Spacecraft size

[tkdguy]

I've been trying to come up with a (mostly) hard science fiction game, set in the near future, around the latter half of this century. I'm stuck on how large spacecraft would be. There will be warships and merchant ships, but the scope will be limited to the solar system, up to Saturn.

 

What would you consider to be realistic sizes (in HERO terms or in terms of length/beam/draft) of these ships? Although I'm going to use the naval designations (cruiser, destroyer, etc.), I imagine these ships will be smaller than seafaring warships.

[L. Marcus]

Re: Spacecraft size

 

. . . I'm a-guessin' that 100 meters long should be about as long as you need to be . . . Battleships (and carriers, maybe, if you'll have them) at 50" x 25", and scale down from that.

[EvilDrPuma]

Re: Spacecraft size

 

Well...while I acknowledge that HERO doesn't build vehicles on a "stuff the parts in a box" principle, perhaps you should consider just what the tech is like before you decide on size ranges. If this is a "harder" SF setting, and especially if you're sticking close to current or projected technologies, then you might have larger power and propulsion systems than a lot of movie and TV spacecraft do.

[BlackSword]

Re: Spacecraft size

 

There would be a number of variables. How long it takes to travel between locations would would determine how much food and fuel stores a ship would need. The technology for getting into orbit might effect it as well, depending on what sort of manufacturing facilities there are in solar system. If most production is still performed on Earth, then ships will be small due to cost of moving manufactured parts into orbit.

 

If and how gravity is generated. Using a spinning system, the size would be influenced by comfort level for the crew. The other option is to accelerate at 1g towards the destination, then flip the ship 180 degrees and decelerate at 1g for the rest of the trip. Will the interplanetary ships be atmosphere capable, or have smaller landing craft (or use general use landing craft at an orbiting space facility).

[Cancer]

Re: Spacecraft size

 

One of your core game-world decisions is how your star-drive (and in-system drive, if that's different) works and how big the power plant has to be for a ship. The answers to those will give you a minimum size (which will be used only by rich folks' pleasure craft and certain military purposes).

 

If piracy doesn't matter, then economics suggests that merchant craft will have the largest cargo capacity for a given engine/power plant rig, and a minimal crew. Humans are expensive to maintain.

 

Assuming manuverability is important and manuverability is direct function of drive power and an inverse function of mass, military vessels will have a much larger powerplant/drive component compared to total ship mass. If manuverability doesn't matter much and all that matters is firepower and armor/defenses, then military vessels will be large semi-mobile gun platforms.

 

You can start with a known item and handwave the rest. If you assume that an insystem merchant craft looks like a Shuttle orbiter vehicle, then it'll have a crew of two to four, an empty weight of 68 tons, and a maximum load weight of 109 tons. The payload bay is roughly cylindrical, 4.5 meters in diameter and 18 meters long. The ship overall length is about 38 meters. The onboard rockets of a Shuttle (if you could magically get the fully loaded ship to space; I am omitting the strap-on solid rocket boosters) generate enough thrust to accelerate the ship at about 1.6 cm/s^2, or 1/60 gee.

 

(Just remember that if you want large accelerations -- like one gee -- then you have to handwave your power plant. To accelerate 100 tons -- that is, a Space Shuttle class object -- at one gee with a laser rocket takes a power plant of order 10^15 watts.)

 

A military craft based on that would be slightly smaller and all that payload mass would be consumed in power plant/drive, armament, and crew accommodation. (Based very roughly on terrestrial submarines, figure each crewman needs 2 to 5 tons of ship to keep him alive, and scale from there.)

 

Depending on your assumptions, all this could be irrelevant, but it gives you something to chew on to start if it's approximately the flavor you're looking for.

[L. Marcus]

Re: Spacecraft size

 

tkd, is there anything written on the tech you'll be using? I mean, design baselines on weaponry and armor, power technology, medicine . . .

[bigdamnhero]

Re: Spacecraft size

 

The single biggest variable is probably: do your ships need to enter the atmosphere or not? If they have to take off and land, then they're probably going to be as small as you can get away with. If they send down launches or dock at orbital stations to send stuff up & down by space elevator, then you can probably get away with much larger ships. So again it depends on what you want.

 

A friend of mine maintains that our tendancy to think of spaceships in terms of naval ships is wrong, or at least several centuries away. He believes that spacecraft are likely to resemble airplanes more than ships for at least the next century or so. Interesting way of looking at it, I thought.

[tkdguy]

Re: Spacecraft size

 

First of all, thanks for the responses I got. I find them quite useful.

 

I realized last night I should have given a few specific designs on the ships. Most of the tech involved is described in the Future Tech Links thread.

 

I'm still deciding whether or not to allow ships to enter the atmosphere. I had planned to get the astronauts to space via beanstalk and board the ship that is docked at the station at the end. I will probably have mass drivers launch ships to space as well, since this would be a military campaign (ex. USA/Europe vs. Russia/China).

