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starship-design: Hull Materials

Hi Lee,

>I don't remember the specifics on Kelly's sail, but most proposals for 
>solar/laser/microwave sails are only 100 micrometers thick at best. I still 
>think most protons, etc. will just blow right through the sail without even 
>impacting much of anything, especially if it isn't a solid sail, and there 
>is no reason why it should be solid.

With not solid, you mean meshlike? That indeed would decrease the problem

Protons, unlike photons can/will loose their energy partially when they
encounter matter.

If the formula I use is valid and my calculations are right then a proton
will loose 7.5MeV in a solid lithium layer of 100 micrometer.

Let me do some calculus to estimate the equilibrium temperature of the sail.
  10 protons per cubic cm, sail velocity 1.2E10 cm/s
  Number of protons per square cm per second: 1.2E11
  Number of protons per square meter per second: 1.2E15
  Power per square meter: 1.2E15*7.5E6*1.6E-19=1440 Watt/m^2
  Sail has 2 sides to radiate the energy away, so that makes 720 Watt/m^2
That gives an equilibrium temperature of 335 Kelvin or 62 Celcius. (Litium
melts at 180 Celcius.)

A reverse calculation tells me that a 600 micron thick solid litium shield
will melt assuming 10 protons per cubic cm at 0.4c.

So indeed the problem may be less than I at first thought. Let's just hope
that this number of 10 protons per cubic cm is a maximum.