Tonnage Rule of Thumb

[tbeard1999]

My Commonwealth campaign is a small ship universe that features a lot of ground actions. The reason for this is drama, of course. The campaign circumstances that create this condition are (a) the Commonwealth is the dominant military/economic power, so it fights lots of "little wars" and few long term "major wars"; (b) the Commonwealth is a democratic, free-enterprise state and spends as little as possible on military forces, which means the Commonwealth Navy is stretched very thin. As a result, there are relatively few large fleet battles; most fighting is ground fighting on various planets. As you can see from my Commonwealth Starships thread, delivering stuff to a planet is a major concern for the Commonwealth Navy.

So, I've had to wrestle with the fact that tonnage in Traveller is a unit of volume, while trying to figure out how much tonnage to allocate to military vehicles and the like. Here's my rule of thumb:

Based on an analysis of standard 40' cargo containers, each container consumes 4.82 dtons and can hold a maximum of 67.5 metric tons. Rounding down for convenience and to account for the fact that every container won't be maxed out on weight, I assume that the average mass of a Traveller dton will be about 4 metric tons. So, for vehicles and the like, I use the higher of the dton volume consumed or the metric tonnage/4.

Comments?

 

[far-trader]

Sounds close to what I'd worked up once. I think I came up with an average of 3mtons per 1dton. But 4mtons per 1dton works too. Though of course taking a cue from the speculative trade rules cargo is shipped with sufficient packing and dead space that it works out to 1mton per 1dton. And my crates are not full volume either to allow some access around them when the cargo hold is filled to capacity (which might be where I lost the 1mton compared to your calculation).

For vehicles I figure that the listed figure is the dtons required for accessible stowage or shipping, which is double the actual dtons.

For example the Air/Raft:

Listed as 4tons, that's dtons required for accessible stowage (about 8 deckplan squares, typically 2 x 4). Actual volume is about 2dtons (open topped, about 1.1m overall height, 2.6m wide, and 5.6m long). The listed cargo capacity of 4 tons is 4 metric tons (so about 1dton (2 squares) and sticking up out of the "top").

 

[Icosahedron]

Ty, sorry to be obtuse (again), but where do you get the 4:1 ratio from the figures you quoted above?

If you divide out the 67.5 by the 4.82 you get 14. This matches the 14m^3 per dTon of canon.

I work on the basis that starships have a density roughly equivalent to terrestrial submarines, ie 1 tonne per m^3, and hence each dT weighs 14 tonnes.

Dense items such as military vehicles will need more space allocation than their size would suggest, as FT noted.

 

[far-trader]

Ty, sorry to be obtuse (again), but where do you get the 4:1 ratio from the figures you quoted above?

If you divide out the 67.5 by the 4.82 you get 14. This matches the 14m^3 per dTon of canon.

I think I can answer this (or not, it's late and I'm tired so... )...

That's 14 metric tons per 1 displacement ton. Or 14,000 kilograms in 14m3. Which works out kind of nicely, 1000kg/m3, kind of rings a bell...

I work on the basis that starships have a density roughly equivalent to terrestrial submarines, ie 1 tonne per m^3, and hence each dT weighs 14 tonnes.

Oh! Hey! There it is

What was the question?

 

[sabredog]

Wouldn't it just be easier to figure out a standard cargo volume for a vehicle as a design limit, say what would fit in the standard combat lander/dropship/cutter ATV module?

I used Striker to redesign the LBB vehicles and from those I had volume and weight. Volume seems more important than mass, since there are no modifiers in the rules involving volume vs. mass in the cargo rules. I design my combat vehicles around what fits in the lander based on the plans for the lander. Otherwise the can of worms of "So why can't my ship go faster when the cargo bay is empty?" opens up. So, the dimensions of the vehicle are what I plan around, not the mass, using Striker.