 

Propulsion will be based on the VASIMR drive, albeit a more advanced version. There won't be any FTL travel, as nobody has discovered how to do it (or whther it is indeed possible). There are colonies/outposts on the Moon, Mars, a few asteroids, and some of the moons of Jupiter and Saturn, so merchant ships and supply ships make regular voyges to those places. I was thinking of using nuclear power, which the engineers maintain using remote-controlled robots.

 

There is no artificial gravity, except when the ship accelerates. The spacemen strap themselves in their couches (I got this idea from Nyrath's site), and often work in free-fall or attached to tethers.

 

The ships will be armored (probably ablative) in order to withstand an enemy's firepower. Weapons on the ships are missiles, rail guns, and autocannons like the Phalanx, although this last one is used mainly for point defense. Unmanned drones may also be used to harass enemy ships. If there are fighter pilots, they're usually stationed in the O'Neill colonies at the L4 and L5 Lagrange points. The fighters would be armed with modified versions of Vulcan cannons and Sidewinder missiles.

 

I had thought of using the Cold Navy ships, although the design may be too advanced for the setting. Maybe the ones from Full Thrust would make a better fit.

 

Edit: I also have been looking at current space tech to see how living in space would be like. Stuff like food, medicine, sleeping quarters, and waste disposal would be important details to remember.

[L. Marcus]

Re: Spacecraft size

 

. . . Dammit, stop being so Reppable!

[Nyrath]

Re: Spacecraft size

 

You've already read my site, but I will point out some tid-bits from it.

 

If you just want something really quick and dirty, figure that every ton of fully loaded and fueled spacecraft has a volume of 5 to 10 cubic meters (m^3), and ten percent of the fully loaded mass spacecraft mass is structural mass. Alternatively, figure that every cubic meter of hull volume masses 100 to 200 kilograms. That corresponds to average pressure compartments being cubes 10 meters on a side, with pressure bulkheads averaging 17 to 33 kg/m^2.

 

For crew quarters: for enlisted men, the US Navy manages to cram twelve crewmen into 100 m^3, or 8.3 m^3 per man. For officers it is a more expansive 10 m^3 per person. (there is a chart at http://www.projectrho.com/rocket/rocket3f.html labeled "Personal space required by people").

 

As a very rough rule of thumb: one human will need an amount of mass/volume equal to his berthing space for three months of consumables (water, air, food).

 

For a back of the envelope calculation, for a military vessel figure roughly 10 to 16 crewmen per kiloton of ship mass.

 

But as with all real-world spacedrives like VASIMR, the ships will be mostly reaction mass. If they are to have a reasonable amount of delta V, they are going to be about 75% of its mass being hydrogen tanks and 25% of the mass will be everything else (crew, structure, habitat module, weapons, missiles, nuclear reactor, heat radiators, food, oxygen, water, everything). If you want to go into the details (figuring specific missions and trying to get away with less reaction mass and more ship mass), you can use the cheat sheet, there are marks for VASIMR in low, medium, and high gears. But for a Star Hero campaign that's probably overkill, just use 75% reaction mass.

[tkdguy]

Re: Spacecraft size

 

Thanks for the links, Nyrath! They help a lot. And bigdamnhero, your friend has a good point. I can see spacecraft taking off from and landing on Earth for a while. That mass driver looks pretty good, although I still like the beanstalk. It looks like the Air Force rather than the Navy will be at the forefront (at least for now) of developing any military spacecraft, so the ship designations may not come into play at all. Maybe I will use a little handwavium there after all...

 

I was looking at the specifications of the space shuttle as a base. Is the Crew Exploration Vehicle designed to replace it reusable? I don't know if it is, which is why I haven't considered it yet.

 

You guys are giving me a lot of great ideas. Thanks again.

[Threnody]

Re: Spacecraft size

 

One thing to ask yourself about:

 

Where's the ore being mined? Where's it being refined? Where are the bits being shaped? Where are the subassemblies being put together? Where's the ship being put together?

 

The more of that you do in high-grav places (Earth, Mars, Moon, L4, L5), the smaller the ships HAVE!! to be.

 

If you mine asteroids, refine in deep space (with big-ass mirrors), sub-assemble in deep space or high orbit, assemble in ditto, then they'll be big. But not able to land even on airless moons in they've got any mass to 'em.

[Threnody]

Re: Spacecraft size

 

Howdy!

 

Sorry 'bout the double post. Boy, is my face red!

 

'Bout getting away from L4 and L5---I once heard an astronomer say it'd be like leaving one of the medium sized moons. He said Iapetus, if I recollect right. Wny ol' how, there's a good bit of "pull" towards them points, so it'll takea bit of doing to get away from them.

[tkdguy]

Re: Spacecraft size

 

Good point. I was thinking of either having them assembled either on earth or on a space station created to manufacture these ships. Considering there hostility is rising among several prominent nations, the latter option may not even be available. Then again, defending the station from foreign spacecraft (or attacking it) may be an adventure scenario.

