Cargo Capacity and volume and mass

[Badenov]

So, something that has been bothering me for a while are dTons. I think they're handled differently in different versions, which makes the issue more confusing, but I will talk to Mongoose 2008, the version I own. Simple version is this, as I understand it (heavy emphasis here, I have been known to misunderstand things): A ship is rated in dtons, the tons of liquid hydrogen that the volume of the ship displaces. Or could contain. There seems to be some confusion, at least on my part, and the result is a paper-thin membrane of hull, with no mass or thickness. Now since armor takes volume, it can be your hull volume, that makes sense at least. If you have a 100dTon hull, with 10dTons of armor, the interior has 90dTons. I am OK so far.

The game seems to lean into physics when it can, so it makes sense to me that the amount of acceleration a ship can develop is a balance between it's mass, regardless of volume (outside an atmosphere, anyhow), and the amount of thrust it's rated for, given helpfully in G's by the engine designers. I had assumed Jumping would be the same, though the existence of drop tanks makes that not the case. So maybe jumping is based on volume rather than mass? It's space magic, either way, but I get confused if Thrust is based on mass and Jump is based on volume.

Now the cargo capacity of the ship is also rated in dTons, which makes sense since it's straight arithmetic and minimizes calculations, but leads to some cases which seem odd to me. If your cargo is liquid hydrogen, of course, your hold is full to the top. But 1 dton of water, by mass, is about a cubic meter, or 1/14 of the space of a dTon. So if you're carrying water as cargo, you can only have your hold 1/14 full, by volume, or you'd be over mass and your thrust 1 engine would now be a thrust 0 engine. You could still Jump, if Jump is based on volume, but you'd never make it to the 100-diameter limit without a tow.

Now as to what you have in your cargo hold, steel is 7.8 times as dense as water, which being 14 times denser than liquid hydrogen, makes steel 109 times as dense as liquid hydrogen. So if cargo is rated by the mass side of dTonnage, a 1 dTon load of steel is 1000kg of steel, which is a very thin, like 1cm thin, layer on the deck. If cargo is rated by the volume side of dTonnage, it fills the entire space, but weighs 109,000kg/109 metric tons.

I am confused because neither case makes much sense to me. The game I'm in uses the first way, and just ignores the silly of tiny patches of cargo on mostly clear deck. Is there some guidance in other Traveller versions? I could see a spur of TL9 grav thruster development being ineria-less cargo bays limited by volume without impacting mass. If done to a fuel tank also, that would make thrust the same whether the tank is empty or full, which seems to be how vessels are assumed to be, though generally most ships thrust are rated for a full fuel tank, and I'm not clear on why when you'd have more thrust at 2/3 your fueled mass and if you're nullifying the mass of the tank.... it all gets twisty and confused when you're doing science babble. Maybe the mass nullifiers have a bleedover effect, such that everything has the effective mass of liquid hydrogen, but that would make firearms and other projectile weapons useless as the bullets/needles/whatever would have no mass behind them.

I have a harder time leaning into M-drive being not only reactionless but totally unaffected by mass and going by volume also. But I'm wildly at a loss when trying to reconcile these issues. Any advice is welcome.

 

[infojunky]

Heh....

Ok, in general a ship's tonnage is rated in the tonnes of liquid hydrogen it displaces, if you drop the ship in that puddle. So straight up its a volume not a mass measure.

Traveller's rule of thumb is a ship weighs 10 metric tons per displacement ton. (laden mass). Also note accretion is limited by compensation...

Now cargo tonnage is a different critter, in that a ton of cargo is both a volume and mass measurement. In that each Dton of cargo has a weight limit on how much it can weigh. (Real world example of a cargo container, they generally have a mass limit of 500 kgs per cubic meter (Think flour if to need a physical example)).

 

[Condottiere]

The simplified version is that engineering performance is based on the amount of enclosed volume they're moving, plus local gravity field.

 

[TamsinP]

Traveller dTons are the equivalent of Gross Register Tonnage, the old system of stating the size of a ship* - it's a direct measure of the permanently enclosed volume of the ship (1 GRT = 100 cubic feet, approximately 2.8 m^3). For wet navy ships, it is not the same as their displacement, which is determined by their mass (and, with the exception of submarines, will be much lower than their GRT).

