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So, for some time I'm doing preliminary work for a story I want to write. I've filled multiple notebooks with my tiny, scratchy-looking handwriting and I'm now getting close to where I feel is a good point to just abandon the rest of the world building for later and start writing the prologue. Now, the story revolves around a warship at the eve of an interstellar war leaving port, the crew not knowing that soon they'll have to fight for their lives. Since there's a lot of military poo poo involved, I spend a lot of time doing dumb poo poo like deciding what kind of space weapons I wanted to use. (One of the main weapons is now the ECB or Electron Beam Cannon: A fancy type of laser, basically.) Then I came to the point where I wanted to visualize how large the battlecruiser at the center of the story should be, and how its mass would relate to the militaries involved in the conflict. You see, I was concerned because authors tend to gently caress up royally when it comes to space ship mass, with ships often ending up with the density of a soap bubble or other crazy poo poo. I wanted to avoid this, so I went through an elaborate process of assumptions to get a number which "feels" right and hopefully won't sound like total nonsense to someone who actually knows a thing or two about physics, engineering, shipbuilding and whatever else. So this process I came up with went a bit like this: 1. Starting Point As a starting point, I chose a small system defense vessel or fast courier (if equipped with FTL engines). Name, class and everything is still undecided. Basically, it's just a thing to help me extrapolate measurements for other ships and vehicles I want to include. This ship was the Corvette-class Place Holder, roughly shaped like an elongated wedge. Think star destroyer, just less ugly and smaller. By using almost random numbers, I decided to make this ship roughly 60m long, 20m wide and 40m high. 2. Block Math I'm very bad at math. So to prevent ugly errors from showing up, I kept everything after point 1 very, very simple. So I first assumed the ship was a simple, rectangular block. This means I could get a volume value by just multiplying 60m * 20m * 40m to get 48000m³. But since I just said the corvette is roughly wedge-shaped, I then assumed about half of this volume would end up in the parts of the wedge not part of this imaginary block of space LEGO, which leaves 24000m³. Now a space ship also isn't just a massive block of matter, so half of this rest would be essentially empty space: Pipes, corridors, the inside of fuel tanks, crew accommodations and whatever else. This meant I could hack the volume into half again to account for this. Still 12000m³ left. And now it was time for density. At this point, I assumed there would be both fancy space materials lighter and heavier than iron in that volume, since it's the future and everything. By another assumption, I decided everything would balance out to roughly the density of iron. So then I looked up the density of iron, rounded down to be on the safe side, and came up with this value for mass: 7000 kg/m³ (density of iron, rounded down hard) * 12000m³ = 84 million kg, or 84000 tons. Welp, that sounds believable, I told myself. 3. Idiot Test The German frigate Bremen F207 isn't wedge shaped, but still a whopping 200m long. It also weighs 3680t. poo poo. On the other hand, the Bismarck is, like all water ships, a long thin line of 250m and weighs a maximum off 53500 tons. A lot closer, but the ship is also still a lot bigger than my example ship. So apparently this method of chaining assumptions together to get a realistic sounding number was absolute garbage, since my smaller space ship shouldn't way that much more then this very real existing ship. Or was it? Any ideas of doing something else (as long as it isn't to math-intensive) to get a believable number for mass? Or what do you think about this process, and if you read in a book that a 60m long ship weighs 84000 tons, how ridiculous would that sound? At what point would you go "Sure, I could imagine a space ship of that size having that kind of mass"? This may sound dumb, but this poo poo has me worked up for half a day now. I'm halfway tempted to just add a a dumbass-divisor of 2 (which would give me a mass of 42000 tons for this space corvette) and then start sketching up sizes and mass for the most important ship classes using this formula.
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Jan 3, 2018 21:20
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Obligatory disclaimer that figuring this out is not at all important for a story and chances are there is no reason to list your spaceship numbers. That said, I like designing spaceships and thinking about them more than is strictly necessary. Incidentally, that's about a 60 meter spacecraft and it only weighs 7k tonnes because it has a solid core of silicon carbide runnning through it. If you need a number, here's some density figures for real world vehicles. Pick something from the table that you like. If you wanted to go for something more complex like what you've done, you'd need to actually know what your ship is made of, but if all you want is a rough figure for a story there's really no need to go that deep.
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Jan 4, 2018 06:21
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It's problematic comparing space ships to water ships anyway, since boats on Earth aren't airtight and don't need to carry their environment around with them. Also note that the inside of your spaceship doesn't need to be made of the same stuff as the hull, so a shell made of solid carbon whateverium would probably still contain walls and fittings of lightweight aluminum. That is, unless it's carved out of a solid asteroid or something. If I wanted to be as accurate as I could be without an engineering degree: I'd start by considering how much the outer skin weighs by finding the surface area, multiplying that by how thick I think it should be on average, and multiplying that by the weight of whatever you're making the hull out of. Then I'd figure the volume, find out how much around a quarter of that would weigh in solid aluminum, and assume that's about it for fittings. If you want to be super duper accurate, add in how much the remainder of the volume weighs in two-thirds air and one-third water. Fudge it up and down depending on whether the ship's engines are big fuckoff rockets or some sort of future reactionless space warp thingy. Round up and call it good. Note that in real spaceships, the hull isn't terribly thick. You don't need tremendous structural strength to hold in one atmosphere of pressure. Of course, (most) modern spacecraft don't have armor or weapons either.
