[3e] Statting a spaceship radiator

[GURPS Fox]

So, my crazy mind has decided to give me more things to do (damn plot bunnies being like their namesake), and one of them is cored around Transhuman Space and Sins of a Solar Empire, of all things, with elements from Tomrrow's War and Traveller. [looks at self in confusion, yes, my brain can scare me at times]

Now, I was prototyping a spaceship (the Alen T Shepard class Corvette), which had been designed in a time period like Transhuman Space instead of the usual 'ignores heat'/'super-efficient hull radiators' that sci-fi usually uses (in-setting, those come in later). This means it'll need radiators... and I haven't found much in converting it to Vehicles 2e, just a blurb in GRUPS THS (3e) and some references to a GURPS 4e Space radiator rule.

Is there a way to convert that, or am I forced to ignore radiators?

[Anthony]

What stats do you need? If we assume radiators operating at a reasonable-ish 500F, and 40% efficient reactors (pretty high, but this is SF), you need about 2 square feet of radiator area per kilowatt of power plant, which will rapidly turn into giant wings if you're powering high power components.

Note that the big energy consumers on a spaceship are drives and weapons, which may have their own methods of cooling. Low specific impulse drives can use their exhaust as coolant (this is why chemical rockets don't melt), nuclear pulse and Orion type rockets produce most of their heat outside of the ship where it directly escapes into space. For weapons, their use is often intermittent enough that expendable heat sinks are the way to go.

[Varyon]

A quick-and-dirty method is to just steal 4e Spaceships' handling of it - assume systems that require radiators include a retractable main radiator array as part of their mass. While extended, said radiator can be targeted as though it were the same SM as the vehicle if engaging from side-on (where its profile is largest) but at SM -5 from if engaging from the front or back. So long as the radiator is extended and functional, the vehicle can function normally; if retracted or disabled, the relevant systems will overheat and shut down in 30 minutes (coolant can extend this; coolant equal in mass to the system is consumed every 30 minutes to keep the system from overheating).

Or if you'd rather actually stat them out with Vehicles, Anthony's numbers should work. I'm not sure about how much they'd weigh, but you could probably do worse than assuming they have the same density as solar panels.

[Anthony]

Note that realistic radiators rapidly turn into "your spaceship is completely immobile on a tactical timescale".

[GURPS Fox]

Originally Posted by Anthony

What stats do you need? If we assume radiators operating at a reasonable-ish 500F, and 40% efficient reactors (pretty high, but this is SF), you need about 2 square feet of radiator area per kilowatt of power plant, which will rapidly turn into giant wings if you're powering high power components.

Note that the big energy consumers on a spaceship are drives and weapons, which may have their own methods of cooling. Low specific impulse drives can use their exhaust as coolant (this is why chemical rockets don't melt), nuclear pulse and Orion type rockets produce most of their heat outside of the ship where it directly escapes into space. For weapons, their use is often intermittent enough that expendable heat sinks are the way to go.

Oh boy, that would make things problematic, given that I'm already in the gigawatts for both propulsion and power requirements (thanks to the FTL drive alone, though I might have to rejigger that as it is).

Originally Posted by Anthony

Note that realistic radiators rapidly turn into "your spaceship is completely immobile on a tactical timescale".

Well, I am thinking of basing them on Transhuman Space radiators at the TLs I'm working at (TL10, for the most part).

[GURPS Fox]

Originally Posted by Anthony

What stats do you need? If we assume radiators operating at a reasonable-ish 500F, and 40% efficient reactors (pretty high, but this is SF), you need about 2 square feet of radiator area per kilowatt of power plant, which will rapidly turn into giant wings if you're powering high power components.

Note that the big energy consumers on a spaceship are drives and weapons, which may have their own methods of cooling. Low specific impulse drives can use their exhaust as coolant (this is why chemical rockets don't melt), nuclear pulse and Orion type rockets produce most of their heat outside of the ship where it directly escapes into space. For weapons, their use is often intermittent enough that expendable heat sinks are the way to go.

Well, if Children of a Dead Earth taught me anything, it is that the cooler the system, the less efficient the radiators are.

