tooperate. In Solid Rocket Motors I have been there now pushing 25years. They had a different attitude on O-rings when I came there,and it is not just Thiokol, it is universal.

**Dr. Covert**:By universal, you mean the solid rocket industry?

**Mr. Ebeling**:The entire solid rocket industry. It gets around from one, thecompetitors' information eventually gets to me by one track oranother, and mine to them, but my experience on O-rings was and is tothis date that the O-ring is not a mechanism and never should be amechanism that sees the heat of the magnitude of our motors, and Ithink before I do retire, I'm going to make sure that we discontinueto fly with round seals which I am against round seals anyway. Ithink seals with memories, not pressure-activated, but energizedthrough mechanical means, and in all cases, keep the heat of ourrocket [**142**] motors away from those seals. Whatever it is, you donot need chamber pressure to energize a seal.

**Dr. Covert**:In this regard, then, did you have an increasing concern as you sawthe tendency first to accept thermal distress and then to say, well,we can model this reasonably and we can accept a little bit oferosion, and then etc., etc. ? Did this cause you a feeling of if notdistress, then betrayal in terms of your feeling aboutO-rings?

**Mr. Ebeling**:I'm sure sorry you asked that question.

**Mr. Covert**:I'm sorry I had to.

**Mr. Ebeling**:To answer your question, yes. In fact, I have been an advocate, Iused to sit in on the O-ring task force and was involved in the sealssince Brian Russell worked directly for me, and I had a certainallegiance to this type of thing anyway, that I felt that weshouldn't ship any more rocket motors until we got it fixed.

**Dr. Covert**:Did you voice this concern?

**Mr. Ebeling**:Unfortunately, not to the right people.**136**

** **

**The Closure Issue**

On December 6, 1985, Thiokol's Brian Russellwrote Al McDonald, Thiokol Solid Rocket Motor Project Director,requesting "closure of the Solid Rocket Motor O-ring erosion criticalproblems."**137** He gave 17 reasons for the closure, including testresults, future test plans and the work to date of Thiokol's taskforce. Four days later (December 10) McDonald wrote a memorandum toNASA's Wear asking for closure of the O-ring problem. All O-ringerosion problems, including the problem containing the July 1985launch constraint, were among the referenced matters that Thiokolsuggested should be closed. McDonald noted that the O-ring problemwould not be fully resolved for some time, and he enclosed a copy ofThiokol's August 30 plan for improving the motorseals.**138**

Brian Russell described the problem trackingprocess and gave the reason for the closure recommendation during thefollowing exchange:

** **

> 
**Mr. Russell**: We have our reliability engineering department, whois responsible to complete the monthly problem report, and inaddition to that we have our monthly problem review board telephoneconference with NASA and the contractors, of which we are a part, andthe monthly problem review or the monthly problem report thatreliability prepares, they get the information from engineering orfrom the office as necessary to complete their status of what hashappened during that month, whether the problem originated that monthor what has been done to close the problem out, and that is submittedevery month, and I for one do review that before it is submitted tothe Marshall Space Flight Center, and so much of the information thatI would read in these reports would be the same information that wehad given in that monthly problem report or over the telephone on theteleconference.

**Chairman Rogers**: Mr. Russell, when you say close the problem out, whatdo you mean by that? How do you close it out normally?

**Mr. Russell**:Normally, whether it takes engineering analysis or tests or somecorrective action, a closeout to the problem would occur after anadequate corrective action had been taken to satisfy those on theproblem review board that the problem had indeed been closed out.That is the way that that happens; for example, we had found a loosebolt on the recovery one time, and we had to take corrective actionin our procedures and in the engineering to make sure that thatwouldn't happen again, and then to verify that corrective action, andat that point that problem would be ready to be closed out. Itgenerally involves a report or at least a mention by the review boardstating what had been done to adequately close it out, and then it isagreed upon by the parties involved. .

**Question**:What do you understand a launch constraint to mean?

**Mr. Russell**:My understanding of a launch constraint is that the launch cannotproceed without adequately-without everyone's agreement that theproblem is under control.

[**143**] **Chairman Rogers**: Undercontrol meaning what? You just said a moment ago that you wouldexpect some corrective action to be taken.

**Mr. Russell**:That is correct, and in this particular case on this 51-B nozzleO-ring erosion problem there had been some corrective action taken,and that was included in the presentation made as a special addendumto the next Flight Readiness Review, and at the time we did agree tocontinue to launch, which apparently had lifted the launchconstraint, would be my understanding.

**Chairman Rogers**: But really my question is: Did you gentlemen realizethat it was a launch constraint?

**Mr. Russell**:I would like to answer for myself. I didn't realize that there was aformal launch constraint on this one, any different than some of theother erosion and blow-by that we had seen in the past.

**Mr. Ebeling**:I agree. .

**Question**: .. . Mr. Russell, you wrote a letter, did you not, or a memorandumindicating that the problem should be closed.

Could you explain to the Commission what youmeant by that?

