When the and other similar space capsules were returning to Earth, it was important for the larger end of their bell-shape to face downwards. This is because the protective shield that resisted the intense heat created on re-entry by atmospheric friction as the spacecraft slowed was positioned there. How were the capsules designed so that they would always keep the larger, protective face towards the Earth and not flip over so that the pointed end faced earthwards? It seems to me that this would be likely to happen as this orientation would minimise air resistance. Or is my grasp of space flight a bit flimsy?
鈥� It is a common misconception that spacecraft entering the atmosphere do so while going straight down, towards the Earth. This is perpetuated by just about every space movie ever made. The truth is that the spacecraft are going nearly horizontal as they enter the atmosphere, even when returning from the moon. They remain within 5 degrees of horizontal until they have lost 75 per cent of their speed.
鈥淪pacecraft enter the atmosphere horizontally and stay that way until they slow by 75 per cent鈥�
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What determines which end points into the wind is where the centre of mass is located. In this case it is very close to the heat shield. The centre of mass of the space capsules was aligned with its central axis and these craft made a ballistic re-entry, meaning there was no lift.
With the and Apollo capsules, the centre of mass was offset from the central axis. This made the heat shield tilt slightly so that it was not perpendicular to the relative wind. This provided a small amount of lift, which made re-entry a little longer but reduced the peak acceleration from 10-12g to around 3-4g.
Stephen Wood, Orlando, Florida, US
鈥� The orientation of an unguided body moving through a fluid depends approximately on the relative positions of the centre of mass and the centre of pressure. The centre of mass is the point about which the weight of the object would balance. The centre of pressure is the point about which aerodynamic pressures balance and, broadly speaking, the body will orient itself so that its centre of mass is ahead of its centre of pressure.
A classic example is an arrow. If you throw an arrow sideways, it will rotate until the head is foremost. This is because the heavy arrowhead places the centre of mass towards the front, while the fletching (or flight vanes) places the centre of pressure towards the rear.
The Apollo capsule was designed with the heavy equipment cradled in the deep, rounded bottom of the spacecraft, while the crew compartment 鈥� much of which is empty space 鈥� was towards the pointed top. This placed the centre of pressure behind the centre of mass, which stabilised the capsule as it fell through the atmosphere. The centre of buoyancy (which is related to the centre of pressure) was also above the centre of mass, thus keeping the capsule upright as it bobbed in the sea after landing.
鈥淭he Apollo capsule was designed with heavy equipment in its deep, rounded bottom鈥�
You can encounter a dangerous example of this with a poorly designed model rocket. If the rocket鈥檚 fins are too small, or the mass of the engine and fuel too far to the rear, the centre of pressure will actually be ahead of the centre of mass. This will make the rocket highly unstable at launch, often spinning like a top as soon as it rises off its launch pad and tower.
However, as the fuel burns the rear of the rocket will get lighter, moving the centre of mass steadily forward. If this moves the centre of mass ahead of the centre of pressure (where it should have been in the first place) the rocket will suddenly stabilise and start moving in a straight line, although in a random 鈥� and perhaps extremely hazardous 鈥� direction.
Dan Griscom, Melrose, Massachusetts, US