Starship Weight

[Thunderbolt]

Watching Cowboy Bebop again, and listening to news item recently about flying boats, got me thinking about starships that can float.

A waterworld might well encourage a such concept because you could land anywhere.

As anybody who was awake in Physics will remember, an object can float if it able to displace its weight in water.

Being a CT fan, has anybody formulated a way of determining weight (as opposed to displacement) for CT ships?

 

[GypsyComet]

From later editions, the general result is that an "unarmored" starship will float readily, if rather low in the water. Very much armor, however, tips the scales and the ship will sink instead.

Without attaching a mass to every component, there is no "easy" way to do an actual calculation.

 

[PFVA63]

I don't have anything for CT but other systems...

Hi,

I don't have anything for CT but I know that some of the other systems do provide weight data. Specifically, in GURPS Traveller: Interstellar Wars )GT:IW) which is the system I've currently been messing around with;

Hull Mass = total surface area of the hull (in 1000's of sq ft)
x 1.5 tons @ GURPS TL 9
or x 1.0 tons @ GURPS TL 10
or x 0.75 tons @ GURPS TL 11

Armor Mass = total surface area of the hull (in 1000's of sq ft) x Armor Rating
x 0.75 tons @ GURPS TL 9
or x 0.5 tons @ GURPS TL 10
or x 0.3 tons @ GURPS TL 11

Stealth Mass = total surface area of the hull (in 1000's of sq ft) x 0.25 tons (regardless of TL)

Maneuver Drive Mass = basically 4 tons Mass for each dTon of Space it takes up

Jump Drive Mass = basically 4 tons Mass for each dTon of Space it takes up

Fuel Tank Mass (Empty) = 0.025 tons per dTon

Fuel Processor Mass = 4 tons Mass per dTon of Space

Bridge Mass =
- 3 tons for a small cockpit of 0.5 dton size
- 4 tons for a small cockpit of 1.0 dton size
- 8 tons for a small bridge of 1.5 dton size
- 12 tons for a standard bridge of 2.5 dton size
- 24 tons for a command bridge of 5.0 dton size

Sensor Mass =
- 6 tons for a Mod-0 sensor of 0.5 dton size
- 12 tons for a Mod-1 sensor of 1.0 dton size
- 18 tons for a Mod-2 sensor of 1.5 dton size
- 24 tons for a Mod-3 sensor of 2.0 dton size
- 36 tons for a Mod-4 sensor of 3.0 dton size
- 48 tons for a Mod-5 sensor of 4.0 dton size
- 72 tons for a Mod-6 sensor of 6.0 dton size
- 110 tons for a Mod-7 sensor of 9.0 dton size
- 170 tons for a Mod-8 sensor of 14.0 dton size
- 240 tons for a Mod-9 sensor of 20.0 dton size
(Note here that the GT:IW book indicates that they assume that the massive computers in early versions of Traveller actually also included the sensors and other electronics)

Turret Mass =
- a light turret (which should be the equivalent of a CT turret I think) = 1 ton

Weapon Mass =
- Beam Lasers = 3 tons
- Pulse Laser = 3 tons
- Missile Rack = 1 ton
- Sandcaster = 1 ton
(There's a lot more on lareger weapons, which I guess would be similar to HG: LBB5 stuff, as well)

Mass of Equipment for Vehicle Bays & Hangars =
- 0.5 tons per vehicle bay
- 1.0 tons per Hangar

Power Plant Mass =
- 4 tons per dton

Quarters Mass =
- 1 ton per stateroom
- 4 tons per low berth unit (2 berths total)

Other Stuff
- Workshops = 15 tons Mass (for a 2.5 dTon space)
- Labs = 10 tons (for a 2 dTon space)
- Survey Module = 12 tons (for a 4 dTon space)
- Sickbay = 1 ton (for a 1 dTon space)
- Cargo Hold = negligible mass per dTon of space
- Notional Assumed Cargo Wt = 5 tons per dTon of space

Using this data, for a HERO Class 200 dTon Private Merchant (which appears to be basically a Beowulf class ship) they calc an empty mass of 240 tons or a loaded mass of 720tons. Similarly for an Iiken Class 100 dTon Scout/Courier (which appears very similar to an S Class Scout from CT) they calc an empty mass of 680 tons or a loaded mass of 740 tons.