In relation to the Traveller dTon being the volume of liquid hydrogen which would be displaced, that would be if submerged. It's probably simpler to think in terms of how many tonnes of liquid hydrogen the hull could contain.

* the current method (Gross Tonnage; GT) isn't a direct measure of volume, and tends to be much lower than the GRT value, but the difference reduces with increasing size.

 

[Badenov]

Traveller's rule of thumb is a ship weighs 10 metric tons per displacement ton. (laden mass). Also note accretion is limited by compensation...

Now cargo tonnage is a different critter, in that a ton of cargo is both a volume and mass measurement. In that each Dton of cargo has a weight limit on how much it can weigh. (Real world example of a cargo container, they generally have a mass limit of 500 kgs per cubic meter (Think flour if to need a physical example)).

I have never seen either of these statements before. Where do they come from?

The simplified version is that engineering performance is based on the amount of enclosed volume they're moving, plus local gravity field.

This works, I guess, even if it hurts my brain and seems contrary for a system that is otherwise very hard sciencey.

In relation to the Traveller dTon being the volume of liquid hydrogen which would be displaced, that would be if submerged. It's probably simpler to think in terms of how many tonnes of liquid hydrogen the hull could contain.

That agrees with my initial statement that the inside and outside volumes mandated a hull of zero thickness, so that the volume displaced when submerged would equal the volume contained. They both mean basically the same thing in this context.

 

[whartung]

FF&S using a 15 tonnes per dTon number as a base number, with the caveat that most ships do not mass more than 15 tonnes.

For calculating the loaded weight of the ship, it suggests 1 tonne (1000kg) per 1m^3 of cargo volume as a guideline, this is the mass of water. Liquid Hydrogen is 70kg (.07 tonne) per m^3.

 

[kilemall]

Well, except the hulls aren’t zero thick, if anything they are far thicker then RL.

 

[mike wightman]

FF&S using a 15 tonnes per dTon number as a base number, with the caveat that most ships do not mass more than 15 tonnes.

For calculating the loaded weight of the ship, it suggests 1 tonne (1000kg) per 1m^3 of cargo volume as a guideline, this is the mass of water. Liquid Hydrogen is 70kg (.07 tonne) per m^3.

FF&S assumes 10 tonnes per displacement ton. If your final design is over 15 tonnes per displacement ton then recalculate maneuver g performance.
In CT a cargo ton was 1000kg by mass but the displacement was 14 cubic metres, so a ship that is mostly cargo and fuel is going to be close to 1 tonnes per displacement ton for the fuel/cargo component and 10 tonnes per displacement ton for the not fuel+cargo.

Warships that carry weapons, screens and armour rather than cargo will be around 15 tonnes per displacement ton of not fuel+cargo

 

[AnotherDilbert]

So, something that has been bothering me for a while are dTons. I think they're handled differently in different versions, which makes the issue more confusing, but I will talk to Mongoose 2008, the version I own.

It's a simplification, it's much easier to dimension drives for a fixed size of ship, rather than a fluctuating mass.

Basically from 1979 forward, 1 Dt is about 14 m³ (sometimes rounded to 13.5 m³, two deck squares).
Mass is about 1 tonne (metric) per m³, machinery and armour is heavier, fuel is much lighter, it generally evens out.
Mass is often swept under the rug for simplicity.


The confounding factor is that in LBB2'77 there were no volume, just mass.

 

[AnotherDilbert]

1980:

CT Striker, B3, p10:
The vehicle is assumed to be carrying a full load of ammunition and a full load of cargo, at 1 ton per m³.

1987:

MT RM, p85:
To compute the average weight of a full cargo hold, multiply the volume of the cargo hold in kiloliters by 1000 kg (one metric ton).

1 kilolitre = 1 m³.

1993:

TNE FF&S, p14:
Calculate two masses, loaded and empty. Loaded includes a full load of fuel, full load of cargo (assume 1 tonne per m³), ...

It isn't really complicated or inconsistent. The rules just tries to simplify the design process.
TNE FF&S, where they didn't simplify this, was generally considered too complicated, so later editions generally simplified this again.


