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# Re: FT-Airless bodies

From: "Oerjan Ohlson" <oerjan.ohlson@t...>
Date: Wed, 10 Jan 2001 18:35:46 +0100
Subject: Re: FT-Airless bodies
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Nyrath the nearly wise wrote:

>An atmosphere does reduce the range of beam weapons, but
>even in airless space such weapons are subject to the
>
>This is because a beam *spreads* as it travels.
>
>The law says that if you double the distance, the
>beam strength drops by one quarter.  Doubling means
>a factor of two, two squared is four.
>In other words, the beam strength is 1/(d^2)

Um... well. Yes, the beam will spread out with range if the range is
long
enough, but it isn't as simple as "twice the range means four times the
cross section".

It was six years since I studied ray optics so my memory is a bit hazy;
in particular I don't remember what effect beam *dispersion* has
(different phenomenon from diffraction IIRC), but from what I remember:

If the beam is perfectly "parallell" (the focal point is infinitely far
away), then it will behave as your quote describes. (I snipped the
quote for brevity; can repost it if someone missed it)

However, if you focus the beam at some point *closer* than infinity,
then the focussing and the diffraction will counteract each other until
the focal point has been reached, so the beam will "spread out" slower
than the single-aperture diffraction formula suggests. If the focal
point is close enough to the projector, then the beam will get
*narrower* until it has reached the focal point - not wider. The
cross-section will always be a finite area though; the diffraction (or
was it the dispersion?) stops it from becoming infinitely small at the
focal point.

Beyond the focal point the beam goes out of focus - ie., the focussing
and the diffraction "cooperates", so the beam diverges faster than the
diffraction formula says.

So, the big question is: where do you put the focal point of your beam?
You *want* it to be exactly at the target's location, but can you
manage that...? <g>

KH Ranitzsch wrote:

>>What would be the requirements for fighters climbing out of a small
>>gravity well, on a airless body?
>
>These would depend critically on the size of the planet and the
density
>of the athmosphere, and, frankly, I don't know the formulas for
dealing
>with athmospheric resistance.

On an airless body, the formula for dealing with air resistance is
actually *very* simple ;-)

Regards,

Oerjan Ohlson
oerjan.ohlson@telia.com

"Life is like a sewer.
What you get out of it, depends on what you put into it."

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