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Jan 4, 2018 17:06
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Elukka posted:Obligatory disclaimer that figuring this out is not at all important for a story and chances are there is no reason to list your spaceship numbers. Thanks, that was super-helpful! Looking at submarine density (0,9t/m³), it seems my idea of using rounded-down iron was wildly of the mark there! I've already revised my lazy math system and now it shoots out believable numbers left and right! (Now I'm trying to get something similar for crew numbers, but my crew system stops working after a certain threshold: Bigger ships having more mass is just physics, after all. Living beings needed to control a ship are a lot harder to nail down in an easy formula -my current attempt works mostly, but very large ships generate very silly crew numbers.) Mirage posted:It's problematic comparing space ships to water ships anyway, since boats on Earth aren't airtight and don't need to carry their environment around with them. Also note that the inside of your spaceship doesn't need to be made of the same stuff as the hull, so a shell made of solid carbon whateverium would probably still contain walls and fittings of lightweight aluminum. That is, unless it's carved out of a solid asteroid or something. Don't worry, I'm aware of the problem with water ships not being space ships. To account for the differences, I took submarine density (since a submarine is closer to a spaceship than normal ships) and doubled it to generate a simple "space ship density". For military ships, that is. For civilian ships I just straight up use submarine density on account of lightweight future materials. The doubled density for military ships is more to account for stuff like redundant structures and armor. Also please note that while it is true that you don't need much structural strength to hold in atmosphere, keeping out radiation and other deep space hazards is an entirely different thing. (Also also, in this fictitious universe, FTL-travel is rather hard on ships, so they can't be built too flimsy, anyway.  )The new system I refined thanks to this thread now allows me to generate new ships and ship classes on the fly, so if I need a character to talk about a ship, I can spit out reasonable sounding values for mass and poo poo in a couple seconds! Just in case others are interested in reducing accuracy without destroying believability, this is my new instrument of making up poo poo: Step 1 Write down stuff like name, class, hull size, measurements. Give your ship a shape. Example: HFN cruiser Bahrain, 150m long, max width 50m, max height 50m. Spindle/cylindrical. Step 2 Calculate volume as if your ship is a rectangular block with the measurements you came up with. Example: 375000m³ Step 3 Apply what I call a "form factor" to get close to the space the actual ship inside that block-shaped volume fills out. Example: Since cylinders are already close to the real volume, and only the parts of the "block" where the spindly bits are will have lots of empty air, the form factor is 2. (The closer you get to 1, the more of that imaginary block gets stuffed with ship, and the higher the form factor is, the thinner the ship will be. An elegant method to create volumes for everything between bulky freighters and tiny, thin couriers.) Actual volume of the ship: 375000m³/2 = 187500m³ Step 4 Multiply with density to get mass. 0,9t/m³ for civilian ships like freighters, 1,8t/m³ for military vessels. Example: 187500m³ * 1,8t/m³ = 337500 tons. And there you have it! Just to test I made basically a dozen ships in a couple minutes this way. And as an idiot test, I looked at what happens if I put in numbers of a real water ship and a submarine. (Bismarck and Typhoon-class) Astonishingly, the numbers came pretty close, basically the space versions of those ships were about twice as heavy, which I chalked up to the differences between space ships and water ships. The crew formula by the way, is a lot simpler: You just take the volume after the form factor is applied, and divide by 800 (because when trying this out, I got the most believable numbers when dividing by 800m³). In the idiot test, the crew numbers for wet ships were wildly off (duh) and for submarines it nearly mapped, which I took as a good sign. Edit: Actually, I did the idiot test with 400m³ per crew member and decided to account for differences between submarines and space ships by further doubling the space per person. The example cruiser above for example has 234 crew members. In a similar way, I looked at what my other examples got me, and up to a point this second formula worked. The point where I stopped being satisfied is very large ships. The thing is, obviously when you make a ship larger, it's mass is going to skyrocket, that's simple physics. At the same time, crew numbers won't be following a similar rate for long, since after a certain point you get ships so huge, most of the new volume and mass you gained by making a ship huge will be taken up by more machinery, structural support, larger engines and the like. The number of people to maintain that poo poo isn't going up at the same rate, however: It's the space future, you don't need more people to swab all the decks, after a certain point you either automatize a lot more than usual, or your new super-ship gets so economically unwise in operation all your 70k crew members will die a horrific death after the first day of travel because the life support systems choke. Which they will do because it's not physically possible to operate a ship that size by just stuffing more people into it. Anyway, as I see it, you can't just blindly take whatever this second formula gives you, at least not for larger ships. Personally, I've written down some modifiers to reduce crew numbers. A battlecruiser for example has a crew mod of 0,9 to reduce the artificial number generated from the formula by 10%. After that the mod gets harsher and harsher, as the crew numbers spiral out of control. And at a certain point I put my foot down and declared this ship to be the largest you can build before economics and physics catch up with you, even in the far future. (This Leviathan-class is over a mile long, weighs something insane like a billion tons and has, even after my "crew mod", 40k people on board just to keep everything running. This will literally be the largest ship class for the entirety of the three story arcs I have mapped out. And if I'm still writing new stuff after that, I'll probably still not go beyond that.) Now that all this math crap is out of my way, I can finally start writing the story. Well, more of it. Way back before I started collecting notes, I wrote a short intro and then at one point a character opens his mouth to talk about his ship and I realized I had no idea how that ship was supposed to look. Or how large it was. What weapons it was supposed to have. And so on. All of this thread is essentially me trying to compromise between realistic worldbuilding and lazy hack writing.
Libluini fucked around with this message at 20:21 on Jan 4, 2018 |
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Jan 4, 2018 20:17
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