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From John Glenn: A Memoir, Chapter 18:

Liftoff was slow. The Atlas's 367,000 pounds of thrust were barely enough to overcome its 125-ton weight. I wasn't really off until the forty-two-inch umbilical cord pulled loose. That was my last connection with Earth. It took the two boosters and the sustainer engine three seconds of fire and thunder to lift the thing that far. From where I sat the rise seemed ponderous and stately, as if the rocket were an elephant trying to become a ballerina. Then the mission elapsed time clock on the cockpit panel ticket into life and I could report, "The clock is operating. Were're under way."

I could hardly believe it. Finally!

The rocket rolled and headed slightly north of east. At thirteen seconds I felt a little shudder. "A little bumply along about here," I reported. The G forces started to build up. The engines burned fuel at an enormous rate, one ton a second, more in the first minute than a jet airliner flying coast to coast, and as the fuel was consumed the rocket grew lighter and rose faster. At forty-eight seconds I began to feel the vibration associated with high Q, the worst seconds of aerodynamic stress, when the capsule was pushing through air resistance amounting to almost a thousand pounds per square foot. The shaking got worse, then smooth out at 1:12, and I felt the relief of knowing I was through max Q, the part of the launch where the rocket is most likely to blow.

At 2:09 the booster engines cut off and fell away. I was forty miles high and forty-five miles from the Cape. The rocket pitched forward for the few seconds it took for the escape tower's jettison rocket to fire, taking the half-ton tower away from the capsule. The G forces fell to just over one. Then the Atlas pitched up again and, driven by the sustainer engine and the two smaller vernier engines, which made course corrections, resumed its acceleration toward top speed of 17,545 miles per hour in the ever-thinning air. Another hurdle passed. Another instant of relief.

Pilots gear their moments of greatest attention to the times when flight conditions change. When you get through them, you're glad for a fraction of a second, and then you think about the next thing you have to do.

The Gs built again, pushing me back into the couch. The sky looked dark outside the window. Following the flight plan, I repeated the fuel, oxygen, cabin pressure, and battery readings from the dials in front of me in the tiny cabin. The arc of the flight was taking me out over Bermuda. "Cape is go and I am go. Capsule is in good shape," I reported.

"Roger." Twenty seconds to SECO." That was Al Shepard on the capsule communicator's microphone at mission control, warning me that the next crucial moment -- sustainer engine cutoff -- was seconds away.

Five minutes into the flight, if all went well, I would achieve orbital speed, hit zero G, and, if the angle of ascent was right, be inserted into orbit at a height about about a hundred miles. The sustainer and vernier engines would cut off, the capsule-to-rocket clamp would release, and the posigrade rockets would fire to separate Friendship 7 from the Atlas.

It happened as programmed. The weight and fuel tolerances were so tight that the engines had less that three seconds of fuel femaining when I hit that keyhole in the sky. Suddenly I was no longer pushed back against the seat but had a momentary sensation of tumbling forward.

"Zero G and I feel fine," I said exultantly. "Capsule is turning around." Through the window, I could see the curbe of Earth and its thin film of atmosphere. "Oh," I exclaimed, "that view is tremendous!"