On reactionless drives

> From: [email removed]
>
> Bertil, I don''t think your idea solves anything. What sort of potential
> energy would such a device create? Engines, by definition, turn potential
> energy (the chemical or nuclear energy in the fuel, or electrical potential
> in a battery) into kinetic energy and waste heat. The fact that a drive
> is reactionless means that you make an object move without waste, in fact
> with the use of less chemical energy than the kinetic energy you require.
> Thus, you're creating energy out of nothing.

I think what Bertil was trying to say is, what is wrong with turning a ship
from having X megajoules of energy stored as potential energy, to a ship
having .9 X megajoules of kinetic energy and .1 X megajoules of waste energy
(e.g. heat) without having to dump a load of stuff out of the back to satisfy
Newton?  This doesn't create energy out of nothing, and doesn't violate
conservation of energy.  Having said that, see below.

>						My personal wish was that the
> folx at GDW and DGP had just left the maneuver drives as fusion torches,
> which follow reasonable laws and so forth. There have been mumblings to the
> effect of "Oh, they were *always* reactionless, we just made it official
> recently," from the designers, or so I've heard.

Let the mumblers take a look at some GDW drawings of starships.  The old
supplements 7 (Traders and Gunboats) and 9 (Fighting Ships - not to be confused
with the new one) have pictures of ships with exhaust nozzles.

All my models of ships (made back in Ye Olde Traveller days) still have exhaust
nozzles, which should tell you a bit about my faith in reactionless drives. :-)

- --
 "Keyboard?  How quaint!" - M. Scott

 Adrian Hurt			     |	JANET:  [email removed]
 UUCP: ..!ukc!cs.hw.ac.uk!adrian     |  ARPA:   [email removed]

Adrian Hurt writes:
>I think what Bertil was trying to say is, what is wrong with turning a ship
>from having X megajoules of energy stored as potential energy, to a ship
>having .9 X megajoules of kinetic energy and .1 X megajoules of waste energy
>(e.g. heat) without having to dump a load of stuff out of the back to satisfy
>Newton?  This doesn't create energy out of nothing, and doesn't violate
>conservation of energy.   ^^^^^^^^^^^^^^^

Yes it does.  I posted a detailed example showing why recently.  Even if you
use Bertil's idea of  (Energy needed for drive = KE added to ship) it doesn't
work because you need a non-accelerating reference frame to measure KE.

Example: Suppose you have your ship between planets A and B, which
are moving with respect to one another.  We now have to choose a frame
of reference, say A.  Say the velocity of B with respect to A is 1e3 m/s.
The KE of ship relative to A is 0, and relative to B is 5e5*M joules.
The ship now accelerates to 1e3 m/s relative to A, in the direction
opposite to B's motion.  Now the KE of the ship relative to A is K1 =
5e5*M joules, where M is the mass of the ship, so we expend
(K1 + waste) joules to power the drive.  However our velocity relative
to B is now 2e3 m/s, giving a KE of K2 = 2e6*M joules, which
is an increase of 1.5e6*M joules, creating 1e6 joules of "free" energy
with respect to B.  With a rocket, that energy wouldn't appear since
is has to spend energy to accelerate the reaction mass and it masses
less afterwards.

I worked all out with a friend for fun when we were building MT
perpetual motion machines. (So I'm weird - sue me)

					W. Dow Rieder

 	When the only tool you have is a hammer, all your problems
start to look like nails...

William Dow Rieder [email removed] writes:
> Adrian Hurt writes:
> >I think what Bertil was trying to say is, what is wrong with turning a ship
> >from having X megajoules of energy stored as potential energy, to a ship
> >having .9 X megajoules of kinetic energy and .1 X megajoules of waste energy
> >(e.g. heat) without having to dump a load of stuff out of the back to satisfy
> >Newton?

>								   Even if you
> use Bertil's idea of  (Energy needed for drive = KE added to ship) it doesn't
> work because you need a non-accelerating reference frame to measure KE.

Oh, *@!~.  Relativity strikes again. :-)

After reading this, I did a bit of working out on some scrap paper myself,
which started out with conservation of energy and ended up with conservation
of momentum.  If you really want a long, boring load of ASCII, I can post it
here.  Suffice to say, I was right to leave all those exhaust nozzles on my
model ships!

There is one possible loophole, which someone on rec.arts.sf-lovers suggested.
So-called "reactionless" drives are nothing of the sort; they use the rest of
the universe as a reaction mass.  Someone else can figure out how, or if, this
can be done.  I'm sticking with rockets - they make the models look better.

> I worked all out with a friend for fun when we were building MT
> perpetual motion machines. (So I'm weird - sue me)

You want perpetual motion machines?  Try assuming that a black globe absorbs
gravitons and see what happens.

- --
 "Keyboard?  How quaint!" - M. Scott

 Adrian Hurt			     |	JANET:  [email removed]
 UUCP: ..!ukc!cs.hw.ac.uk!adrian     |  ARPA:   [email removed]