**Mr. Russell**:Yes. In our December telephone call on the Problem Review Board-and Ican't remember the date-it was around the 9th or so-there was arequest to close the problems out and particularly the ones that hadbeen open for a long time, of which this was one, and a long timemeaning six months or more.

There was a request from the Director ofEngineering, as I recall it, that we close these problems out..

**Dr. Walker**:That was the Director of Engineering at Marshall?

**Mr. Russell**:Yes, at Marshall Space Flight Center. Now, he wasn't in that call. Myunderstanding is what they told us and my recollection was that Mr.Kingsbury would like to see these problems closed out.

Now, the normal method of closing them out isto implement the corrective action, verify the corrective action, andthen the problem is closed, it comes off the board and is no longerunder active review. .

**Chairman Rogers**: What was being done to fix it?

**Mr. Russell**:Well, we had a task force created of full-time people at Thiokol, ofwhich I was a member of that task team, and we had done someengineering tests. We were trying to develop concepts. We haddeveloped some concepts to block the flow of hot gas against theO-ring to the point where the O-ring would no longer be damaged in anew configuration.

And we had run some cold gas tests and somehot gas motor firing tests and were working toward a solution of theproblem and we had some meetings scheduled with the Marshall SpaceFlight Center. We had weekly telephone calls where we statused ourprogress and there was a team at Marshall also of engineering peoplewho were monitoring the things that we were doing to fix the problemwith the goal of implementing a fix in our qualification motor No. 5,which was scheduled at that time in January, this timeframe beingabout the December timeframe of last year.

**Chairman Rogers**: Can I interrupt? So you're trying to figure out howto fix it, right? And you're doing some things to try to help youfigure out how to fix it.

Now, why at that point would you close it out?.

**Mr. Russell**:Because I was asked to do it.

**Chairman Rogers**: I see. Well, that explains it.

**Mr. Rummel**:It explains it, but really doesn't make any sense. On one hand youclose out items that you've been reviewing flight by flight, thathave obviously critical implications, on the basis that after youclose it out, you're going to continue to try to fix it.

So I think what you're really saying is,you're closing it out because you don't want to be bothered. Somebodydoesn't want to be bothered with flight-by-flight reviews, but you'regoing to continue to work on it after it's closed out.**139**

 

[**144**] Marshall receivedthe Thiokol letter asking for the closure and an entry was placed onall Marshall Problem Reports referenced in McDonald's December 10letter indicating"contractor closure received" on December 18, 1985.**140** On January 23, 1986, another entry was placed on thesame reports indicating the "problem is considered closed."**141** Lawrence Mulloy and Lawrence Wear testified thoseentries were "in error." They said:

** **

> 
**Mr. Mulloy**: The problem assessment system was put in place toprovide visibility throughout the Shuttle system for the types ofproblems that do occur, not just in flight, but also in qualificationtests, and in failure of hardware that is back for refurbishment at avendor or whatever. And it is a closed loop tracking system thatlists the anomaly .

Now, the entry that is shown in there that theproblem was closed prior to 51-L is in error. What happened therewas, one of your documents here which we did not discuss is theletter from Mr. McDonald to Mr. Wear which proposed that this problembe dropped from the problem assessment system and no longer betracked for the reasons stated in Mr. McDonald's letter.

That letter was in the review cycle. Theletter, I believe, was dated 10 December 1985. It came into thecenter, it was in the review cycle. After Mr. Wear brought thisletter to my attention, my reaction was, we are not going to dropthis from the problem assessment system because the problem is notresolved and it has to be dealt with on a flight-by-flightbasis.

Since that was going through the review cycle,the people who run this problem assessment system erroneously entereda closure for the problem on the basis of this submittal fromThiokol. Having done that then for the 51-L review, this did not comeup in the Flight Readiness Review as an open launch constraint, soyou won't find a project signature because the PAS system showed theproblem was closed, and that was an error.

**Chairman Rogers**: Who made the error? Do you know?

**Mr. Mulloy**:The people who do the problem assessment system.

**Mr. Wear**:Mr. Fletcher, and he reports within our quality organization at theFlight Readiness Reviews, . . . as I think have been described to youbefore. There is one from Thiokol to me, and there is one from mygroup to Larry, and then Larry, of course, does one with the Shuttleproject office, and so forth, on up the line. At my review and atLarry's review, here is a heads up given to the qualityrepresentative at that board for what problems the system has open,and they cross-check to make sure that we address that problem in thereadiness review.

On this particular occasion, there was noheads up given because their Problem Assessment System consideredthat action closed. That is unfortunate. **142**

 

Project Manager Mulloy was asked duringCommission hearings about the original response to O-ringerosion:

** **

> 
**Mr. Hotz**: Mr. Mulloy, I would like to try to understand this insomewhat simpler terms than you people are used to using.

Is it correct to state that when youoriginally designed this joint and looked at it, that you did notanticipate erosion of any of the O-ring during flights?

**Mr. Mulloy**:That is my understanding. I entered this program in November of 1982and I wasn't there on the original design of the joint, but when Itook over the program there was no O-ring erosion anticipated.

