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# Re: [GZG] Batteries and CG

From: Samuel Penn <sam@g...>
Date: Tue, 14 Sep 2010 12:12:21 +0000
Subject: Re: [GZG] Batteries and CG
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On Tue, 14 Sep 2010 06:15:54 +0430, John Atkinson
<johnmatkinson@gmail.com>
wrote:
> Yes, and Yes.  But how much the energy expended in space travel is
> getting it from point A to point B, and how much is expended in
> getting it out of A's gravity well and controlling it's entry into
> B's?	I'm not saying mass is utterly irrelevant, but I am saying that
> if space travel is economical enough to be commonplace, and economical
> enough to be launching an invasion, you aren't going to be worried
> about a couple kilos here and there.

I'm more thinking of the difference between shipping balsa wood (<
1t/m^3)
and tightly packed depleted uranium shells (~19t/m^3). Even between
water and iron, there's a seven fold difference in mass. You're right
that
a few kg, or even a few tonnes, isn't going to make much difference in
the sort of backgrounds we're interested in.

In terms of what difference CG would make, it's mostly going to be for
low acceleration ships (close to 1g) close to the planet. Assuming an
Earth-like world, gravity will cut (outward) acceleration by 1g, so
a ship with an acceleration of 1.1g will see a ten-fold improvement
when using CG. At the high-end of Traveller, a 6g ship 'only' sees a
20% improvement.

For that 100 diameter trip to a safe Jump distance, that's 1,280,000km.
After only 6,400km (a mere 5% of the trip), the planet's gravity has
dropped to .25g. It's down to 1/16th at 10%. I haven't done the maths,
but even for a relatively low acceleration ship, most of the advantage
of CG is during initial take off and landing. After that, it's
contributing very little.

It would allow cheap, low thrust ship designs however if people aren't
worried about how long the journey takes.

Picking up on the smuggling comments - CG might make it easier to
ship stuff between orbit and the surface. If CG units are small and
relatively cheap, they may be harder to detect than rockets or large
parachutes, making it possible to drop small packages from orbit
without detection.

In my Traveller, CG (actually described as repulsor beams) had a
relatively short range. It was used to land/hover spacecraft without
having to burn up the landing pad with hot thrusters. Altitude was
limited to a few tens of metres for most ships.

>> Would this lead to spherical ship design?
>
> Perhaps, if there aren't other considerations.  Like the fact that
> minis makers and gamers don't like 'em.  So you have to come up with
> some PSB to fix that.

Agreed.

>> If you have a Traveller-like 100 diameter limit for FTL, then
>> mass will affect acceleration and how quickly you can get to
>> a safe jump point.
>
> And if you don't, you don't have organized political bodies very long.

:)

I don't recall the B5 episode where White Stars jumped out of Hyperspace
directly above a Martian base to provide air support, but yeah, the lack
of such a limit would be a game changer.

--
Samuel Penn
sam@glendale.org.uk
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