# Re: Re[2]: Starship rotors & Drilling

```A few Heat Transfer and Thermodynamics comments:

All other factors equal (materials, surface and surrounding temperature),
a surface with 2X area will transfer heat at twice the rate of a surface
with X area.

Fins are added to increase cooling without increasing the size of the
entire object.  They increase the surface area many times more than a
solid object in the same space.

Caveot for cooling fins - Adding a cooling fin of X area to an object of X
area will NOT transfer heat at twice the rate of an object of X area.  Why
you ask, because the further you travel along the fin away from the heat
source, the cooler the fin.  The cooler the fin, the smaller temperature
difference between the fin and the surrounding fluid/gas.  The smaller
temperature difference, the lower the heat transfer rate.  Engineers have
methods of calculating the optimal length, spacing, etc.... email me if
you are really into this kind of thing.  You can come to class with me.

Another factor that enters into cooling is that the fluid will flow due to
the rising of the fluid/air as it is heated by the object.  Thus, you can
have convection increasing as the fluid/gas is heated, it flows away and
new/cooler fluid/gas replaces it.

Finally, two factors affect the amount of heat taken away from an object-
How well heat is transferred from the surface
How well heat is transferred from the core to the surface.

I posted a detailed discussion of this several months ago, but the short
of it is that some materials can hold more heat energy than others (ie
steel over Aluminum), while others can transfer heat energy from the core
to surface better (ie Aluminum over steel).  Note that the surface cooling
does not depend upon the surface being cooled, just the temperature
difference and the cooling fluid/gas.

Happy motoring,

Eric Schumacher
es61@prism.gatech.edu

>
> Mike, > With an extremely limited understanding of thermodynamics, is it
not safe > to say that given mass with 2X surface area will cool more
quickly than the > same mass with X surface area.  Isn't that why heat
sinks increase area by > adding fins, ie. why vented rotors and shotgun
barrels etc where brought > about, to aid in cooling regardless of air
flow? (though agreed more is > better)?  > Mike
>

```