**Mr. Hotz**: Sothat when you did run into signs of O-ring erosion, this was a badsign.

**Mr. Mulloy**:Yes, sir. .

**Mr. Hotz**: Sothen you decided to introduce a standard based on the measurement orthe possibility of the limits of O-ring erosion. And as those limits,as the experience went up, your criteria for, say, flight went uptoo.

In other words, when you experienced more thanmaximum anticipated O-ring erosion, you waived the flight and said"Well, it's possible to tolerate that. We still have a marginleft."

**Mr. Mulloy**:Are you speaking of the case where we did not have a primaryseal.

**Mr. Hotz**:Yes.

[**145**] **Mr. Mulloy**: Yes, sir.That is correct. .

**Mr. Hotz**:Then you finally, you're talking about these margins of safety, and Iwonder if you could express in either percentages or actualmeasurement terms-you have used the term "wide margin." I wonder ifyou could give us a quantitative measurement as to what you considera wide margin?

**Mr. Mulloy**:Yes, sir. Well, as I said we had demonstrated that we could stand 125thousandths of erosion and still seat. The maximum erosion that wehad seen in the case joint was on STS-2, which was 53 thousandths, sothat is a factor of two and a half .

**Dr. Keel**: .. . I think, Larry, if you go back and look at your Flight ReadinessReviews, that you were relying on less margins than that.

You were arguing in the Flight ReadinessReviews where you briefed the problems of primary O-ring erosion thatfor the worst case for the field joint also that it would be 90thousandths.

**Mr. Mulloy**:That is correct.

**Dr. Keel**: Atthat point you were pointing out that's okay, because you can seal at95, not at 125 but at 95. It wasn't until later on during the processthat you determined you could seal at 125.

**Mr. Mulloy**:That is when we got the hot gas test data.

**Dr. Keel**: Sothat's a five percent margin, roughly, five and a half.

**Mr. Mulloy**:On the 90 to 95 on a max predictable, yes. **143**

** **

**Temperature Effects**

The record of the fateful series of NASA andThiokol meetings, telephone conferences, notes, and facsimiletransmissions on January 27th, the night before the launch of flight51 -L, shows that only limited consideration was given to the pasthistory of O-ring damage in terms of temperature. The managerscompared as a function of temperature the flights for which thermaldistress of O-rings had been observed-not the frequency of occurrencebased on all flights (Figure 6). In such a comparison, there is nothing irregular inthe distribution of O-ring "distress" over the spectrum of jointtemperatures at launch between 53 degrees Fahrenheit and 75 degreesFahrenheit. When the entire history of flight experience isconsidered, including"normal" flights with no erosion or blow-by, thecomparison is substantially different (Figure 7).

This comparison of flight history indicatesthat only three incidents of O-ring thermal distress occurred out oftwenty flights with O-ring temperatures at 66 degrees Fahrenheit orabove, whereas, all four flights with O-ring temperatures at 63degrees Fahrenheit or below experienced O-ring thermaldistress.

Consideration of the entire launch temperaturehistory indicates that the probability of O-ring distress isincreased to almost a certainty if the temperature of the joint isless than 65.

** **

**Flight Readiness Reviews**

It is clear that contractor and NASA programpersonnel all believed that the O-ring erosion/blow-by anomaly, andeven the launch constraint, were problems that should be addressed inNASA's Flight Readiness Review process. The Flight Readiness Reviewis a multi-tiered review that is designed to create an informationflow from the contractor up through Level III at Marshall, then toLevel II officials from Johnson and Level I at Headquarters. Withregard to the Solid Rocket Booster, the process begins at the elementlevel and culminates in a coordinated Marshall position at thesubsequent Levels II and I Flight Readiness Review.**144**

NASA policy manuals list four objectives ofthe Shuttle Projects Flight Readiness Review, an intermediate reviewbetween Level III and Level I, when contractors and Level III programpersonnel consider the upcoming launch. The stated objectivesare:

> 
"1.To provide the review team withsufficient information necessary for them to make an independentjudgment regarding flight readiness.

"2. Review solved problems and previous flightanomalies and establish confidence in solution rationale.

** **

**** ****
******[**146**] ******
****
**[top] Figure 6. Plot of flights withincidents of O-ring thermal distress as function oftemperature.**
**[bottom] Figure 7. Plot of flightswith and without incidents of O-ring thermal distress. NOTE: Thermaldistress defined as O-ring erosion, blow-by, of excessiveheating.******
** **
> 
[**147**] "3. Address allproblems, technical issues, open items and constraints requiringresolution before flight.

"4. Establish the flight baselineconfiguration particularly as it differs from previous missions."**145**

The Commission has reviewed the variousdocumentary presentations made by Thiokol and NASA program people forFlight Readiness Reviews on all Shuttle flights. The O-ringpresentations in those Flight Readiness Reviews have been summarizedin an Appendix to this report.

