Challenger Disaster

An investigation through the technical descriptions and analysis behind the structural failure ,

An evolutionary desire to journey to the unknown is the most distinctive characteristic of mankind from other species. Leading to that role, NASA’s space shuttle Challenger was the second shuttle to reach space and completed 9 missions with success. On the other hand, Challenger was also NASA's darkest tragedy. On its 10th launch, on Jan. 28, 1986, the shuttle exploded 73 seconds after liftoff, killing the seven crew members. In addition to several impacts on different subjects, the disaster became a crucial milestone for humans examination process about understanding the nature. Regarding these significant impacts of the 1986's Challenger Spacecraft disaster on American society and the entire human race, this attempt of fooling the nature should be investigated through the technical descriptions and analysis behind the structural failure, the engineering ethics concerning the social relationships and forward looking regulations in addition to their remarkable impacts on future space technologies. However, in this "relatively" short article, we will mainly focus on the technical analysis.

Instead of concluding with clear decisions and arguments about the structural failure that the shuttle had to encounter, the technical descriptions and analysis of the basic parts of the spaceship should be investigated through the scientific approaches. Basic mechanical concepts of the shuttle consist of several installments which are the critical portions of the construction. The space shuttle Challenger basically consists of the fuel tank, flanked by two solid-fuel rocket boosters, attached to the orbiter, whose main engines burn liquid hydrogen and liquid oxygen (see Fig.1), and one of the most vital pieces of sealing materials are called  ‘o rings’. When scientists at the Jet Propulsion Laboratory discovered that The Thiokol Chemical Company’s polymers made ideal binders for solid rocket fuels, manufacture of o rings and these polymers became Thiokol’s initial business (Feynman, 1988). The solid rocket boosters (or SRBs) are key elements in the operation of the shuttle that produces enough thrust to overcome the earth's gravitational pull and achieve orbit, thus, to produce much more thrust per pound than their liquid fuel counterparts (NASA, 1986) they have to be designed properly. As it is investigated by the Thiokol engineers, the night before the launch the temperatures were unexpectedly cold and the effects of these temperatures on the rockets boosters, joint rotation and o-ring seating was the main concern. In spite of their unstable conditions and the risks, the management decided that the engineers’ data was inconclusive and gave permission for the launch (Lewis, 1988). On the launch day the weather at the Cape Canaveral was recorded as “the coldest on which NASA had ever attempted to launch a spacecraft.”

Figure 1.  The space shuttle Challenger. The fuel tank, flanked by two solid-fuel rocket boosters, is attached to the orbiter, whose main engines burn liquid hydrogen and liquid oxygen. National Aeronautics and Space Administration. (1986). Pre-launch activities, technical description and analysis. Retrieved from http://history.nasa.gov/rogersrep/v2appf.htm http://history.nasa.gov/rogersrep/v2appf.htm. (© NASA.)

            As it is indicated in the official presidential report (U.S. Government Printing Office, 1986)  at .678 seconds into the flight, it has been approved by the photographic data that a gray smoke was effused from the joint on the right solid rocket booster (see Fig.2).In addition to that data, computer graphic analysis from other cameras (see Fig 3.) indicated the initial smoke came from the 270 to 310-degree sector of the circumference of the aft field joint of the right Solid Rocket Booster which faces the External Tank. The vaporized material streaming from the joint pointed out there was not a complete sealing action within the joint (NASA, 1986). As it is pointed out, this sealing action is provided by o ring materials and each SRB joint is sealed by two o-rings: the bottom ring known as the primary o-ring, and the top known as the secondary o-ring (see Fig.4). The purpose of the rings is to prevent hot combustion gasses from escaping from the inside of the motors that is given in Figure 2. Detailed investigations of the launch activities showed that the temperature was outside the operation specifications of the rings and the cold weather could pose a safety hazard to the joint rotation and o-ring seating of the shuttle as the Thiokol engineers worried. Number of incidents with respect to the calculated joint temperature can also give an appreciable idea of the effects of the temperatures (see Fig. 5). All in all, knowing the basic mechanical concepts that keep a space shuttle ‘safely’ flying is the first step to gain an understanding of the structural failure, moreover, looking for the fundamental sciences such as basic physics of materials should have been the primary action of Thiokol engineers instead of determining the safety factors that this paper will be investigating in detail.

