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Thread: Armoring a radiator.

  1. #31
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    Quote Originally Posted by Noclevername View Post
    Right. I suggested as much in the OP.

    But retraction is only viable until it isn't. If your heat sinks are at capacity, you have no options but extend the radiators or vent coolant, and coolant runs out real fast.
    Shutdown to a bare minimum. Wait for the danger to pass. Then redeploy.

    Convert the extra heat into something useful like electricity using a thermoelectric generator. You could then store the electricity in spare batteries and discharge them when all is safe. Or use the extra electricity as a weapon and zap the incoming missile.
    Last edited by DaCaptain; 2018-Sep-26 at 11:48 AM.
    I know that I know nothing, so I question everything. - Socrates/Descartes

  2. #32
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    Quote Originally Posted by DaCaptain View Post
    Shutdown to a bare minimum. Wait for the danger to pass. Then redeploy.
    In combat?

    Convert the extra heat into something useful like electricity using a thermoelectric generator.
    Yes, but thermodynamically, that's ultimately a dead end. Generates more waste heat than it consumes, I'm told.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  3. #33
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    Quote Originally Posted by Noclevername View Post
    In combat?
    You can't always be throwing punches.
    I know that I know nothing, so I question everything. - Socrates/Descartes

  4. #34
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    Quote Originally Posted by Noclevername View Post
    Before I click on it, what is it?
    Directed Energy Interception of Satellites
    Harrison Shea, Will Hettel, Phillip Lubin (just released, 2018)
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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  5. #35
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    Quote Originally Posted by DaCaptain View Post
    You can't always be throwing punches.
    You can't always be shooting in a firefight, should you unload your gun in the middle of one?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  6. #36
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    Quote Originally Posted by Noclevername View Post
    Lasers would have to be thermodynamically almost 100% efficient to act as heat dumping, wouldn't they?
    Not in this instance. It uses a temperature gradient to generate photons which can then be radiated off ship. So thermodynamically it is allowed I think.

    The problem is, it needs a temperature gradient. In other words you will need a radiator of some sort

  7. #37
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    Quote Originally Posted by kzb View Post
    Not in this instance. It uses a temperature gradient to generate photons which can then be radiated off ship. So thermodynamically it is allowed I think.

    The problem is, it needs a temperature gradient. In other words you will need a radiator of some sort
    I certainly don't have the knowledge or experience to offer a valid opinion, but judging by some discussions I've read between people who do it doesn't sound like you need a radiator in order to create a temperature gradient. Or rather the laser is the radiator. The emission of photons from the laser supposedly cools the device. The temperature gradient you need to run the radiator, as I understand it, could be achieved by collecting and storing the waste heat internally, inside the ship. It's pretty straightforward to create two heat sources of different temperature.

    Waste heat is stored in an on-board heat sink of some sort. The gradient between that heat sink and another at ambient ship temperature powers the quantum cascade refrigeration laser assembly, which is cooled by the process of lasing. Could a laser refrigeration system like this actually work? I'd say unknown but possible. Some experts say the thermodynamics look like it could work. Some experts say the thermodynamics don't work out. Its firmly in the realm of speculative right now.

  8. #38
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    Quote Originally Posted by Noclevername View Post
    Well, the ship is a 70x40x30 meter arrowhead/wedge with room for crew & some cargo, assume power output equivalent to 3x the best closed cycle nuclear achievable in that volume with today's tech.

    (Tech level is efficient small fusion reactor, so the 3x factor is kind of arbitrary. No hard numbers available. Hard rad/neutron emissions are handwaved away.)
    Sublimating water into vapor in the vacuum of space gives you 2.8 MW of cooling for each kg/s of water expended.

    Your craft has a volume of ~~0.333*70*40*30=28000 m^3. An American 688 class submarine has a volume of ~~10000 m^3 and 165 MWt reactor. Assuming 3x the power density, we get 165*28000*3/10000=1400 MW of heat, requiring 500 kg/s of emergency open-circuit cooling water to maintain at full power if the normal closed-circuit radiator is shut down or disabled, so each 1 m^3 of water would provide 2 seconds of backup. 10 minutes of emergency cooling would take about 1% of the ship's volume in water storage.

    Quote Originally Posted by Darrell View Post
    I certainly don't have the knowledge or experience to offer a valid opinion, but judging by some discussions I've read between people who do it doesn't sound like you need a radiator in order to create a temperature gradient. Or rather the laser is the radiator. The emission of photons from the laser supposedly cools the device. The temperature gradient you need to run the radiator, as I understand it, could be achieved by collecting and storing the waste heat internally, inside the ship. It's pretty straightforward to create two heat sources of different temperature.

    Waste heat is stored in an on-board heat sink of some sort. The gradient between that heat sink and another at ambient ship temperature powers the quantum cascade refrigeration laser assembly, which is cooled by the process of lasing. Could a laser refrigeration system like this actually work? I'd say unknown but possible. Some experts say the thermodynamics look like it could work. Some experts say the thermodynamics don't work out. Its firmly in the realm of speculative right now.
    No, that pesky 2nd law of thermodynamics gets in the way. Two heat sources of different temperatures would rapidly equalize (ie, you'd warm up the colder source) if you tried to use them as a heat sink. Ultimately, what your radiator is offloading is entropy, and a laser is not going to do that for you.

  9. #39
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    Quote Originally Posted by VQkr View Post
    Sublimating water into vapor in the vacuum of space gives you 2.8 MW of cooling for each kg/s of water expended.