I add .25 to the space used for combat loaded vehicles (roll-on, roll-off), and .75 to those that are containerized or factory loaded (some assembly required). So for example, my standard assault lander for AFV's/ IFV's has a cargo bay with 8m wide x 3.5m high doors. The standard MBT is 6m wide and 3m high so it will fit through the doors. 4 of them will fit on the lander (or 6 IFV's) with more or less waste space - but then there always needs to be a little of that for practical reasons. The MBT described has a volume that works up to 16 ship tons, but since it's not exactly a compact thing, the cargo area needs to accommodate the actual dimensions, not the generic tonnage.

Just wondering, it was an issue I wrestled with a long time ago so a fresh perspective might be a good thing.

 

[tbeard1999]

Ty, sorry to be obtuse (again), but where do you get the 4:1 ratio from the figures you quoted above?

If you divide out the 67.5 by the 4.82 you get 14. This matches the 14m^3 per dTon of canon.

I work on the basis that starships have a density roughly equivalent to terrestrial submarines, ie 1 tonne per m^3, and hence each dT weighs 14 tonnes.

Dense items such as military vehicles will need more space allocation than their size would suggest, as FT noted.

My calculations are based on the standard 40' shipping container. It is 67.5 cubic meters, which equates to 4.82 dtons. It can hold a maximum of 26.5 metric tons of cargo, or about 5.5 metric tons per dton. I reduced this to 4 metric tons per dton to account for the fact that not every container would contain its maximum mass.

This yields the rule of thumb -- use the greater of dtons or metric tons divided by 4.

Heavily armored vehicles may require more dtons than their volume suggests as you note. The M1A2 Abrams, for instance, weighs about 60 metric tons and has a volume of about 12 dtons. Using my rule of thumb, it will consume the higher of 60/4 or 12 in dtons. So it would be 15 dons.

 

[tbeard1999]

Wouldn't it just be easier to figure out a standard cargo volume for a vehicle as a design limit, say what would fit in the standard combat lander/dropship/cutter ATV module?

It would be easier, but it would not necessarily be realistic. Armies seldom have the luxury of designing vehicles that are optimized for any particular transport. Even if they do, transports change and military needs change. A tank designed for a 1941 cargo ship (like the US Sherman) would have been utterly outclassed 10 years later. A cargo ship designed to carry Western tanks in the 1970s would have envisioned ~45 metric tons per tank. 20 years later, the typical Western tank masses 60 metric tons. Etc.

And military vehicles must perform a variety of missions to boot, which can be compromised by overly restrictive mass/volume limits. The US Sherman, which was designed for transportability and reliability, was badly outclassed towards the end of WWII by the latest German and Soviet tank designs. More recently, the US Striker armored personel carrier is not airmobile when equipped with the armor package designed to defeat the most likely threat (infantry portable antitank weapons). The armor packages are flown in separately and attached in theater.

And even where a vehicle is designed to fit a particular transport, it's always possible that military needs will require it to be transported by non-standard transports. Ad hoc operations happen all the time in warfare. Imagine a local general commandeering (or chartering) the PC's Free Trader to help transport an armored battalion to a neighboring system.

Otherwise the can of worms of "So why can't my ship go faster when the cargo bay is empty?" opens up.

<shrug>

It's a fair question, but not that big a deal IMHO.

So, the dimensions of the vehicle are what I plan around, not the mass, using Striker.

Well, not everyone has Striker. And not everyone who has Striker wants to mess with it. My approach would allow extremely heavy cargo to be accurately assessed without using Striker. It also seems pretty simple to use (and it will generally only be necessary with heavily armored vehicles).

 

[leo knight]

I've been using English units, being a Luddite . I had figured 1 Dton volume= 5 tons mass, since nautical gross tons are 100 cubic feet. But I like your 4:1 ratio, and the way you figure the air raft.

edit: 1 Dton would be 5' x 10' x 10', or 500 cubic feet, just to be clear.

 

[Icosahedron]

It can hold a maximum of 26.5 metric tons of cargo, or about 5.5 metric tons per dton.

Got it now. That bit was missing from the original post. Cheers.