[Nyrath]

Re: Spacecraft size

 

I was looking at the specifications of the space shuttle as a base. Is the Crew Exploration Vehicle designed to replace it reusable? I don't know if it is' date=' which is why I haven't considered it yet.[/quote']

Parts of it are.

 

The Crew Exploration Vehicle will be a glorified Apollo command and service module, AFAIK the service module is not re-usable. It will be launched on a huge version of the shuttle solid rocket booster, which is re-usable.

 

The Constellation cargo vehicle will be launched on a modified Shuttle engine, and is not re-usable.

[McCoy]

Re: Spacecraft size

 

The more of that you do in high-grav places (Earth' date=' Mars, Moon, L4, L5), the smaller the ships [u']HAVE!![/u] to be.

L4 and L5 are high-grav places?

[L. Marcus]

Re: Spacecraft size

 

Sure, compared to, say, Jupiter . . .

[bigdamnhero]

Re: Spacecraft size

 

Considering there hostility is rising among several prominent nations...

Good point: the political situation will also impact ship design. Right now, spaceships are designed to be as light as possible, which if you look at something that doesn't need to enter an atmosphere could be pretty darn flimsy. (Think of Apollo Program LEMs.) But the minute people start shooting at one another in space, all of a sudden survivability becomes a big factor and structural integrity a huge concern. Would it even be feasible, let alone practical, to launch any kind of armed-and-armored spaceship from Earth's surface? Or would it have to be built in orbit?

[Sundog]

Re: Spacecraft size

 

Good point: the political situation will also impact ship design. Right now' date=' spaceships are designed to be as light as possible, which if you look at something that doesn't need to enter an atmosphere could be pretty darn flimsy. (Think of Apollo Program LEMs.) But the minute people start shooting at one another in space, all of a sudden survivability becomes a big factor and structural integrity a huge concern. Would it even be feasible, let alone practical, to launch any kind of armed-and-armored spaceship from Earth's surface? Or would it have to be built in orbit?[/quote']

 

It would certainly be possible to launch a warship from Earth's surface - but that would be hideously inefficient, expensive and wasteful, both of fuel and of ship mass, since you'd have to build it to survive terrestrial conditions, and the launch, as well as space, rather than just space.

 

So, not practical, save in emergency situations (the only such I can think of in litaerature is the Archangel Project spacecraft Michael in Footfall - and that's only possible using an Orion Drive to launch!)

 

Orbital shipyards fed by orbital factories fed by mined asteroids are orders of magnitude more efficient than any other system. And could be primarily either automated or telefactored, reducing costs still further.

[Nyrath]

Re: Spacecraft size

 

The more of that you do in high-grav places (Earth' date=' Mars, Moon, L4, L5), the smaller the ships [u']HAVE!![/u] to be.

L4 and L5 are high gravity places? Are you sure?

[tkdguy]

Re: Spacecraft size

 

I don't know abot L4 and L5, but definitely the Earth, the Moon, and Mars. I'm toying with the idea that there are ship yards in geosynchronous orbit near the space elevators. Materials will be transported via the space elevators. I briefly thought about putting the ship yards orbiting the moon, but that seems impractical. AFAIK, there's no useable (for spacecraft, at least) material there. Perhaps ore mined from asteroids will provide the material. That would give a reason for those colonies to exist.

 

I have written a few notes about the range and crew. Here's my rough draft:

 

Fighters: Hold 1-2 people; range is limited to orbital flight (maybe Earth-Moon) distance

 

Corvettes: Up to 20 crew members; Earth-Mars range

 

Frigates: Up to 50 crew members; can travel throughout the colonized solar system

 

Destroyers: Up to 80 crew members; can travel throughout the colonized solar system

 

Cruisers: Up to 120 crew members; can travel throughout the colonized solar system

 

Battleships: Up to 150 crew members; can travel throughout the colonized solar system

[Susano]

Re: Spacecraft size

 

I don't know abot L4 and L5' date=' but definitely the Earth, the Moon, and Mars. I'm toying with the idea that there are ship yards in geosynchronous orbit near the space elevators. Materials will be transported via the space elevators. I briefly thought about putting the ship yards orbiting the moon, but that seems impractical. AFAIK, there's no useable (for spacecraft, at least) material there. Perhaps ore mined from asteroids will provide the material. That would give a reason for those colonies to exist.[/quote']

 

Actually... there does seem to be a lot of useable raw materials on the moon. Water ice, gases, and the like, you just need to grind up a lot of lunar matrix to get at it. And if you are bringing rocks in from the asteroid belt for processing, it's easier to work around the moon then Earth. Less debris, less gravity.

[tkdguy]

Re: Spacecraft size

 

Good point. I was simply thinking about the metal. But definitely water and some gases are useful. Having processing plants on the Moon would make sense after all then. I'll have to pay more attention to the economics of my campaign.

[Susano]

Re: Spacecraft size

 

There is some talk of using the moon as a staging point for a Mars mission.

[tkdguy]

Re: Spacecraft size

 

I found some info about future spacecraft designs here.