History Spotlight, 1/28

73 seconds after liftoff on January 28, 1986, the Space Shuttle Challenger suffered a catastrophic failure of one of its right solid rocket booster o-rings. The jet of burning fuel leaking out ignited the main rocket booster and caused the destruction of the orbiter. The destruction of the Challenger is a textbook case of what happens when a bureaucracy is more interested in achieving results than listening to warnings.

So, what went wrong? The contractor who made the O-ring, Morton Thiokol, had only certified the O-rings to work at temperatures above 53 degrees and it was 31 degrees at the launch site. The evening before launch, engineers for Morton Thiokol made their case to NASA to scrub the launch and await favorable weather conditions.

Well, maybe a diversion is in order to explain what the O-rings were supposed to do. The two solid rocket booster (the two on either side of the big one) are made in six sections and welded in to three sections at the factory. At the launch site, the three sections are made into one big booster using the O-rings as joints. So, these rubber O-rings have to withstand temperatures of many thousands of degrees.

When the engineers at Morton Thiokol expressed their reservations at launching in such cold weather, the NASA managers countered that there were two O-rings for each joint and that, if one failed, the other would kick in. This was exactly the wrong kind of thinking. The O-rings were designated as a "Criticality 1" component, meaning that if they went, the shuttle and its occupants would be destroyed. During the investigation afterwards, when Sally K. Ride was grilling NASA managers before Congress, she pointed out that they were not supposed to rely on the backup for Criticality 1 components. The backup was there for an unforeseen failure, sure, but it was not supposed to take the place of the primary and leave no backup at all. In the end, the Morton Thiokol engineers were overruled by their management and NASA management and the launch went as scheduled.

From about a half-second after liftoff to about three seconds after liftoff dark grey puffs of smoke could be seen coming from near the bottom of the right solid rocket booster. This was caused by the casing of the booster buckling under the stress of ignition allowing hot gases of 5,000 degrees to shoot out. This had actually happened several times before and every time, the primary O-ring had fallen out of its place and sealed the breech, so NASA and Morton Thiokol said, "It works well enough!" and wrote it into the design specifications If you're thinking that this is a rather half-assed way to run the most advanced space program in history, you're right. "Well enough" is good for lawn mowers, not for space shuttles.

Puff of smoke

Because of the cold weather, neither O-ring could not fall out of its groove and seal the breach and both were soon mostly vaporized from all the hot gasses rushing past. At that point, corrosion from the burning solid rocket fuel had built up enough to seal the breach and the shuttle continued lifting off, with no one the wiser. As the shuttle lifted off, it throttled up to 104% power, standard operating procedure. Unfortunately, the force of the atmosphere blowing by the solid rocket booster ripped off the "scab" of corroded rocket fuel and allowed the burning fuel to spray out through the joint.

Plume of flame.

The burning fuel jet caused an unbalanced thrust in the rocket and it slammed in to the main booster, causing it to dump its entire load of hydrogen at once. The aerodynamic forces (upwards of 20g, while the orbiter had been designed to only handle 5g) caused the orbiter to break up. It is possible that the astronauts were alive afterward, as several of the Personal Egress Air Packs had been activated, but there was no way they could have survived impact with the Atlantic Ocean at 200g.
The O-rings were the direct cause of the tragedy, but the root cause was NASA and Morton Thiokol's dependence on "well enough."