Figure 2. Detailed picture, taken from the launch pad, of the "gray smoke. ". National Aeronautics and Space Administration. (1986). Retrieved from http://history.nasa.gov/rogersrep/v2appf.htm. (© NASA.)

Figure 3. Three examples of o-ring erosion. Such erosion would occur unpredictably along 2 or 3 inches of the 37 foot o-ring. . National Aeronautics and Space Administration. (1986). (© NASA.)

Figure 4. Joint rotation is caused by pressure from inside the rocket pushing the walls out farther than the joints. A gap opens, and gas flows past one or both of the 0-rings. Primary and secondary o-rings are marked with circles.

Figure 5. Number of incidents with respect to the calculated joint temperature give an appreciable idea of the effects of the tempratures. . National Aeronautics and Space Administration. (1986). Pre-launch activities, technical description and analysis.

To gain a basic understanding of the disaster, the structural failure should be examined with the fundamental sciences. The science of materials is the main concern regarding the aggressive environments and presence of specific temperatures. Under the aggressive environments and presence of specific temperatures the tensile stresses or strains on the outside of the surface become important issues. The rate of diffusion in the material increases as the stress increases, which can give rise to the problem of reduction in material strength. As a result of the tensile stresses, any flaw or crack in the material can propagate under these stresses and may make the situation worse (Singh, Dillard, Hyer, Case & Thangjitham, 2009). Due to the wide range applications and direct relevance to the usage of the seals in Challenger, elastomeric structure should be examined in detail. An elastomer is a polymer with the property of “elasticity”, generally having notably low Young’s modulus and high yield strain compared with other materials (McKeen, 2014). The term is often used interchangeably with the term rubber. Stress relaxation and aging in strained polymer networks can be attributed to two distinct processes. The process causing stress relaxation may be physical or chemical in nature, and under normal conditions, both processes can occur simultaneously. Physical relaxation involves the motion of molecular chains towards new configurations in equilibrium, chemical relaxation determines these effects with the breakage and formation of covalent bonds (Singh, Dillard, Hyer, Case & Thangjitham, 2009).

            If the physical and chemical relaxation types are considered together, it can be clearly claimed that the most important approximation about the temperature dependence and aging exposures is based on the Arrhenius relationship, and it suggests that a chemical erosion process is controlled by an exponential reaction rate  (Patterson et al., 2014). This relationship can be used to estimate the seal material lifetime under the specific environments.

      It has been stated in this article that one of the most vital pieces of sealing materials, used to seal SRB’s joints are the o rings and their manufacturing steps, as well as material properties, should be investigated particularly. Regarding the sensitive product properties o rings, manufacturing steps include particular examinations, especially in the quality control of the material. Determination of the tensile strength, hardness, relaxation rate, specific gravity, and compression is vital to obtain reliable products. On the other hand, the proper mixture of the compound and production of the rubber is crucial to prevent any squeezing and molding that can damage the entire process of manufacture (Chandrasekaran, 2010). Most of the seals are intended to be used under compression (static or dynamic applications) and when a certain amount of compressive strain is applied. Understanding the viscoelastic behavior of an elastomeric seal under compression is crucial to determine the dynamic effects on Challengers o rings. For an elastomeric o-ring, the sealing capability depends on the contact stresses that occur between the o-ring and the surfaces with which it comes into contact. The leakage can be observed when the peak contact stress, reduces to a value lower than the pressure differential across the seal.

      For the purpose of understanding the deformation behavior of an o-ring, a sample can be placed between chambers under the pistons and the decay of stress, which is a direct measure of the molecular chains of the network that can be determined with developing contact pressure, contact width, and distribution of stress and strains profiles of a custom seal cross-section using finite element analysis techniques along with theoretical and experimental validation of finite element analysis results. A simulation of the seal compression between two plates, with the two-dimensional, nonlinear, axisymmetric analysis is given in Figure 6 (Singh, Dillard, Hyer, Case & Thangjitham, 2009). Illustration of the o-ring under compression is shown with original and deformed configurations together in Figure 7 (retrieved from www.highpowermedia.com). To sum up, it is obvious that the material science behind the o ring structure gives an appreciable and undeniable clue about the structural failure of the shuttle. The reaction rate of material decreases as the temperature decreases. Even if the mathematical dependencies between other parameters can be effective to determine the deformation (or erosion), by the way of inference, the relation between the temperature dependence of o rings and the night that shuttle was launched can be seen clearly.

