[spaceships] economies of scale, particularly for power points

muduri · 06-03-2019, 08:19 AM

Hi all, we've been having a ton of fun over here playing demolition derby with Spaceships tactical combat. Definitely did some tweaking handwaves to make TL9 tech cheap but just barely viable against TL10 and TL10^ (and some crazy discounts to give each player access to like one TL11^ component) but it's worked out pretty well.

One of the things that makes Spaceships accessible is the scaling, the 6-steps-to-10 system for most attributes except say cost. However, we've been chewing on the idea of further incentivizing different builds at different scales to maximize variety for the derbies.

So if I'm reading it right, guns and launchers increase in damage more slowly than other weapons but make up for it with more ammunition. Relatedly, maybe we'll charge a kind of scaling "miniaturization tax" for say particle weapons, reactionless drives and reactors below a certain size, because science.

So apart from the obvious "is this a crazy idea?" question, which I'm also curious to hear responses on, this is all leading around to: just out of curiosity, how do fission reactors do these days at say TLs 7 and 8? How easy / cheap has it been to scale a reactor to say the Nimitz - and for that matter how many component slots is it using? And how steeply do we approach a lower logistics / cost limit on reactor sizes these days for like a cruiser or destroyer?

Of course by TL10 we should have "Mr. Fusion" blenders, but it seems useful to have some current reference points to play with. Thanks for thoughts!

Rupert · 06-03-2019, 06:22 PM

I didn't find the mass of Nimtz' A4W reactors on a quick search, but the D2Gs that were used for nuclear 'destroyers' (later reclassified as cruisers) mass 1,500 tons and produce about 150MW thermal output, powering propellers with about 25MW, plus an unknown amount of electricity. These numbers seem suspect though, as they give a very low efficiency compared to the A4W's, which seem to be about 50% efficient.

The ships the D2Gs were placed in each had a pair, and the ships displaced about 13,000 tons. Thus each reactor used about 2.3 slots, the pair used ~4.6 slots, and counting propellers, generators, and miscellaneous machinery you can probably assume their powerplants used 5 slots.

Note that you can get comparable output from a couple of gas turbines weighing 5 tons each, though of course all the mounting propeller shafting and gearing and so on increase the mass enormously. And then there's the fuel comsumption - 0.2-0.25 tons of fuel per hour per MW, not counting the air/oxygen requirements.

AlexanderHowl · 06-04-2019, 09:51 AM

No nuclear reactor reaches 50% efficiency, 30% efficiency is normal for large reactors, with smaller reacts having lower efficiencies.

muduri · 06-05-2019, 02:22 PM

Quote:

Originally Posted by Rupert

I didn't find the mass of Nimtz' A4W reactors on a quick search, but the D2Gs that were used for nuclear 'destroyers' (later reclassified as cruisers) mass 1,500 tons and produce about 150MW thermal output, powering propellers with about 25MW, plus an unknown amount of electricity. These numbers seem suspect though, as they give a very low efficiency compared to the A4W's, which seem to be about 50% efficient.

The ships the D2Gs were placed in each had a pair, and the ships displaced about 13,000 tons. Thus each reactor used about 2.3 slots, the pair used ~4.6 slots, and counting propellers, generators, and miscellaneous machinery you can probably assume their powerplants used 5 slots.

Note that you can get comparable output from a couple of gas turbines weighing 5 tons each, though of course all the mounting propeller shafting and gearing and so on increase the mass enormously. And then there's the fuel comsumption - 0.2-0.25 tons of fuel per hour per MW, not counting the air/oxygen requirements.

AHA, very useful, many thanks. Thanks also Alexander for note on efficiency. So sounds like at our TL8 we can get a single reactor down to ~2 slots of an SM10 vessel, presumably one slot of SM11.

Very curious how easy it is to miniaturize down to that size or whether the plant is much more expensive per MW than a larger unit... but that's presumably something that would take some serious engineering design expertise, may not show up on the interwebs.

Varyon · 06-06-2019, 06:45 AM

Quote:

Originally Posted by muduri

Very curious how easy it is to miniaturize down to that size or whether the plant is much more expensive per MW than a larger unit... but that's presumably something that would take some serious engineering design expertise, may not show up on the interwebs.

While it's now probably a bit outdated, the old GURPS Vehicles for 3e had most power plants above 5 kW have a set weight per kW, plus a constant (which for those that allowed <5 kW plants, was the weight of a 5 kW plant). This worked out to mean the larger your reactor, the less it weighed per kW, and price was linear with weight.

Of course, the Vehicles fission reactors are different from those in Spaceships, as they need refueled every 2 years rather than every ~100 or so. I did some very amateur calculations for my personal Vehicles to Spaceships conversion. A Spaceships-style fission reactor would be 5 kW per ton of vehicle mass at TL 8, 10 kW per ton of vehicle mass at TL 9, and 30 kW per ton of vehicle mass at TL 10. Minimum SM is (16-TL), although you can technically get 1 SM smaller but get 1/2 output. A PP is roughly 5 kW per ton of vehicle mass, and Spaceships assumes higher-TL reactors are derated to increase how long they last.

