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Re: Real Life Thrust (was: hyperspace)

From: daryl.lonnon@C... (Daryl Lonnon)
Date: Tue, 27 Aug 1996 18:24:17 -0400
Subject: Re: Real Life Thrust (was: hyperspace)

This is my first post to this list ... so be kind :-).

> >  > Nuclear power is great for generating
> >  > large, sustained amounts of steam/electricity without the need
for 
> >  > refueling. But unless you can use electricity to generate
gravitic
> fields or some 
> > >  such, how do you translate this into thrust? 
> >  
> >  Actually nuclear power is great for generating large amounts of
HEAT.
> >  Ground based nuclear power plants use this to heat water of gas and
use
> >  the resulting steam or high preasure gas to drive turbines to
produce
> >  electricity. 
> 
> Fine, get technical on me!   ;-)   That was what I meant, but thanks
for
> clarifying. Nuclear ships use the same principle, with steam driving
the
> prop, right?
> 
> >  There's no reason why you can't pump water through the
> >  core, let the core heat it into high preasure steam, and allow the
steam
> >  to escape through the back of your ship - and there you have it:
THRUST
> 
> Very true, but then you still have an expendable fuel: water! When you
run
> low on water, you have to head back to dock. And since water can't be
> compressed, a large chunk of your mass is going to be taken up with
water
> storage. I'm trying to dream up a system that doesn't require frequent
> re-fueling. 

Doesn't one of Newton's Laws state that the net kinetic
velocities must remain constant in a system.

So I'm not sure what it's possible to have a system that follows are
current day view on how physics works and still not consume any fuel.

With that said, I think it might be possible to create a system that
doesn't use much fuel.

In particular, I seem to remember reading about a system that used
radioactive decay for it's thrust.  You took a chunk of radioactive
material and surrounded it on three sides by shielding and just let it
decay ... the particles thrown off during it's "decay" resulted in a
miniscule amount of thrust which would last a long long time.

Also, something to think about, the net kinetic energy stays the same
in a system ... so if you could throw a miniscule amount of mass out
at close to the velocity of light you could probably keep a sizable
amount of thrust up for a pretty good period of time before you ran
out of fuel (sorta a particle accelerator/rail gun down the center
of your ship).	Actually, thinking about it, a perfectly efficient
laser (converting mass directly to light) would probably be the most
efficient thrust mechanism (but that's entering the realm of
speculation).

I suddenly have the urge to the do the math on this.
Constants:
C=300,000,000 m/s or 3x10^8 (or thereabouts)
1G=9.8 m/s^2 (round to 10 m/s^2)
The formulas used:
force = mass * velocity (therefore velocity = force / mass)
(average) acceleration = velocity / time
So for our starship we've got
(average) acceleration = 
  ([force of ejected material] / [mass of starship]) / time
  == (FoEM / (MoS * time))
Now if we assume the force of the ejected material is being pushed out
at C or thereabouts so FoEM = Mass of Ejected Material * C
We have
(average) acc = ((MoEM * C) / (MoS * time))

So our variable here is MoEM (we can always throw more out the back)
... time would be a second (to calculate average acceleration over a
longer period of time ... it would just increase the MoEM linearly)).

So let's assume we have a 1x10^6 kg ship which we want to accelerate
at 1 G (10 m/s^2), and we have a really cool rail gun mechanism that
can accelerate things to close to the speed of light (3x10^8) ... it
would require us to throw out per second:
10 = ((MoEM * 3x10^8) / 1x10^6)
10 / 3x10^2 = MoEM
MoEM = 0.0333... kg (or roughly 33.3 grams per second).

(which would give you 30 million seconds of fuel before you completely
expended your ship (almost a year) :-) (of course the ships weight
would decrease as you traveled :-). (Note: This 30 million seconds is
a constant for all masses of ships accelerating at 1G).

If you have a less efficient drive (ie unable to throw
things out the back at light speed) your mass of emitted material
goes up proportionally. (So if you could only accelerate things
to 1/3 light speed you'd have to throw out 0.1 kg per second).

Hope I'm not wrong on the above (since it's been about 5 years since
my last physics class and I'm working on memory) ... feel free to
correct me if I am.

Daryl

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