(1) a displacement of 20 tons is about 270 cubic meters (13.5 cubic meters
per ton). This would drop the density you figured by over 90% (about 1.79
g/cc i think...). I don't remember the periodic table well enough to know what
this corresponds to (if someone wants to check my math, please feel free...).
(2) well, at least in the discussion of battle dress, we haven't gotten into
nanotechnology (right now on GMAST-L, we are discussing (very) high-tech
combat armor, and the prevailing opinion seems to be for nanotechnology,
which allows the suit to reconfigure into whatever patterns are stored in
its computer's memory). This would certianly make for battle dress that
would fit just about anyone, but is pretty far out there...
(3) there should be a difference between Astrogation and Navigation;
this is not an unnecessary skill split.