 

[sabredog]

It would be easier, but it would not necessarily be realistic. Armies seldom have the luxury of designing vehicles that are optimized for any particular transport. Even if they do, transports change and military needs change. A tank designed for a 1941 cargo ship (like the US Sherman) would have been utterly outclassed 10 years later. A cargo ship designed to carry Western tanks in the 1970s would have envisioned ~45 metric tons per tank. 20 years later, the typical Western tank masses 60 metric tons. Etc.

Its not a luxury, it's a requirement. Weight is a moot point if the tank won't fit in the doors, or the landing craft won't have enough room to hold the thing. While transports get bigger, roads, tunnels, and rail transport has to be accounted for, too. Thats why the dimensional requirements are there.

And military vehicles must perform a variety of missions to boot, which can be compromised by overly restrictive mass/volume limits. The US Sherman, which was designed for transportability and reliability, was badly outclassed towards the end of WWII by the latest German and Soviet tank designs. More recently, the US Striker armored personel carrier is not airmobile when equipped with the armor package designed to defeat the most likely threat (infantry portable antitank weapons). The armor packages are flown in separately and attached in theater.

And here, with the Sherman example you prove my point. However, the German designs might have been better, but it had nothing to do with being transported easilt on a ship - the Germans built heavier tanks as the war went on because their design philosophy was dictated by their side being forced in a more and more defensive role. The days of being able to move quickly were long gone and tanks needed heavier guns and armor to withstand the Allied advances and air power. Moving pillboxes were needed more than the light Mk-III's of 1940.

Now IYTU vehicles might be able to fulfill a wide range of mission requirements, but that last I checked all the examples you've cited here were only designed for one mission or another. It's unrealistic to say that any military vehicles need to be capable of a wide range of missions.

If you want to design a one-size-does-all MBT/IFV/SPA/APAA type of vehicle then it would also seem logical that it would be a lot easier to just design a lander/transport made to fit it.

The Striker slat armor wasn't even part of the original design - it was developed long after the vehicle was built - originally to be air transportable. But that's what happens all the time.

And even where a vehicle is designed to fit a particular transport, it's always possible that military needs will require it to be transported by non-standard transports. Ad hoc operations happen all the time in warfare. Imagine a local general commandeering (or chartering) the PC's Free Trader to help transport an armored battalion to a neighboring system.

True, but what if the Free Trader (not designed for this sort of thing) can't fit a tank on board, not because of weight but because of dimensions? Just because a lot of trawlers might be available to cross a channel doesn't mean they could be used if the tanks you need them to carry would hang over the gunwales. Ad hoc has realistic limitations, too.

But I suppose, this being sci-fi n' all.....maybe you could sling a couple tanks under the Free Trader hull and use the ....wait for it....

...demountable tank rules for mass vs. perfomance.

Not arguing mind you...just throwing out ideas.

 

[far-trader]

Heh Drop "tanks" I have sooo got to work that into a game sometime

 

[tbeard1999]

Its not a luxury, it's a requirement. Weight is a moot point if the tank won't fit in the doors, or the landing craft won't have enough room to hold the thing. While transports get bigger, roads, tunnels, and rail transport has to be accounted for, too. Thats why the dimensional requirements are there.

And yet we have innumerable examples of military vehicles being too large/heavy for the most common forms of transport. Sorry, but I stand by my assertion.

Now IYTU vehicles might be able to fulfill a wide range of mission requirements, but that last I checked all the examples you've cited here were only designed for one mission or another. It's unrealistic to say that any military vehicles need to be capable of a wide range of missions.

Well, my Traveller universe does features a military that is not particularly lavishly funded. Therefore -- like most militaries in the Real World today -- vehicle designs need to be capable of a wide variety of missions. The high cost of small numbers of highly specialized vehicles precludes most such vehicles.

If you want to design a one-size-does-all MBT/IFV/SPA/APAA type of vehicle then it would also seem logical that it would be a lot easier to just design a lander/transport made to fit it.

