Starship Weight

[Captain Midnight]

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.

I think you meant to say "toroid" -- putting a plasma-spewer in the middle of a spheroid would just ensure that the plasma burns a hole through the ship once you activate the drives. :devil:

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.

That's going to be hard to do without it being designed to do so from the drawing board. In that case, your steward/purser/cargomaster is going to have to have some loadmaster experience as well, or you're in trouble if you're carrying anything in the cargo bay.

With those cargo doors <on a Beowulf>, that might not be a good thing to try.

I'd certainly not recommend submerging to any significant depth, but bobbing on the surface might work out. There's a difference between "holds air pressure in vacuum" and "remains watertight at depth". :smirk:

 

[Icosahedron]

Sorry, guys, but I'll just stick with my CT handwavium flotation devices that are deployed automatically from the skimming/dipping gear and are tailored to the ship to ensure a level ride. Saves a lot of headaches IMHO.
Any ship designed for fuel skimming/dipping is watertight. mega:

 

[deniable]

That would be an interesting image. Think of a free trader coming down to the water surrounded by a big rubber raft /life jacket.

 

[Drakon]

That's going to be hard to do without it being designed to do so from the drawing board. In that case, your steward/purser/cargomaster is going to have to have some loadmaster experience as well, or you're in trouble if you're carrying anything in the cargo bay.

I agree that it should be designed at the start for water landings. And you should load your cargo evenly, regardless of the type of expected landing.

I'd certainly not recommend submerging to any significant depth, but bobbing on the surface might work out. There's a difference between "holds air pressure in vacuum" and "remains watertight at depth". :smirk:

Don't I know. I used to ride submarines. But if you can remain watertight at depth, you are set for vacuum.

As for the Beowulf, I don't know, those cargo doors are pretty low. It might be possible to reverse those side pods, put the cargo locks on top and the fuel tanks/scoops on the bottom. Would make loading and off loading cargo a bit different on land, but...

 

[Icosahedron]

That would be an interesting image. Think of a free trader coming down to the water surrounded by a big rubber raft /life jacket.

And why not? They do it with tanks IIRC.

 

[aramis]

Heck, they used inflatable airbags to land the rovers on mars... can't get much sillier looking than that!

And some proposal for the Mars Manned Mission includes using a kevlar dome... so I can easily see same said dome serving as a water-landing flotation system.

And let us not forget that the Apollo capsules used flotation bags, too.

 

[Straybow]

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

That would be extraordinarily light. 1.5 tons/1000ft² is 3 lb/ft², equivalent to 14 guage sheet metal. By comparison, a coastal vessel of only 100 tons displacement (water, not H2, so about 20 dT) would use 3/16th inch steel minimum, 2½ times as heavy. An aluminum hull might be about that weight at that size, but spacecraft have to be sturdier to withstand micrometeorites and orbital debris.

The GURPS method also ignores the fact that larger surfaces need to be thicker for the same degree of rigidity. A large surface ship has hull thickness about one inch, up to two inches for really big ships, just for structural integrity. That's more than 10-20 times the GURPS hull mass given. Allowing for advanced materials might take half of that mass away, but then you add in heat shielding for atmospheric entry, thermal insulation for all exterior surfaces, rad shielding for engineering compartment, etc and you'll get most of that back.

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.

Just based on shipping container dimensions, you'd get about 5 tons/dT, not counting armor or engines.

 

[PFVA63]

Hi

That would be extraordinarily light. 1.5 tons/1000ft² is 3 lb/ft², equivalent to 14 guage sheet metal. ... An aluminum hull might be about that weight at that size...

Hi,

As you note Aluminum does alter things. Looking on the internet, the density of Aluminum appears to be about 0.098 lb/cubic inch (2.70g/cubic cm) [http://http://my.execpc.com/~rhoadley/magconda.htm]. As such, 1.5mt/1000sq ft is roughly the equivalent of a uniform shell about 0.23inch thick. In addition to this though, there are also other options such as composites and/or foamed aluminum ( http://http://www.metalfoam.net/papers/simancik1998b.pdf ) which might also have some potential.

Another thing also to keep in mind is that in the GURPS Traveller: Interstellar Wars rules, if a craft has an airframe type hull it must have a minumum of an armor rating of 10. As such, the 200dton Hero class I mentioned previously actually has a hull structure weight of 84mt. Dividing the hull weight by hull volume suggests a weight per voulme of 0.42mt/dton for this craft. For the 100dton Scout in the GT:IW rule book, it has an armor rating of 50 giving it a hull weight of 480mt (if I did the math right) for a weight per volume of 4.8mt/dton.

