Measurement

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evo
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Measurement

Postby evo » Sat Oct 15, 2011 9:58 am

Hi,

I tried to create my first ship, but I encountered an issue. I use the French corebook, and read the OV corebook, but I am not informed either. Is there someone who can explain *clearly* what is this [self-censoring] "D-Ton", and how it works ?

That's a bit confusing, because, the UK/US "ton" unit seems to be a Volume *AND* a Mass unit (why to do things simpler when it can be complicated, right ?). In my mind, the "Ton" is a mass unit, thus :

Mass :
1 ton = 1.102 metric tons

I think that the D-ton used in Traveller is a volume unit, because it's written page 106 (French) : 1 Dton = 14 cubic meters. No mass into play, there. Thus :

Volume :
1 Dton = 14 cubic meters

But the fret, goods, article, gears are labeled in Tonne (mass unit). How can I guess how much goods I can load in a 200 D-ton (volume unit) cargo ? it depends on the density of the object. A ton of feathers or a ton of gold do not take the same place !

The use of D-ton in Traveller is unnecessarily complicated. Why not use an abstract unit (a "space" for example), the same for all, for the construction of vessels and vehicles, and then use an approximate conversion as 1 space = 1 tonne, or 10 cubic meters, or 300 Cauliflower (etc. .) ?
BFalcon
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Re: Measurement

Postby BFalcon » Sat Oct 15, 2011 12:43 pm

As I understand it...

A d-ton (dton, dt) is the volume a metric ton of hydrogen would occupy (under normal atmospheric pressure and gravity).

It measures 1.5m x 1.5m x 3m roughly. As for how much it actually weighs, I'm not sure, someone else will have to help you there.

Edit: also, all cargos listed in the book are also listed as dtons, not mass tons, if that helps... it seems to be the standard unit of measurement of the setting... so unless you hear otherwise, assume 1 ton = 1dton, not mass...
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Re: Measurement

Postby BP » Sat Oct 15, 2011 2:37 pm

dton stands for displacement ton in Classic Traveller - a unit of volume.

It has nothing to do with mass* - it is based, as BFalcon stated, on a volume occupied by liquid hydrogen, but really that is just background fluff.

To compound matters, the 14 cubic meters does not fit well (pun intended) with the 1.5 meter deckplan measures. The standard being 13.5 cubic meters = 1 dton. Given that this is a game, that should have been the definition to begin with. (I.e. - one 'design ton' = 13.5 cubic meters and an abstract unit that didn't use the word ton would arguably have been much better, but displacement ton is used in nautical terms which the original authors where a fan of for spaceships.)

Mongoose intentionally decided to drop the 'd'. Making matters worse.

* - since gravitics is assumed and the effects of mass are thus pretty much ignored in the game mechanics for spaceships.
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Re: Measurement

Postby AndrewW » Sat Oct 15, 2011 2:48 pm

BP wrote:Mongoose intentionally decided to drop the 'd'. Making matters worse.
Inconsistent usage, sometimes the d is given, sometimes not...
BP
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Re: Measurement

Postby BP » Sat Oct 15, 2011 4:51 pm

Quite - have seen d-ton, dton and ton :roll:

Core even included a side box with the definition of 'displacement ton' and used d-ton (or dton), but then also used tons in the example, IIRC.

In a book or on these forums, I recall seeing an official source state that it was their intent to drop the 'd'. The ship tables thus use 'Tons' pretty consistently - though I'm sure an exception can be found...
evo
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Re: Measurement

Postby evo » Sat Oct 15, 2011 5:06 pm

Ok, I understand : "Dtons" or "Tons" are the same unit of volume.

Then, when I read "tonne" or "tonneau" in the French corebook, it's in fact the same unit : a d(isplacement)ton. In a same way, I assume that the "Tons" used in the Trade section (P. 165-166) are in fact "Dtons". And that's why I was confused ; it's mistranslated by "Tonne" (mass unit).

In short :
1 dton = 13.5 cubic meters (dimensions : 1.5 x 1.5 x 3 meters)

Thank you !
:D
far-trader
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Re: Measurement

Postby far-trader » Sat Oct 15, 2011 5:17 pm

And to further complicate the issue I could post examples from past versions of the rules where the Dton is in fact equal to a mass ton (in use) and stated as being 1,000kg :)

So 1Dton of feathers ready for shipping would weigh 1,000kg including the crate and packing. It would be compressed (being some 400million cu cm uncompressed on a quick google, about 400 cu m, some 30Dtons) to fit into 1Dton.

