Historical accuracy in recounting the "fundamentally flawed" SRM joints of the Space Shuttle Challenger Home | About Us | Apr 22nd 2018 06:29pm

Understanding the Challenger Disaster

Alfred L. Newberry, PE
Fiberglass Engineering Mechanics

Hans Christian Anderson’s 1837 story “The Emperor’s New Clothes” is a metaphorical tale of collective denial. Collective denial aptly describes the repeated failures over a period of years by NASA and Morton Thiokol to recognize and correct what Commissioner David Acheson so aptly termed the “fundamental error” in the SRM joint design.

James Thomas, President of ASTM, wrote “if you say something long enough and often enough, people will believe it.”1 The fundamental message which came out of the Presidential Commission on the Space Shuttle Challenger Accident was that the main cause for the destruction of STS-51-L was cold temperatures resulting in cold O-Rings. This claim has been repeated “long enough and often enough” that people believe it. However, the facts are “cut and dried”. The Challenger disaster occurred because of a fundamental error in the joint design. Cold temperature was only a factor because of the joint misdesign.

A metaphorical tale as far fetched as the Emperor’s New Clothes” and in the genre of a typical “engineer joke” may prove useful in communicating the breakup of STS-51-L. This tale is based on Russian Roulette which Wikipedia defines as “a potentially lethal game of chance in which participants place a single round in a revolver, spin the cylinder, place the muzzle against their head and pull the trigger.” Russian Roulette, while not commonplace, occurs often enough to prompt a medical journal article2.

An Engineering Team Which Tackles Russian Roulette

It comes to the attention of a civic minded engineer that a group of teenagers in his community are planning a game of Russian Roulette in an abandoned warehouse. Alarmed, the engineer calls a group of his friends who mobilize an ad hoc task force to address the problem.

The engineers form a strategy.

  1. They measure the time it takes for the average teenager to pull the gun’s trigger
  2. They measure the time it takes for the trigger to release the firing pin.
  3. They measure the time it takes for the firing pin to reach the primer.
  4. They measure the time it takes for the primer to ignite and in turn ignite the powder.
  5. They plot the pressure versus time curve in the cartridge.
  6. They plot the acceleration, velocity and displacement versus time curves of the bullet.
  7. They measure the time it takes for an average teenager to rotate the gun so the bullet misses.
  8. Being thorough as they are, the engineers note that cold temperatures affect the “timing functions” and create time versus temperature curves for all of the above. The engineers note the warehouse is not heated, so cold temperatures are possible, but feel it is unlikely the teenagers will play the game when it is cold in the warehouse.
  9. The engineers design an energy absorbing backstop so the escaping bullet won’t damage the wall or other furnishings.
  10. The engineers also research the issue of hearing damage and ultimately recommend the small foam ear plugs. While not as effective as over-the-ear protectors, the plugs won’t interfere with rotating the gun away from the head.

The engineers then set about to educate the teenagers on all these parameters and to monitor the progress of the rounds of Russian Roulette. While there are many close calls, grazed scalps, etc. the game is played over twenty times without a major mishap. The engineers continue to tweak their methods and refine their timing functions.

All is well until one day the engineers learn that the teenagers have decided to bundle up and play on a particularly cold day. With great alarm the engineers review their “timing functions” and also note with great concern that the heavy coats will slow the rotation of the gun away from the head. The engineers convene with the parents and express their worries and concerns and recommend that the game be delayed a couple of days as the temperatures will improve. The parents don’t want to upset the kids so allow the game to proceed. A teenager is killed. The DA charges the engineers with contributing to a homicide and they are brought to trial.

An absurd and ridiculous and bizarre story for sure. Then again so is the Emperor’s New Clothes. An Engineering Team Which Tackles Russian Roulette is a metaphorical tale or parable3 illustrating the absurdity of creating a science out of a fundamentally flawed situation. What should the engineers have done? – everything possible to stop the very first game of Russian Roulette. Rather than studying timing functions and the effects of cold temperatures and worrying about hearing damage they should have, in the words of the venerable Churchill, “done what is necessary” to stop an incredibly dangerous and fool hardy game. The “fundamental flaw” is the game of Russian Roulette itself and no amount of little fixes and timing function studies offset that inherent danger.

The Russian Roulette tale is very analogous to the approach Morton Thiokol (MTI) and NASA took toward the extremely dangerous Space Shuttle Solid Rocket Motor (SRM) flawed joint. As is discussed in the “History of Space Shuttle SRM Field-Joint Capture Feature by Fred Policelli” section of this website, it was well known before the first shuttle flight that the joint opened up.

It is safe to say that any objective person, upon understanding the real problem in the SRM joints, would agree that the MTI engineers should have “done what was necessary” to stop Space Shuttle flights. To create a bizarre science of what Acheson called “little fixes” rather than fixing the “fundamental flaw” was unethical to the ultimate degree.

An important chapter in the SRM joint issues is overlooked and/or minimized in most all of the reports and publications dealing with the Challenger disaster, namely, the role of Hercules Aerospace. Hercules had a contract with NASA Marshall Space Flight Center to develop the Filament Wound Casing Solid Rocket Motor (FWC SRM) to reduce weight and improve shuttle performance.

During the course of their meetings with MSFC and Thiokol and during the course of their own engineering analysis, Hercules became aware of the seriously flawed SRM field joint.

After a Technical Information Meeting at MSFC held December 2 thru 4, 1981, Fred Policelli, the Hercules FWC SRM Technical Manager, invented the Capture Feature which greatly reduces joint rotation and eliminates the O-ring problem.

Fred Policelli's Capture FeatureThe Capture Feature is the major element of the redesigned SRM joint and flies on the Shuttle today.

The structural analysis team at Hercules Aerospace used Finite Element Analysis4 to further understand the joint, the opening problem and the benefits of the capture feature. Hercules brought this work to the attention of both MTI and NASA. Some of the Hercules FEA models are published on pages 471 and 472 of Volume 1 of ASM’s Engineered Materials Handbook5.

It is the opinion of this author, a licensed professional engineering in 37 states, that the MTI engineers did not act ethically because they did not vehemently oppose the flawed SRM joint design from the time frame Commissioner Acheson calls “early on”. Parabolically they were “An Engineering Team Which Tackles Russian Roulette”. According to the NCEES6 Model Rules for engineers “the first and foremost responsibility is the public welfare”.

Did the fictitious Russian Roulette engineers act ethically in not opposing the dangerous game? Did their complex studies of the dangerous game constitute ethical conduct?

Did the MTI Shuttle SRM engineers act ethically in not opposing the fundamentally flawed booster joints? Did their complex studies of “little fixes” constitute ethical conduct?

1Thomas, James “Plain Talk for a New Generation – If You Say It Long Enough”, ASTM Standardization News, February 2002

2Shields LB, Hunsaker JC, Stewart DM (March 2008). "Russian roulette and risk-taking behavior: a medical examiner study". Am J Forensic Med Pathol 29 (1): 32–9

3The word “parable” comes from two Greek words “para” which means along side and “ballo” which means to throw. Hence, a fictitious narrative told to illustrate an important truth.

4Finite Element Method (FEA) is perhaps the greatest advancement in solid mechanics since the development of calculus. See http://en.wikipedia.org/wiki/Finite_element_analysis for more information on FEA. There are many such articles on the internet.

5Dropek, Richard “Numerical Design and Analysis of Structures”, ASM International Engineered Materials Handbook -- Composites Volume 1, 1987 ISBN 0-87170-279-7

6NCEES – National Counsel of Examiners for Engineers and Surveying

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