- EXTERNAL LINK: Why does traveller use displacement tons?
- EXTERNAL LINK: David's DGP: Traveller Q&A: Official Answers To Your Questions
- EXTERNAL LINK: What does 1 ton of space means in square metres?
Megatraveller Q&A (Feb 2019)
Why is the 'ton' used to measure both displacement (volume) and physical weight? For example, the G-Carrier discussed in Grand Census displaces less than 9 'tons', yet weighs 72 'tons' when empty. Is starship performance affected by cargos of excessive weights, even though they fit in the ship? - J. N.
MegaTraveller craft design clarifies the definition of the various types of tons on page 57 of the Referee's Manual.I agree with you that the terms can be confusing - which is why I tried to get away from using "tons" for volume, and using kiloliters instead for vehicle and spacecraft design. Our preference here at DGP is to clearly specify when we are talking about a displacement ton. For example: a Scout/Courier has a size of 100 displacement tons. To recap, here's the definitions given in MegaTraveller:
Tons Displacement: A widespread method of specifying a space vessel's size is to give its volume in terms of the amount of liquid hydrogen it would displace (as if it were immersed in a vast sea of liquid hydrogen). Tons displacement is not to be confused with the craft's weight in metric tons (that is, its actual mass). A starship that displaces 100 tons may actually weigh over 1,000 metric tons. A displacement ton is a measure of volume rather than weight; one displacement ton equals 13.5 kiloliters of volume.
Volume: A craft's volume is the amount of space it takes up. Volume is measured in kiloliters - a kiloliter equals one cubic meter. Thus a cube that is one meter on a side has a volume of one kiloliter. A kiloliter contains 1,000 liters; 13.5 kiloliters equals one ton of displacement.
Weight: A craft's weight is measured in metric tons. One metric ton equals 1,000 kilograms. To answer your question about excessive cargo weight affecting starship performance, in MegaTraveller, loaded weight is computed using the mass of a cargo hold totally filled with water (i.e., 1 metric ton per kiloliter). This, clearly, is more massive than a typical cargo load.
Clarifying the distinction between displacement and mass has several interesting effects on the MegaTraveller craft design system. MegaTraveller ship performance (agility) is based on true ship mass, not displacement, so ship performance is realistically affected by changes in the ship's mass. I think this makes starship design in MegaTraveller both more fun and more challenging.
For example, if you want to armor your ship's hull to the hilt, you'll pay the price. Your ship's mass is going to sky-rocket, and your ship's agility will suffer. The ship's displacement remains unaffected.
- - Joe D. Fugate Sr.
Volume should be 14.114m3? (2019)
The volume of a dTon is normally specified as between 13.5m3 and 14m3. The density ofis given as 70.85g/l or 70.85kg/m3. One thousand kilograms (aka a tonne) of liquid hydrogen would therefore occupy 14.114m3. (Liquid hydrogen is light!) Anyway, 14.114m3 is way off 13.5m3. And this doesn't include any containering, pressure vessels, pipes, etc. Am I missing something?
- - Tim
- The original definition of a "ton" was two 1.5 meter squares on a ship deck plan map, each 3m tall, which is where the 13.5m3 value comes from. The 14m3 value derives from, as you note, a rounded value of the tonne of liquid hydrogen, which is the fuel used by the jump drive. I have read papers published by NASA about compressing or freezing liquid hydrogen which can achieve values of around 11m3 per tonne. NASA was very interested in this because making rocket fuel smaller means a large lift mass. The different versions of Traveller vary about which value is used.
- - Tjoneslo (talk) 22:54, 17 September 2019 (EDT)
Another discussion of size of a Ton
The 13.5m^3 dTon probably arises from mapping conventions used in Classic Traveller, where two 1.5m x 1.5m squares were stated as being one dTon. 1.5m x 1.5m x 3m (presumed deck height) x 2 (squares) = 13.5m^3. This was changed to 14m^3 in later versions, probably because 14 is a nice round number that's closer to the actual volume of a metric ton (1000kg) of liquid hydrogen, which is about 14.114m^3 [assuming Traveller's displacement ton is based on the metric ton (1000kg), not the Imperial (long, 1016.05kg) ton or the US (short, 907.185kg) ton]. Liquid Hydrogen is supposedly 70.85 g/L = 70.85 kg/m^3. 1000/70.85 = 14.11433. To there's around 14.114 cubic meters in a metric ton. If a dTon in Traveller were assumed to be then same as a metric ton (around 14.11433m^3), then the vertical deck spacing on Traveller starships would be around 3.14m [14.11433/(1.5x1.5x2)]. User:Jimv (talk) 06:49, 23 September 2021 (EDT)