Author Topic: MCT Speculation and Discussion Thread 4  (Read 570079 times)

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1760 on: 02/24/2016 05:03 PM »
TomH,
If you melt 1 kg of ice you will be transforming it from lower potential, a solid, to higher potential, a liquid. To reuse it you will need to re freeze it.  To do so on a spaceship you will need a radiator. Same thing with chemical potential.
I've done a bit of math and I find about 10  kW of lighting, 10 kW of crew heat, another 10 kW of electronics and 20 kW of miscellaneous gains such as pumping, conductor resistance, water heating,  food heating, washing, air scrubbing, etc  So at least 50 kW of thermal load on the radiators, since all this heat comes from combustion of food, or the solar cells. As the hull needs to be insulated for re entry, it will be at low temperture and likely not a good radiator, so we will need a lot of radiators.

Offline RocketmanUS

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Re: MCT Speculation and Discussion Thread 4
« Reply #1761 on: 02/24/2016 05:26 PM »
{snip for length }
We are considing the possible design for removing excess heat from the MCT , that is part of the MCT's design.

I did put in the disclosures in my post above of what you replied to, agreeing to some or all of what you stated.

Thermopile and Stirling_engine, yes I know they both need to be cooled on the other side. However it could possible have an over all reduction the temperature in the cabin.

And I did say help in powering the heat pump, meaning it would not be able to supply all the needed power. The rest of the power would need to come from the MCT's power source ( solar panels ).

Research still would need to be done. However it is possible to be done to reduce the heat in a given area, but do to size and mass it may not be possible on something like the MCT. Radiators would most likely for now be the most mass efficient way to get ride of the excess heat in the cabin.

Offline Jim

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Re: MCT Speculation and Discussion Thread 4
« Reply #1762 on: 02/24/2016 05:53 PM »

And I did say help in powering the heat pump, meaning it would not be able to supply all the needed power. The rest of the power would need to come from the MCT's power source ( solar panels ).


The only way of removing heat from the MCT is by radiators or dumping mass.  There is no way around it.   A "heat pump" by definition moves heat from a cold region to a hot region and it consumes energy in the process which generates more heat.  A heat pump does not use direct heat for power.
« Last Edit: 02/24/2016 06:31 PM by Jim »

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1763 on: 02/24/2016 06:19 PM »
Here is a little spreadsheet about radiators, solar cells and energy use.

Basically, I propose 100 kW of electric peak power at Mars, so almost 200 at Earth; 70 kW of radiators, about 50 kW of peak energy use in the ship habitat.

Anyone have a suggestion about the energy required to recycle CO2 back into O2?

Radiators would be attached to cargo bay door, as for the shuttle, or extend out like the ISS.

Offline RocketmanUS

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Re: MCT Speculation and Discussion Thread 4
« Reply #1764 on: 02/24/2016 06:45 PM »

And I did say help in powering the heat pump, meaning it would not be able to supply all the needed power. The rest of the power would need to come from the MCT's power source ( solar panels ).


The only way of removing heat from the MCT is by radiators or dumping mass.  There is no way around it.   A "heat pump" by definition moves heat from a cold region to a hot region and it consumes energy in the process which generates more heat.  A heat pump does not use direct heat for power.
Jim in an above post I did stay a Stirling engines and not heat heat pump for producing power from the waste heat, the heat pump just moves heat from one point to another. For the tech we have today I agree with you that the heat pump would most likely produce more heat than the Stirling engine would convert to electrical power. That making this not work on a space craft meant to land on a planet. But that does not mean sometime in the future it will not be possible to have such a system be able to work on a craft such as the MCT concept. People are working on the concept, it may or may not ever work.

And some heat pumps do use heat for power. Look up motor home refrigerators and how they use heat from propane to work ( this would not work for MCT ).

So for now I agree the MCT would be using radiators for getting rid of excess heat.

Offline Jim

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Re: MCT Speculation and Discussion Thread 4
« Reply #1765 on: 02/24/2016 07:00 PM »

1.  Jim in an above post I did stay a Stirling engines and not heat heat pump for producing power from the waste heat,

2.   the heat pump just moves heat from one point to another. For the tech we have today I agree with you that the heat pump would most likely produce more heat than the Stirling engine would convert to electrical power. That making this not work on a space craft meant to land on a planet. But that does not mean sometime in the future it will not be possible to have such a system be able to work on a craft such as the MCT concept. People are working on the concept, it may or may not ever work.


