Space Shuttle Challenger, 1986: A Joint Seal and a Launch Decision

What happened

The Space Shuttle Challenger (mission 51-L) was lost shortly after launch on January 28, 1986. The Presidential Commission on the Space Shuttle Challenger Accident, often called the Rogers Commission, reported its findings to the President on June 6, 1986.

What the Commission found

• Physical cause: the Commission concluded that the loss was caused by a failure in the joint between the two lower segments of the right Solid Rocket Motor. The specific failure was the destruction of the seals meant to prevent hot gases from leaking through the joint during the propellant burn. The Commission found that no other element of the Shuttle system contributed.
• The Commission examined the other possibilities in detail, including the External Tank, the Space Shuttle Main Engines and the payload, and ruled each of them out.
• Decision making: Chapter V of the report states that the decision to launch was flawed. Those who made it were unaware of the recent history of problems with the joint seals, of the contractor's initial written recommendation against launching at temperatures below 53 degrees Fahrenheit, and of the continuing opposition of the contractor's engineers after its management reversed its position. The Commission found failures in communication that led to a decision based on incomplete and sometimes misleading information, and concluded that if the decision-makers had known all the facts, it is highly unlikely they would have decided to launch on January 28, 1986.
• The ambient temperature before launch was the coldest of any launch up to that time.

Apply the lesson

• Think of a design limit your team relies on that was set from past successful use and not from testing at the extremes. What conditions are outside the experience behind it?
• When an engineer says "I do not think this is safe", where in your process does that statement go, and who must answer it in writing?
• Write the decision rule for a go/no-go call so that the burden of proof is on showing the system is safe, not on proving it is unsafe.

Engineering lesson: a concern raised by the people closest to the hardware has to reach the decision-maker in clear terms, and the default when the data are uncertain must be the safe one.

What the Commission recommended

The Commission adopted recommendations to help assure the return to safe flight. The first concerned the design:

• The faulty Solid Rocket Motor joint and seal must be changed, either by a new design that eliminates the joint or by a redesign of the current one. No design option should be precluded because of schedule, cost or reliance on existing hardware.
• The joints should be fully understood, tested and verified, and the integrity of the joints should be insensitive to dimensional tolerances, handling, assembly, inspection, environment and operating pressure.
• Certification should include tests that duplicate the actual launch configuration as closely as possible, over the full range of operating conditions including temperature.
• NASA should ask the National Research Council to form an independent oversight committee to oversee the design effort.

Other recommendations dealt with how the program was run. The Commission found that project managers for the Shuttle elements felt more accountable to their center management than to the Shuttle program organization, and it recommended giving the Program Manager clear authority for all STS operations. It also recommended a Shuttle Safety Advisory Panel reporting to the Program Manager.

Sources

• Report of the Presidential Commission on the Space Shuttle Challenger Accident (NASA History)
• Chapter IV: The Cause of the Accident
• Chapter V: The Contributing Cause of the Accident