Ship's Fabricator

[MasterGwydion]

The litre is a derived unit.

It is the result of a cube of side 10cm, the cm being defined as one hundredth of a meter which itself is defined as... you get the point.

The metre is a unit of length, a length x length x length is a volume.

The "correct" unit for volume is the m³

Just out of curiosity... Why doesn't Traveller just use the metric system without changes? Could have made cubic decameters the standard for starships. You know what the abbreviation is for decameters? (dam) So volume would be dam3. lolz! This would have been amazing! Opportunity lost.

 

[rinku]

I mean, we could revert to MegaTraveller or TNE and account for mass, volume and surface area. That is the realistic, crunchy way. There is NOTHING wrong with that approach for people that like that stuff. Especially if it's important to the campaign assumptions (i.e. no magic reactionless drives or artificial gravity to smooth the edges)

But as anyone that has done much work with those sequences knows, it's a lot of accounting for not very much difference. Mass vs thrust determines the acceleration in a vacuum. Components are all sorts of mass per volume, but it turns out you have some that are quite dense (drives, full cargo holds) and some that are not (staterooms, control areas, access areas, empty cargo holds). And the 10-60% of the ship that's liquid hydrogen storage turns out to be a fair average. In the end you have a kilogram value and a cubic metre value that often agrees with a density of liquid hydrogen.

CT and Mongoose ship design is overtly a simplified process to let you put components together and end up with a vessel with usable characteristics. Of course a ship has mass; it's put together using volume, but it's also sufficient to say that a drive of sufficient size can propel the average mass for a given ship volume, at a particular rate.

And deckplans? You can't usually work out exact volume or mass from a diagram of where the crew can walk around. Most of that is just air, boxed in by sturdy but thin metal. Except for the simplest hulls (and especially for streamlined ones) there's going to be varying heights between deck and ceiling, curved space that might be useful to store fuel or machinery in, or maybe not. The old rule of thumb is still used - 2 squares per dTon, within plus or minus 10%... but maybe don't bother showing the fuel tanks or volume that's unusable due to streamlining.

As I said earlier, mass is the most important parameter for a spacecraft (and it's probably more important than volume for aircraft. Volume is only more important for watercraft). One approach anyone can take is to remove the "displacement" from dTons and think of the ships just in terms of mass most of the time. When volume is needed, such as for deckplans, use two 1.5m squares, or approximately 14m3, per metric ton (with a +/-10% variance). Which I suspect was precisely what was intended when the idea was first published around 1978 (Snapshot?)

 

[rinku]

A further point... if you want to be realistic, you need to recalculate as the payload changes. Even with reactionless drives, a ship has a significantly lower mass after jumping, or if it has an empty hold compared to a full one. Every passenger and their belongings is mass. Every missile fired is lost mass. Is the ship's boat docked or not? Different ship mass.

It's generally not worth bothering with. Even if you are using reaction drives and this will definitely affect the final thrust value. It rarely matters exactly how many minutes it takes to climb to orbit or how many hours it takes to make 100D. A 10% better or worse thrust isn't likely to change the number of combat rounds the patrol ship will have to shoot at you, or if you'll need to risk a 10D jump or not, which is what actually matters.

 

[MasterGwydion]

A further point... if you want to be realistic, you need to recalculate as the payload changes. Even with reactionless drives, a ship has a significantly lower mass after jumping, or if it has an empty hold compared to a full one. Every passenger and their belongings is mass. Every missile fired is lost mass. Is the ship's boat docked or not? Different ship mass.

It's generally not worth bothering with. Even if you are using reaction drives and this will definitely affect the final thrust value. It rarely matters exactly how many minutes it takes to climb to orbit or how many hours it takes to make 100D. A 10% better or worse thrust isn't likely to change the number of combat rounds the patrol ship will have to shoot at you, or if you'll need to risk a 10D jump or not, which is what actually matters.

This is why we use volume and not mass in Traveller, simplicity.