[extropy-chat] unidirectional thrust

[Hal Finney]

Mike Lorrey writes:
> I don't suppose I have to explain to everyone else how wrong you are. A
> Joule being equal to 1 kg m2 s-2, or 1/4.184 of a calorie, is a unit
> that involves a time term, ergo 100 kJ for 10 seconds is 1 MJ. No over
> unity, Dirk, just bad math and a misunderstanding by you of energy and
> work.

I thought Dirk was wrong at first, in fact I had a message all typed up,
but then I thought... what if I'm wrong?  And then I decided that I *was*
wrong!  I think Dirk's analysis is correct.  Let me make it more concrete.

Let's suppose a lifter will work in outer space.  We'll use a photovoltaic
panel and some kind of DC-to-DC pulsed transformer to get the 30 kV
we need.

Looking at the first chart on the lifter page at
<http://jnaudin.free.fr/lifters/data/index.htm>, he can lift about
35 grams with Lifter v4.0, for 132.9 Watts.  That's a force of
.035 kg times 9.8 m/s/s or about .3 Newtons.  A random solar panel,
<http://www.wholesalesolar.com/products.folder/module-folder/kyocera/KC167.html>,
can generate 167 Watts and weighs 16 kg.  We'll add 4 kg for the
transformer (this is all hand-wavey) and get a 20 kg device that we can
shoot into space and it will start accelerating all by itself.

.3 Newtons over 20 kg is about .015 m/s/s acceleration that our device
will deliver.  It's small, but it builds up.  After 1 day, it's going over
a kilometer per second!  And velocity increases by a km/s every day.
Meanwhile we are putting in 132.9 Watts the whole time, which is 132.9
J/s times 86400 s/day or 11.5 MJ per day.  Well, after two days we are
at 2 km/s so our kinetic energy is m*v*v/2 or 40 MJ.  But we've only
used 2 * 11.5 MJ or 23 MJ.  So Dirk is right, we are over unity already,
even with a primitive lifter like Naudin has designed.  And it gets
worse every day.

It's interesting to compare this with a rocket.  The difference is that
with a rocket, most of the energy goes into the exhaust, which constantly
decreases the mass of the rocket.  The result is that there's a limit
to how much delta V you can get, depending on your exhaust velocity
and the mass ratio of fuel to payload.  But these lifters, or any such
"space drive", don't have exhaust shooting out, so they can go on forever,
constantly increasing their kinetic energy as the square of the velocity.
As Dirk says, eventually they will go over unity, and as this example
shows, it could happen suprisingly quickly.

Hal