Apollo 13's launch: a crew swap and an engine cutoff

Apollo 13 launched from Kennedy Space Center on April 11, 1970, at 2:13 p.m. EST (1:13 p.m. in Houston), carrying Jim Lovell, Jack Swigert and Fred Haise toward the third crewed Moon landing — and two things had already gone wrong before the famous oxygen-tank explosion.

The first was that Swigert was not supposed to be aboard. In the last week before launch, backup lunar module pilot Charlie Duke fell ill with rubella (German measles), which he had caught from one of his children’s friends, after working closely with the prime crew. Blood tests showed Lovell and Haise had had the disease as children; command module pilot Ken Mattingly’s results were inconclusive, and doctors feared he could fall ill during the critical lunar-orbit phase. Despite Lovell’s pleas to keep him, Mattingly was grounded days before launch and his backup, Swigert, took the seat.

The second problem came 5½ minutes into the flight. The crew felt a vibration, and the centre engine of the Saturn V’s second stage shut down two minutes early. It was pogo, a self-reinforcing vibration between engine thrust and propellant flow that had dogged that stage; this time the engine was shaking on its mounts at about ±33.7 g before a sensor cut it off.

The rest of the rocket made up the shortfall. The other four engines burned 34 seconds longer and the third stage nine seconds longer to make orbit, and the mission carried on.

Two days later, about 56 hours in and some 200,000 miles from Earth, a damaged oxygen tank ruptured and took the second tank with it, ending any hope of landing. The crew used the lunar module as a lifeboat, swung around the Moon, and splashed down safely in the Pacific on April 17.