Since in GT:IW a dTon = 500 cubic feet and the density of sea water on Earth is 35 cubic feet per ton, then a dTon of Sea Water would weigh 14.3 t on Earth. For a 100 dTon vessel then if its loaded weight is less than 1428 t it should float (if I've done my math right) or similarly for a 200 dTon ship it will float if its weight is less than 2857 t. As such both these vessel's appear to be able to float in an ocean on Earth. On other planets though, I guess you'd have to adjust the calcs for the density of the liquid and the strength of the gravity on that planet.

I have a spreadsheet that I put together to design ship's using the GT:IW rules and it includes some of the data I left out above for non-CT type weapons and stuff. It's located here (http://members.cox.net/psjn/GT%20IW%20SD%20Rev%2004.xls)if you are interested.

Regards

PF

 

[Thunderbolt]

Thank you for such a detailed answer, this is very useful !

 

[Icosahedron]

Detailed and useful, but perhaps a tad complex for some of us. I'm a CT fan myself and I made a simple assumption many years ago that in all probability, spacecraft have a similar density to terrestrial submarines, ie unity.
Therefore,
mass in tonnes = volume in cubic metres = displacement in dTons*14.
in general.

If I wanted more detail, I figured Drives and machinery have spaces around them, and armour will probably be layered with bracing between, so I guesstimated a typical density of 5 tonnes / cubic metre for such components, and a simplified 0.2 tonnes / cubic metre for accommodation space.

However, the overall density will remain close to unity and unless the ship is completely shut down, it could support itself in water anyway, all you need do is keep the grav generator ticking over, or if you want a complete shutdown, deploy your float system (installed on any craft with fuel scoops).

There is a detailed discussion of floating elsewhere on the site. I recall one citizen actually threw some machinery into a swimming pool to test it out, see if you can run a search for it.

 

[BigBadRon]

MegaTraveller used to include the actual mass of components in their design sequences.

For the examples in the Imperial Encyclopedia it generally worked out to somewhere between 5 and 7 tons per dton of the ship.

 

[PFVA63]

Almost forgot

Hi,

I almost forgot a couple things.

First, there is a table and some other data in the GT:IW book for determining hull surface area based on ship size and configuration, for use in the equations I mentioned above.

Second, the book almost provides a simple estimate for guessing at loaded weight, where loaded mass is assumed equal to hull mass + 2 x the size of the vessel in dTons.

Overall, I tried plotting up the vessel's included in that book and found that the above equation could be off by a bit depending on the type of vessel you are designing so I plotted up the information I had for GT:IW type ships to see if I could ID any better trends.

Here's a plot of Fully Loaded Mass vs Ship Size (in dTons) for some of the different ship types provided in the book.

Also, here's a plot of Empty Mass vs Size for the same ships.

And finally here are some plots of Loaded Mass vs Empty Mass for both starships and small craft.

Hope this helps.

Regards

PF

 

[Andrew Boulton]

As a general rule of thumb, warships sink, traders float. Ignoring contragrav, obviously.

 

[Thunderbolt]

More grist for the mill !!

If I've understood the unity point correctly, if a ship displaces exactly its own weight in water, it would only just stay at the surface, with the top of the ship awash.

To truly float like a ship or barge, it's volume must displace a greater weight of water than its own weight, so that part of its superstructure protrudes above the surface.

Assuming I can determine that the ship floats, my next task is to determine where the water line would be.

I'm working on a 3D model and need to make sure that access doors and so forth are not placed such that they would be submerged when the ship is on water.

 

[atpollard]

Assuming I can determine that the ship floats, my next task is to determine where the water line would be.

Even if the fully loaded ship 'just' floated, how much is empty will alter the water line. If a 200 ton ship has its 100 ton cargo hold half empty, then it will weigh 50 tons less than when they are full.

(assuming weight = volume as mentioned above)

 

[Ishmael]

I think it should be figured during the ship design sequence...
which makes it rules dependent.

Its also intersting to note that tech 11 superdense has a worse 'toughness' per unit density than kevlar/spectra when using material list made by R. O'Connor from the E. Fok list and others. The advantage of superdense, for example, would be that it takes up less volume for the same toughness, but that depends on which rules you choose to use.

One rule of thumb that seems accepted is:
1 dton= 10 tonnes, or a ship's density of ~.7 for a typical ship.

