On January 28th 1986, the scheduled launch for NASA’s space orbiter- the Challenger took place as planned. However, 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. After extensive failure analysis, it was determined that the failure of 2 rubber O-rings used to seal sections of the right side Solid Rocket Booster (SRB, the 2 white pillars attached on either side of the shuttle stack) caused the disaster.
To understand why the O-rings failed, it becomes important to understand a basic concept of polymeric 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.
A good way to understand why this transition occurs is to use an analogy 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.
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 and lost its ability expand during the heat of the launch to seal the fuel tanks properly. This allowed leakage of exhaust gases from the SRB which heated and compromised the External Tank (ET, the large orange structure of the shuttle stack). This caused uncontrolled combustion between the liquid hydrogen and oxygen, spontaneously combusting most of the propellant, resulting in the destruction of the fuel tanks and causing the shuttle to break apart.
This was the first time NASA launched shuttles at such low temperatures. The coldest temperature of a previous launch was 11oC higher. However, this did not go unnoticed. 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.
As it turned out, the culture at NASA back then was such that they were far more concerned with the public relations issues that would arise by not launching the shuttle that day than they were about the technical issues that would arise by launching that day. Christa Mcauliffe was a civilian teacher who was flying in the space shuttle as part of the ‘Teachers in Space’ program by NASA. She would give 2 live 15 minute lessons from space on the 4th day of the mission on Friday when all school children would still be in school to see the lessons. A delay in the launch by a day would mean no children would be in the school on a Saturday to see it, making delays a huge PR problem for NASA. They also had to ensure the launch stuck to schedule because the proposed flight rate of up to 50 launches per year was nowhere near the case in reality, on account of its workforce size, with an average of about only 5 launches happening per year. All of this meant that the concerns of the engineers were not properly conveyed to the authorities higher up and the team responsible for assessing launch readiness. This was a recipe for disaster just waiting to happen.
Almost a billion US dollars and, more importantly, seven lives were lost because of human negligence. Lawsuits 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 not only goes on to prove the importance of materials used when designing structures but also the necessity to prioritise safety by a large margin over just about any other issue.
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