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Neither Cooper nor the Mercury controllers at Canaveral were terribly worried about this prospect. Scott Carpenter, although overshooting his landing target by some 250 miles, had achieved re-entry with the same maneuvers. Cooper himself had often practiced them on a ground simulator.
The Dead Inverters. But the Canaveral experts were taking no chances. Rushing to a training capsule in Hangar S, they set up the trouble that Cooper was experiencing on a simulator, assured themselves that their diagnosis was right.
Everyone breathed a bit easier—but not for long. Over Zanzibar on his 22nd orbit, panel lights indicated that one of Cooper's three inverters had gone dead. He tried to start a second, but could not.
The inverters convert battery power to alternating current, are needed to operate the autopilot system. Cooper's sole remaining inverter was needed to power cabin cooling gear on reentry. Now Cooper would have no automatic aids at all in bringing his capsule down. No inverters had ever failed on a Mercury flight before; on the possibility that the third inverter might also go, engineers dug feverishly into their capsule simulator, checked out a plan to get Cooper down on the capsule's battery power if necessary. "We would have found some way to fire the retros." said Capsule Designer John Yardley later, "if it meant telling him what wires to twist together."
It was now up to Cooper—with some dramatic help from the calm, crisp voice of John Glenn on the Coastal Sentry. Cooper and Glenn ran swiftly, surely, down a check list of the operations Cooper must perform for reentry. Cooper skillfully steadied his craft by a manual control stick with his right hand, prepared to punch the retrofire button with his left.
Like a rifleman with a crosshair sight, he lined up a horizontal mark on his window with the horizon, which brought the narrow neck of his capsule pointing down 34°. He lined up a vertical mark with predetermined stars to provide proper heading. Asked Glenn: "How does the attitude check?" Replied Cooper: "Right on the old kazoo."* "Thataway boy. O.K. on procedures, Gordo. I'll give you a one-minute mark before retrofire, and then I'll give you a ten-second countdown to what would normally be your sequence time."
"Good Show." The pair counted in unison, and Cooper pushed his button. Because of his electrical problems, he got no light signals that his retrorockets had fired. But he could feel them. "Roger, you're green," said Glenn, indicating that telemetry signals on the Coastal Sentry had confirmed the firing. "How is your attitude, Gordo?" "Real great." "Good show. boy. Looks like you came out right on the money, on time."