Starship Length, Volume and Mass


General Discussion


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Let's say your average starship is about five times as long as it is tall and wide.

A starship's density should be about 0.25 metric tons per cubic metre. A jet airliner has an average density of 0.27, but 0.25 is a nicer number to work with for approximations.

So for the above starship with proportions of 5:1:1, to calculate its total mass in metric tons, all you need is:

m = l³ / 100

Where l is the ship's length in metres along its longest axis. If you start with the mass and want length, it's the same thing in reverse:

l = ∛(m * 100)


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That's interesting. I grew up on Traveller, so I'm used to their version of tonnage, where a 3m cube of space is 2 helium tons. Because that's how much it displaces helium. Or was it hydrogen? I forget.


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A 3m cube of 2 tons is equal to about 0.22 tons/cubic metre. Pretty close!


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Pathfinder Adventure Path, Rulebook, Starfinder Roleplaying Game Subscriber

That's a pretty darn nice and simple ratio to use, UR. I'd actually expect a slightly higher density than in an aircraft, but ease of maths means I'm happy to handwave it.


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Useful points of reference:

1 cubic metre of water is 1 metric ton (tonne).

1 cubic metre of air is 0.0013 metric tons.

1 cubic metre of solid steel is 7.8 metric tons.

1 cubic metre of bone is 1.85 metric tons.


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Odraude wrote:
That's interesting. I grew up on Traveller, so I'm used to their version of tonnage, where a 3m cube of space is 2 helium tons. Because that's how much it displaces helium. Or was it hydrogen? I forget.

Wasnt it Hydrogen since that is the most basic atom and acts as a decent universal measuring point?


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I've seen similar discussions on the "Atomic Rockets" web site.

They propose using a design more akin to a nuclear submarine than air craft for determining 'ship density'. Since a submarine has to withstand greater and repeated pressure variances.

Yes, I understand a star ship is invariably only experianceing an atmorpheric difference of about one atmosphere to nill and back per trip... but still, you'd want serious redundancy in something that might pop a leak half way between places in the middle of nothing.

Dark Archive

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, wrote:

I've seen similar discussions on the "Atomic Rockets" web site.

They propose using a design more akin to a nuclear submarine than air craft for determining 'ship density'. Since a submarine has to withstand greater and repeated pressure variances.

Yes, I understand a star ship is invariably only experianceing an atmorpheric difference of about one atmosphere to nill and back per trip... but still, you'd want serious redundancy in something that might pop a leak half way between places in the middle of nothing.

Yep, in many regards underwater engineering is a lot harder than space systems engineering -- and that's one of the main reasons. The pressure goes up *very* quickly as you get deeper underwater. The direction of the pressure's easier to work with underwater than in space, but the magnitude's the killer.

Dark Archive

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Umbral Reaver wrote:
Let's say your average starship is about five times as long as it is tall and wide... A jet airliner has ...

Keep in mind that a cylindrical shape matters far less outside an atmosphere, unless you need to worry about aerodynamics or presenting a small cross-section (stealth and/or combat) a sphere's far better.


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I know that spheres are better, but I'm going by most common science fiction ships, which tend to be longish.


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Callum Finlayson wrote:
Umbral Reaver wrote:
Let's say your average starship is about five times as long as it is tall and wide... A jet airliner has ...
Keep in mind that a cylindrical shape matters far less outside an atmosphere, unless you need to worry about aerodynamics or presenting a small cross-section (stealth and/or combat) a sphere's far better.

Or keeping the engines far away from the crew members...


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If I recall, I believe that rocket propelled 'sky scrapers' are pretty good for a 'burn and turn' ship. Since the ship can go about 1G, it can replicate Earth gravity. So the ships are built more like buildings, where the ceiling is facing the nose of the fuselage and the floor is directed towards the rear. Something like the ships from The Expanse.


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Umbral Reaver wrote:
A starship's density should be about 0.25 metric tons per cubic metre. A jet airliner has an average density of 0.27, but 0.25 is a nicer number to work with for approximations.

Knowing nothing about... well anything even tangentially related to starship engineering why is that the density it should be? Is it a "humans and their stuff and their air take up so much space and have so much mass" sort of thing?


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Pathfinder Adventure Path, Rulebook, Starfinder Roleplaying Game Subscriber
pocsaclypse wrote:
Umbral Reaver wrote:
A starship's density should be about 0.25 metric tons per cubic metre. A jet airliner has an average density of 0.27, but 0.25 is a nicer number to work with for approximations.
Knowing nothing about... well anything even tangentially related to starship engineering why is that the density it should be? Is it a "humans and their stuff and their air take up so much space and have so much mass" sort of thing?

Pretty much, yes. People need a certain amount of room (and air), and bulkheads and cables and doors and things need a certain amount of room and when you factor in all of the things together, the overall density of the container needed to cart all of those things around is "a number". I'd have to look it up to make sure UR is correct, but I have no reason to doubt that figure (it's in the right order of magnitude for sure).


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Chemlak wrote:
Pretty much, yes.

You ever do that thing where in the process of asking a question you realize that youve answered it? I realized too late after posting that yes actually, how much stuff we have in a certain amount of space is the literal definition of density. *face palm*


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Extra complexity if you even care! It probably doesn't matter:

If your ship is much narrower than a 5:1:1 box, reduce the total mass by an appropriate amount. Does your ship take up about half that volume? Halve the mass from the equation.

If your ship is much wider and/or taller, increase the mass accordingly. It's probably not important to get an accurate measurement at all.

Grand Lodge

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i love this, you just dont know how much i really love this. when people throw out math and other fun stuff i just grin.Although i almost killed one of my old DMs for using a moving bird as a point of origen. every round i would wonder why my math wasnt adding up a cart a hill and the enemys would change in distance. so i asked....wish i hadnt.BTW i only gave him a hard time cause he swore up and down he was smarter than us and was a rules lawyer. ok that was a bit must to say i lvoe when people use math but hey.

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