Flying without wings

The first generation of manned spacecraft - for example, the American Mercury and the Soviet Vostok ships - were designed to re-enter the atmosphere in a purely ballistic fashion. Once they were set on their Earthward trajectory, they had no ability to change their flight path and steer towards a landing site. Later the Gemini and Apollo spacecraft, and the Russian Soyuz, could fly in a controlled manner and managed to do so without wings.

Although the Apollo command module had a symmetrical shape, its internal weight distribution set its centre-of-gravity towards the crew's feet. This was a deliberate arrangement in order to make it adopt an aerodynamically stable attitude that leaned one way as it ploughed through the atmosphere. Such a lopsided presentation to the hypersonic airflow turned the stubby spacecraft into a crude wing, giving it the ability to generate lift in a direction towards the crew's feet. Simply by performing a roll manoeuvre, the spacecraft could then aim this lift vector up or down, or side to side, allowing the re-entry to be flown in a controlled manner, usually by the computer.

Note that although it is always referred to as a lift vector, the vector is relative to the spacecraft and in use, the direction of the so-called 'lift' could be downwards. If the spacecraft was a little high in the re-entry corridor and was going to overshoot the landing site, the roll thrusters could fire to turn the spacecraft around to a heads-up attitude, aiming the lift vector towards Earth and forcing it into a lower flight path where the thicker atmosphere reduced its speed further. The meagre lift that such a poor wing could generate was amplified by the huge speed of re-entry to the extent that, for a few minutes, the spacecraft would typically fly around a constant 60-kilometres altitude and, in some cases, even manage to rise away from Earth.

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