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RE: starship-design: Hull Materials

On Sunday, November 09, 1997 3:36 PM, Timothy van der Linden 
[SMTP:TLG.van.der.Linden@tip.nl] wrote:
> Good, then everybody must be able to make you wiser.  (Oops ;)
Reeaall funny... <G>

> Actually this deuterium ice may melt away quickly. Assuming 10 protons
> per
> cubic cm and a velocity of 0.3c, that means an inpact power of about 0.6
> Watt per square cm. That makes 6000 Watt per square meter. At Earth orbit
> the Sun shines at a mere 1400 Watt per square meter!

In other words we can forget the giant iceball idea...

> As far as I know: If we shield for protons, then the density of the
> material
> doesn't matter much if the purpose is to stop the protons. The weight
> however does. So whether you uses 10cm thick Aluminium or 2.4 cm of Lead,
> both will be as much mass and both will shield almost equally well.
> The reason to use lead in earthly applications is usually because we're
> short of room.

Actually we use an alloy of Tungsten in most earthly applications...if I 
remember correctly, it is Tungsten, Chromium and something else.

> High melting points may indeed be handy if we're going much higher than
> 0.3c, then the temperatures can be similar to those of the Shuttle's
> shield
> when entering orbit.

My thought exactly

> I wonder: The X-rays you are worrying about, are those the ones that
> "float"
> around in space already, or the ones that are generated when protons
> impact
> on the shield?

Actually, both.

> X-ray shielding is quite dependant on the energy of the x-rays. If the
> graph
> that I'm looking at doesn't lie, then it seems that above 1 MeV the kind
> of
> matter doesn't make a huge difference anymore. Again then all that counts
> is
> the weight.
> (At 100 MeV you only need 6 times more mass when you use air as shielding
> than when you use lead. Of course this difference is not insignificant,
> but
> surely small when you compare it to shielding against lower energy 

Yes, but density per unit volume is significantly different for various 
metals, and obviously so for water, etc. The X-ray density number is the 
same thing as the density of the material I believe, since it is expressed 
in gm/cm^3.