1.  The Stirling still needs a radiator to work. 

2. Technology today or tomorrow isn't going to change things. 

There is no way around it, there always will be radiators on spacecraft.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1766 on: 02/24/2016 07:25 PM »
And some heat pumps do use heat for power. Look up motor home refrigerators and how they use heat from propane to work

Yes, look up how they work. Study it until you intuitively understand it. You should eventually realise why you are wrong about being able to replace radiators with "something like a sterling engine"; why that concept doesn't make any sense whatsoever.

Propane- or kerosene-burning fridges exploit the difference between the high-grade heat of the burner and the lower-grade heat-sink of the radiator coil/fins in order to do work. If you removed the radiator, the inside of the fridge would just heat up.

A sterling engine likewise exploits the difference between a heat source and a heat-sink. The heat-sink is normally produced via a radiator coil/fins radiating lower-grade heat into the environment. Something must take the heat away from the system or else the two sides will simply come to equilibrium and the engine will stop working.

You can't have a refrigeration system that doesn't have some kind of heat sink to eliminate the waste heat.

Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1767 on: 02/24/2016 07:52 PM »
Before we get to all the tricky techno solutions (BTW still waiting for my waterless ultrasonic clothes cleaner) could I suggest a simple, clear heavish grade plastic bag you stick your stuff in with a waterproof closure, hook up to water & soap connectors then knead the water & soap in to hand wash.
Now the rinsing and drying stuff looks a bit harder. A waste connector to flush the water then transfer to a warm dry air cabinet to gradually suck the water off the fabric?

There's a big gap between a sci-fi waterless ultrasonic cleaner and "stick it in a bag with some soapy water".
True. Mostly that the bag solution already exists, which I'd forgotten when I originally posted.  Something like it is sold as a gadget for backpackers and lightweight travelers.
Quote
A water efficient combination washer/dryer seems pretty trivial compared to everything else that'll need to work. It doesn't need to be as fast or heavy as anything we use in the home. Hell, if you must, it can even be spun by hand (or foot)
Well there was something in the 2011 ISS budget for this but it never happened.  :(

You're right it can be slower and hence lower power than an Earth unit.

But I'd caution that one recurring lesson of all the ECLSS development efforts is that it was the "little things" that caused the most trouble, such as catalytic beds releasing dust into the environment, or valves needing redesign to ensure they seal properly (to be fair quite a bit of that was due to the dust).
« Last Edit: 02/24/2016 08:10 PM by john smith 19 »
BFS. The worlds first Methane fueled FFORSC engined CFRP structured A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP structured booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C Apply So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline dror

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Re: MCT Speculation and Discussion Thread 4
« Reply #1768 on: 02/24/2016 07:54 PM »

And I did say help in powering the heat pump, meaning it would not be able to supply all the needed power. The rest of the power would need to come from the MCT's power source ( solar panels ).


The only way of removing heat from the MCT is by radiators or dumping mass.  There is no way around it.  A "heat pump" by definition moves heat from a cold region to a hot region and it consumes energy in the process which generates more heat.  A heat pump does not use direct heat for power.

There are methods which convert heat directly to electricity (though with low efficiencies so far). See this:
http://spectrum.ieee.org/nanoclast/semiconductors/materials/optical-rectenna-could-doube-solar-cell-efficiency
If it could be used to convert some IR to DC, wouldn't it would count as "a way around it"?
*I realise that lowering the working temperature may necessitate a bigger radiator, but technically less heat will be lost so less electricity needs to be produced.
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Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1769 on: 02/24/2016 08:09 PM »
You can vacuum dry most things, at low pressure the water will boil away from its own heat. You can put the water back into the eclss. Cleaning a sleeping bag may be a chore though.
Agreed. The challenge is to retain the water for reuse.

Bizarre as it may seem I think the simplest option may be to use some vacuum accumulator tanks. Vent them to vac for use, then use them to collect the vapor for later reuse on a pulse basis.

