THE SPACE SHUTTLE CHALLENGER AND COLUMBIA ACCIDENTS
The NASA Space Shuttle Disasters
The space shuttle is one of the most complex engineered systems ever built. The challenge of lifting a space vehicle from earth into orbit and have it safely return to earth presents many engineering problems. Not surprisingly, there have been sev- eral accidents in the U.S. space program since its inception, including two failures of the space shuttle. The disasters involving the space shuttles Challenger and Columbia illustrate many of the issues related to engineering ethics as shown in the following discussion. The space shuttle originally went into service in the early 1980s and is set to be retired sometime in 2011 or 2012.
The Space Shuttle Challenger Disaster
The explosion of the space shuttle Challenger is perhaps the most widely written about case in engineering ethics because of the extensive media coverage at the time of the accident and also because of the many available government reports and transcripts of congressional hearings regarding the explosion. The case illustrates many important ethical issues that engineers face: What is the proper role of the engineer when safety issues are a concern? Who should have the ultimate decision- making authority to order a launch? Should the ordering of a launch be an engi- neering or a managerial decision? This case has already been presented briefl y, and we will now take a more in-depth look.
Background
The space shuttle was designed to be a reusable launch vehicle. The vehicle consists of an orbiter, which looks much like a medium-sized airliner (minus the engines!), two solid-propellant boosters, and a single liquid-propellant booster. At takeoff, all of the boosters are ignited and lift the orbiter out of the earth’s atmosphere. The solid rocket boosters are only used early in the fl ight and are jettisoned soon after takeoff, parachute back to earth, and are recovered from the ocean. They are sub- sequently repacked with fuel and are reused. The liquid-propellant booster is used to fi nish lifting the shuttle into orbit, at which point the booster is jettisoned and burns up during reentry. The liquid booster is the only part of the shuttle vehicle that is not reusable. After completion of the mission, the orbiter uses its limited thrust capabilities to reenter the atmosphere and glides to a landing.
The accident on January 28, 1986, was blamed on a failure of one of the solid rocket boosters. Solid rocket boosters have the advantage that they deliver far more thrust per pound of fuel than do their liquid-fueled counterparts, but have the dis- advantage that once the fuel is lit, there is no way to turn the booster off or even to control the amount of thrust produced. In contrast, a liquid-fuel rocket can be con- trolled by throttling the supply of fuel to the combustion chamber or can be shut off by stopping the fl ow of fuel entirely.
In 1974, NASA awarded the contract to design and build the solid rocket boost- ers for the shuttle to Morton Thiokol. The design that was submitted by Thiokol was a scaled-up version of the Titan missile, which had been used successfully for many years to launch satellites. This design was accepted by NASA in 1976. The solid rocket consists of several cylindrical pieces that are fi lled with solid propellant and stacked one on top of the other to form the completed booster. The assembly of the propellant-fi lled cylinders was performed at Thiokol’s plant in Utah. The
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8 1.8 Case Studies
cylinders were then shipped to the Kennedy Space Center in Florida for assembly into a completed booster.
A key aspect of the booster design are the joints where the individual cylinders come together, known as the fi eld joints, illustrated schematically in Figure 1.1a . These are tang and clevis joints, fastened with 177 clevis pins. The joints are sealed by two O-rings, a primary and a secondary. The O-rings are designed to prevent hot gases from the combustion of the solid propellant from escaping. The O-rings are made from a type of synthetic rubber and so are not particularly heat resistant. To prevent the hot gases from damaging the O-rings, a heat-resistant putty is placed in the joint. The Titan booster had only one O-ring in the fi eld joint. The second O-ring was added to the booster for the shuttle to provide an extra margin of safety since, unlike the Titan, this booster would be used for a manned space craft.