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THE LAST WORD

Point the way

When the Apollo 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?

■ 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.

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 Mercury space capsules was aligned with its central axis and these craft made a ballistic re-entry, meaning there was no lift.

With the Gemini 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 WoodOrlando, Florida, US

■ 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 – much of which is empty space – 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.

You can encounter a dangerous example of this with a poorly designed model rocket. If the rocket’s 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 – and perhaps extremely hazardous – direction.Dan GriscomMelrose, Massachusetts, US

This week’s questions

JERSEY OF MANY COLOURS

When I look at an electric light bulb through the fabric of my maroon, acrylic school jumper, why does it seem to have a rainbow halo around it?Thomas O’Hare (aged 6)London, UK

BRAINY BREEDING

Dog breeding often gets a bad press, including the apparently unfounded assertion that breeding for looks has an adverse effect on intelligence in dogs. But has anyone ever bred dogs, or any other species, purely for intelligence? Just how intelligent could any species get through selective breeding? And how quickly?John SchofieldLondon, UK

BLANK AT THE BANK

I was trying to take money out of an ATM and I couldn’t remember my four-digit PIN. I use these machines most weeks and I don’t have to think about my PIN – I just enter it.

About half an hour later the number came back to me and I immediately knew that it was correct. What caused this brief memory lapse and what happened later to allow me to remember my PIN?Tomek RejCarlingford, New South Wales, Australia

Last words past and present, plus questions, at www.last-word.com

“The Apollo capsule was designed with heavy equipment in its deep, rounded bottom”

“Spacecraft enter the atmosphere horizontally and stay that way until they slow by 75 per cent”