[most of the common fission reactor designs made by players tend to be in the 2000K range in terms of external coolant heat... usually via sodium]

EDIT:

Using GVB, and assuming that the old fission reactor is TL8 and using long-term access, a 1-space old fission reactor is a 1090kW reactor, a 4-space old fission reactor is a 7340kW reactor, and an 8-space old fission reactor is a 15,660kW reactor. According to the radiator appendix in THS's original book, you'll need 1,000sf per 8-spaces of old fission reactor and fission drives. Assuming new fission reactors are at least TL9 and using the long-term access option, a 1-space new fission reactor is 7,350kW, a 2-space new fission reactor is 15,700kW, and a 4-space new fission reactor is 32,300kW. With these and the old fusion reactors, you'll need 1,000sf per 4 spaces of reactor and engine.

Now, if CoaDE taught me anything, this suggests that we're dealing with radiators spec'ed to high temperatures (likely 2000 Kelvin) as a whole with very high-end materials (a radiator that I 'quick build' on CoaDE using 3D-Graphine can radiate GIGAwatts of heat for 53173.71745855sf using 60-degree tapering fore and aft) or consist of things like liquid droplet radiators.

[Anthony]

Originally Posted by GURPS Fox

Well, if Children of a Dead Earth taught me anything, it is that the cooler the system, the less efficient the radiators are.

Yes and no. The problem with a high temperature radiator is that you need to get your heat to the radiator, which requires power (and additional heat production) if the radiator is hotter than the system it's intended to cool. If you want a 2,000K radiator... all your heat-producing components need to be able to run at 2,000K. There are a few components that really can operate at that kind of temperature, but most machinery really cannot (a 2,000K sodium cooled reactor will probably explode violently if you look at it funny).

[Verjigorm]

Originally Posted by Anthony

Yes and no. The problem with a high temperature radiator is that you need to get your heat to the radiator, which requires power (and additional heat production) if the radiator is hotter than the system it's intended to cool. If you want a 2,000K radiator... all your heat-producing components need to be able to run at 2,000K. There are a few components that really can operate at that kind of temperature, but most machinery really cannot (a 2,000K sodium cooled reactor will probably explode violently if you look at it funny).

Couldn't we use different radiators for different systems though? Like, if our fusion engine produces a 1500k worth of heat to dispose of, we run it on some dedicated radiators, while our habs and their associated stuff use lower temperature radiators. Which probably gives us a really big set of radiators for our habs, and a much smaller one for our drive system.

[Anthony]

Originally Posted by Verjigorm

Couldn't we use different radiators for different systems though?

Yes, but you start running out of viable materials to make your reactors out of, and also you either need a heat differential or heat pump to get your heat to the radiator fast. You also need to be running the reactor core at a significantly higher temperature than the radiator -- how much higher depends on how much power output and efficiency you want (there's a hard limit on efficiency of (1 - ratio of temperatures), but there's some tradeoff in power density vs efficiency; a reasonable ratio is about 2:1), and you really start running out of reasonable materials somewhere in the 1,000-1,500K range, outside of some very specialized applications that generally don't produce electric power. If you're feeling really generous you might see 1,000K radiators that need only about half a square foot per kilowatt of power output.

[Verjigorm]

Originally Posted by Anthony

Yes, but you start running out of viable materials to make your reactors out of, and also you either need a heat differential or heat pump to get your heat to the radiator fast. You also need to be running the reactor core at a significantly higher temperature than the radiator -- how much higher depends on how much power output and efficiency you want (there's a hard limit on efficiency of (1 - ratio of temperatures), but there's some tradeoff in power density vs efficiency; a reasonable ratio is about 2:1), and you really start running out of reasonable materials somewhere in the 1,000-1,500K range, outside of some very specialized applications that generally don't produce electric power. If you're feeling really generous you might see 1,000K radiators that need only about half a square foot per kilowatt of power output.

Yeah, so I generally figure around 1200 K as the operating temp for the engines. Hotter would help for a lot of things, but as you point out: material science starts to limit our options.

I don't know how many kilowatts the habitats of a manned vessel would have, but considering they need to maintain around 300K, then it's probably around 4 square foot per kilowatt?