Looking at some data on ocean going ships suggests that for a relatively small high speed ship like the PHM hydrofoils or the Russian Tarantul class Fast attack craft or the US PG-84 fast gunboat might have a structural weight per total craft volume of about 0.60 to 0.67 mt/dton, while ships like the Sa'ar IV & PG511 fast attack craft or the Claud Jones small frigate might have a weight per volume of 1.10 to 1.15 mt/dton.

Finally, it might be better to compare the weight of a space craft to airplanes or the space shuttle rather than ocean going ships. I have some data on aircraft structural weights and if I get a chance I might try and look through them later to see how they compare.

Regards

PF

 

[far-trader]

For what it's worth comparison wise from memory when I was doing up a Traveller version of the Space Shuttle it weighs about 100,000kg and is around 100dton. So that would be in the ballpark of 1 mass tons to 1dton.

 

[dean]

Titanium is a much more likely material in every case. It has a much higher melting point than aluminum (2000 deg F above aluminum), has a low coefficient of linear expansion (half that of aluminum), and slightly more corrosion-resistant (although both are very good), and while denser (Ti appx 4.5 g/cm^3, Al appx 2.7 g/cm^3), is much more adapted to the application overall. Alloys would be the order of the day.

 

[atpollard]

For what it's worth comparison wise from memory when I was doing up a Traveller version of the Space Shuttle it weighs about 100,000kg and is around 100dton. So that would be in the ballpark of 1 mass tons to 1dton.

The Apollo Era craft were closer to 4 metric tons per displacement ton, IIRC.

Another useful measure would be the density of water (setting an upper limit if we assume that the 'typical' starship will float). What is the mass of a dTon of water?

 

[far-trader]

I should also say I expect and have no problem with Traveller ships being built much much sturdier. They should weigh considerably more. I'd say at least 5mtons per 1dton for civilian craft, and up from there for armored craft. Weight isn't an issue for contra grav so they don't have to be light just to get off the ground. And once in space weight is even less an issue.

Off the cuff I'd probably rate the weight at:

((5mtons per 1dton) + (1mton per 1dton x CT Armor Rating)) x 10/TL

So a 20ton TL9 Lifeboat would be: ((5x20) + (1x20X0)) x 10/9 = 111.1mtons

And a 200ton TL14 SDB with factor 14 armor would be: ((5x200) + (1x200x14)) x 10/14 = 2714.3mtons

Does that seem reasonable?

 

[far-trader]

...(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?).

Not sure how I missed this topic first time around but I've postulated on this bit somewhere once. The old mini of the Type T has some lines on the wings that suggested to me they folded allowing the ship to settle much closer to the ground. The folds would shorten the lower wing tips to the same level as the bottom of the hull, more or less. I've got some pics around somewhere. I'll see if I can hunt them up.

 

[aramis]

The Apollo Era craft were closer to 4 metric tons per displacement ton, IIRC.

Another useful measure would be the density of water (setting an upper limit if we assume that the 'typical' starship will float). What is the mass of a dTon of water?

13.4 to 14.7 Mt/Td depending on purity and temp, and Td size.

Roughly 998g/L to 1050g/L for cool liquids.

Pure water at 4°C is 1000g/L

 

[PFVA63]

Hi

Hi,

Thanks for the input on the Space Shuttle and the Apollo craft. I have recently been messing around with comparing jet aircraft to Traveller small craft, and data that I have on the F-104 Starfighter suggests that it has a volume of about 1.7 dtons (if I am recalling correctly) with an empty weight of 6.35mt, a loaded weight of 9.37mt, and a max weight of 13.17mt. This give the aircraft a weight per volume of about 3.7 to 7.7 mt/dton.

With regards to the 1.5mt/1000 sq ft, I was discussing above, that only included the hull structure and bot any machinery, oroutfitting, etc.

As noted below (from an earlier post in this thread) using the GT:IW rules the all-up weight per volume for the various craft in their rulebook varied from 2to 13.25mt/dton depending on craft type, with stuff like an SDB being the most dense and passenger vessels or yacht's being least dense.

Regards

Pat

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.

...