 

[PFVA63]

Looking through my GT:IW Stuff

Hi,

Looking through my GT:IW Stuff, ships designed to those rules come out about;

• 2.0t /dTon for Passenger Vessels & Yachts
• 2.5t /dTon for Survey & Exploratory Vessels
• 4.0t /dTon for Merchant Ships
• 3.0-6.7t /dTon for non-Fighter Small Craft
• 7.5-13.25t /dTon for Fighters & other Military Small Craft
• 5.9-9.7t /dTon for semi-Military Vessels (like Pickets & Commerce Raiders)
• 6.0-9.0t /dTon for Military Ships
• 3.2t /dTon for a Fast Courier
• 7.4t /dTon for a Regular Courier

The densest vessel in the book is a 400 dTon SDB which comes out at 13.25t /dTon. One thing about the GT:IW Rules is that they don't require fuel for the maneuver drive or power plant; the only fuel onboard is used for jump drives. As such I guess that it shouldn't be surprising that a heavily armed SDB, without any tanks for relatively light fuel, would come out so dense. However, at a density of 13.25t /dTon it should still have about 7% of its hull out of the water.

I seem to recall from "Invasion: Earth" (I think) that SDB's were supposed to be able to hide under the ocean, so that it may be that in the case of the GT:IW vessels they are able to flood some of their void spaces to reduce buoyancy enough to submerge or something.

Anyway, most of the Military vessels in the GT:IW book end up only submerging about 50-65% of their hulls and most of the freighters about 25-30% of their hulls (if I've done my calcs right).

Hope this helps.

Regards

PF

 

[Andrew Boulton]

In TNE, a Dragon SDB weighs 15t/dt.

 

[Captain Midnight]

To truly float like a ship or barge, it's volume must displace a greater weight of water than its own weight, so that part of its superstructure protrudes above the surface.

Assuming I can determine that the ship floats, my next task is to determine where the water line would be.

I'm working on a 3D model and need to make sure that access doors and so forth are not placed such that they would be submerged when the ship is on water.

The problem you're going to run into is that unless you've taken great pains to evenly distribute mass throughout the ship, it's going to be unbalanced, and therefore won't float in a "level" position. For example, if all your heavy drives are located at the back of the hull, then the ship's center of gravity won't be located at the geometric center; instead, the center of gravity (abbreviated as CG) will be somewhere toward the back of the hull.

By definition, the ship's weight can be treated as a force pointed down through the center of gravity. Also by definition, the buoyant force can be treated as a force pointed directly upward through the "center of buoyancy (see this link for a more technical explanation of the concept). Since the ship is presumably floating in a fluid of effectively uniform density, the center of buoyancy (abbreviated CB) is located at the geometric center of the displaced fluid.

What does this mean for your hypothetical floating starship?

She's not going to float like she was grounded and the water started rising evenly around her. Instead, she's going to slant, and the angle of tipping is going to depend on how far the CG is from the CB. In dramatic cases, she might ultimately wind up floating in a near-vertical position. You won't be able to tell the exact angle until you determine the size of the ship and a three-dimensional distribution of mass; it's almost certain to have some "angle of loll", to use the technical term.

It's going to be very rare that you'll try to float without your contragrav or thrusters providing some force to control your orientation in the water. If your universe calls for significant effects outside the ship while those drives are working, you'll have to figure a way to deal with those.

You could also try to use the ship's fuel tanks as ballast tanks, and take on water (or whatever you're floating in) to counter the weight. However, since the tanks are meant for liquid hydrogen, you'd want to be careful to vent them thoroughly afterwards, and to not slosh very much, so you won't overstress any baffles contained in the tankage.

 

[Ishmael]

For me, ships in water are probably designed for being in water and have their center of gravity and center of bouyancy worked out in some fasion, besides other featuers that might be needed as keeping air in is different than keeping water out.

either that, or just don't worry about it.

 

[Rindis]

Also keep in mind that the shape of the ship matters. A classic Type S scout has most of it's surface area near the back, where the center of weight is (especially true if 'riding high'). Thus, the rear will get more support from the water, counterbalancing the weight. Whether or not it'd be "level" would take more math than I think anyone here is prepared to put in (and the odds are probably still against it). But you wouldn't be way out of line to say it's workable (if a trifle unsteady every time someone walks from port to starboard, much less shifts cargo) as GM.

The classic trader types are probably out of luck. :nonono:

 

[Captain Midnight]

Whether or not it'd be "level" would take more math than I think anyone here is prepared to put in (and the odds are probably still against it).

