From: leifmk@Lise.Unit.NO (Leif Magnar Kj|nn|y)
Subject: Re: Unrealistic space scenes
Organization: Norwegian Institute of Technology
Date: Mon, 14 Dec 92 15:52:33 GMT

In article <6X3oVB7w165w@cithara.login.qc.ca>, spongebo@cithara.login.qc.ca (THE EYE-BALL KID) writes:
 > olljmann@vineland.pubs.stratus.com (Jim Mann) writes:
 > 
 > > In article <1992Dec9.193652.1@vax1.umkc.edu> kgassner@vax1.umkc.edu  
 > > writes:
 > > > 5. The propulsion system of ships is always active in the same way,  
 > > regardless
 > > >    what kind of maneuver they are performing.
 > > >    As long as a ship doesn't change course, the propulsion system  
 > > should be 
 > > >    off. (E.g. in those Big-Ship-is-slowly-passing-the-camera-shots)  
 > > But the
 > > >    fancy lights on the end of the ships are just looking so nice. 
 > > > 
 > > 
 > > Why?  We only do that now because fuel is so expensive. If you had
 > > a ship that was actually capable of continuous 1 G acceleration,
 > > you'd probably keep the engine on most of the time.
 > 
 > Yes, but that would assume that 50% of the fuel would be used for acceleration
 > and 50% for deceleration -- right? Or am I missing something here. (sorry if I
 > am)
 > 

Not so, not so.  All known forms of energy production or storage requires some
sort of fuel that has *mass*.  With a rocket, for instance, a *lot* of the fuel
is spent not only to accelerate the payload but also to accelerate *the rest of
the fuel*.  So fuel has parasitic mass.  This means that the longer continuous
acceleration you want, the more extra fuel you need.  With "low-efficiency" fuel
like that used in present-day rockets, this means that if you keep increasing the
time you want to accelerate, you will fairly soon need so much fuel that it will
no longer be practical.  

With futuristic technology, this problem will become less severe.  However, even
if you have total conversion of matter into energy *and* are able to translate
*all* of that matter into kinetic energy for your ship, you *will* notice the
problem of parasitic fuel mass if you want velocities really close to c.  (Don't
ask me to do the math now; it's a little bit hairy and my brain is off math-mode
because all my finals are done, OK?)

 > Also, what is "1G acceleration"? I pressume that's an Earth G, but if you're
 > flying to a planet that's larger -- or smaller -- than Earth you have to
 > re-calculate your "G" (so you can fall into orbit on arrival, right?).
 > 

Pardon?  Yes, by convention, when people speak of "1G acceleration" they mean the
same acceleration as that which the force of gravity will have on an object--any
object--near the surface of the Earth.  (This value is actually slightly dependent
on your location; the "standard" value which I've seen most often is 9.81.. m/s*s,
but around here it's 9.82....)

But what *are* you talking about?  Recalculate "G" to "fall into orbit"?  What's
this supposed to mean?  I'm sorry, I just do not understand what you're referring
to.

 > I could be way out here, and expect to be corrected, but something I would like
 > to know is:
 > 
 > What are the structural peramiters required for "high-speed" outer-space
 > travel. Clearly things like "wind-drag" don't factor in, but there must be a
 > "shape" of ship which flies better. For example, a "sail-ship" might move fine
 > at the speed of the solar wind, but if you burn fuel to accelerate, the sails
 > will collapse under their "lower momentum"... or will they.
 > 
 > I'd appreciate refernce books if anyone has any. This IS a facinating
 > discussion...
 > 
 > 
 > 
 > Spongebo - THE EYE-BALL KID (:::::-)
 > 
 > spongebo@cithara.login.qc.ca

Well, I don't have the specifics, but this mostly depends on such nice things as
how much you want to accelerate, what parts of the ship have how much mass, and
the tensile strengths of various components.  Shape, in itself, is not very
important if you want to build a ship that won't ever have to enter an atmosphere.
Of course, the velocities you want are not important at all--in theory.  In 
practice, you will keep bumping into micrometeors, dust, and the odd hydrogen
atom.  With very high velocities, such things are likely to cause damage.

-Leif Kj|nn|y
Norwegian Institute of Technology
Trondheim, Norway

leifmk@lise.unit.no