Re: starship-design: Hull Materials

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

Hi Lee,

>I would like to start a discussion on a topic which I know very little 
>about.

Good, then everybody must be able to make you wiser.  (Oops ;)

>Any ship we design that is capable of achieving substantial fractions of c 
>will be exposed to enormous amounts of radiation and material impacts. I 
>have seen several proposals for using frozen deuterium ice (fuel) 
>surrounding the ship and providing shielding from impacts. Of course, this 
>assumes that you are using hydrogen for fuel...

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!

>I want to explore alternatives for the actual hull of the craft. What 
>metals are available, what sort of melting points, etc. I think of 
>particular interest would be the X-ray density, ductility, and hardness 
>numbers.

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.

>Some of the most refractory metals with high melting points and high x-ray 
>density are borides and carbides of Tantalum, Tungsten, and Uranium. 

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.

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?

>Unfortunately, these metals are not very ductile, in other words they are 
>extremely brittle. The ideal candidate would possess the advantages of 
>Tungsten Carbide in a more malleable form. Perhaps some sort of Chobham 
>type of armor with layers of metal and ceramics.
>
>As a comparison, what is the X-ray density of deuterium ice? How much more 
>ice would be required to equal the shielding of one of the refractory 
>metals?

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)


Timothy