On the other hand 1Dton of bricks ready for shipping while also weighing 1,000kg and including packing and a crate would be uncompressed (being some 0.5 cu m) taking up only a fraction of the Dton for the actual bricks, with the bulk of the space going to a lot of packing stryofoam (I suppose).

It is all a bit silly isn't it? Not the least of which is who is shipping feathers and bricks between the stars, and for what nefarious purposes!!

;)

In my opinion, one of the simplest ways to deal with it is to ignore it. Or just treat it all as a vague general size. Like your suggested "space" measure, and as someone else here recently named it (iirc, but I can't recall who) a "Design ton" for Dton.

You can also apply some extrapolation. For example if your Dton (13.5 cu m) of freight is valued at Cr10,000, and the characters have a look inside and it is filled with handguns (at say Cr100, boxed in 1liter and 1kg each) then those 100 handguns will have a bit of packing material to make up the remaining volume, rather than there being 135 handguns (13.5 cu m being iirc 13500 liters) and giving the characters 35 free handguns.

NOTE: above subject to pre-caffeinated math and trusting the web for density figures :)
Dan "far-trader" Burns

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Re: Measurement

Postby BP » Sat Oct 15, 2011 6:10 pm

far-trader wrote:...Like your suggested "space" measure, and as someone else here recently named it (iirc, but I can't recall who) a "Design ton" for Dton. ...
Guilty ;)

Chose 'design ton' for my players (back in the '80s) in order to avoid some ambiguity over displacement ton - which has a varying nautical definition and a scientific and literal one. It emphasizes that though originally based on the volume of one ton metric mass of liquid hydrogen (under explicit situations), design ton or dton is precisely defined as 13.5 cubic meters for design and spaceship designation purposes IMTU (and how it is mostly used officially).

Thus, my version of dton ('design ton') is defined on precise metric units (13.5 cubic meters) vs the original physical nature (in the backstory). This is akin to the international inch, which is based on a precise metric equivalent, versus older standards, which based it on a portion of a physical yard. Both are called an inch, though precise usage would append international, survey, etc. As in the realworld, this could 'explain' discrepancies between Traveller designs that used a more literal 14 cubic meter definition. ;)
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Re: Measurement

Postby rust » Sat Oct 15, 2011 6:35 pm

Any attempt to connect Traveller's displacement ton with real
world units is bound to become very ... interesting. To give an
example of what one would have to deal with:
The displacement ton is the standard volume of water representing
one ton displacement. It equates to 35 cubic feet (0.9911 m3) of
sea water at average density, being slightly less than the 224 im-
perial gallons of the water ton. It is usually abbreviated as DT. The
water ton was formerly used in Great Britain and is equal to 224
imperial gallons (35.96 cu ft; 1.018 m3), the volume occupied by a
mass of 1 long ton (2,240 lb; 1,016 kg) under the conditions that
define 1 imperial gallon (1.201 US gal; 4.546 L).
So, yes, call it a "design ton" or use some other abstract term and
description, and do not try to establish a relation with real world
units - that way insanity is waiting ... :lol:
BP
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Re: Measurement

Postby BP » Sat Oct 15, 2011 6:56 pm

Yep, another reason I defined 'design ton' based on RW units - exactly 13.5 cubic meters.

(Funny, till more recent years, I always used 1 meter = 39.37, as I was taught in the 70s & 80s. Thus I ended up with 2.54000508 cm to an inch when I wanted to be really anal about it (mostly when teacher's insisted) - assuming 2.54 was just an approximation. I even complained about the fact that an inch should be redefined as exactly 2.54 cm. However, 1 'international' in = exactly 2.54 cm was officially adopted in the U.S. in 1959 and I am pretty sure most commercial enterprises and tools probably use that definition today.)
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Re: Measurement

Postby G. K. Zhukov » Sat Oct 15, 2011 9:09 pm

evo wrote: In short :
1 dton = 13.5 cubic meters (dimensions : 1.5 x 1.5 x 3 meters)

:D
Well... No. :wink:

In fact, if you perform the math you will see that 1.5 x 1.5 x 3 = 6.75 meters, which is HALF of a displacement ton in the Traveller canon. So when you are designing a starship you will need TWO of those 3D slabs per dton of space.

That is, 1 dton = 13 cubic meters (dimensions 2 x (1,5 x 1.5 x 3 meters)). Or two deck plan squares.
evo
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Re: Measurement

Postby evo » Sat Oct 15, 2011 10:44 pm

that way insanity is waiting...
Image
BP
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Re: Measurement

Postby BP » Sun Oct 16, 2011 1:47 am

Ah - good call G. K. Zhukov... except 2 x 6.75 is 13.5 ;)
  • 1 dton = 13.5 cubic meters = 2 deckplan squares, 1.5 meters on a side, 3.0 meters tall (1.5 m x 3.0 m x 3.0 m)
Standard assumption is 3 meters 'high' decks (with some used for structure and utilities, etc.)

