On Jan. 27, 1986, Allan McDonald stood on the cusp of history. McDonald directed the booster rocket project at NASA contractor Morton Thiokol. He was responsible for the two massive rockets, filled with explosive fuel, that lifted space shuttles skyward. He was at the Kennedy Space Center in Florida for the launch of the Challenger "to approve or disapprove a launch if something came up," he told me in 2016, 30 years after Challenger exploded. His job was to sign and submit an official form. Sign the form, he believed, and he'd risk the lives of the seven astronauts set...