On Mon, 23 Jun 2008 15:15:03 GMT, wrote in
:

Except they can't as battery technology is an order of magnitude short
on energy density to be able to do it.

I've always thought that the energy stored in metallic aluminum might
be harnessed for motive power. If you look at it on the energy
density chart http://en.wikipedia.org/wiki/Energy_density, it would
appear to be a reasonable energy source.

Storage type Energy density Energy density
by mass (MJ/kg) by volume (MJ/L)
----------------------------------------------------------
Gasoline[7] 46.9 34.6
Aluminum (burned in air) 31.0 83.8
Hydrazine (toxic) 19.5 19.3
combusted to N2+H2O

So while aluminum isn't as rich an energy source pound-for-pound as
gasoline, it is significantly better than rocket fuel.