The erosion on STS-2 was not considered on anylevel of the Flight Readiness Review for STS-3.**146** Similarly the heat effect on STS-6's primary O-ring inthe nozzle was not mentioned on the STS-7 Flight Readiness Review in1983. However, the rationale for acceptance of the "secondary sealcondition" for the lightweight case first flown on STS-6 containedthe observation that an O-ring sealed during a Thiokol test under3,000 psi where .125 inches had been cut out of theO-ring.**147**

The inattention to erosion and blow-by anomalychanged when Thiokol filed a problem report on the field jointerosion after STS 41-B. The O-ring problems (field and nozzle) on41-B were briefed as a "technical issue" in the 41-C Flight ReadinessReview. "Probable causes" were defined as:

> 
"Putty blow-through at ignitioncauses cavity between putty and primary O-ring to fill duringpressurization. Inability of putty to withstand motor pressure. Airentrapment in putty during mating. Blow holes in putty during jointleak test."

Thiokol presented the question at its 41-Cpreboard to Marshall, "If primary O-ring allowed a hot gas jet topass through, would the secondary O-ring survive impingement?"**148** At the 41 -C Level I Flight Readiness Review, on March30, 1984, Marshall said the erosion phenomenon was "acceptable" andthat blow holes in the putty were the"most probable cause." Therationale for the acceptance of the possibility of erosion on STS41-C was:

> 
"Conservative analysis indicatesmax erosion possible:

".090 in. (field joint)

".090 in. (nozzle joint)

"Laboratory test of full scale O-ring/jointcross section shows capability to sustain joint sealing integrity at3,000 psi pressure using an O-ring with a simulated .095 in. erosiondepth.

"Recommendation:

"Fly STS 41-C accepting possibility of someO-ring gas impingement." **149**

The next significant treatment of the problemoccurred after the coldest flight, 51-C at 53 degrees in January1985. In part, Thiokol's extensive analysis for the 51-E FlightReadiness Review was due to the fact that four joints on 51-C hadproblems. **150** Additionally, Mr. Mulloy's specific request for arecap of the O-ring history undoubtedly prompted a full treatment.Temperature was highlighted as a concern when Mulloy took Thiokol'sanalysis up to the Shuttle Projects Office Flight Readiness Review.That 18-page briefing concluded with the statement that: "STS 51-Cconsistent with erosion data based. Low temperature enhancedprobability of blow-by. STS 51-C experienced worst case temperaturechange in Florida history. STS 51-E could exhibit the same behavior.Condition is acceptable." **151**

At the Level I Flight Readiness Review for51-E on February 21, 1985, the previous 18-page analysis had beenreduced to a one page chart with the resolution: "acceptable riskbecause of limited exposure and redundancy (Ref. STS 41-C FRR)".**152** No mention of temperature was found in the Level Ireport.

The last major discussion of erosion was atthe Level I Flight Readiness Review for STS 51-F (July 2,1985).**153** An analysis of the failure of the nozzle primaryO-ring to seal due to erosion on flight STS 51-B (April 29, 1985) waspresented. This serious erosion was attributed to leak checkprocedures. An increase in the nozzle leak check to 200 psi wasproposed to be a cure. There was no mention of the fact that .171inches of erosion on the primary O-ring far exceeded a more recentanalysis model prediction of .070 inches maximum possible erosion.This was a revision of the former prediction of .090 inches. Thelaunch constraint activated after STS 51-B was not specificallylisted in the Level I Flight Readiness Review for 51-F. TheCommission has also not found any mention of the duly 1985constraint, or its waiver for subsequent Shuttle flights, in anyFlight Readiness Review briefing documents.

[**148**] The Commission'sreview of the Marshall and Thiokol documentary presentations at thevarious Flight Readiness Reviews revealed several significant trends.First, O-ring erosion was not considered early in the program when itfirst occurred. Second, when the problem grew worse after STS 41-B,the initial analysis of the problem did not produce much research;instead, there was an early acceptance of the phenomenon. Third,because of a belief that in-flight O-ring erosion was "within thedata base" of prior experience, later Flight Readiness Reviews gave acursory review and often dismissed the recurring erosion as within"acceptable" or "allowable" limits. Fourth, both Thiokol and Marshallcontinued to rely on the redundancy of the secondary O-ring longafter NASA had officially declared that the seal was a non-redundantsingle point failure. Finally, in 1985 when temperature became amajor concern after STS 51-C and when the launch constraint wasapplied after 51-B, NASA Levels l and II were not informed of thesedevelopments in the Flight Readiness Review process.

** **

**Findings**

The genesis of the Challenger accident-thefailure of the joint of the right Solid Rocket Motor-began withdecisions made in the design of the joint and in the failure by bothThiokol and NASA's Solid Rocket Booster project office to understandand respond to facts obtained during testing.

The Commission has concluded that neitherThiokol nor NASA responded adequately to internal warnings about thefaulty seal design. Furthermore, Thiokol and NASA did not make atimely attempt to develop and verify a new seal after the initialdesign was shown to be deficient. . Neither organization developed asolution to the unexpected occurrences of O-ring erosion and blow-byeven though this problem was experienced frequently during theShuttle flight history. Instead, Thiokol and NASA management came toaccept erosion and blow-by as unavoidable and an acceptable flightrisk. Specifically, the Commission has found that:

1.The joint test and certification program wasinadequate. There was no requirement to configure the qualificationstest motor as it would be in flight, and the motors were statictested in a horizontal position, not in the vertical flightposition.