Sizewise I guess the sleeping bag would be the challenge for any system. Obviously zero g helps a lot in this but since I presume  people will be staying in the ship on Mars and hence touching the surface a lot during sleep it will need to be washed more frequently.

Consider how long you'd be comfortable sleeping in the same bed linen.
BFS. The worlds first Methane fueled FFORSC engined CFRP structured A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP structured booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C Apply So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline hkultala

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Re: MCT Speculation and Discussion Thread 4
« Reply #1770 on: 02/24/2016 08:15 PM »

There are methods which convert heat directly to electricity (though with low efficiencies so far).

No, there are NOT. There are methods than convert heat DIFFERENCE directly into electricity.

In order to have that considerable heat DIFFERENCE, big radiators are needed.

Offline dror

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Re: MCT Speculation and Discussion Thread 4
« Reply #1771 on: 02/24/2016 09:39 PM »

There are methods which convert heat directly to electricity (though with low efficiencies so far).

No, there are NOT. There are methods than convert heat DIFFERENCE directly into electricity.

In order to have that considerable heat DIFFERENCE, big radiators are needed.
There are methods than convert heat DIFFERENCE directly into electricity. These are called heat pumps and  are disscussed above.
Less discussed are direct methods like IR photovoltaic cells which dont use heat difference and don't require radiators themselves.
Anyway, IMO,  when talking about a system which has a raditor, and someone suggests to add a heat pump,  it is correct to say that the heat pump requires a radiator only if the combined system's raditor is bigger then the original ststem's radiator, which is probably the case, but the total system efficiency may still be better if you count the energy production part too.
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Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1772 on: 02/24/2016 10:11 PM »
You can vacuum dry most things, at low pressure the water will boil away from its own heat. You can put the water back into the eclss. Cleaning a sleeping bag may be a chore though.
Agreed. The challenge is to retain the water for reuse.

Bizarre as it may seem I think the simplest option may be to use some vacuum accumulator tanks. Vent them to vac for use, then use them to collect the vapor for later reuse on a pulse basis.

Sizewise I guess the sleeping bag would be the challenge for any system. Obviously zero g helps a lot in this but since I presume  people will be staying in the ship on Mars and hence touching the surface a lot during sleep it will need to be washed more frequently.

Consider how long you'd be comfortable sleeping in the same bed linen.
It would seem we shed 10grams of skin per day, for a total of 3.6 kg per year.  So 100 crew would produce 1 kg per day of skin flakes, and for a 200 day trip 200 kg.

So basically at the end of the trip you have two extra crew members made of dead skin!!!!  Plus at least the same amount of hair, so you really need to clean the ship and wash the linen.  I guess the skin mites will get a lot of it and transform it into CO2 and skin mite shit.  Beaurk.
Most of the skin will wash off in showers though.  So bacteria will get it instead?

And there is all that urea from sweat that condenses in the linen as well.  Definitively need washing machines ;-)

« Last Edit: 02/24/2016 10:12 PM by lamontagne »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1773 on: 02/24/2016 10:15 PM »

There are methods which convert heat directly to electricity (though with low efficiencies so far).

No, there are NOT. There are methods than convert heat DIFFERENCE directly into electricity.

In order to have that considerable heat DIFFERENCE, big radiators are needed.
There are methods than convert heat DIFFERENCE directly into electricity. These are called heat pumps and  are disscussed above.
Less discussed are direct methods like IR photovoltaic cells which dont use heat difference and don't require radiators themselves.

Nope.  As previously mentioned, there aren't.  We're just far enough from the fundamental laws to make things really confusing for people;  Many things that are not technically impossible end up being practically impossible or impossible by virtue of secondary laws formed out of the core axioms.  Sorry if you misunderstood, but this is not the place for thermodynamic engineering 101;  I would start with the Wikipedia entry on Carnot heat engines and work your way up from there.  It may help to consider how IR photovoltaic cells would work at an equilibrium temperature, or consider where the energy is supposed to go after you 'generate' it with magical IR photovoltaic cells that do work (spent inside, it heats up the interior).  Please seek other resources than this thread.
« Last Edit: 02/24/2016 11:02 PM by Burninate »

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1774 on: 02/24/2016 11:10 PM »

There are methods which convert heat directly to electricity (though with low efficiencies so far).

