#Thermal heatsink/damper

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near trellis
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Copied from #delta-v discussion here #delta-v message.

While taking my shower prior to going to bed yesterday, I had an idea for a new auxiliary power unit. I feel like it must have been suggested before but I can't find it. We've got auxiliary capacitor to run power hungry thrusters, mining tools, MPUs, etc. for short durations, but we don't have any sort of "thermal sink" as an addon. Basically something to increase the thermal mass of your reactor. This should allow smaller reactors to run certain thruster in short burst without nuking the thrust and efficiency, allow higher target temps with less overshoot for more electrical/thermal/thrust/exhaust velocity, allow certain thrusters on certain ships (ex: NPMP on triplet and titan), etc.. Use cases would be limited since the added weight will only be worth it on certain combo of reactors/ships/thrusters. It could make for some interesting builds.

I'm not sure how it could be implemented, it would depend on the technology used in the ship's reactor. Some mentioned that the reactor used water plasma which might require a lot of power to contain, others said super critical water which might require a heavy and strong container. Ideally, the module should be reasonably light and have enough thermal capacity to be an appropriate replacement over using a much larger reactor.

sonic pasture
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Trying to give a short synopsis of what came up in the discussion:

  • A simple and lightweight method would be to have an additional pressure vessel that stores superheated propellant for higher thermal mass
  • Phase change materials work as a thermal battery. Rhenium came up as a possible candidate, because its melting point is close to 3500K
  • An oversaturated salt solution like in hand warmers might work. No idea how that could be combined with supercritical water though
  • Molten salt could also work as storage medium
stark plover
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yes giant hand warmers

sonic pasture
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Ok, sad news: 5 tons of Re would only store 887 MJ of latent heat when melting :(

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And there seems to be something wrong with the heat of fusion listend in the English WP. It says 60.43 kJ/mol, while the German WP and 3 other websites i've found by quick searching say 33 kJ/mol.

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I've used 33 kJ/mol for the calculation

near trellis
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The artbook seems to mention that the ship reactors in the dV universe are fuelled by liquid thorium. Their properties in dV don't really match what I can find on wikipedia though, maybe it's some high tech future version. My initial though would have been to store the heated water in a pressure vessel but that might not be workable.

sonic pasture
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I don't see why that wouldn't work, the pressure won't be higher than in the reactor itself and there are already pipes in the ship that transport the hot propellant. Simplified, it's just a very big pipe.

near trellis
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If the big pressure vessel is workable, the module itself would only be the weight of the container, the sink itself would be the preheated propellant.

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I could see that as either an extension of the thermal mass of the reactor, causing it to rise and drop slower, or as a "capacitor", where hot remass is drawn first and recharged when not under high load. This would provide a fixed-ish temperature output, target capacity could even be set in the tuning menu (50% for dampening dips and peaks, 80% or more for long burst at the cost of wasting more power trying to maintain a large sink).

celest gazelle
sonic pasture
blissful bobcat
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okay, lets crunch numbers

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at 3500K it seems that storing the energy in heat alone will overwhelm anything you can get from phase change

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Highest specific heat I found is Hydrogen, at 14.30 [J/gK]

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at 3500K, 1 ton of hydrogen would store 50GJ

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actually, this looks quite plausible

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with water, you could store 6.5GJ

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The usable energy is way less through, as you won't be cooling it down to 0K

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but you can manipulate the temperature by increasing the pressure in the vessel 🤔

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yeah, this seems plausible

you could have 10,000kg device holding water vapor at 3500K and a piston to control the pressure. You can hold the vapor at lower temperature during storage, and increase the pressure to get that usable 3500K when you need it.

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This could store about 50GJ

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thats 50GW you can discharge in a second, or 5GW over 10 seconds

near trellis
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The weight/thermal capacity of the device can make or break it's usefulness. The nakamura reactors are very power dense and if the sink is too expensive, you may as well fork out for a bigger reactor and just let it cool down when thrusting. 50 GJ isn't too bad though, that's a bit over 2s of NMPD without having to tap into the reactor's thermal mass (not taking into account the reactor going into high gear to prolong the burn time). Assuming the device weight is only the mechanical piston and container (the water is simply taken from the propellant you're already hauling), I wonder if 5000kg for 50GJ of heat capacity would be doable. There could also be multiple model with different weight and heat capacity.

Another thing beside purely holding heat for burst thrusting, is that the sink can help manage reactor temp. A higher temps helps with heat and electrical generation, and I think it also improves thrust and engine efficiency. Being able to run your reactor safely at say 4300k without wide spikes in temps (assuming the reactor and sink can coordinate with each other to avoid overshooting) can help offset the increased weight. It could have some value in the current meta for bigger ships with an already high dry mass.

The only very thermally intensive engine with have right now are the NMPD and NPMP. Of the two, I'd only consider the first one to be good. The sink might be worth considering if we get more high heat engine in the future.

near trellis
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Been doing some googling (quick disclaimer, I don't understand everything I read), it seems like compressing water at above 22MPa and above 647k turns it into a supercritical fluid, which we could keep on heating up to operating temperature. Small change of pressure and temperature have large effects on the density (and I assume also specific heat, maybe some more energy again with a phase change?). This could potentially mean that a compact high pressure vessel with extremely hot water in it, would be quite compact and energy dense.

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The container would be the sci-fi fudge here mostly. Considering all the near future stuff packed into our ships, it wouldn't be too much of a stretch to have some sort of container for it, maybe?

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Not sure if the reduction in density is a good thing. It gets really messy. Pressure will go up for a given volume when increasing the temperature. The increase in specific heat does sound like a nice thing though, 600°C and it's already over 8 times water and standard atmospheric pressure.

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This should be fixed pressure.

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Not sure how to read those and if it's workable in the dV lore.

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I feel like the two graph don't agree.

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Maybe that's what they meant with "small changes in pressure have huge effects on density and specific heat".

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And I gave myself a headache...

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Lots of thumbs up for the people at Kodera doing the lore building and research. I'd have tried fudging long ago. I'll let the others chime in.

sonic pasture
sonic pasture
near trellis
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If that's already what's used by the reactor then it doesn't seem too unimaginable for me to have a second container dedicated to storing and releasing the heated propellant.

blissful bobcat
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#science-behind-delta-v message

Okay, seems I found a promising material to use as heatsink/TES: Beryllium Oxide (BeO)

The relevant number is Latent Heat of Vaporization at ~6500 kJ/kg with boiling point of 4173K. This gives ~6.5GJ storage per metric tone with a boiling temperature that might actually be viable for auxiliary equipment for our reactors.

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Assume we have 20,000kg device. With reactor producing say 10GW for simplicity, once it hits 4200K it can absorb 13 seconds of full reactor output - more than enough to allow disengagement of fuel rods even on damaged rector. It can then give back that heat still at 4200K.

We could probably control the phase change temeprature by mixing in some material.