2. Prior to the accident, neither NASA norThiokol fully understood the mechanism by which the joint sealingaction took place.

3. NASA and Thiokol accepted escalating riskapparently because they "got away with it last time." As CommissionerFeynman observed, the decision making was:

> 
"a kind of Russian roulette..

[The Shuttle] flies [with O-ring erosion] andnothing happens. Then it is suggested, therefore, that the risk is nolonger so high for the next flights. We can lower our standards alittle bit because we got away with it last time.... You got awaywith it but it shouldn't be done over and over again like that . "**154**

4. NASA's system for tracking anomalies forFlight Readiness Reviews failed in that, despite a history ofpersistent O-ring erosion and blow-by, flight was still permitted. Itfailed again in the strange sequence of six consecutive launchconstraint waivers prior to 51-L, permitting it to fly without anyrecord of a waiver, or even of an explicit constraint. Tracking andcontinuing only anomalies that are "outside the data base" of priorflight allowed major problems to be removed from, and lost by, thereporting system.

5. The O-ring erosion history presented toLevel I at NASA Headquarters in August 1985 was sufficiently detailedto require corrective action prior to the next flight.

6. A careful analysis of the flight history ofO-ring performance would have revealed the correlation of O-ringdamage and low temperature. Neither NASA nor Thiokol carried out suchan analysis; consequently, they were unprepared to properly evaluatethe risks of launching the 51-L mission in conditions more extremethan they had encountered before.

 