Where we do keep track of mass, starships end up being about 10 tonnes per Dt, or somewhat less than 1 tonne per m³:
MT:

TNE:

 

[Condottiere]

In theory, steel is cheap and air is free (and somewhat weightless).

In the end, hull armour and engineering would probably be the most mass intensive components, assuming spinal mounts are a series of tubes.

 

[TamsinP]

10 tonnes per dTon would be incredibly high. WW2 battleships work out around 4.5 tonnes/dTon.

 

[AnotherDilbert]

10 tonnes per dTon would be incredibly high. WW2 battleships work out around 4.5 tonnes/dTon.

It's not the same displacement tons... Wet ship doesn't float in hydrogen.

 

[TamsinP]

It's not the same displacement tons... Wet ship doesn't float in hydrogen.

I have calculated the approximate volumes of all the WW2 US warship classes, converted from m^3 to Traveller dTons and then calculated the density from the displacement.

I've done something similar for the current US Navy fleet. The volumes actually much more accurate because I was able to convert their GRT/GT values (they're all registered with the US Coast Guard) to m^3 and then to dTons. Most surface vessels are in the 4-5 tonnes/dTon range.

 

[AnotherDilbert]

I have calculated the approximate volumes of all the WW2 US warship classes, converted from m^3 to Traveller dTons and then calculated the density from the displacement.

OK, I couldn't find any accurate volume or GRT figures for warships with a fast search.

A Libery ship at ~7000 GRT ≈ 20 000 m³ and 14 500 tonnes would have a density of ~0.7 tonnes/m³ ≈ 10 tonnes per Traveller Dt.

A supertanker (Bellamya) with a GT of ~275 000 ≈ 865 000 m³ and a mass of ~630 000 tonnes would have a density of ~0.7 tonnes/m³ ≈ 10 tonnes per Traveller Dt.

 

[TamsinP]

OK, I couldn't find any accurate volume or GRT figures for warships with a fast search.

A Libery ship at ~7000 GRT ≈ 20 000 m³ and 14 500 tonnes would have a density of ~0.7 tonnes/m³ ≈ 10 tonnes per Traveller Dt.

Liberty Ship - 7176 GRT (=1450.6 dTons), empty weight 3326.8 tonnes (= 2.3 tonnes/dTon), full load 14245 tonnes (= 9.8 tonnes/dTon).

Batillus class supertanker - 275,268 GT (=61,389 dTons), empty weight 77,300 tonnes (= 1.25 tonnes/dTon), full load 630,962 tonnes (10.2 tonnes/dTon).

Given those figures, I'm happy to admit that 10 tonnes/dTon is a reasonable estimate for a fully laden cargo ship.

It's interesting that even heavily armoured WW2 warships have a lower density than cargo ships, but I assume that helps with sea-keeping at higher speeds.

 

[Badenov]

So, the place my confusion originated was in working out how cargo ships could carry more cargo. Since their holds, rated as I thought in tons of liquid hydrogen, were carrying things that were much denser, and using the 'one ton = 1000kg or 14m^3 which ever comes first' method of determining how much you can fit in a ton means the holds are mostly open space with a few things tied down.

With the assumption that drive ton ratings were mass and ship size ton ratings were volume, I designed a ship in High Guard at 20,000 tons, with Power Plant, M-Drive, and J-drive (and fuel) rated for 60900 mass-tons of hauling. The resulting ship had only 378 dtons of cargo volume, the 20,000 d-ton space being mostly fuel and power plant, but could move along with 41,278 tons of steel at 7800kg/m^3 as cargo, which is about 109 tons per d-ton, much higher than the quoted rating, but for only a small part of the ship. Over all, the ship is only about 6 tons per d-ton, as fuel, being liquid hydrogen, is 1 ton per dton and is most of the ship . This circles back to drives being rated by volume rather than mass, which as I said makes my inner engineer cringe, and means I wouldn't have needed to overrate my drives because mass is hand-waved in the cargo hold.

