Initial Conceptblunt Body Concept 1953

Fig. 5.2 Shadowgraph research images of various reentry vehicle shapes, illustrating the bow shock waves experienced by returning spacecraft (courtesy NASA)

MISSILE NOSE CONES 1953-1957 MANNED CAPSULE CONCEPT 195?

Fig. 5.2 Shadowgraph research images of various reentry vehicle shapes, illustrating the bow shock waves experienced by returning spacecraft (courtesy NASA)

began to accelerate more and more rapidly. This caused a gradual build-up of G forces. Starting at the normal 1 G just before liftoff, Commander Shepard experienced 6 G's, or six times the pull of gravity, before the engines cut off. He and his spacecraft then experienced about 5 min of weightlessness as they arced through apogee at 116^ miles altitude. This is well above 100 km, the official border of space. The fun was just about to begin, though, for when the bell-shaped spacecraft turned around and reentered the atmosphere (Fig. 5.2), its low lift-to-drag ratio and greatly reduced mass (the launch vehicle was no longer attached) meant that it would slow down much more rapidly than the Redstone had accelerated it initially. With the blunt end pointed forward, the astronaut experienced a crushing 11 G's of deceleration. He was therefore pushed into his seat both on the way up and on the way down. Shepard's capsule made the final descent by parachute and splashed down in the Atlantic, to await recovery by helicopter (Fig. 5.3).

On July 21 of the same year, Gus Grissom made a similar flight, reaching an altitude of 118.3 miles, experiencing 5 min 18 s of weightlessness, and 11.1 G's on reentry. These were the first Americans to enter space, and the only human beings,

Fig. 5.3 America's first spaceman, the suborbital Alan Shepard, being recovered by helicopter from the Atlantic Ocean. He later went on to become the only Mercury astronaut to walk on the Moon, as Commander of Apollo 14 in February 1971 (courtesy NASA)

to date, to be launched on suborbital trajectories by ballistic missiles. The two Russian cosmonauts who entered space that year - Yuri Gagarin on April 12 and Gherman Titov on August 6 - were boosted by bigger rockets that threw them all the way into elliptical orbits about our planet. Has anyone else been to space in a suborbital vehicle? Yes, they have, in a baby spaceplane called the X-15.

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