[**149**] **References**

- 1. Letter, Dorsey to   Hardy, November 7, 1978.    2. Report, "STS-3   through STS-25 Flight Readiness Reviews to Level III Center   Board," NASA. 
  - 3. Ibid.      4. Report, "Selection   of Contractor for Space Shuttle Program SRM," NASA. December 12,   1973; GAO Report B-17367, page 339.
    - 5. Ibid., page   6.        6. Ibid., pages 21 and   22.
      - 7. Ibid., page   18.          8. Ibid., page 7
        - 9. Ibid., page   20            10. Chart, "SRM and   Titan III Clevis Joint Comparison," from Pelham presentation to   Commission Development and Production Subcommittees, March 17,   1986, page 3, PC 73978.
          - 11. Chart,   "Segment/Segment Interface," from Pelham presentation to   Commission Development and Production Subcommittee, March 17,   1986, page 2, PC 73977              12. Report, Thiokol   Space Shuttle Joint Review`" Thiokol, February 25, 1986, PC   021453.
            - 13. Ibid.                14. Ibid.
              - 15. Report,   "Presidential Commission Development and Production Panel,   Response to Panel Question/Special Actions- SRM and Titan III   Clevis Joint Comparison," Thiokol, April 3, 1986, PC   073979.                  16. Report, 'Original   Design of Joint Assembly SRB Motor Thiokol," Thiokol, July 13,   1973, PC 009350, and Commission Work Session, Panel on Development   and Production, April 17, 1986, page 18.
                - 17. Report, "1974   Proposal Write Up On Case Design." Thiokol, 1974, page 4. 3-3, PC   010957.                    18. Ibid page 4. 3-19,   PC 010973.
                  - 19. Commission Work   Session, Development and Production Panel, April 7, 1986, page   118.                      20. Commission   Interview Transcript, McIntosh, H., April 2, 1986, page 5.
                    - 21. bid.                        22. Letter, Brian   Russell to Bob Ebeling, Thiokol, April 9, 1986, PC 091702 and   Commission Hearing Transcript, May 2, 1986, pages   2653-2658.
                      - 23. Commission Hearing   Transcript, February 6, 1986, page 30.                          24. Report, "Space   Shuttle Case Burst Test Report." Thiokol, December 21, 1977, PC   049551-049648, TWR-11664.
                        - 25. Ibid.                            26. Report,   "Analytical Evaluation of the Space Shuttle Solid Rocket Motor   Tang/Clevis Joint Behavior," Thiokol, October 6, 1978, TWR-12019;   and Report, "SRM Clevis Joint Leakage Study," NASA, October 21.   1977.
                          - 27. Commission Hearing   Transcript, February 25, 1986, page 1435.                              28. Commission Hearing   Transcript, May 2, 1986, page 2784.
                            - 29. Report, "SRM   Clevis Joint Leakage Study," NASA, October 21, 1977, PC   102337.                                30. Letter, Miller to   Eudy, January 9, 1978, PC 009923.
                              - 31. Letter, Miller to   Eudy, January 19, 1979, PC 009921.                                  32. Ibid.
                                - 33. Ibid., footnote   31.                                    34. Commission Hearing   Transcript, May 2, 1986, page 2782.
                                  - 35. Report, "Visit to   Precision Rubber Products Corporation and Parker Seal Company,"   NASA, February 6, 1979.                                      36. Ibid.
                                    - 37. Commission Hearing   Transcript, May 2, 1986, page 2782.                                         38. Report, "Phase I   Design Certification Review," Thiokol, March 23, 1979,   TWR-12230.
                                      - 39. Report,   "Analytical Evaluation of the Space Shuttle SRM Tang/Clevis Joint   Behavior," Thiokol. October 17, 1978. PC 102302.                                          40. Report, "Space   Shuttle Verification/Certification Review Propulsion Committee   Cognizant Engineers 5th Meeting,'' NASA, July 10, 1980, pages   C-7-22.
                                        - 41. Commission Hearing   Transcript, May 2, 1986. page 2307                                            42. Report,   Verification/Certification Space Shuttle Program Response to   Assessment, " NASA, September 1980, page 59, PC 094010.
                                          - 43. Report, "SRM   Program Response." NASA, August 15, 1980. PC 102359.                                              44. NASA Handbook,   NASA. 52300.4(1D-2). Appendix A, page a-1.
                                            - 45. Commission   Interview Transcript, A. Aldrich, April 8, 1986, page 13.                                                46. Memorandum,   Abrahamson to Beggs, December 8, 1981.
                                              - 47. Ibid.                                                  48. Report. ''Erosion   of SRM Pressure Seals," Thiokol. August 19, 1985, Rev. A (February   10, 1986), page A- 4a.
                                                - 49. Commission   Interview Transcript, Thomas J.W., April 10, 1986, pages 64-66:   and Reports, Marshall Space Flight Center Problem Assessment   Reports, NASA.                                                    50 Report,   "Post-flight Evaluation of STS-2 SRM Components," Thiokol,   January. 1983, part 1, page 2, TWR 13286.
                                                  - 51. Report, "NASA   Response to Commission Request DP-006." NASA, March 17, 1986, PC   074021.                                                      52. Report, "STS-8,   SRB Pre-Board Flight Readiness Review", Thiokol, July 29,   1983.
                                                    - 53. Report, "Retention   Rationale, SRM Simplex Seal," Thiokol, December 1, 1982, page 4.   and Report, 'Critical Items List." NASA, December 17, 1982.                                                        54. Report. "SRB   Critical Items List," NASA, December 17, 1982.
                                                      - 55. Report, "Retention   Rationale, SRM Simplex Seal," Thiokol. December 1, 1982, page   5.                                                          56. Ibid., page   4.
                                                        - 57. Commission   Interview Transcript, McIntosh. H., April 2, 1986, page 66.                                                            58. Commission Hearing   Transcript, May 2, 1986, page 2729-30.
                                                          - 59. Ibid., footnote   31, page 1.                                                              60. Commission Hearing   Transcript, February 26, 1986, pages 1700- 1701.
                                                            - 61. Commission Hearing   Transcript, February 26, 1986, pages 1514-1516.                                                                62. Report, "Space   Transportation System Level I Change Request-Report, SRB Critical   Items List Requirements," NASA, March 2, 1983, page 1; and Report,   "Space Shuttle Program Requirements Control Board Directive -Level   II- SRB Critical Item List Requirements for SRM Case Joint   Assemblies." NASA, March 2, 1983, page 1.
                                                              - 63 Report, "Space   Shuttle Program Requirements Document Level I," NASA, June 30,   1977, page A-8.                                                                  64. Commission Hearing   Transcript, May 2, 1986. pages 2842 -2844.
                                                                - 65. Commission Hearing   Transcript, May 2. 1986, page 2845: and Report, "Space Shuttle   Program Requirements Control Board Directive-Level II, SRB   Critical Item List Requirements for SRM Case Joint Assemblies."   NASA, March 2, 1983, page 1.                                                                    66. Commission Hearing   Transcript, May 2, 1986, page 2852
                                                                  - 67. Commission   Interview Transcript, A. Aldrich. and R. Kohrs, April 8, 1986,   pages 19-20.                                                                      68. Commission   Interview Transcript, L. Weeks, April 7, 1986. page 16.
                                                                    - 69. Ibid., footnote   54.                                                                        [
                                                                      150
                                                                      ]   70. Report, "MSFC Problem Assessment Report-   O-Ring Erosion in the Case to Nozzle Joint," February 26, 1986,   page 1 of 3; and Commission Interview Transcript, W. Hankins,   April 2, 1986, page 11.