PF

 

[Drakon]

It seems to me that one could assign different weights/volumn for the various systems. Staterooms would have one value, while jump drive and power plants would be denser. Then, when you design the ship, it is easy to figure total weight from the weighted sum of the designed systems.

I am not sure how much to assign though.

 

[Straybow]

Another thing also to keep in mind is that in the GURPS Traveller: Interstellar Wars rules, if a craft has an airframe type hull it must have a minumum of an armor rating of 10. As such, the 200dton Hero class I mentioned previously actually has a hull structure weight of 84mt. Dividing the hull weight by hull volume suggests a weight per voulme of 0.42mt/dton for this craft. For the 100dton Scout in the GT:IW rule book, it has an armor rating of 50 giving it a hull weight of 480mt (if I did the math right) for a weight per volume of 4.8mt/dton.

I don't know how Gurps does armor, ship-to-ship combat damage, etc. I don't know what an armor rating of 50 is: light, medium, heavy?

Passenger airframes and conventional spacecraft are minimized for weight, sacrificing protection for economy and/or maneuverability. Trav spacecraft are intended to have very long service lives. They are rugged, not fragile.

 

[PFVA63]

Hi

Hi,

I see I messed up something in my previous posts. Previously I had noted that in GT:IW you need a minimum armor rating of 10 for an airframe type craft, but in reality I should have indicated that this is for either an airframe or streamlined craft. The only craft that would have an armor rating less than 10 would be unstreamlined craft.

With regards to armor ratings in GT:IW, the merchant ships and small craft in that book all have an armor rating of 10 to 12, while military small craft have a rating from 13 to 33, and large military ships have a rating anywhere from 50 up to 400 (for the biggest battleship).

For craft capable of regularly operating within an atmosphere, they would have a hull structure weighing 1.5t/1000 sq ft plus another 10 x 0.5t/1000 sq ft for the armor assuming a TL of 10, or 10 x 0.75t/1000 sq ft assuming a TL of 9. This gives a notional weight per sq ft of surface area of about 14.3lb/sq ft at TL 10 or 19.8lb/sq ft at TL 9. Assuming a density of 169lb/cubic foot for Aluminum would give equate to thickness of just over 1 inch for the TL 10 craft or 1.4inch for the TL9 craft (if I did the math right).

In reality, I'd suspect that not all structural weight would go into the surface of the craft, as some would be needed for the decks and bulkheads, etc. Based on some data I found on the internet for a frigate sized ocean going naval vessel, the weight of the shell and supporting structure on a real world ocean going ship might be maybe on the order of about 1/3rd the ship's total structural weight. If we assume that a Traveler space ship is similar to an ocean going naval vessel then a TL10 ship would have a hull surface equivalent in weight to about a 1/3rd an inch of Aluminum while a TL9 ship would have a hull surface equivalent in weight to about 0.47 inches of Aluminum.

Regards

PF

 

[PFVA63]

Hi

Hi,

I inadvertantly forgot something in my last post. What I had meant to add was that, based on what I had previously posted I could see how a minimum hull weight of 1.5t/1000 sq ft might seem kind of light for a space craft, but since that was only meant for ships not intended to operate regularly in an atmosphere (which I hadn't made clear in my earlier posts) most ships would have much heavier hull weights, along the lines of what I posted last night.

I figured that the minimum 1.5t/1000 sq ft weight is something that would allow you to build things similar to our real life space stations (Mir, Skylab, the ISS, etc) and/or the moon lander which I believe are generally fairly lightly constructed and not meant to operate within the Earth's atmosphere.

Finally, all this was just based on the stuff in GT:IW which I posted for reference and if they seem light to anyone they could easily assume something heavier.

One thing I like about the GT:IW rules is that they seem reasonably comprehensive and they result in most merchant ships floating fairly high in the water with most military ships floating abit lower, but still on the surface, with only really dense craft like System Defense Boats (SBDs) close to being neutrally buoyant. This seems close to what is implied for ships in Traveller as I understand it from the books, games and articles that I have looked at. Specifically it allows most ships to refuel from the oceans of water worlds without sinking, while potentially allowing SBDs to hide beneath the surface.

Anyway, sorry for any confusion due to my sloppy posting.

Regards

PF

 

[Straybow]

Our modern space vehicles are fairly small and don't have to resist multi-G forces. They have 0 armor value.

They are designed to tolerate micrometeor impacts, not to be proof of them as would a Trav spacecraft designed to operate in junk-heavy orbit or dust-heavy asteroidal regions.