[TML] Tonnes, Tons and Tuns (long pedantic and possibly boring)

[Jerry W Barrington]

I would not "cut part of the volume off".  The shapes categories are
guidelines, not necessarily literal.  So, I would assume the size of the
base "sphere" would be enlarged such that the volume remains at the designed
amount.  At the extreme, this would be a hemisphere sitting atop the
circular nozzle.

Also, the basic rule you are reaching for is well known.  It's why animals
don't get too large.  The mass of the ship goes up as the cube of the scale,
but the area (of the whole or of any given part like "the aft") only goes up
as the square of the scale.  Thus, doubling the diameter of the sphere
multiples the weight (and needed thrust) by 8, but only multiplies the area
(including potential nozzle area) by 4.  So obviously, you will reach a
point that there is not enough area to put nozzle.  The exact point will
depend on configuration, but a sphere is going to have the largest aft cross
section, unless you mount the nozzle under a wide flat ship, instead of to
the rear as most people envision it.

So, fast ships are going to be broad, flat spheroids or domes.  Needles are
horrible.