My normal high guard cargo ships of the same 20,000d-tons could carry a third to a quarter of that 41,278 mass-tons, at with between 11,000 and 13,000-tons 'cargo' depending on what else they carried (passenger cabins, defensive weapons, etc). Of course cargo is much vaguer than 'Steel', we have Basic Raw Materials, Precious Metals, and so on. Of course with nothing listed for density. All we know are that the tons they are listed in are the same as the tons that our cargo hold is rated in. Lucky for us!

But if my cargo hold is rated for steel, it can presumably quite a lot of things made mainly from steel, or things denser than steel, (I'm looking at you, radioactives). Things less dense than steel, for instance Basic and Uncommon Ore that might come closer to the density of a terrestrial planet (~5515kg/m^3) will be capped by my very limited volume and I could only carry 29,185 mass-tons of that, but Uranium, at 18,700kg/m^3, I could pile up until the drives lost performance, 41,278 mass-tons is only 2207m^3. I could double the load, get half-performance from my rating 2 engines, and still not fill my hold to the top.

So the real question is asking 'Has any consideration been paid to cargo mass, or do we blithely assume mass nullifiers in our cargo bays?' Or 'Are our engines rated in how much mass they can push or how much volume they can push?' The answer seems to change by version, which hurts my overtaxed brain even more.

It would have taken one sentence to make this clear, and that sentence was skipped (in this version of the rules). Judging from the discussion, it was not skipped in other versions, but that seems to make the physics worse, or at least very twisty. It seems like mass is -never- part of any meaningful spaceship calculation?

 

[TamsinP]

My assumption is that, until MegaTraveller, the authors decided to ignore mass and just use the volume to keep things simple. I'm pretty sure that various people have come up with their own explanations.

 

[Badenov]

My assumption is that, until MegaTraveller, the authors decided to ignore mass and just use the volume to keep things simple. I'm pretty sure that various people have come up with their own explanations.

I am afraid you are correct.

So, I reworked my old 20,000-ton hauler (with the 13,650 d-ton hold) with the 10-tons per d-ton assumption for all the non-fuel bits, and ran the numbers to see what I could get in the cargo bay while keeping the overall ship to 200,000 mass-tons. My 13,650-ton cargo bay could only be bumped up to about 12.8 tons per d-ton (my brain made me work it out to 12.8244 tons/dton) to keep the ship at 200,000 mass-tons. That does mean my 13,650-dton cargohold can carry 175,000 mass-tons of goods, though that only works out to 916kg/m^3, so a load of steel is still limited by the mass (175,000 tons of steel, 175,000,000kg, at 7800kg/m^3 works out to 22,442 m^3 or 1603 d-tons, so hold is still mostly empty) and a full hold of water won't weigh anything close to 175,000 tons.

 

[AnotherDilbert]

With the assumption that drive ton ratings were mass and ship size ton ratings were volume, I designed a ship in High Guard at 20,000 tons, with Power Plant, M-Drive, and J-drive (and fuel) rated for 60900 mass-tons of hauling. The resulting ship had only 378 dtons of cargo volume, the 20,000 d-ton space being mostly fuel and power plant, but could move along with 41,278 tons of steel at 7800kg/m^3 as cargo, which is about 109 tons per d-ton, much higher than the quoted rating, but for only a small part of the ship. Over all, the ship is only about 6 tons per d-ton, as fuel, being liquid hydrogen, is 1 ton per dton and is most of the ship .

LBB5 never deals in or mentions mass.

LBB5'80, p22:
... one ton equals 14 cubic meters ...

A "20 000 displacement ton" ship has a volume of 280 000 m³, and an unknown mass.
The roughly 10 tonnes per Dt estimate comes from later editions.

A 20 000 Dt ship needs a 2% = 400 Dt jump drive to be able to do J-1. Mass is never involved, it is swept under the rug.

This circles back to drives being rated by volume rather than mass, which as I said makes my inner engineer cringe, and means I wouldn't have needed to overrate my drives because mass is hand-waved in the cargo hold.

It's a simplification. Mass is hand-waived everywhere.
Whats the mass of the power plant? Who cares, we know the volume and cost.



Traveller uses plain old physics, except for a few explicit exceptions.
Ships obey Newton's Laws. M-drives produce thrust in the drive itself, that propels the ship by F=ma.
Generally that is simplified, in FF&S it's explicit.