The cause was complications from a recent fall, said his daughter Lora McDonald.
For the millions of Americans who turned on their television sets to watch the Challenger take off on Jan. 28, 1986, the image of the space shuttle blowing apart in midair — killing seven astronauts, including New Hampshire schoolteacher Christa McAuliffe — was seared into their memory. The disaster is often described as an event on the order of the assassination of President John F. Kennedy in 1963 or the terrorist attacks of Sept. 11, 2001: Those who lived through it will never forget where they were when it occurred.
Mr. McDonald was in Cape Canaveral, Fla., at NASA’s Kennedy Space Center, where the Challenger was set to take off. He was the senior on-site representative of his company, contractor Morton Thiokol, where he oversaw the engineering of the rocket boosters used to propel the shuttle into space. Among colleagues, the New York Times reported, Mr. McDonald had a reputation as one of the most skilled rocket engineers in the country.
It was unseasonably cold in Florida, with weather forecasts predicting that temperatures might drop as low as 18 degrees Fahrenheit in the hours before the Challenger was scheduled to lift off. That cold snap became the crux of vociferous debate among Mr. McDonald and other engineers, Morton Thiokol executives and NASA officials about whether the mission should go forward.
Citing the cold, Mr. McDonald insisted that takeoff be postponed, according to accounts of the deliberations that later emerged in news reports. A critical component of the rocket booster was the O-ring, a rubber gasket that served to contain burning fuel. Because of their composition, O-rings were highly vulnerable to temperature drops, and engineers warned that their effectiveness could not be guaranteed below 53 degrees Fahrenheit.
Mr. McDonald relayed these concerns in what he described as increasingly frenzied conversations the night before the launch. NASA officials, upset by the last-minute complication, were eager to move forward with the mission; company executives, according to later findings by a presidential commission on the Challenger disaster, appeared to feel pressure to “accommodate a major customer.”
In addition to the matter of the O-rings, Mr. McDonald said he raised weather-related concerns including the danger that ice might damage the shuttle’s exterior.
“If anything happened to this launch, I told them I sure wouldn’t want to be the person that had to stand in front of a board of inquiry to explain why I launched this outside of the qualification of the solid rocket motor,” he would later testify.
Protocol required the senior engineer to sign off on the launch. When Mr. McDonald refused, his supervisor signed for him. The Challenger lifted off at 11:38 a.m. on Jan. 28 and disintegrated approximately 72 seconds later, its remains streaking across the sky.
“My heart just about stopped,” Mr. McDonald later said in a public lecture, according to the Commercial Dispatch of Columbus, Miss.
President Ronald Reagan convened a commission to investigate the catastrophe. Led by William P. Rogers, a former U.S. secretary of state and attorney general, it included astronauts Neil Armstrong and Sally Ride and Nobel Prize-winning physicist Richard P. Feynman.
Mr. McDonald was present at a closed session of the commission — watching from what he called the “cheap seats” — when he heard what he considered misleading testimony by a NASA official about the debate leading up to takeoff.
“I was sitting there thinking, ‘That’s about as deceiving as anything I ever heard,’ ” Mr. McDonald said in an interview aired on NPR. “So I raised my hand. I said, ‘I think this presidential commission should know that Morton Thiokol was so concerned, we recommended not launching below 53 degrees Fahrenheit. And we put that in writing and sent that to NASA.’ I’ll never forget Chairman Rogers said, ‘Would you please come down here on the floor and repeat what I think I heard?’ ”
Mr. McDonald soon testified in public proceedings, describing a culture in which, in the past, engineers had been required to demonstrate to officials that their products were safe for flight.
“In this case, we had to prove it wasn’t, and that’s a big difference,” he testified. “And I felt that was pressure.”
Mr. McDonald was demoted at Morton Thiokol after his testimony, then reinstated after Congress moved to end the company’s federal contracts if he was not returned to his job.
“I really expected to be going out the door,” he later recalled. “And I would have, if it had not been that the presidential commission and certain members of Congress found out about it and really read the riot act to the management of my company. That saved my job, frankly.”
Allan James McDonald was born on July 9, 1937, in Cody, Wyo., and grew up in Montana, where his father was a deputy county assessor in Bozeman. His mother was a homemaker.
Mr. McDonald received a bachelor’s degree in chemical engineering from Montana State University in 1959, then joined what was then Thiokol Chemical Corp. His early projects included designing external insulation for the Minuteman missile. In 1967, he received a master’s degree in engineering administration from the University of Utah.
Survivors include his wife of 57 years, the former Linda Zuchetto of Ogden; four children, Greg McDonald of Ogden, Lisa Fischer of Barrington, R.I., Lora McDonald of Clayton, Calif., and Meghan McDonald Goggin of White Plains, N.Y.; and nine grandchildren.
After his reinstatement at Morton Thiokol, Mr. McDonald played a principal role in a redesign of the booster rockets. He retired in 2001 as a vice president at the company. With James R. Hansen, he wrote the book “Truth, Lies, and O-rings: Inside the Space Shuttle Challenger Disaster” (2009) and spoke frequently to scientific, corporate and government audiences about the role of ethics in professional life.
He often cited an aphorism with particular resonance for him. “Regret for things we did is tempered by time,” he would tell his listeners. “Regret for things we did not do is inconsolable.”
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