AlexanderHowl · 06-06-2019, 11:46 AM

Fission needs a critical mass of fuel, anywhere from 10 kg to 100 kg, depending on the design, so that is a hard limit. Radiation shielding is also a hard limit, though it is a bit less linear. In general, every 100 MW of fission energy produces 85 MW of kinetic energy and 15 MW of radiation (neutron and gamma ray). Of the kinetic energy, it turns to heat, which you use to boil water. Of the radiation, well, that is what shielding is for, and it turns to heat after absorption, which is used to boil water. With a proper design, you get 35 MW of electricity and 65 MW of waste heat for every 100 MW of fission energy.

Now, shielding roughly works by absorbing half the energy per X thickiness, X depending on the energy of the radiation, the type of radiation, and the type of shielding material. In general, you will want a hydrogen rich material (like water) to absorb the neutron radiation and dense materials to absorb the gamma rays. Since 15 MW of radiation translates roughly to 1.5 million rads of radiation per second at 1 meter distance, you really want a lot of shielding. Sufficient shielding to reduce to the legal limit of 5 rads per year of exposure would be around 45 layers of shielding, which could be anything from air to lead, depending on your design.

06-03-2019, 08:19 AM	#1
muduri Join Date: Oct 2009 Location: Harlem, New York	[spaceships] economies of scale, particularly for power points Hi all, we've been having a ton of fun over here playing demolition derby with Spaceships tactical combat. Definitely did some tweaking handwaves to make TL9 tech cheap but just barely viable against TL10 and TL10^ (and some crazy discounts to give each player access to like one TL11^ component) but it's worked out pretty well. One of the things that makes Spaceships accessible is the scaling, the 6-steps-to-10 system for most attributes except say cost. However, we've been chewing on the idea of further incentivizing different builds at different scales to maximize variety for the derbies. So if I'm reading it right, guns and launchers increase in damage more slowly than other weapons but make up for it with more ammunition. Relatedly, maybe we'll charge a kind of scaling "miniaturization tax" for say particle weapons, reactionless drives and reactors below a certain size, because science. So apart from the obvious "is this a crazy idea?" question, which I'm also curious to hear responses on, this is all leading around to: just out of curiosity, how do fission reactors do these days at say TLs 7 and 8? How easy / cheap has it been to scale a reactor to say the Nimitz - and for that matter how many component slots is it using? And how steeply do we approach a lower logistics / cost limit on reactor sizes these days for like a cruiser or destroyer? Of course by TL10 we should have "Mr. Fusion" blenders, but it seems useful to have some current reference points to play with. Thanks for thoughts!

06-03-2019, 06:22 PM	#2
Rupert Join Date: Aug 2004 Location: Wellington, NZ	Re: [spaceships] economies of scale, particularly for power points I didn't find the mass of Nimtz' A4W reactors on a quick search, but the D2Gs that were used for nuclear 'destroyers' (later reclassified as cruisers) mass 1,500 tons and produce about 150MW thermal output, powering propellers with about 25MW, plus an unknown amount of electricity. These numbers seem suspect though, as they give a very low efficiency compared to the A4W's, which seem to be about 50% efficient. The ships the D2Gs were placed in each had a pair, and the ships displaced about 13,000 tons. Thus each reactor used about 2.3 slots, the pair used ~4.6 slots, and counting propellers, generators, and miscellaneous machinery you can probably assume their powerplants used 5 slots. Note that you can get comparable output from a couple of gas turbines weighing 5 tons each, though of course all the mounting propeller shafting and gearing and so on increase the mass enormously. And then there's the fuel comsumption - 0.2-0.25 tons of fuel per hour per MW, not counting the air/oxygen requirements. __________________ Rupert Boleyn "A pessimist is an optimist with a sense of history."

06-04-2019, 09:51 AM	#3
AlexanderHowl Join Date: Feb 2016	Re: [spaceships] economies of scale, particularly for power points No nuclear reactor reaches 50% efficiency, 30% efficiency is normal for large reactors, with smaller reacts having lower efficiencies.

06-06-2019, 11:46 AM	#6
AlexanderHowl Join Date: Feb 2016	Re: [spaceships] economies of scale, particularly for power points Fission needs a critical mass of fuel, anywhere from 10 kg to 100 kg, depending on the design, so that is a hard limit. Radiation shielding is also a hard limit, though it is a bit less linear. In general, every 100 MW of fission energy produces 85 MW of kinetic energy and 15 MW of radiation (neutron and gamma ray). Of the kinetic energy, it turns to heat, which you use to boil water. Of the radiation, well, that is what shielding is for, and it turns to heat after absorption, which is used to boil water. With a proper design, you get 35 MW of electricity and 65 MW of waste heat for every 100 MW of fission energy. Now, shielding roughly works by absorbing half the energy per X thickiness, X depending on the energy of the radiation, the type of radiation, and the type of shielding material. In general, you will want a hydrogen rich material (like water) to absorb the neutron radiation and dense materials to absorb the gamma rays. Since 15 MW of radiation translates roughly to 1.5 million rads of radiation per second at 1 meter distance, you really want a lot of shielding. Sufficient shielding to reduce to the legal limit of 5 rads per year of exposure would be around 45 layers of shielding, which could be anything from air to lead, depending on your design.