P.S. - that guy needs to brush his teeth evo.
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Re: Measurement

Postby Reynard » Sun Oct 16, 2011 2:33 am

Page 105 in the Core rulebook states, to paraphrase, that size is measured in displacement tons or d-tons... one d-ton roughly equals 14 cubic metres. After that the book doesn't bother detailing what it is. In other words, a d-ton is a unit of volume describing the whole of a ship and the sum of the parts one can place inside. If the units don't exceed the total allowed, it works. That simple. After looking over my 1977 edition Starship rulebook, this is an improvement. Mongoose seem to copy over a lot and could have cleaned and clarified better.

They don't even bother explaining how the deck plans are created. Neither classic nor Mongoose edition mention 1.5 x 3 x 3 except the deck graphic show a grid of 1.5 on a side. Otherwise pretty hard to picture a ship's interior. I looked at classic High Gaurd and that's where '14 cubic meters' is first mentioned. Even Snapshot doesn't go into any detail. The actual descriptor of a d-ton must have been in a Journal article. It sounds a lot like 'we know what we ment and thought you did too'.

To enlighten where they got their measures I checked out http://mysite.du.edu/~jcalvert/tech/fluids/cargo.htm

To quote relative info: "The tonnage of a ship is not a weight, but a volume. One ton is 100 cubic feet. The total internal volume of a ship is its gross tonnage, and if we subtract all the volume not used for cargo, we get the net tonnage. " and "The total weight of the ship and everything in it is the displacement, measured in long tons of 2240 lb. A long ton is only a little larger than a metric ton of 1000 kg, but is considerably larger than the U.S. short ton of 2000 lb....The deadweight is the difference in displacement when the ship is completely unloaded, of cargo, fuel, crew and so forth. The cargo deadweight is the weight of the cargo alone.". What's not mentioned is 'displacement' means the surrounding volume of water outside the ship. That's replaced in Traveller with the measure of the common liquid fuel internally massing, coincidentally and conveniently, at 1000kg.

So yeah, a starship is measured as would a sea going vessel with smaller measures to fit easier in the rules.
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Re: Measurement

Postby Wil Mireu » Sun Oct 16, 2011 6:05 am

I guess one thing I never understood about all of this is why displacement tons exist at all. Ocean ships displace water so there's some logic for having them in that context. Spaceships don't displace anything at all though; they aren't floating in an ocean of liquid hydrogen after all.

I think it would have made much more sense to use cubic metres as the unit of volume.
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Re: Measurement

Postby G. K. Zhukov » Sun Oct 16, 2011 7:32 am

BP wrote:Ah - good call G. K. Zhukov... except 2 x 6.75 is 13.5 ;)
Which is exactly what I meant to say - sorry for not being able to properly convey the idea. :?
Wil Mireu wrote:I think it would have made much more sense to use cubic metres as the unit of volume.
Too late for that. :cry: Traveller's assumptions are supported since 1977 by tons of canon material all of it using dtons for starship volumes.

Some incarnations of Traveller under other rules (i.e. GURPS IIRC) used the term "spaces" instead of "dtons". But the volume represented by one of those "spaces" was roughly the same as one "dton".

Incidentally, the size of a displacement ton also depends on the size of the grid used to represent a starship deckplan. For instance, under the Classic Traveller, MegaTraveller, T20 and Mongoose Traveller, each grid square is 1.5 x 1.5 meters, with a height of 3 meters, so you would have TWO grid squares of deck plan per displacement ton (13.5 cubic meters) of internal volume (check my calculations above). But Traveller: The New Era worked under a different set of rules (GDW's House Rules) which used a 2 x 2 meter square grid, so deck plans were built on the assumption that each 2 x 2 meter grid was ONE displacement ton... and this was performed by changing the height to 3.5 meters and stating that one displacement ton was 14 cubic meters (2 x 2 x 3.5 = 14 m2).
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Re: Measurement

Postby Reynard » Sun Oct 16, 2011 12:34 pm

I am assuming they did it for flavor. Using historic measurements for a modern application isn't unusual. Scifi has often associated naval terminology to space and starships. Why reinvent the wheel? Using the simplest terms in the driest way is sterilized and boring.

As I just woke up I didnn't have time to read all my Journal if the TAS back ussues but I did have my MegaTraveller Referee's Manual handy. Under Craft Design they detail ton displacement. Again, to paraphrase: "...to give its volume in terms of the amaont of 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, actual mass). A starship that displaces 100 tons may actually weigh over 1000 metric tons... one displacement ton equals 13.5 Kiloliters of volume."

