Thing 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 generations of spacecraft like Gemini and Apollo, and the Soyuz, could fly in a controlled manner even without wings.

Although the Apollo command module had a symmetrical shape, its internal weight distribution was deliberately offset to place its centre of mass towards the crew;’s feet. This made it adopt an aerodynamically stable attitude that leaned to one side as it ploughed through the atmosphere because the lighter side of the spacecraft tended to succumb to the atmospheric drag to a greater extent. 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. Therefore, simply by performing a roll manoeuvre, the spacecraft could aim this lift vector in any direction perpendicular to the flight path which allowed the re-entry to be flown in a controlled manner, usually by the computer.

This term, ‘lift vector’ can be confusing as it is borrowed from the aeronautical world where it is applied to a wing’s ability to provide a lifting force. But just like an aerobatic aircraft that rolls and loops, that force can be in any direction perpendicular to the airflow. It is perfectly possible for the direction of the so – called ‘lift’ to be downwards, towards Earth. 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 and aim the lift vector towards Earth. This would force it into a lower flight path where the thicker atmosphere would reduce 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 at about a constant 60-kilometres altitude and, in some cases, even manage to rise away from Earth.