                                                                      - 71. Report, "Erosion   of SRM Pressure Seals," Thiokol, February 10, 1986, TWR-15150   Revision A; and Chart, "History of O-Ring Damage in Field Joints"   from Thiokol's presentation on February 25, 1986 to Commission, PC   072076 and PC072077.                                                                          72. Report, "Case and   Nozzle Joint Configuration Review," Thiokol, July 2, 1980, pages 2   and 5; and Report, "Erosion of SRM Pressure Seals," Thiolol,   August 19, 1986, pages A-4 and A-6, TWR-15150.
                                                                        - 73. Report, "Char   Condition on O-Ring Seal in Forward Field Joint of SRM A57 of   STS-11 Flight Mission 41-B," Thiokol entry of March 12, 1984, page   5, TWR-14283; and Report, "MSFC Problem Assessment System-Segment   Joint Primary O-Ring Charred," NASA, February 17, 1984.                                                                            74. Report, "MSFC   Problem Assessment System-Segment Joint Primary O-Ring Charred,"   NASA, entry of March 12, 1984, page 5, TWR-14283.
                                                                          - 75. Letter, Morefield   to Mulloy, March 9, 1984.                                                                              76. Report, "Flight   Readiness Review- 41C Level I," NASA, March 30, 1984.
                                                                            - 77. Hans Mark 41-C   Programmatic Action Item, NASA, March 30, 1984.                                                                                78. Letter, Abrahamson   to Lucas, January 18, 1984, PC 008191.
                                                                              - 79. Letter, Wear to   Kilminster, April 13, 1984, pages 1 and 2.                                                                                  80. Report,   "Protection of Space Shuttle SRM Primary Motor Seals," Thiokol,   May 4, 1984, PC 014053.
                                                                                - 81. Report, "Erosion   of SRM Pressure Seals Presentation to NASA HQ," Thiokol, August   19, 1985, page 1, and "STS 41-C Action Item Closeout," L. Mulloy,   S. Reinartz, NASA, February 20, 1986.                                                                                    82. Routing slip,   Miller to Hardy, NASA, February 28, 1984, PC 0266494.
                                                                                  - 83. Report, "SRM FIeld   Joint Zinc Chromate Vacuum Putty Test Report," Thiokol, April 21,   1983, page 13.                                                                                      84. Report, "SRM Joint   Putty, O-Ring, and Leak History," Thiokol, April 9, 1984, page 1,   TWR-13484.
                                                                                    - 85. Ibid.; and Report,   "Erosion of SRM Pressure Seals," Thiokol, TWR-15150, page D-16, PC   002963.                                                                                        86. Ibid.; and Report,   "SRB SRS 9 Flight Readiness Review," NASA, November 4, 1983, page   35.
                                                                                      - 87. Report, "Erosion   of Solid Rocket Motor Pressure Seals," Thiokol, August 19, 1985,   TWR-15150, PC 021767.                                                                                          88. Commission Hearing   Transcript, May 2, 1986, page 2687.
                                                                                        - 89. Commission Hearing   Transcript, May 2, 1986, page 2621.                                                                                            90. Memorandum,   B.Russell, Thiokol, June 13, 1984, "Minutes of Telecon with NASA   MSFC on June 7, 1984," page 1, PC 102463.
                                                                                          - 91. Memorandum,   R.Russell, Thiokol, June 1, 1984, "Vacuum Putty/O-Ring Test   Results," page 1, PC 102460.                                                                                              92. Memorandum,   J.Miller, March 6, 1985, NASA.
                                                                                            - 93. Memorandum, Young   to Director, Flight Crew Operations, March 3, 1986.                                                                                                94. Commission   Interview Transcript, M. Silveira, April 16, 1986, page 30.
                                                                                              - 95. Commission   Interview Transcript, J.Beggs, May 1, 1986, pages 5-6.                                                                                                  96. Commission Hearing   Transcript, February 27, 1986, page 1899.
                                                                                                - 97. Report, "Flight   Readiness Review STS 51-E SRM-16," Thiokol, February 12, 1985,   pages 3-1 through 3-17.                                                                                                    98. Commission Hearing   Transcript, February 25, 1986, page 1392.
                                                                                                  - 99. Commission Hearing   Transcript, Febrtuary 14, 1986, page 1202.                                                                                                      100. Report, "Problem   Assessment System Record #A07934," NASA, page 3, PC 037598.
                                                                                                    - 101. Memorandum,   "51-C O-Ring Erosion Re: 51-E FRR," Mulloy to Wear, 03/31/85, PC   102482.                                                                                                        102. Report, "STS   51-E Flight Readiness Review," Thiokol, February 8, 1984,   TWR-14740 Rev. B Section 1, page 4.
                                                                                                      - 103. Ibid., Section   6, page 4.                                                                                                          104. Report, "SRM   Seal Erosion Problems," NASA, March 19, 1986, PC 10235.
                                                                                                        - 105. Report, "O-Ring   Resiliency Testing," Thiokol, June 3, 1985, PC 102509; and   Memorandum, "Actions Pertaining to Field Joint Secondary Seal,"   B.Russel, Thiokol, August 9, 1985, PC 102543.                                                                                                            106. Ibid.
                                                                                                          - 107. Commission   Hearing Transcript, May 2, 1986, page 2591.                                                                                                              108. Commission   Hearing Transcript, May 2, 1986, pages 2606-2607.
                                                                                                            - 109. Commission   Hearing Transcript, May 2, 1986, page 2591.                                                                                                                110. Memorandum,   Lindstrom to Distribution, NASA, September 15, 1980, page   1.
                                                                                                              - 111. Report, "MSFC   Problem Assessment System," February 26, 1986, PC 037710.                                                                                                                  112. Report, "SRM   Seal Erosion Problem, Revised," March 19, 1986, PC 037593.
                                                                                                                - 113. Commission   Hearing Transcript, May 2, 1986, page 2735.                                                                                                                    114. Letter, McDonald   to Wear, Thiokol, December 10, 1985, PC 49701.
