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Point the way
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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
“Spacecraft enter the atmosphere horizontally and stay that way until they slow by 75 per cent” 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
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
shield. The centre of mass of the Mercury space capsules was aligned with its central axis and these craft made a ballistic reentry, 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 Wood Orlando, Florida, US
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
■ The orientation of an unguided body moving through a fluid depends approximately on the relative positions of the centre of “The Apollo capsule was designed with heavy mass and the centre of pressure. equipment in its deep, The centre of mass is the point rounded bottom” about which the weight of the object would balance. The centre of pressure is the point about centre of mass. This will make which aerodynamic pressures the rocket highly unstable at balance and, broadly speaking, launch, often spinning like a top the body will orient itself so that as soon as it rises off its launch its centre of mass is ahead of its pad and tower. centre of pressure. However, as the fuel burns the A classic example is an arrow. rear of the rocket will get lighter, If you throw an arrow sideways, moving the centre of mass it will rotate until the head is steadily forward. If this moves foremost. This is because the the centre of mass ahead of the heavy arrowhead places the centre of pressure (where it centre of mass towards the should have been in the first front, while the fletching (or place) the rocket will suddenly flight vanes) places the centre stabilise and start moving in of pressure towards the rear. a straight line, although in a The Apollo capsule was random – and perhaps extremely designed with the heavy hazardous – direction. equipment cradled in the deep, Dan Griscom rounded bottom of the spacecraft, Melrose, Massachusetts, US
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
Do Polar Bears Get Lonely? A brand new collection ight serious enquiry, brilliant insight cted and the hilariously unexpected Available from booksellers m/ and at www.newscientist.com/ polarbears
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 Schofield London, 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 Rej Carlingford, New South Wales, Australia
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
“Spacecraft enter the atmosphere horizontally and stay that way until they slow by 75 per cent” 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
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
shield. The centre of mass of the Mercury space capsules was aligned with its central axis and these craft made a ballistic reentry, 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 Wood Orlando, Florida, US
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
■ The orientation of an unguided body moving through a fluid depends approximately on the relative positions of the centre of “The Apollo capsule was designed with heavy mass and the centre of pressure. equipment in its deep, The centre of mass is the point rounded bottom” about which the weight of the object would balance. The centre of pressure is the point about centre of mass. This will make which aerodynamic pressures the rocket highly unstable at balance and, broadly speaking, launch, often spinning like a top the body will orient itself so that as soon as it rises off its launch its centre of mass is ahead of its pad and tower. centre of pressure. However, as the fuel burns the A classic example is an arrow. rear of the rocket will get lighter, If you throw an arrow sideways, moving the centre of mass it will rotate until the head is steadily forward. If this moves foremost. This is because the the centre of mass ahead of the heavy arrowhead places the centre of pressure (where it centre of mass towards the should have been in the first front, while the fletching (or place) the rocket will suddenly flight vanes) places the centre stabilise and start moving in of pressure towards the rear. a straight line, although in a The Apollo capsule was random – and perhaps extremely designed with the heavy hazardous – direction. equipment cradled in the deep, Dan Griscom rounded bottom of the spacecraft, Melrose, Massachusetts, US
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
Do Polar Bears Get Lonely? A brand new collection ight serious enquiry, brilliant insight cted and the hilariously unexpected Available from booksellers m/ and at www.newscientist.com/ polarbears
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 Schofield London, 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 Rej Carlingford, New South Wales, Australia