In article , Bob
Martin wrote:


That number of 9 million kg seems a little large... we've sent much
larger things out of the gravity well on much less fuel, using less
efficient processes.

The energetics are different.

If you are halfway up the beanstalk, then burning a kg of fuel will
give you the same amount of energy as it would at the base. think of
the energy wasted as you drop the ash from your fuel over the side and
let it just fall back to Earth. (If you could use it as a
counterweight to pull you up, then you'd be better off.)

In a rocket, the amount of kinetic energy you get from burning a
certain amount of fuel (thrusting along the velocity vector) is
proportional to your speed. Intuitively, this is because the fuel in
your tanks has kinetic energy that you partially recover--if your
exhaust velocity is equal to your speed (in some frame of reference)
then the burned fuel ends up with zero kinetic energy and the rocket
gets it all.

--
David M. Palmer (formerly @clark.net, @ematic.com)