On January 28th 1986, the scheduled launch for NASA’s space orbiter- the Challenger, took place as planned. But the launch was the only part of the mission that stuck to plan. 73 seconds after liftoff, the shuttle exploded and broke up, bringing a devastating end to the spacecraft’s 10th mission, claiming all 7 lives onboard. All because of a tiny, rather overlooked aspect of a structure failing due to a very basic property of the given material.
After extensive failure analysis, it was determined that the failure of 2 rubber O-rings caused the disaster.
To understand why the O-rings failed, the answer lies in a basic concept of polymer materials- the glass transition temperature. When a polymer is cooled below this temperature, it becomes hard and brittle, like glass. The temperature at which this happens is called glass transition temperature( Tg). Each and every polymer material has a glass transition temperature that varies with its structure, substituent groups, molecular mass etc.
An analogy to make things clear
A good way to understand why this transition occurs is to use an anology of snakes. Polymers exist as highly coiled chains. Consider a room full of snakes. Let each snake represent a polymer chain. We know that snakes are cold blooded. When it’s warm, snakes move more, slither and slide over each other easier. The snakes move randomly. Now, when the temperature is reduced, snakes generally don’t move much. They slow down and tend to stay still. They’re still wrapped around, over and under each other, but there is no motion. Polymer chains work similarly. This lack of movement in the polymer chains causes it to become a brittle mass.
Scientific background to the problem
The O-Ring in the Challenger shuttle was made of nitrile rubber- a synthetic variant of natural rubber. The glass transition temperature of nitrile rubber used was around 1-2o C. Due to the cold ambient temperature along with the ice collected on the launch pad due to some unusual temperatures, the rubber became a brittle mass losing its ability to seal the fuel tanks properly. This allowed leakage of exhaust gases, which caused uncontrolled combustion between the liquid hydrogen and oxygen, destroying the fuel tanks and causing the shuttle to break apart.
It is interesting to note that the engineers involved in the design of the shuttle did warn their superiors about certain components of the shuttle failing at low temperatures but, for whatever reasons, these warnings were unheeded. This was also the first time NASA launched shuttles at such low temperatures. The coldest temperature of a previous launch was 11oC higher.
Impact of the disaster
Almost a billion US dollars and, more importantly, seven lives were lost because world renowned scientists and engineers forgot to keep in mind basic concepts of simple materials. Law suits were filed against both NASA and the company responsible for designing the rocket boosters and the tanks- Morton Thiokol.
To this day, this incident is a case study in many discussions of engineering safety and workplace ethics. This incident only goes on to prove the importance of materials used when designing structures.