For some of us, we've done those sorts of calculations before; buoyancy and stability are generally covered in fluid dynamics courses in engineering school. The key is finding simplifying assumptions. For example, you'll probably neglect any lateral discrepancies in mass distribution (assuming that the mass is equally distributed along the vessel's fore-and-aft centerline); this means that there won't be any roll to consider from buoyant forces. Also, most hull designs can be approximated by geometric solids, such as ovals, spheres, or pyramids, and there are equations already created to calculate the centroids. This means that finding the submerged area is just a question of figuring how much of the hull is submerged in order to provide neutral buoyancy; that comes when the mass of displaced fluid is equal to the mass of the floating object.

If you really want to make sure the ship will float more-or-less level, keep your center of gravity close to the geometric center of the object. (If you rule that thrusters spit out hot plasma, then obviously you won't be able to put all of your drives near the center of the ship; it won't look as spiffy when you're running your M-drive, though.) There will need to be some sort of idea about what components weigh, and where they are concentrated; there isn't a law that says the power plant and the jump drive have to be at the back of the ship, so that can help with your balance issues. You can also just keep the thrusters (or the contragrav, if you want to give it a handwave to allow fiddling with the ship's weight) going all the time you're floating. You could also say that some (or all) fuel tanks are designated "trim tanks", able to pump in water to adjust buoyancy.

It's a challenge that ship designers could certainly handle, but if a spacecraft isn't specifically designed to float level, it almost certainly won't. I'd allow it for any standardized design with a reasonable degree of symmetry -- probably for a Beowulf or a cutter, but almost certainly not for something like a Type T Patrol Cruiser (I'm still not certain that the Type T is ever really intended to enter atmosphere -- what would the landing gear look like, with those funky fins?).

 

[atpollard]

(I'm still not certain that the Type T is ever really intended to enter atmosphere -- what would the landing gear look like, with those funky fins?).

Isn't it obvious?
... being a military ship, it is always powered up and hovers above the ground. An overhead cable supports it in the 'shed' for maintainence.
That's why TCS has such high operational costs.

 

[roninkelt]

According to Archimedes Rule, a ship will displace a volume of water with equal weight to it's own weight. The only question is whether that amount of water is smaller than the volume of the ship (it will float) or larger (it will sink). With the amount of fuel tankage and empty space in the typical traveller ship, it is my opinion that most would float. 1 ton of lhyd occupies 14 cubic meters (a specific gravity of 0.07) - the same weight of water only occupies just over 1 cubic meter (a specific gravity of 1.0) - that is plenty of volume to offset even the densest armor, given the volumes of armor in the source books.

Whether or not the ship will be stable and seaworthy is much more difficult question, but looking at the designs of most ships, I would say not. However, to the occupants of the ship it doesn't really matter - the internal grav field would function to keep everyone happy. My gut feeling is that most ships would bob like corks, so trying to do a loading or unloading at sea would be difficult at best.

 

[Drakon]

If you really want to make sure the ship will float more-or-less level, keep your center of gravity close to the geometric center of the object. (If you rule that thrusters spit out hot plasma, then obviously you won't be able to put all of your drives near the center of the ship; it won't look as spiffy when you're running your M-drive, though.)

I think it would depend on the shape chosen. On something like a cylinder, having your plasma spewer in the center would look silly. However, if you started off with a spheroid, it might spiffy enough.

There will need to be some sort of idea about what components weigh, and where they are concentrated; there isn't a law that says the power plant and the jump drive have to be at the back of the ship, so that can help with your balance issues. You can also just keep the thrusters (or the contragrav, if you want to give it a handwave to allow fiddling with the ship's weight) going all the time you're floating. You could also say that some (or all) fuel tanks are designated "trim tanks", able to pump in water to adjust buoyancy.

Wicked refs could use this to make power failures at sea more interesting. Just my personal preference, but I want to be able to shut the ship down completely, without sinking or listing terribly.

Trim tanks are better, and, if you have a power system that used water instead of hydrogen, (or uses water as a storage medium for hydrogen), all the better. Your fuel tanks are your trim tanks. You might want to submerge before blasting off for the stars, but that would be a small price to pay.

I'd allow it for any standardized design with a reasonable degree of symmetry -- probably for a Beowulf...

With those cargo doors, that might not be a good thing to try.