    Your craft has a volume of ~~0.333*70*40*30=28000 m^3. An American 688 class submarine has a volume of ~~10000 m^3 and 165 MWt reactor. Assuming 3x the power density, we get 165*28000*3/10000=1400 MW of heat, requiring 500 kg/s of emergency open-circuit cooling water to maintain at full power if the normal closed-circuit radiator is shut down or disabled, so each 1 m^3 of water would provide 2 seconds of backup. 10 minutes of emergency cooling would take about 1% of the ship's volume in water storage.
    Thanks!
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  10. #40
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    Quote Originally Posted by Darrell View Post
    I certainly don't have the knowledge or experience to offer a valid opinion, but judging by some discussions I've read between people who do it doesn't sound like you need a radiator in order to create a temperature gradient. Or rather the laser is the radiator. The emission of photons from the laser supposedly cools the device. The temperature gradient you need to run the radiator, as I understand it, could be achieved by collecting and storing the waste heat internally, inside the ship. It's pretty straightforward to create two heat sources of different temperature.

    Waste heat is stored in an on-board heat sink of some sort. The gradient between that heat sink and another at ambient ship temperature powers the quantum cascade refrigeration laser assembly, which is cooled by the process of lasing. Could a laser refrigeration system like this actually work? I'd say unknown but possible. Some experts say the thermodynamics look like it could work. Some experts say the thermodynamics don't work out. Its firmly in the realm of speculative right now.
    You still need a hot end and a cold end to make it work.

    Don't get me wrong I am not discounting it entirely.

    Collecting and storing waste heat at two different temperatures in itself uses energy and therefore creates even more waste heat. On the other hand if you had a higher temperature heat source in the ship, such as a nuclear reactor, it is possible this heat powered laser could carry away some of the reactor waste heat and reduce the heat load on the radiators.

    I think the best you could hope for is the radiators could be reduced in size. In a combat situation this would be worthwhile because it reduces the target area.

  11. #41
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    So using an online calc and assuming light water as the coolant, I get a size of... way too big! Square miles. Even cutting power level to regular nuclear-sub output isn't helping enough.

    Reducing the area needed would require a hotter coolant. Any suggestions for the coolant??
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  12. #42
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    Quote Originally Posted by Noclevername View Post
    So using an online calc and assuming light water as the coolant, I get a size of... way too big! Square miles. Even cutting power level to regular nuclear-sub output isn't helping enough.

    Reducing the area needed would require a hotter coolant. Any suggestions for the coolant??
    Hydrogen gas. Your panels are going to need to be hot to radiate this kind of power from panels similar in area to the surface of the craft (as opposed to massive fold-out arrays like on the ISS). Energy dissipated from a radiator increases with the fourth power of absolute temperature, so increasing radiator temperature is the most efficient way to increase the thermal power (though a warmer cold sink decreases the amount of useful work you can get from a heat source).

    BTW, you mentioned a NTR (nuclear thermal rocket) in your OP. All this radiator talk presumes a closed system (ie, a reactor driving a turbine to produce electrical power for weapons or ion engines). If most of your power is going toward thermal propulsion such as an NTR, your waste heat leaves with the exhaust - you don't need a large radiator for that.
    Last edited by VQkr; 2018-Sep-26 at 06:22 PM. Reason: Note about NTRs and radiators

  13. #43
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    I've got an idea for a dramatic scene involving repairing a coolant leak in a running battle. A hot hydrogen leak inside the ship will lead to big bonfire and probable total loss, I'd like to avoid quite that much drama. Would helium work instead?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  14. #44
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    Quote Originally Posted by Noclevername View Post
    I've got an idea for a dramatic scene involving repairing a coolant leak in a running battle. A hot hydrogen leak inside the ship will lead to big bonfire and probable total loss, I'd like to avoid quite that much drama. Would helium work instead?
    NaK. This was used in fast breeder reactors as coolant and has already been used in space.

    A leak onboard would give a spectacular fire but without blowing up the whole thing.

    https://en.wikipedia.org/wiki/Sodium-potassium_alloy

  15. #45
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    Quote Originally Posted by VQkr View Post
    <EDIT>



    No, that pesky 2nd law of thermodynamics gets in the way. Two heat sources of different temperatures would rapidly equalize (ie, you'd warm up the colder source) if you tried to use them as a heat sink. Ultimately, what your radiator is offloading is entropy, and a laser is not going to do that for you.
    I was thinking last night, from a thermodynamics point of view is there any difference between this laser idea and a thermocouple?

    A thermocouple could generate electricity from a temperature difference. That electricity then powers a resistor which radiates energy to space at a high temperature.

    The high temperature means it can be much smaller than a radiator at room temperature.

    But there is still this problem of needing a hot end and a cold end to make it work.

  16. #46
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    If I understood the article correctly the configuration of quantum cascade laser that the researchers where speculating about cools down by the process of lasing. I've no idea of the efficiency or the basic principles of how it is supposed to work. But, again if I understand it correctly, this laser is powered directly by heat and as a result of producing photons it is cooled. The researches were specifically speculating about using this type of laser for cooling, initial applications considered were cooling of electronics.

  17. #47
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    Quote Originally Posted by Noclevername View Post
    So using an online calc and assuming light water as the coolant, I get a size of... way too big! Square miles. Even cutting power level to regular nuclear-sub output isn't helping enough.

    Reducing the area needed would require a hotter coolant. Any suggestions for the coolant??
    Liquid metals -- lead-bismuth eutectic, sodium, sodium-potassium eutectic, and mercury have all been used as high-temperature heat transfer fluids -- molten salts, such as thorium fluoride, or gases.

    All have been used in ground and some in space-based systems.

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