At the end of the construction rules is info about crreating deck plans: "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 3.0 meters) equals6.75 cubic meters or 6.75 kiloliters. Two floor squares equals 13.5 kiloliters or one displacement ton."

This may have been the first official and consolidated discription for Traveller measurement. MegaTraveller made much more use of kiloliters and kilograms in its complex construction rules. Mongoose Traveller, as did classic Traveller kept it simple which is fine. The only thing, as this thread suggests, missing was a clearer definition for measurements used especially for people who want to create their own deckplans. Now you can.

One more bit of related info. The average 21st century male is about 1.8 meters tall while deck heights are 3 meters. From what I remember the average ceiling is about eight feet or 2.45 meters so a about half a meter is where the wiring and ductwork is hidden. The old books also mention a small portion of each units volume is used for halls and common areas. Now go build a ship.
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Re: Measurement

Postby rust » Sun Oct 16, 2011 2:46 pm

Wil Mireu wrote:I guess one thing I never understood about all of this is why displacement tons exist at all.
I think the authors just wanted to use a familiar shipbuilding
term (although they modified the meaning), in the same way
they decided to use familiar astronomical terms - in the "real"
Third Imperium it would probably seem rather strange to use
the Astronomical Unit and Lightyear based on Terra instead of,
for example, the equivalent units of Capital or Vland.
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Re: Measurement

Postby BP » Sun Oct 16, 2011 5:56 pm

Reynard wrote:... I looked at classic High Gaurd and that's where '14 cubic meters' is first mentioned. Even Snapshot doesn't go into any detail. The actual descriptor of a d-ton must have been in a Journal article. It sounds a lot like 'we know what we ment and thought you did too'. ...
Yeah - I went a long time with only the boxed 3 rulebooks. Book 2, at least the CT-reprints edition, states 'As a rough guide, one ton equals 14 cubic meters (the the volume of one ton of liquid hydrogen).' (Bk 2 pg13 - after tons has been mentioned several times...).

All my original designs were in feet and I calculated 14 cubic meters the hard way. Don't recall ever reading the 2 deck squares to a ton thing before maybe ~2000! However, its what I ended up with when I picked up the Forms Supplement, which stated a grid scale of 1.5 m. I never bought any journals or deckplans so everything was my own.
...But Traveller: The New Era worked under a different set of rules (GDW's House Rules) which used a 2 x 2 meter square grid, so deck plans were built on the assumption that each 2 x 2 meter grid was ONE displacement ton... and this was performed by changing the height to 3.5 meters and stating that one displacement ton was 14 cubic meters (2 x 2 x 3.5 = 14 m2).
That is funny to me... when I 'derived' a 13.5 cubic meter and decided to define dton as a 'design ton' for my players, it was because I couldn't come up with an elegant way making a 14 cubic meter volume that would fit on a square grid nicely and not be too high and still use the 1.5 meter or some variant (.5, .75).

Being raised with family who worked at a shipyard, even 3 meter decks seemed high - my earlier designs used 7 feet. Being around ships, 'displacement ton' seemed natural to use and I never questioned its applicability as I was used to ships being referred to by their 'tonnage' (which varies all over in meaning, and actually is often only good as a vague relative comparison). Gravitics meant mass could be ignored, so volume 'tons' seemed quite applicable.

Re: rust's last comment. Yeah, don't generally play 3I, but I use different terminology in game for distances, time and mass, though I decided to keep them equal to real world units for simplicity - makes for nice roleplay flavor.

Always spend some time making up terms, phrases, and curses for a setting so players have something to work with.
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Re: Measurement

Postby far-trader » Sun Oct 16, 2011 6:14 pm

BP wrote:
Reynard wrote:... I looked at classic High Gaurd and that's where '14 cubic meters' is first mentioned. Even Snapshot doesn't go into any detail. The actual descriptor of a d-ton must have been in a Journal article. It sounds a lot like 'we know what we ment and thought you did too'. ...
Yeah - I went a long time with only the boxed 3 rulebooks. Book 2, at least the CT-reprints edition, states 'As a rough guide, one ton equals 14 cubic meters (the the volume of one ton of liquid hydrogen).' (Bk 2 pg13 - after tons has been mentioned several times...).
...fwiw, CT Book 2 (possibly not in the first printing c1977 but since revised printing c1981) page 21: Deckplans:
The preferred scale for the interior should be 1.5 meters per square, with the space between decks put at about 3.0 meters. One ton of ship displacement equals approximately 14 cubic meters. Therefore one ton equals about two squares of deck space.
Dan "far-trader" Burns

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