But as noted, the specialized lander/transports may not be available for a variety of reasons. In that case, you need to be able to assess how it can be transported by unspecialized transports.

The Striker slat armor wasn't even part of the original design - it was developed long after the vehicle was built - originally to be air transportable. But that's what happens all the time.

Well, it's true that the LAV family of vehicles was deployed 20+ years ago. But my research indicates that the original design specs for the medium infantry carrier (for which the Striker ultimately selected) included the heaviest protection levels. The fact that the vehicle is not easily transportable was ultimately accepted as an unavoidable tradeoff. In any case, the Striker is a perfect example of a vehicle that cannot easily be deployed in combat condition on normal transports.

 

[sabredog]

Well, my Traveller universe does features a military that is not particularly lavishly funded. Therefore -- like most militaries in the Real World today -- vehicle designs need to be capable of a wide variety of missions. The high cost of small numbers of highly specialized vehicles precludes most such vehicles.

Huh, since most armed forces use tanks as tanks, APC's as APCs, and so on I don't see what you are getting at. Maybe it's your terms...battlefield conditions or environment might be better? An MBT makes a lousy SPAA unit, but you can stack infantry on it kinda like a truck. Thats not a bad option, either, considering the ablative qualities of the troopers. So I guess you might be right there.

Though it makes me wonder why everyone keeps buying MBTs, APC's, AFV,s instead of something like a Merkava with a bunch of missile rails added to it. And a mortar in back. And maybe a Stinger launcher....

While you could have some cross-capability in weapons platforms, through swapping out turrets or such, having only one type fits all isn't what anyone does in this reality if it can be avoided. Some nations have built AFV's that are tailored to the type of fighting and environment they operate in like, say the SA Rooikat, but even as laudable as that vehicle is in its particular niche, it still is really only suited to shooting at light-medium armor. It can't carry troops, and it has indirect or AA fire capability.

Well, it's true that the LAV family of vehicles was deployed 20+ years ago. But my research indicates that the original design specs for the medium infantry carrier (for which the Striker ultimately selected) included the heaviest protection levels. The fact that the vehicle is not easily transportable was ultimately accepted as an unavoidable tradeoff. In any case, the Striker is a perfect example of a vehicle that cannot easily be deployed in combat condition on normal transports.

Actually the Stryker can be deployed easily with most transports. One of the reasons it was selected was because it fit (dimensions not just weight) in the USAF's new (at the time on the boards) C-17, and it can be transported on any RO-RO merchant ship, a wide variety of military naval vessels used for AFV transport, basically anything that can carry an LAV or larger. It can't be hauled under a helicopter, but not much can anyway other than guns and hummers nowadays. There is a limit to modern day choppers that a Free Trader would exceed.


I was only trying to point out my method for figuring out what fits in where is based on dimensions, not just volume.


Far-Trader:

Just imagine a small grav armor unit that has to insert fast and low with some tanks to take the enemy by surprise. They hire a couple of Free Traders, sling the tanks under them, then come flying down as innocent merchants until they get close to the bad guys. Then it's tanks-away.

Maybe some daring tank riding desant types in battledress with grav belts could be clinging to the armor to lend support. Scary, but just think how good a story it'd make later.

 

[tbeard1999]

Huh, since most armed forces use tanks as tanks, APC's as APCs, and so on I don't see what you are getting at.

I don't think it's terribly complicated. Even in the lavishly equipped US Army, many "specialized" vehicles use the chasis of a common vehicle type (the M113 chasis is used on a wide variety of vehicles; the Bradley chasis is being used more often). And while I can see a military having certain design standards for width and suchlike (and even standards for transports), I can also see such standards being rendered obsolete as time goes by. As noted, a transport designed to carry Shermans would have been inadequate to carry the M26/47/48/60 series (to say nothing of the M1/IPM1/M1A1/M1A2/M1A2HA series. And standards that were met by the Sherman would not have been met by later series of tanks. Therefore, I do not think it's reasonable to assume that *all* military vehicles will perfectly fit *all* transports.

