Re: grav potential

>> A first approximation calls M(Sol) about the same as M(AC), so the net
>> cost of the jump is ~= 0, plus the drive energy to make a jump of twice
>> the distance.
>
>Why twice the distance?  (I assume that in this sentence, "distance" means
>"distance from Sol to Alpha Centauri".)  If the energy require to make a
>jump of this distance is E, then you're saying you need to expend 2*E.

I meant twice the distance of the hypothetical jump to the midpoint
between Sol and AC.

>>		On the other hand, GmM/r is the escape energy of Sol,
>> which is the kinetic energy required to produce a velocity of ~30 km/s.
>> For a ship weighing say, 1 T, that's 4.5e11 J, or 450 000 MJ. Not to
>> be sneezed at.
>
>A ship accelerating at 1-G will achieve that velocity in less than an hour,
>using just its manoeuvre drive.  (To get to 30000 m/s using an acceleration
>of 9.8 m/s^2 will take 3061 seconds, i.e. 51 minutes.)  Maybe not to be

You realise what you just said?  A spacecraft with a 1-G maneuver drive
can produce 4.5e11 J in 3000 s, for an average power output of 150 MW.
Constant-acceleration drives are scary.

>So the ship expends its 4.5e11 J, and get into jump space.  One week later,
>it emerges from jump space near Alpha Centauri, and gets 4.5e11J back -
>what happens to this energy?  Presumably it gains this energy as it emerges
>from jump space, and the energy ends up back in the ship's jump capacitors.

Depending on how jumpspace is supposed to work, I shouldn't think the
hundred billion joules of GPE would have to be 'spent'.  The only
relevent number ought to be the difference in initial and final
energies.  If the energy did have to come from somewhere, it would be
a very blatant violation of the 2nd law of Thermodynamics to get it back
in the capacitors. More plausible would be to get it as kinetic energy,
but physical reality would almost certainly produce it as heat. Which
would vapourise the ship, very quickly.

   roald.

- --
Scientific Progress goes 'BOINK'?	-- Hobbes