Spaceships--how much IS a power point

[YankeeGamer]

Originally Posted by Johnny1A.2

Yeah, that's the kind of power serious spacecraft have to have. Depending on how you measure it, a Saturn V first stage generated somewhere between 60,000 and 200,000 megawatts (for a few minutes, granted). The thrust power of some hypothetical engine designs could easily get into terawatt levels.

Now 40 gigawatts is an awful lot for a land vehicle. For comparison again, a Nimitz class aircraft carrier has a power plant that runs in the range of 200 megawatts.

A land vehicle needs more power to move than a sea vehicle, and will be traveling over irregular terrain. It only has 2 PP, and the fabricator uses one when it's running. The 30Gigajoule laser for terrain modification uses 1/3 power point. (The craft is on a peaceful mission, so the possible of terrain modification being used for modifying hostiles from "solid" to "vapor" isn't discussed much.)
Theoretically, its drivetrain needs only a partial power plant to achieve its cruise of 5 MPH and its top speed of 10-12 mph. Due to extra rough terrain, and being geared LOW for super high torque, it has much more power plant than 12 mph should need.

[AlexanderHowl]

No land vehicle would be able to radiate the waste heat without superscience, 3 GJ of electricity would mean a minimum of 3 GJ of waste heat, which would melt the vehicle in a few minutes.

[YankeeGamer]

Originally Posted by AlexanderHowl

No land vehicle would be able to radiate the waste heat without superscience, 3 GJ of electricity would mean a minimum of 3 GJ of waste heat, which would melt the vehicle in a few minutes.

I might have to "rule of cool" it away, with a solid dose of narativium and large quantities of PNAMBC science. (Pay No Attention to the Man Behind the Curtain) or just put big heat radiators on it. It's traveling on a vacuum world, and even hard sci-fi oft ignores the radiator issue.

[AlexanderHowl]

Why would you need an SM+12 land vehicle on a vacuum world though? Just have a SM+12 stationary base and use SM+6 mecha to explore the surface. Heck, it would not even need any power points, as it could use minifac cabins instead of fabricators unless it was meant to be an industrial center.

[ericbsmith]

Originally Posted by AlexanderHowl

Heck, it would not even need any power points, as it could use minifac cabins instead of fabricators unless it was meant to be an industrial center.

If you plan on having more than a few of minifacs that's kind of a cheat, and minifacs aren't going to let you build very large things anyway (like the exploration Mecha you mentioned). Spaceships 7 gives Factories a limit on the size of a vessel they can build/assemble of the Ships SM-6 (p. 4); that is a Factory in a SM+12 ship can construct craft up to SM+6, so you'd need a full sized factory to build them efficiently (Spaceships 7 also gives rules for building vessels that are too large to build in the factory itself, using parts from the factory; efficiency and cost suffer badly).

[AlexanderHowl]

Why would you need to build the rovers in the facility? That would be a rather atrocious waste of resources unless you were making a colony base (even then, anything above a SM+10 fabricator would probably be excessive). Better to bring a few dozen rovers and to replace them as needed.

[Ulzgoroth]

Even in a vacuum, a ground vehicle is by definition very close to a great big heat sink. I'd think you could find a way to take advantage of that. A couple notions:

A) Towed surface heat conductor - basically, drag a bunch of heat-conducting wires along the ground and sink heat through them.

B) Ground-penetrating heat-conductor plus on-board heat reservoirs for when you're moving and can't have the heat sink planted.

[Varyon]

Originally Posted by ericbsmith

Using that method we get approximately:
Fission 2MW/ton 1 Power Point
Fusion 10MW/ton 2 Power Point
Antimatter 40MW/ton 4 Power Point
Total Conversion 200MW/ton 5 Power Point

Using power per ton of power plant and supporting structures (rather than power per ton of vehicle mass), I get approximately 100 kW/ton for fission*, 2 MW/ton for fusion, 10 MW/ton for antimatter, and 20 MW/ton for total conversion. That's at minimum TL for each (Vehicles had power plants give more power per unit weight as TL increased, while Spaceships has them last longer before refueling as TL increases instead), and works out to 100 kW/ton per PP for fission, 1 MW/ton per PP for fusion, 2.5 MW/ton per PP for antimatter, and 4 MW/ton per PP for total conversion.

*The Vehicles fission reactors are actually 300 kW/ton, but only last 2 years before they need refueling. I don't remember how I worked it out, but I came up with a variant that more closely matched Spaceships longevity with 1/3rd output, but cost a good deal more than in Vehicles (and cost more to refuel; I think it was basically just having a lot more fuel rods to start with but burning through them much more slowly).

Originally Posted by YankeeGamer

I might have to "rule of cool" it away, with a solid dose of narativium and large quantities of PNAMBC science. (Pay No Attention to the Man Behind the Curtain) or just put big heat radiators on it. It's traveling on a vacuum world, and even hard sci-fi oft ignores the radiator issue.

A passable option might be, since there's an abundance of frozen atmosphere beneath the tracks of the vessel, you pull ice up as you chug along, soak up waste heat by melting and boiling it, then shoot steam out behind you. I assume the planet's gravity will be enough to keep the superheated gas from escaping, so it will eventually settle back down as snow (frozen planet and all that). Something else to keep in mind is that the components may not really need a full PP - sure, an SM+13 fusion power plant may output up to 30 GW of usable power, but the treads may not really need a full 15 GW, and neither might the factory module. If you just want a rough estimate, I feel each PP being 5 kW per ton of total vehicle mass is workable (this works out to 100 kW/ton of power plant, matching fission above). For your crawler, that's 1,500,000 kW, or 1.5 GW (just a touch more than is required for Doc Brown's time machine).

Originally Posted by AlexanderHowl

Why would you need an SM+12 land vehicle on a vacuum world though? Just have a SM+12 stationary base and use SM+6 mecha to explore the surface. Heck, it would not even need any power points, as it could use minifac cabins instead of fabricators unless it was meant to be an industrial center.

"Why would you play the game you want, instead of this completely different one?" Less snarky, if my above suggestion is workable, keeping on the move is actually a decent way to stay cool without rapidly drilling your way down through frozen atmosphere to the surface. Additionally, on a planet with a proper atmosphere a meteor shower calls for a nighttime picnic with some astronomy equipment. On a vacuum planet, a meteor shower calls for bravely running away. That latter is easier to do with a mobile base. There are, of course, other reasons to want a mobile base.

[AlexanderHowl]

Frozen surfaces can only absorb so much waste heat before they evaporate (remember, it is a vacuum). If we assume 3 GJ of waste heat per power point for an SM+12 vehicle, that is probably sufficient to evaporate three metric tons of water ice per second per power point (over ten thousand metric tons per hour per power point). In order to radiate the heat away, you would probably need around 2 square kilometers of radiators to avoid causing transportation issues, so you cannot just drag a tow line behind and expect it to work.

[Rupert]

Originally Posted by AlexanderHowl

Frozen surfaces can only absorb so much waste heat before they evaporate (remember, it is a vacuum). If we assume 3 GJ of waste heat per power point for an SM+12 vehicle, that is probably sufficient to evaporate three metric tons of water ice per second per power point (over ten thousand metric tons per hour per power point). In order to radiate the heat away, you would probably need around 2 square kilometers of radiators to avoid causing transportation issues, so you cannot just drag a tow line behind and expect it to work.

That seems very large - 2 square kilometres for 3GJ is only 1.5KW/m^2, and I think that means the radiator is only running at about 130C.