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Old 9th June 2018, 03:19 AM   #41
Ziggurat
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Originally Posted by The Great Zaganza View Post
The point/fact is that dissipating heat through radiation is much less efficient than through Brownian motion.
That depends on the temperature. Brownian motion cooling scales linearly with temperature differential. Radiative cooling scales as T4.
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Old 9th June 2018, 04:11 AM   #42
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Many science fiction authors have wrestled with the idea of how to get rid of excess heat, one way is to "drop" a heat sink off the craft when it reaches a certain temperature (presumably something dense like a metal?) once it reaches almost molten you drop it.

If you were on a regular route that had other spacecraft following the same route you could have space ship A dropping its heat sink, time passes and the heatsink cools, space ship B is scheduled to reach the now cool heat sink, space craft B drops its now almost molten heat sink and picks up space ship A's now cooled heat sink.
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Old 9th June 2018, 04:18 AM   #43
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Originally Posted by Darat View Post
Many science fiction authors have wrestled with the idea of how to get rid of excess heat, one way is to "drop" a heat sink off the craft when it reaches a certain temperature (presumably something dense like a metal?) once it reaches almost molten you drop it.

If you were on a regular route that had other spacecraft following the same route you could have space ship A dropping its heat sink, time passes and the heatsink cools, space ship B is scheduled to reach the now cool heat sink, space craft B drops its now almost molten heat sink and picks up space ship A's now cooled heat sink.
Cute idea, but it wouldn't actually make sense. Radiative cooling works well enough, the heat capacity per unit mass of matter isn't high enough, there wouldn't be any actual advantage.
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Old 9th June 2018, 04:30 AM   #44
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A lot of this discussion depends on how much heat is being generated against what needs to be dumped, what are the mass requirements/limitations and so on.
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Old 9th June 2018, 04:33 AM   #45
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Originally Posted by Ziggurat View Post
Cute idea, but it wouldn't actually make sense. Radiative cooling works well enough, the heat capacity per unit mass of matter isn't high enough, there wouldn't be any actual advantage.
That's easy to fix in science-fiction, they dropped the nano-superconducting-infinite-magneto-fusion heat sink...
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Old 9th June 2018, 05:26 AM   #46
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Originally Posted by Roboramma View Post
Thinking more about what I was trying to achieve with this idea, I guess it's about trying to get the mass/unit area of the radiator down. If we have some liquid that can be sprayed over a large distance to a collector, the area of cooling surface is whatever the distance to the collector is multiplied by the size of the collector. Theoretically that distance could be quite large. The mass is based on that distance and the density of the coolant that fills up that space, plus the mass of the collector.
To be pedantic, you’re describing a volume, not an area. But my problem is that I imagine much of the hot coolant would disperse before reaching the collector, and what reached it would be frozen droplets that would simply bounce off anyway.

Originally Posted by Roboramma View Post
This is just a guess but I'm thinking that the coolant could be less massive because we don't need to include the pipes to pump it through. It's just travelling through empty space. The mass of the collector is a problem, but conceivably we can get that relatively low (handwave, handwave).

All good point. Probably beats my handwaving.

To add to what you are saying, you also need some sort of system to get the coolant back to the original ship. All this complication adds mass and probably makes it less efficient than just a simple radiator. But maybe it's efficiency would scale with size?
It would, but ignoring the likely loss of coolant in your giant evaporator, you’re having to fly a lot more mass just to keep your cooling system with you, and added a stationkeeping headache.

Originally Posted by Roboramma
You are probably right. To sum up what I said above, I think the idea is basically to lower the mass of the radiator by removing the pipes. Probably doesn't work though.
It’s still a clever and innovative idea.
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Old 9th June 2018, 08:00 AM   #47
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Originally Posted by Roboramma View Post
Thinking more about what I was trying to achieve with this idea, I guess it's about trying to get the mass/unit area of the radiator down. If we have some liquid that can be sprayed over a large distance to a collector, the area of cooling surface is whatever the distance to the collector is multiplied by the size of the collector. Theoretically that distance could be quite large. The mass is based on that distance and the density of the coolant that fills up that space, plus the mass of the collector.

This is just a guess but I'm thinking that the coolant could be less massive because we don't need to include the pipes to pump it through. It's just travelling through empty space. The mass of the collector is a problem, but conceivably we can get that relatively low (handwave, handwave).

All good point. Probably beats my handwaving.

To add to what you are saying, you also need some sort of system to get the coolant back to the original ship. All this complication adds mass and probably makes it less efficient than just a simple radiator. But maybe it's efficiency would scale with size?



