“You don’t get in it, you put it on.” The Mercury spacecraft in which America’s first astronauts rode into space was so small and cramped that none of them could be taller than 5’11”.
Why so small? The rockets of that era were neither reliable enough (they blew up with an alarming frequency) nor powerful enough to boost large payloads into space, much less human beings. The Mercury capsule was just large enough to hold one person and provide enough life support to keep him alive for relatively short periods. The longest Mercury mission lasted less than a day and a half, and by its end its onboard systems were beginning to fail one by one.
https://en.wikipedia.org/wiki/Mercury_13#/media/File:JerrieCobb_MercuryCapsule.jpg
This image serves two purposes. One is to show the scale of the capsule. The other is to show Jerrie Cobb, a superb pilot, one of a group of women who underwent the same testing as the men selected to be the first American astronauts. But it had been decreed that these first space travelers would all be military test pilots. Since women were excluded from their ranks, they could not qualify. Hers is a story well worth learning, of a woman denied her goal but paving the way for others to follow.
https://upload.wikimedia.org/wikipedia/commons/f/f6/Mercury-capsule.jpg
You’ll notice that the shape of the Mercury capsule is quite different from the spherical Vostok of the Soviets. You might think that the best shape to re-enter the atmosphere at 17,000 miles per hour and protect your craft from the resultant frictional heating would be a streamlined needle nosed craft. Perhaps something like this supersonic experimental aircraft, the X-3?
https://en.wikipedia.org/wiki/Douglas_X-3_Stiletto#/media/File:Douglas_X-3_NASA_E-1546.jpg
Well, that’s fine for an aircraft whose speeds never even reached 1000 mph. (Despite its sleek appearance, the X-3 was a severe underperformer, and only one was ever built.) But not for a re-entering spacecraft.
With a blunt shape, the air hitting the leading surface cannot get out of the way fast enough; instead it creates an air cushion and a boundary layer to keep superheated air and a shock wave away from the surface, and allow them to move around the craft.
This explains in some greater detail.
This is what it looks like from the inside.
That leading surface still got very hot however, and further heat protection was provided by an ablative heat shield. Without getting overly technical, this protective layer chars and burns away, carrying heat energy with it. This is an image of the heat shield of John Glenn’s Friendship 7 Mercury capsule after its return to Earth. The circle in the center is where a package of retrorockets used to deorbit the spacecraft was located.
https://airandspace.si.edu/multimedia-gallery/nasm2016-1808jpg
Remember those underpowered rockets? America’s first two crewed spacecraft never entered orbit; that requires being boosted to speeds of 17,500 mph. Instead, they flew suborbital missions, up and down like an artillery shell, with maximum speeds of 5100 mph.
https://www.nasa.gov/sites/default/files/images/542198main_IMG_0128-raw_full.jpg
Two such suborbital flights (Alan Shepard and Gus Grissom) were followed by John Glenn’s historic three orbit flight in February 1962. Another 3-orbit mission (Scott Carpenter), a 6-orbit flight (Wally Schirra), and a final 22-orbit mission with Gordon Cooper wrapped up the Mercury program.
Alan Shepard lifts off atop a Redstone rocket on May 5, 1961.
Nearly a year later, the more powerful Atlas rocket was considered reliable enough to launch John Glenn into orbit on February 20, 1962.
America had shown it could launch humans into space and return them safely to the earth. Now it was time to practice the skills necessary to travel to the moon.
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