Though it makes me wonder why everyone keeps buying MBTs, APC's, AFV,s instead of something like a Merkava with a bunch of missile rails added to it. And a mortar in back. And maybe a Stinger launcher....

<blink>

I don't think that I ever stated that one vehicle should be able to do everything. Although it's common to use an MBT chassis for specialized vehicles--ARVs, SPAAA (Flakpanzer Gepard is a Leopard 1 chassis; the cancelled Sgt. SP AAA system was an M48 chassis), being common. APC chassis are often used for a variety of lighter specialized systems -- the M113 is the basis for the US Army's M106 and M125 SP mortar carriers, M901 ITOW vehicles, the M58A3 SP smoke generator; the "M150" SP TOW vehicle; M548 cargo carrier; M577 mobile command post; M981 FIST-V forward observer vehicle; M1108 Universal Carrier; and literally dozens of other variants.

I see no reason that the same economic and logistical factors that argue for such an approach will somehow disappear in the far future.

So maybe the issue is terminology -- I don't necessarily consider variants to be different vehicles.

 

[PFVA63]

Hi

My calculations are based on the standard 40' shipping container. It is 67.5 cubic meters, which equates to 4.82 dtons. It can hold a maximum of 26.5 metric tons of cargo, or about 5.5 metric tons per dton. I reduced this to 4 metric tons per dton to account for the fact that not every container would contain its maximum mass.

This yields the rule of thumb -- use the greater of dtons or metric tons divided by 4.

Heavily armored vehicles may require more dtons than their volume suggests as you note. The M1A2 Abrams, for instance, weighs about 60 metric tons and has a volume of about 12 dtons. Using my rule of thumb, it will consume the higher of 60/4 or 12 in dtons. So it would be 15 dons.

Hi,

Looking over some data I found a while back on the internet on some trials the US did on the HSV-1X, suggests that for a Stryker Infantry Vehicle, the 4mt/dton that you mention is probably a pretty good guess.

Specifically, the paper gave a weight for the vehicle of 19 ST (or 17.2mt), and dimensions of;

L = 284in (7.2m)
W = 110in (2.8m)
H = 109in (2.8m)

Which gives a Density of 0.305mt/cubic meter or 4.1 to 4.3mt/dton (depending on if you use 13.5 cubic meters per dton or if you use 14 cubic meters per dton, respectively).

Alternately, if you add a couple inches around the vehicle for access, etc, the numbers will come out even closer to about 4mt/dton (if I did the calcs right).

One last thing to consider though, is that in the original Traveller books (if I am recalling correctly), when laying out deck plans they seem to assume a 0.25m thick deck, which appears to be included in the volume of the space being laid out. As such, in a stateroom, even though the height is 3m or so from the line of one deck to the next, the clear height in the space is anly possibly 2.75m or so (or did they assume 0.5m for structure and 2.5m clear space, I can't seem to recall right now).

Anyway, this kind of implies that you might want to also include an allowance for ship's structure etc. But, either way, the value you proposed based off of container type weights seems to be a pretty good guess for vehicles like the Stryker, at least when in a transport to configuration (they might be heavier fully loaded).

Hope this helps.

Regards

PF

 

[sabredog]

I don't think it's terribly complicated. Even in the lavishly equipped US Army, many "specialized" vehicles use the chasis of a common vehicle type (the M113 chasis is used on a wide variety of vehicles; the Bradley chasis is being used more often). And while I can see a military having certain design standards for width and suchlike (and even standards for transports), I can also see such standards being rendered obsolete as time goes by. As noted, a transport designed to carry Shermans would have been inadequate to carry the M26/47/48/60 series (to say nothing of the M1/IPM1/M1A1/M1A2/M1A2HA series. And standards that were met by the Sherman would not have been met by later series of tanks. Therefore, I do not think it's reasonable to assume that *all* military vehicles will perfectly fit *all* transports.