Figure 6. Finite element simulation of an o-ring under compression with a surface crack. Singh, K. H., Dillard, A., Hyer, W., Case & Thangjitham. (2009). Lifetime prediction and durability of elastomeric seals for fuel cell applications. (p.121). Unpublished doctoral dissertation Virginia Polytechnic Institute, Virginia, USA.

Figure 7. Illustration of the o-ring under compression is shown with original and deformed configuration. Singh, K. H., Dillard, A., Hyer, W., Case & Thangjitham. (2009). Lifetime prediction and durability of elastomeric seals for fuel cell applications. (p.184). Unpublished doctoral dissertation Virginia Polytechnic Institute, Virginia, USA.

       As a result of the engineering failure that pushed the limits of a natural occurrence, the significance of fundamental sciences has become a focal point of social discussions. The most objective and cold investigations about the disaster were conducted by a well-known anti-authoritarian physicist Richard P. Feynman. The outcomes learned from the unwillingly prepared report (1986) by Feynman are vital to see a physicist’s point of view on the Challenger subject. As it is mentioned by Feynman, the main problem with the construction of the joint in the ship was the wrong utilization of o-rings. In general cases, the o-rings are fixed at some position while the gaps between the sliding and rotating parts remain constant, but in the case of the shuttle, these gaps increase as the pressure increases. In this condition, the resilience and reaction rate of the material or the rubber itself become important issues to maintain the seal, which has to return its initial position fast enough to close the gap. In spite of the fact that these problems were discovered by Thiokol engineers and the company that was responsible for the manufacture of o-rings (Parker Seal) warned them about the usage of the rings, the program kept going and the shuttle kept flying (Feynman, 1988).

Despite the different results obtained from previous samples, officials behaved as if they understood it, explaining it to each other with the logical arguments. They assigned a “safety factor of three” instead of being very concerned about these poorly understood conditions that could have created a deeper erosion. The boosters of the shuttle were not designed to erode and erosion was not something that can determine the “safety factor”, this was a clue that something was wrong (Feynman, 1988). Being very concerned and uncomfortable around the officials (Leighton, 2000, for more information), at the public meeting of the Presidential Commission on 11 February 1986, Feynman easily demonstrated the effect of a low temperature on a specific o-ring by dipping pieces of rubber in a pitcher of ice water (see Fig.8). To conclude, it is encouraging to see a physicist’s approach on pure engineering concepts. As a result of the misconceptions about the symmetry in the laws of nature and persistence of not understanding the laws of physics perfectly, engineering problems interfered with public relations are becoming more and more destructive. It has to be understood that nature cannot only be interpreted by numerical calculations but also the meanings of this calculations. As it is argued by Feynman (1986), “For a successful technology, reality must take precedence over public relations, for nature cannot be fooled.”

Figure 8. The O-ring ice-water demonstration of Richard Feynman at the public meeting of the Presidential Commission on 11 February 1986. National Aeronautics and Space Administration. (1986) (© NASA.)

The only way of avoiding the destructive results of poorly understood subjects by engineers and politics is appreciating the role of the fundamental sciences. The engineering ethics and non-technical effects such as policy should be analyzed carefully since the technical analysis is required to be considered with respect to the ethics and these political effects. Moreover, history repeats itself as long as the implementations are considered to be temporary actions to cast a shadow on the investigations. Considering the Challenger’s impacts on future space technologies, we must confront the reality of interstellar “challenge” as the next step of the search for the unknown.

References

            Boisjoly, R. P., Curtis, E. F. & Mellican, E. (1989). Roger Boisjoly and the Challenger   Disaster: The ethical dimensions. Journal of Business Ethics, 8(4), 217-230. Retrieved from http://link.springer.com/article/10.1007%2FBF00383335#page-1

Chandrasekaran, C. (2010). Rubber seals for fluid and hydraulic systems. (p.100). USA:      Elsevier. doi:10.1016/B978-0-8155-2075-7.10015-2

Feynman, G., Leighton, R. (1988). What do you care what other people think: further adventures of a curious character. USA: W. W. Norton & Company. 

