“Skirts up” — the interstage skirt is jettisoned by the troubled second stage of the Saturn V.

It began as a good day, the culmination of a relatively smooth countdown.  Ignition, launch commit and liftoff, liftoff of Apollo 6, fifty years ago today, the start of the second flight, unmanned, of the 363-ft.-tall Saturn V moon rocket, liftoff at 7:00:01 a.m. into clear skies.  And it has cleared the tower.

It was a good day, until two minutes into first stage burn.   Longitudinal oscillations, up and down the vehicle, the dreaded pogo vibrations, like riding a pogo stick, began shaking the vehicle.  The pulses built for thirty seconds, becoming large enough to jar the crew, if one had been aboard, with forces of more than half a “g” — in excess of flight limits.  At the same time, several pieces of the Saturn Launch Adapter surrounding the Lunar Module (a test mock-up for this flight) peeled away.

It was a still good day, the stack maintained structural integrity.   First stage burnout at nominal velocity 148 seconds after launch.  Second stage ignition, thrusting on five hydrogen-fueled J-2 engines.

The day tilted toward bad two and a half minutes into the second stage.  Hydrogen flow to engine #2 began to decrease, thrust dropping.  Seven minutes after launch, with more than a minute and a half left in the burn, #2 shutdown.  A second later, engine #3 quit, too.  Guidance, although not programmed for a two-engine-out scenario, attempted to compensate, burning the remaining three engines to fuel depletion, 58 seconds longer than planned.  The third stage, with its single J-2 engine, took over the task of achieving obit.  It burned for 29 seconds longer than planned.

The bad got worse.  The third stage still had enough fuel to push Apollo 6 on a lunar-like trajectory, from which it would simulate a quick-return abort, firing its own SPS engine to drive itself into the atmosphere at lunar-return speeds.   “Standing by, 3 hrs. 14 min. 52 sec. [into the flight].  We did not have re-ignition.”   The engine refused to start.

Making the best of the bay, controllers implemented a back-up plan, using Apollo’s SPS engine to lift it into an orbit with a high point of 13,893 mi. from which it dove towards earth.  Not enough propellant remaining to accelerate it to lunar-return speeds, it re-entered over the Pacific at a speed of 33,000 ft. per second instead of the lunar velocity of 36,500 ft. per second.  Apollo 6 successfully splashed down in the Pacific after a flight of 9 hrs. 57 min. 20 sec., shortly before 5 p.m. EST.

It wasn’t a good day, but it wasn’t a bad one, either.  That is, until it turned very bad about an hour after Apollo 6 splashed down:  Dateline, Memphis, Tennessee, the Lorraine Motel. . .   Apollo 6 was instantly forgotten.  Except, that is, by the engineers charged with finding and fixing the faults in the Saturn V so that the schedule could be maintained and the next Saturn V launched with a human crew aboard.