<blink>

I don't think that I ever stated that one vehicle should be able to do everything. Although it's common to use an MBT chassis for specialized vehicles--ARVs, SPAAA (Flakpanzer Gepard is a Leopard 1 chassis; the cancelled Sgt. SP AAA system was an M48 chassis), being common. APC chassis are often used for a variety of lighter specialized systems -- the M113 is the basis for the US Army's M106 and M125 SP mortar carriers, M901 ITOW vehicles, the M58A3 SP smoke generator; the "M150" SP TOW vehicle; M548 cargo carrier; M577 mobile command post; M981 FIST-V forward observer vehicle; M1108 Universal Carrier; and literally dozens of other variants.

I see no reason that the same economic and logistical factors that argue for such an approach will somehow disappear in the far future.

So maybe the issue is terminology -- I don't necessarily consider variants to be different vehicles.

Ah, now I see. Yes, the issue was just the terms used. Instead of wide variety of missions I should have read, common chassis's adaptable to wide variety of vehicles. Yeah, that's how I do it too.

BTW: an idea I've toyed with along those lines was working up a standardized turret that can be adapted to carry a wide assortment of mission specific weapons/equipment, and then plugged into a standard chassis as needed. So a ship could carry a couple of vehicle chassis ready to run, and have racks with several turrets to drop in as would fit the mission. The turrets take less room than more vehicles do, and it would maximize flexibility in the field.

Since I use...God help me sometimes ...Striker I was able to come up with such a system for MTU without too much heartache. It also makes it easy for players to buy a standard chassis with a basic cargo box for customization at a low price, then they can aquire add-on turrets for later use.

Just a thought, since your Commonwealth Navy spends so much time out in the boonies and flexibility seems a must.

 

[Fritz_Brown]

Far-Trader:

Just imagine a small grav armor unit that has to insert fast and low with some tanks to take the enemy by surprise. They hire a couple of Free Traders, sling the tanks under them, then come flying down as innocent merchants until they get close to the bad guys. Then it's tanks-away.

Maybe some daring tank riding desant types in battledress with grav belts could be clinging to the armor to lend support. Scary, but just think how good a story it'd make later.

And, just think how effective they'll be as heat shields during atmospheric entry........ :smirk: o:

 

[coliver988]

And, just think how effective they'll be as heat shields during atmospheric entry........ :smirk: o:

Well, if you've grav tech, then you can probably go really slow in re-entry. IIRC the air/raft can reach orbit in planet size hours (size 8 world = 8 hours). Better hit the restroom first!

Of course, going that slow gives more time to fix on the trader visually and say something along the lines of "That ain't no trader" or else "I've got a bad feeling about this."

Grav tech + effectively unlimited power changes a lot of assumptions in how things work, especially in terms of space travel.

 

[far-trader]

Well, if you've grav tech, then you can probably go really slow in re-entry. IIRC the air/raft can reach orbit in planet size hours (size 8 world = 8 hours).

I've said it before, I'll repeat it again, as you so kindly practically quoted the bit, note that is says "reach" orbit. As in climb to orbit. Not "de-orbit" as in come down from orbit. Guess what happens to an air/raft that tries to de-orbit? :toast:

Of course, going that slow gives more time to fix on the trader visually and say something along the lines of "That ain't no trader" or else "I've got a bad feeling about this."

Grav tech + effectively unlimited power changes a lot of assumptions in how things work, especially in terms of space travel.

In terms of space travel yes. In terms of aerodynamics no. But that's another hot topic for another thread (and currently in the TNE yahoo one for that matter).

Of course for the matter here with respect to externally mounted vehicles entering atmosphere is that your choice is unstreamlined and fast or unstreamlined and slow if you accept that CG will allow it. Either way the vehicles are smoked and/or ripped off. By the atmosphere in the first case, by missiles in the second.

 

[aramis]

And, just think how effective they'll be as heat shields during atmospheric entry........ :smirk: o:

As was recently shown IRL, one doesn't need heat shields to reenter. one merely needs a low enough velocity.