Thermodynamics of the Challenger Explosion
“It is my honest and very real fear that if we do not take immediate action to dedicate a team to solve the problem?we stand in jeopardy of losing a flight along with all the launch pad facilities.” This warning was given in a memorandum from Roger M. Boisjoly, an engineer at Morton Thiokol, NASA?s rocket contractor, to Robert K. Lund, vice president of engineering. Unfortunately, this “very real fear” came true on January 28, 1986, when the Space Shuttle Challenger exploded just 73.63 seconds after ignition of the rocket boosters. As third graders, we watched this horrendous explosion on television at school. It was an experience none of us will forget. This paper will discuss the thermodynamic explanation for the explosion of the Challenger, which was the failure of the rubber O-rings that seal the joints of the solid rocket booster sections.
In order to understand the mechanics of the Challenger, the O-rings must be seen with respect to the framework of the shuttle. The solid rocket boosters are located on either side of the external fuel tank, which is attached to the orbiter. A solid rocket booster (SRB) consists of four segments, held together by three field joints. These field joints are sealed by both primary and secondary O-rings. On the Challenger, the O-rings are twelve feet in diameter, with a thickness of 0.280 inches (7.11 x 10-3 m). The function of O-rings in an SRB is to seal the gaps between the tang and clevis joint, which are joints that hold the segments of the SRB together. O-rings are sealed by gas pressure that is produced upon combustion, inside the tang and clevis joint. The gas pressure pushes on the O-ring from all angles, forcing it into the gap between the tang and clevis, therefore, sealing the joint .
The temperatures before the launch made the O-rings too brittle to fill the gaps in the joint. During the 38 days the rocket sat on the launch pad at Kennedy Space Center, the O-rings had been flattened into their grooves. In this respect, the O-rings were unable to regain their original shape, which allowed fuel to blow past them in the bottom segment of the right SRB when the SRBs were ignited. The fuel eroded the O-rings, and burned them to the point where a flame burst through the joint. According to the Glass Transition Temperature theory, when an elastomer is exposed to extremely low temperature, the elastomer becomes brittle and loses its elasticity. In the case of the Challenger, the low temperature was 28o F. This was low enough to make the O-ring rigid and brittle.