Historical accuracy in recounting the "fundamentally flawed" SRM joints of the Space Shuttle Challenger Home | About Us | Apr 24th 2014 06:07am

STS-51-L Space Shuttle Challenger Explosion

Challenger ORing Explosion

A Fundament Flaw in SRM Joint Design

One of the most astute summaries of the cause of the Challenger explosion came from a lawyer, not an engineer. David Acheson, one of members of the Presidential Commission on the Space Shuttle Challenger Accident, said the following during the hearings.

"A lot of material we have received, one reads that the designers, presumably both the corporate designers and the NASA supervisors, believe the joint was designed to compress and seal in the gas tight under combustion pressure. And it turns out very quickly in the joint history that it did the opposite. It opened up. I just don't understand why the program then decided to go into a lot of little fixes to see if you could compensate for the fundamental error"

“It is easier to blame a thing than to blame people” was the response of a Navy SEAL and son of one of the Apollo capsule design engineers when told about the joint rotation flaw in the Space Shuttle solid rocket motor (SRM). Indeed, it is easier to blame cold temperatures and a decision made at the last minute and under management pressure rather than to blame an engineering design flaw which was ignored by the engineering departments of Morton Thiokol and NASA’s Marshall Space Flight Center. This flaw was ignored for years, in spite of numerous warnings both from the SRM’s and from engineers who flagged the problem.

Parker Hannifin is one of the world’s leading authorities on O-Ring design. In the Parker O-Ring Handbook ORD 5700 paragraph 4.0 says “It has been said that O-rings are ‘the finest static seals ever developed.’ Perhaps the prime reason for this is because they are almost human proof …. If the gland has been designed and machined properly”.

The disaster of Challenger mission STS-51-L was the result of human engineering error. The simple “no spin” truth is the O-ring gland was not properly designed. While this issue surfaced a number of times during the hearings of “Presidential Commission on the Space Shuttle Challenger Accident”, the Commission focused on cold temperatures and the last minute decision to launch during cold temperatures. Perhaps the consequences of highlighting the fundamental problem would not have been in what Chairman Rogers called “the national interest”.

The theme of Allan McDonald’s book “Truth, Lies and O-Rings” is that cold temperature was the major culprit in the Challenger explosion. However, McDonald admits repeatedly the joint design was flawed and this was the primary problem.

The flaw is simply this: when pressurized, the joint opens up and the O-ring gland becomes grossly improperly configured. Cold O-rings may well aggravate this condition but a lot of things aggravate an incorrect O-ring gland configuration.

Following are brief excerpts from “Truth, Lies and O-Rings”. Note that emphasis has been added. Also note that FWC is an acronym for Filament Wound Case. FWC were composite cylinders which reduced SRM weight. Hercules Aerospace was developing the FWC SRM design for NASA Marshall Space Flight Center and their use would have significantly improved Space Shuttle performance.

Page 61 – “However, if the rapid deflection of the joint during pressurization prevented O-ring contact with the metal, then all bets were off. …. Furthermore, any significant amount of erosion directly on the sealing surface would probably result in a continuously leaking joint that would eventually lead to a catastrophe.

Page 66 & 67 – “I made the presentation at NASA headquarters on August 19, 1985….One of my viewgraphs, entitled “Primary Concerns” was “Field Joint – Joint Deflection and Secondary O-ring Resiliency. ….Another recommendation that I made in the presentation was to include a capture-feature (a metal lip on the tang side) of the field-joint to restrict the motion in the joint during case pressurization at ignition.”

Page 31 – “The FWC field-joints included a metal-capture lip on the tang side of the joint that significantly reduced the rotation, or opening, of the joint during pressurization. … It was a unique arrangement that was part of the Hercules design for the FWC.

Page 69 – “In September 1985, the task force submitted its first recommendation to NASA Marshall for solving the O-ring seal problem: to incorporate the capture-feature in the field-joint.”

Page 217 – “The original analysis of the joint, which had been conducted back in the mid-1970’s, indicated that seal redundancy was maintained, because the joint closed during pressurization. It wasn’t until after considerable hardware had been fabricated and tested that it was realized that the joint didn’t close during pressurization; it actually opened.

Pages 288 & 289 – “Thiokol’s senior management … calling a meeting in late February 1986 …Jack Kapp’s speech was effective, because Jack was the first one to be promoted in the new organization, chosen to be Director of Engineering Design for the redesign effort – even though some twelve years earlier he had been most responsible for the poor SRB field-joint design in the first place…..it had been his analysis that said the joint should close during pressurization, when in reality the joint opened. That mistake was known by MTI and NASA management and should have been corrected long before any Shuttle was ever flown.

Page 290 – “Even though the design had been developed in the mid-1970’s with the best analytical tools available in the industry at the time, the fact remained that Thiokol’s original analysis indicated the joint would close during pressurization when, in fact, early hydro-tests indicated that it really opened. This was the most fatal flaw in the SRB design….”

Allan McDonald omits the fact that the capture feature was invented by Fred Policelli, who was the Program Technical Manager at Hercules. In fact, as his book progresses it becomes “our capture feature” rather than Mr. Policelli’s capture feature.

“It is easier to blame a thing than to blame people”. Indeed! It was easier and more expedient to blame an abstract thing like cold temperatures and to blame a hasty decision, than it was to blame the engineers who incorrectly designed the SRM field-joint and the engineers who, over a period of about 10 years, ignored the myriad of warning signs and the engineering proof of this “most fatal flaw”. STS-51-L Space Shuttle Challenger exploded because of the fatal flaw in the joint design. The loss of the Challenger Crew and the destruction of the Challenger Space Shuttle were the result of almost a decade of collective denial, cognitive dissonance, ignored warnings and a fundamental breakdown of engineering ethics.

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