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Re: A good altitude for Ortillery? Math and astrophysics guys help out

From: Tom Anderson <thomas.anderson@u...>
Date: Tue, 30 Nov 1999 23:42:39 +0000 (GMT Standard Time)
Subject: Re: A good altitude for Ortillery? Math and astrophysics guys help out

On Tue, 30 Nov 1999 wrote:

> Given:
> 1) Geosynch is roughly 22,000 km (if I'm not way off base)

umm ... about 40 000 km last time i checked. have they changed the
universal gravitational constant again? Arthur C Clarke, in the essay in
which he proposes the geosync commsat, puts it at 26 000 M, which is 42
000 km. i believe Arthur. he da *man*.

> 2) Given geosynch is stationary wrt a point on the earth
> 3) Given a lower orbit will be (of necessity) faster? I think
> 4) Given the lower the orbit, the faster the orbit - so the further
> off geosynch, the less time you'll be able to see a given point on the
> earth before going out of arc

all true.

> Can some of the math whizzes calculate an altitude where a ship
> wishing to supply ortillery support can see a given point on the earth
> (we'll call it the battlefield) for a period of at least an hour -
> we'll call this the support-fire window.

it's a little more complex than that: when the ship is on the target's
horizon, there is a line of sight, but it's through a lot of atmosphere
and the surface at the target is parallel to the line of fire - not good
shooting conditions. what's actually needed is the length of time the
is within a certain angular distance of the target point: ie, the time
when the ship is within a certain distance of the midpoint of the sky at
the target (the zenith?).

> I figure an hour is a few DS2
> turns, and at least 12 SG2 turns. Probably long enough to lay down
> enough ortillery to matter.

i'll drink to that.

> So.... what altitude would be reasonable and give us at least an hour
> as a support-window? If someone had someone is gutsy enough to bash up
> two formulas, one for the support window
> delta-t(support window)= ? (some f(altitude))
> and
> t (inbound transit) = ? (some f(altitude, probably launch v and acc))

these functions should be easily invertible if at all possible!

> This might be instructive for all of us - it'll give the FTers a
> feeling for how close to the surface you have to get

too close! 8)

> It may be we can evolve some simple rules which let players choose a
> lower altitude support mission (with a smaller window of availability)
> as a trade off for faster response on the fire missions, or vice
> versa - trade delay time for a longer ortillery support mission.

i have/had an ancient pc game where you play an alien fleet commander
attacking planets; you have teleportable shock troops and landerborne
heavy troops (the shock troops have to beam down, build the landing
ans survive long enough for the heavies to arrive), and orbital fire
support from your BB flagship and some cruisers: you could do just this
with the orbits, and it also affected how much planetary defence fire
got. given that you were generally fighting battles at six sites on the
planet at once, whilst also doing strategic bombardment, it was rather
tricky. a fantastic game!

i would add two things ...

one, you're not considering the possibility of OTH supporting fire;
all, classic artillery is indirect, so why not ortillery? okay, so the
travel time goes way up, but ICBMs from ships at 1000 km altitude should
be a rather effective weapon.

two, you're assuming firepower is concentrated in ships. i think the
javelin idea (explained well in NivenPournelle's 'footfall' - BUY YOUR
COPY TODAY!) is that you plaster a low orbit with autonomous javelins,
then call down the closest ones when you need them. thus, there is
fire support almost directly overhead. oh, and Peter F Hamilton's 'the
reality dysfunction' (bad sf, good space opera) has 'kinetic harpoons'
used to great effect. haDOOOOKEN!


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