No, there are NOT. There are methods than convert heat DIFFERENCE directly into electricity.

In order to have that considerable heat DIFFERENCE, big radiators are needed.
There are methods than convert heat DIFFERENCE directly into electricity. These are called heat pumps and  are disscussed above.
Less discussed are direct methods like IR photovoltaic cells which dont use heat difference and don't require radiators themselves.

Nope.  As previously mentioned, there aren't.  Sorry if you misunderstood, but this is not the place for thermodynamic engineering 101;  I would start with the Wikipedia entry on Carnot heat engines and work your way up from there.  It may help to consider how IR photovoltaic cells would work at an equilibrium temperature, or consider where the energy is supposed to go after you 'generate' it with magical IR photovoltaic cells that do work (spent inside, it heats up the interior).  Please seek other resources than this thread.

Well , perhaps just a bit of thermo: The Carnot equation is Work=Heat*(1-Tcold/Thot)

So for a ship wall at 20C, or 293K, and exchanging heat with an exterior at 200K, the maximum work that could be done (or rate of energy extraction and transformation) is 1-200/293=32%  so the rest of the energy, 68% would leave the ship at external radiators.  This is for an ideal Carnot cycle.  Real cycles are about 50% of these efficiences or less.  so we might extract 15%.  Of course the radiators themselves would exchange heat with deep space according to the Stephan boltzman law of radiation, and would need to be very large since the temperature difference with space (200K to 180 K) would be very small.  In fact these radiators would be much larger than if we had just radiated away all the heat in the first place.  How much larger?  Well radiateor surface is to the fourth of the temperature difference, so 293 to 180 vs 200 to 180 is 5 times more, so 5^4 is 625 times.
So we would get 15% back, at the cost of radiators 625 times larger.
Not worth it, is it?

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1775 on: 02/24/2016 11:46 PM »

There are methods which convert heat directly to electricity (though with low efficiencies so far).

No, there are NOT. There are methods than convert heat DIFFERENCE directly into electricity.

In order to have that considerable heat DIFFERENCE, big radiators are needed.
There are methods than convert heat DIFFERENCE directly into electricity. These are called heat pumps and  are disscussed above.
Less discussed are direct methods like IR photovoltaic cells which dont use heat difference and don't require radiators themselves.

Nope.  As previously mentioned, there aren't.  Sorry if you misunderstood, but this is not the place for thermodynamic engineering 101;  I would start with the Wikipedia entry on Carnot heat engines and work your way up from there.  It may help to consider how IR photovoltaic cells would work at an equilibrium temperature, or consider where the energy is supposed to go after you 'generate' it with magical IR photovoltaic cells that do work (spent inside, it heats up the interior).  Please seek other resources than this thread.

Well , perhaps just a bit of thermo: The Carnot equation is Work=Heat*(1-Tcold/Thot)

So for a ship wall at 20C, or 293K, and exchanging heat with an exterior at 200K, the maximum work that could be done (or rate of energy extraction and transformation) is 1-200/293=32%  so the rest of the energy, 68% would leave the ship at external radiators.  This is for an ideal Carnot cycle.  Real cycles are about 50% of these efficiences or less.  so we might extract 15%.  Of course the radiators themselves would exchange heat with deep space according to the Stephan boltzman law of radiation, and would need to be very large since the temperature difference with space (200K to 180 K) would be very small.  In fact these radiators would be much larger than if we had just radiated away all the heat in the first place.  How much larger?  Well radiateor surface is to the fourth of the temperature difference, so 293 to 180 vs 200 to 180 is 5 times more, so 5^4 is 625 times.
So we would get 15% back, at the cost of radiators 625 times larger.
Not worth it, is it?
8)

Thank you for doing what I (or someone else) should have done a couple pages ago to put this topic to bed. You probably were clearer than I would've been, too.
« Last Edit: 02/24/2016 11:46 PM by Robotbeat »
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Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1776 on: 02/25/2016 03:40 AM »
Agreement on my part. Not impossible, but extremely impractical. Doesn't violate thermodynamics, but surely not worth extra mass. I am satisfied with that.