                                                                                                                  - 115. Commission   Hearing Transcript, May 2, 1986, excerpt beginning pages 2590   through 2646.                                                                                                                      116. Commission   Hearing Transcript, May 2, 1986, page 2589.
                                                                                                                    - 117. Commission   Hearing Transcript, May 2, 1986, page 2635.                                                                                                                        118. Commission   Hearing Transcript, May 2, 1986, page 2867.
                                                                                                                      - 119. Memorandum,   Winterhaler to Weeks, June 28, 1985.                                                                                                                          120. Memorandum, R.   Boisjoly, July 22, 1985.
                                                                                                                        - 121. Memorandum, R.   Boisjoly, July 31, 1985.                                                                                                                            122. Letter, "Actions   Pertaining to SRM Field Joint Secondary Seal," Russell to Thomas,   August 9, 1985.
                                                                                                                          - 123. Report, "Erosion   of Solid Rocket Motor Pressure Seal Updated from August 19, 1985-   Revised February 10, 1986," Thiokol, TWR-15150, PC 000769.                                                                                                                              124. Ibid.
                                                                                                                            - 125. Memorandum, Lund   to Sayer, August 20, 1985.                                                                                                                                126. Memorandum, "SRM   Flight Seal Recommendation," Thompson to Stein, August 22,   1985.
                                                                                                                              - 127. Commission   Hearing Transcript, February 14, 1986, page 1220.                                                                                                                                  128. Report, "Program   Plan Improvement of Space Shuttle SRM Motor Seal," Thiokol, August   30, 1985, page 6.
                                                                                                                                - 129. Report, "Erosion   of SRM Pressure Seals," Thiokol, September 10, 1985, pages A-1 to   C-5.                                                                                                                                    130. Letter,   Kingsbury to Mulloy, September 5, 1985.
                                                                                                                                  - 131. Report,   "Activity Report- Solid Rocket Motor Seal Problem Task Team   Status," Thiokol, October 4, 1985.                                                                                                                                      132. Memorandum,   Ebeling to McDonald, October 1, 1985, page 1.
                                                                                                                                    - 133. Ibid., page 2;   and Report, "Erosion of SRM Pressure Seals, Update," February 10,   1986, pages A-4a, A-6a, TWR-1510, PC000760.                                                                                                                                        134. Report, "Level I   STS 61-C Flight Readiness Review," NASA, December 11, 1985.
                                                                                                                                      - 135. Ibid., footnote   133; and Report, "Solid Motor Branch Significant Events," NASA,   December 12, 1985.                                                                                                                                          136. Commission   Hearing Transcript, May 2, 1986, pages 2746-2747.
                                                                                                                                        - 137. Memorandum,   "Closure of SRM O-Ring Erosion Critical Problems," Russell to   McDonald, December 6, 1985.                                                                                                                                            138. Letter, McDonald   to Wear, December 10, 1985.
                                                                                                                                          - [ 151 ]   139. Commission Hearing Transcript, May 2,   1986, pages 2682-2695                                                                                                                                              140. Report, "Problem   Assessment System," NASA, entry dated December 18, 1985.
                                                                                                                                            - 141. Ibid., entry   January 23, 1986.                                                                                                                                                142. Commission   Hearing Transcript, May 2, 1986. pages 2589-2590.
                                                                                                                                              - 143. Ibid., pages   2619-2623.                                                                                                                                                  144. Commission   Hearing Transcript, February 11, 1986, pages 65O-653.
                                                                                                                                                - 145. Report, "Shuttle   Project Flight Readiness Review (Prelaunch Activities Team   Report)," NASA, December 29, 1983, page 75.                                                                                                                                                    146. Reports, "STS-3   Flight Readiness Review for Levels I, II, III and   Contractor."
                                                                                                                                                  - 147. Commission   Hearing Transcript, February 26. 1986, page 1639.                                                                                                                                                      148. Report, "STS-13   Solid (41-C) Rocket Motor Flight Readiness Review," Thiokol, March   2, 1984, page 2, TWR-14231.
                                                                                                                                                    - 149. Report STS 41-C   Flight Readiness Review Solid Rocket Booster," NASA, March 30,   1984, page 39.                                                                                                                                                        150. Report, "O-Ring   Erosion on SRM-15," Thiokol, February 12, 1985, pages 1 through   17.
                                                                                                                                                      - 151. Report, "STS   51-E Flight Readiness Review," Thiokol, February 12, 1985,   TWR-14740 Rev. D., Section 3, page 17.                                                                                                                                                          152. Report, "STS 51-E   Flight Readiness Review, Level 1," NASA, February 21, 1985, page   4.
                                                                                                                                                        - 153. Report, "STS 51-F   Flight Readiness Review, Level 1,'' NASA, July 2, 1985, page   5.                                                                                                                                                            154. Commission   Hearing Transcript, April 3, 1986, page 2469.

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