RE: starship-design: Hull Materials

[TLG.van.der.Linden@tip.nl (Timothy van der Linden)]

>> Good, then everybody must be able to make you wiser.  (Oops ;)
>
>Reeaall funny... <G>

Well, believe it or not, but at first I had intended that phrase to make
other less hesitant to reply.

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

Yes, unless, you can somehow radiate that energy at an other place. Eg. put
a grid of superconducting material in the ice and connect it with a large
radiator (that doesn't radiate towards the ice). This however seems to be
not very practical (and certainly throws away the simplicity of an 'iceball').

>> 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.

Oh, could be. It's density is about 1.7 times higher than lead. And its
strength is much bigger.  This seems to confirm that for Earthly
applications we seek those that use least room.
However, I now suddenly recall that for most Earthly applications we don't
shield against protons but against low energy X-rays. Against these rays,
dense metals indeed seem a lot better.

>> 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

Unless we use some cooling system. 

>> 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.

Actually I don't know much about both.. <G>

>> 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 
>> x-rays)
>
>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.

The density may be different, but since we're not short of room, that does
not really matter.
My theory and tables mainly talk about mu/rho [gm/cm^2] (mass attenuation
coefficient) when considering X-ray shielding. For low X-ray energies the
photo electric absorption by the K,L,M electronshells seems to be the
dominating factor. It looks like we once again need tables to know what's best.


Timothy