[LINK] Finally up to date on the Shuttle

Antony Barry tony@tony-barry.emu.id.au
Fri, 14 Feb 2003 17:17:17 +1100

On Friday, February 14, 2003, at 02:01 PM, Chirgwin, Richard wrote:

> There seems, to someone whose physics needs work, to be a contradiction
> here. Anyone good enough at aerospace to sort me out?

IWAPLO (I was a physicist long ago) but I'm pretty rusty. Gravitational 
attraction between two bodies is GmM/r^2 where m and M are the masses 
of the bodies, G is the universal gravitational constant and r is the 
distance between them. "^" is just there to indicate a superscript for 
exponentiation). As they approach the force gets rapidly greater, 
inversely proportional to the square of the distance to the centre of 
the earth in the case of an orbiting body. For the moon the force is 
small. For something in a 200km orbit much greater. (384000/6400)^2 = 
2500 times less for instance. So the moon takes a month to go round the 
earth and the shuttle 90 minutes. The acceleration to sustain circular 
motion is (v^2)/r where v is the velocity so you can derive v^2 = GmM/r.

Thence when the retrorocket brakes the shuttle it starts to drop which 
increases it's speed. The more you slow it the faster it ends up going 
and it hits the atmosphere at about 5 miles per second. Friction does 
the rest. The trick is to convert the energy of motion into hot air 
without breaking the shuttle and frying the occupants. Remember that 
the speed of the shuttle is the gravitationally and kinetically stored 
energy of the 100 tonnes of fuel that put it into orbit. It didn't all 
just disappear when the shuttle got to orbit. It's represented by it's 
height and velocity and has has to go someplace when the shuttle comes 
down. Sadly this time some of it went into destroying the shuttle and 
killing it's occupants.


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