Offline BobCarver

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Re: MCT Speculation and Discussion Thread 4
« Reply #1777 on: 02/25/2016 01:02 PM »
Forgive my naivety.

In the novel Sundiver (David Brin), they dumped excess heat by converting to electricity, and using that to power a laser that they simply shone in to space.

Is that even theoretically possible?

Nope because the electrical side gets hot and that needs its own set of radiators. Basically you make more waste heat than you throw overboard.

I read a paper ages ago which described a "laser radiator" which basically a very complex gasdynamic laser. Essentially it produced a beam of light with a blackbody radiation curve which carried the waste heat away (so not really a radiator which shoots heat away as a laser beam).

Actually, it can be done, but some claim it's a violation of the 2nd law. However, in reality, it is not. An electroentropic or magnetoentropic device can do it. It generates electricity without using a temperature differential by directly converting heat into electricity.

Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1778 on: 02/25/2016 02:10 PM »
It would seem we shed 10grams of skin per day, for a total of 3.6 kg per year.  So 100 crew would produce 1 kg per day of skin flakes, and for a 200 day trip 200 kg.

So basically at the end of the trip you have two extra crew members made of dead skin!!!!  Plus at least the same amount of hair, so you really need to clean the ship and wash the linen.  I guess the skin mites will get a lot of it and transform it into CO2 and skin mite shit.  Beaurk.
Most of the skin will wash off in showers though.  So bacteria will get it instead?
Keep in mind the ISS has been in continuous occupation for 1-2 decades and this is not a show stopper.

However I think managing this issue needs to become a bit more sophisticated than " clean the place with wipes, stick them in the cargo vehicle, let them burn on re-entry."  :(

Good point about the skin mites. All humans carry a substantial micro climate of assorted creatures with them. True most of them need an electron microscope to see clearly but they are  there and long term operations away from Earth will need to take this into account.
Quote
And there is all that urea from sweat that condenses in the linen as well.  Definitively need washing machines ;-)
I kind of like the washing bags idea because
a) it's likely to be much easier (and cheaper) to engineer this as a low risk solution for what is a non core (but still pretty important) task and
b)At a subconscious level it would get people used to the idea that they are personally responsible for their own well being.

In some ways the more interesting question is what do  you do with the waste from such bags? IE the dead skin, urea etc.

Incidentally people seem to be thinking that a 100 day journey is it and things will get a lot easier once landed.

Once landed it would be possible to deploy much bigger solar arrays and radiators. [EDIT So power and thermal control would not be show stoppers. ] Inflatable habitats could  increase habitable area substantially as well.

But now anything that's been floating in the atmosphere will settle on the nearest surface.

What happens next will depend on how abrasive, chemically or bio active the are.

The other joker is that we know Martian surface dust is very chemically aggressive. I'm not sure there any area on Earth quite like it.  The design must therefor ensure either the dust cannot foul mechanisms or that such mechanisms can be cleaned. Otherwise if you can't close all doors and re-stow all equipment you may not be able to take off.
« Last Edit: 02/25/2016 02:26 PM by john smith 19 »
BFS. The worlds first Methane fueled FFORSC engined CFRP structured A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP structured booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C Apply So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

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Re: MCT Speculation and Discussion Thread 4
« Reply #1779 on: 02/25/2016 04:10 PM »
Forgive my naivety.

In the novel Sundiver (David Brin), they dumped excess heat by converting to electricity, and using that to power a laser that they simply shone in to space.

Is that even theoretically possible?

Nope because the electrical side gets hot and that needs its own set of radiators. Basically you make more waste heat than you throw overboard.

I read a paper ages ago which described a "laser radiator" which basically a very complex gasdynamic laser. Essentially it produced a beam of light with a blackbody radiation curve which carried the waste heat away (so not really a radiator which shoots heat away as a laser beam).

Actually, it can be done, but some claim it's a violation of the 2nd law. However, in reality, it is not. An electroentropic or magnetoentropic device can do it. It generates electricity without using a temperature differential by directly converting heat into electricity.
Explain how heat can conduct or radiate without a temperature differential.
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To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

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