Feynman, M. (Ed.). (2005). Don't you have time to think?. (  ed.). (pp.397-399). England:  Clays Ltd, St Ives plc.

Feynman, R. (1988, February). An outsider's inside view of the Challenger inquiry. Physics   Today, 41(2), 26-37. doi: 10.1063/1.881143  

           Hastings, D. (2003). The Challenger Disaster. Retrieved from Massachusetts Institute of       Technology website: http://ocw.mit.edu/courses/engineering-systems-division/esd-10-introduction-to-technology-and-policy-fall-2006/readings/challenger.pdf

Leighton, R. (2000). Tuva or Bust!: Richard Feynman's last journey. (p.39). New York: W.  W. Norton & Company. 

Lewis, R. S. (1988). Challenger: The final voyage. (pp.4-8). New York: Columbia University  Press.

Logsdon, J. (1998). Return to flight: Richard H. Truly and the recovery from the Challenger accident. In Pamela M. (Ed.), From engineering science to big Science: The NACA   and   NASA Collier trophy research project winners (pp. 345-364). Washington, D.C.:   NASA.

Martin, M. & Boynton,  A. (2005, February). From liftoff to landing: NASA’s crisis communications and resulting media coverage following the Challenger and  Columbia tragedies. Public Relations Review, 31, 253–261.

          Martin, M. W. & Schinzinger, R. (2005). Ethics in Engineering (4^th ed.). New York: McGraw Hill.

McKeen, W. (2014). The effect of long term thermal exposure on plastics and elastomers.     PDL handbook series. (pp.239-241). doi:10.1016/B978-0-323-22108-5.00011-4

          Moomaw, B. (2003, February 2). The space age born of the Cold War is over. Space Daily.  Retrieved from http://www.spacedaily.com/news/oped-03e.html

National Aeronautics and Space Administration. (1987). Implementation of the recommendations of the presidential commission on the space shuttle Challenger accident. Retrieved from http://history.nasa.gov/rogersrep/v6index.htm

National Aeronautics and Space Administration. (1986). Actions to implement the recommendations of the presidential commission on the space shuttle Challenger accident.            Retrieved from http://history.nasa.gov/rogersrep/actions.pdf

National Aeronautics and Space Administration. (1986). Pre-launch activities, technical description, and analysis. (pp.11-14). Retrieved from                                     http://history.nasa.gov/rogersrep/v2appf.htm

Patterson, A., Ferreira, P., Banks, E., Skeene, K., Clarke, G., Nicholson, S. & Rawlinson-  Malone, C. (2014, November). Modelling drug degradation in a spray dried polymer dispersion using a modified Arrhenius equation. International Journal of     Pharmaceutics, 478,     348–360.

Robbins, J. (Ed.). (1999, July). The pleasure of finding things out. (pp.22-25). USA: Perseus       Books. 

Sealing Elements, technical handbook. (n.d). Retrieved from www.eriks.info  

Singh, K. H., Dillard, A., Hyer, W., Case & Thangjitham. (2009). Lifetime prediction and durability of elastomeric seals for fuel cell applications. Unpublished doctoral dissertation, Virginia Polytechnic Institute, Virginia, USA.

Teacher Christa McAuliffe’s husband since Challenger. (2011, January 28). USA Today. Retrieved from http://usatoday30.usatoday.com/tech/science/space/2011-01-28-RW_challenger_mcauliffe_ST_N.htm

U.S. Government Printing Office. (1986). Report of the presidential commission on the space shuttle Challenger accident. (Report No: 99-1016). Retrieved from      http://er.jsc.nasa.gov/seh/explode.html

Watson, T. (2011, January 30). 25 years later: How the Challenger disaster brought NASA down to earth. USA Today. Retrieved from http://usatoday30.usatoday .com/tech/science/space/2011-01-26-1Achallenger26_CV_N.htm

Watson, T. (2011, January 28). U.S. human spaceflight and the road ahead. USA Today. Retrieved from http://usatoday30.usatoday.com/tech/science/space/2011-01-26-1Achallenger26_VA_N.htm