On 23Sep2021 0730, Phil Pugliese - philpugliese at yahoo.com (via tml
list) wrote:
>   I vaguely recall reading that the 'extra' 1/2 DT was retconned by including wall thickness, etc.
It's not really necessary to retcon it - Traveller's always said that
floorplans only need to be within 10-20% of the 'exact' tonnage/area to
be perfectly acceptable. The rest can be gained or lost in walls, fuel
tankage, higher or lower than normal ceilings and under-floor/over-head
crawlspaces, and so on.

Anyway, I thought I'd check what each version I've got handy says:

CT 1977 doesn't really say how big a 'ton' is, but as it refers to a ton
of fuel as also massing a ton, you can tell it uses the same assumption
that later rules do, it just doesn't spell it out or say what this means
in terms of volume per ton displacement.

CT 1981 says "As a rough guide, one ton equals 14 cubic meters (the
volume of one ton of liquid hydrogen)", so it's not trying to be precise.

MT says "A displacement ton is actually a measure of volume rather than
weight; one displacement ton equals 13.5 kiloliters of volume." MT very
much preferred to work in kilolitres.

Also: "Craft plans may have a scale of 1.5 meters per square. Optimum
space between decks is 3 meters: one floor square (1.5 meters by 1.5
meters by 3 0 meters) equals 6.75 cubic meters or 6.75 kiloliters. Two
floor squares equal 13.5 kiloliters or one displacement ton.

When allocating space within the craft, assume only a portion of the
specified volume for extended accommodation (staterooms, bunks, low
berths, and so on) is used; the remainder should be used in common areas
and other accomodations for the crew.

A leeway of 10% to 20% should be allowed. If the final craft plan comes
within 20% of the craft's UCP volume, the plan is acceptable.

TNE says that "Each displacement ton is drawn as a grid square 2 meters
by 2 meters, with a vertical height of 3.5 meters. Much of this vertical
height is taken up by deck thickness, lighting, environmental ducting,
etc., so that free height is only about 3 meters", matching its use of
2m x 2m squares for combat maps. It then continues with the same text as
MT on the matter of allocating space and margin of 'error'.

T4 says "Hull size is measured in standard displacement tons (tons),
equivalent to 14 cubic meters, or the volume of one metric tonne
(1000kg) of liquid hydrogen."

GURPS Traveller swapped to US customary measure (because that's what
GURPS uses) and thus says "Displacement tons measure size (500 cf per
dton)." That's 14.16 cubic metres, making it actually the closest to
being exactly a tonne of liquid hydrogen in volume.

MgT 2e says that "A displacement ton displaces a volume equal to one ton
of liquid hydrogen. One displacement ton is roughly equal to 14 cubic
metres."

T5 says "Hull sizes are expressed in displacement or volume Tons (= 13.5
cubic meters)".

So historically speaking the use of 13.5 cubic metres is the anomaly,
and I'm sure it's to make it fit perfectly with two 1.5m x 1.5m grid
squares.

As an aside, I noticed whilst flipping through the texts that spaceships
in TNE cost 25-50% or so more than they did in CT and MT. The same
things in MgT - 25-50% extra. T5 doubles the cost, or more.

--
Rupert Boleyn <xxxxxx@gmail.com>