You are probably right. To sum up what I said above, I think the idea is basically to lower the mass of the radiator by removing the pipes. Probably doesn't work though.
You should check out that Project Rho link I posted. It's probably one of the best lay discussions of the subject. Among many other considerations, it covers exactly the kind of radiation scheme you describe here.
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Old 10th June 2018, 04:20 AM   #48
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Originally Posted by The Great Zaganza View Post
The point/fact is that dissipating heat through radiation is much less efficient than through Brownian motion.
So we could bounce our spacecraft off lots of other spacecraft and make the heat their problem.
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Old 10th June 2018, 11:36 AM   #49
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Originally Posted by Lothian View Post
Ok this is the last help I will give you. After this you are on your own. Here is a modern spacecraft with its big propeller to get it into space.
http://www.internationalskeptics.com...ac0b08f27d.jpg
This is the clever bit. Turn the propeller around and you have a cooling fan.
No need to thank me.
Don't be ridiculous. You've got the blades right in front of your rocket engines. You need to put the propeller/fan on the other end:

http://www.thespacereview.com/archive/8a.jpg

Seriously, liquid droplet radiators are a real approach which has been studied. A low vapor pressure coolant fluid is ejected through one set of nozzles and collected after cooling. The main advantages are lots of radiating area for little mass (no radiator tubing/fins) and immunity to punctures, plus simpler deployment (a sprayer arm and a collector arm, instead of a folded up radiator). A disadvantage is there's likely to be some loss. Variations of the idea include electrostatically charging the droplets to make them easier to collect with minimal loss.
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Old 11th June 2018, 07:26 AM   #50
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The Rotary Rocket actually flew. It did several rocket ascents to altitude, and used to rotor system to hover, translate, and land. A neat idea for a single stage to orbit (SSTO) System, but they went out of business before getting a flight vehicle into space.
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Old 11th June 2018, 08:21 AM   #51
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Originally Posted by sts60 View Post
Yep, “radiators” in automobiles are nothing of the sort.
True, but if you told a mechanic that your convector is leaking he would have no idea what you're talking about.
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Old 11th June 2018, 08:58 AM   #52
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Old 11th June 2018, 03:16 PM   #53
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Originally Posted by sts60 View Post
The Rotary Rocket actually flew. It did several rocket ascents to altitude, and used to rotor system to hover, translate, and land. A neat idea for a single stage to orbit (SSTO) System, but they went out of business before getting a flight vehicle into space.
The test vehicle that was built didn't have rocket engines, all flights were entirely under rotor power. It was also manually piloted, and almost impossible to control...they only managed sustained flight after adding automatic throttle control.

Notably, after they failed to get investment for developing their own oddball spinning rocket engine, they planned to use a derivative of the Fastrac. SpaceX's Merlin is a rather more successful derivative of the same engine, though little more than the use of a pintle injector remains of the original.
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Old 11th June 2018, 04:29 PM   #54
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Dadgumit, that’s right. I think I was swapping in memories of DC-X flights. The Rotary Rocket did look cool as it hovered and translated, and it was a neat idea, but yeah, it never actually flew as a rocket. Thanks for the correction.
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Old 13th June 2018, 05:47 AM   #55
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Sorry if I'm late to the party, but how about just make the spacecraft into a flying heat sink? Minimize the non-engine / radiation-producing part of the ship, clump it all together, wrap it up in copper/aluminium/gold, and there you go. A flying burrito.

If this has already been mentioned, ignore the post and carry on.
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Old 13th June 2018, 10:27 AM   #56
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The problem will become far more relevant if and when spaceships become nuclear powered.
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Old 13th June 2018, 12:45 PM   #57
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Originally Posted by 3point14 View Post
The problem will become became far more relevant if and back when spaceships become became nuclear powered.
FTFY

https://www.space.com/13702-nuclear-...fographic.html
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Old 13th June 2018, 01:29 PM   #58
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Originally Posted by Darat View Post
Many science fiction authors have wrestled with the idea of how to get rid of excess heat, one way is to "drop" a heat sink off the craft when it reaches a certain temperature (presumably something dense like a metal?) once it reaches almost molten you drop it.

If you were on a regular route that had other spacecraft following the same route you could have space ship A dropping its heat sink, time passes and the heatsink cools, space ship B is scheduled to reach the now cool heat sink, space craft B drops its now almost molten heat sink and picks up space ship A's now cooled heat sink.
Originally Posted by Ziggurat View Post
Cute idea, but it wouldn't actually make sense. Radiative cooling works well enough, the heat capacity per unit mass of matter isn't high enough, there wouldn't be any actual advantage.
I've encountered the idea, but not for cooling. Mostly as a way to hide an IR signature in stealth mode...and in a case like that I could see this being something that might help. After all, you produce heat no matter what, so if you block your heat signature that has to go somewhere.

Ideas for similar problems usually involve selective radiation (heat is shunted in the direction away from what you're hiding from).
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Old 13th June 2018, 01:58 PM   #59
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Originally Posted by 3point14 View Post
The problem will become far more relevant if and when spaceships become nuclear powered.
Originally Posted by GodMark2 View Post
RTGs dispose of their own heat by sticking out from the spacecraft where their fins can radiate the excess heat away. However, MSL (and its twin upcoming Mars 2020 rover) use a cooling loop to use some of that heat to warm the rover.

Another bit not generally known is that even the smaller solar-powered rovers (Sojourner and Spirit and Opportunity) used tiny 1-watt plutonium heaters in various places to keep them alive during those cold Martian nights. Cassini and Huygens together had well over a hundred. They’re tiny and purely passive, the spacecraft heating equivalent of one of those little self-stick battery LED lights you stick in a cupboard.
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