Author Topic: MCT Speculation and Discussion Thread 4  (Read 610393 times)

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1740 on: 02/23/2016 04:34 PM »
How was the electricity produced?  Conversion of energy produces heat.

The question is  good one, however conversion of energy in an endothermic reaction absorbs heat. I do not know what their supposed mechanism is, however people who continue to claim that it is impossible due to violation of second law of thermodynamics are incorrect. When you smash a cold pack and put it on a sprained ankle, it gets cold due to ambient heat driving the endothermic reaction. Ambient heat is converted into potential energy and locked up in the molecules created. Most mechanical conversions usually are exothermic due to inefficiencies of the conversion mechanism, however endothermic chemical reactions do exist. Photovoltaic panels convert visible electromagnetic energy into electricity. I would think that conversion of the infrared portion of the electromagnetic spectrum into electricity should be theoretically possible. A heat pump might assist in concentrating the thermal energy prior conversion, but that is a separate mechanism with its own set of issues.

Offline Krevsin

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Re: MCT Speculation and Discussion Thread 4
« Reply #1741 on: 02/23/2016 04:47 PM »
The cold pack dumps its heat to the surrounding air. The heat is still present, it's just been moved, namely from the pack and to the surrounding air. It's why the backs of fridges are hot. Because the fridge is merely dumping the heat, not unmaking it.

"Endothermic" does not mean "laughs in the face of entropy", rather it states that a certain reaction requires heat to happen and it most certainly does not remove the heat, merely moves it around.
« Last Edit: 02/23/2016 05:04 PM by Krevsin »

Offline stoker5432

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Re: MCT Speculation and Discussion Thread 4
« Reply #1742 on: 02/23/2016 05:37 PM »
Just because we COULD do AG doesn't mean we SHOULD. If your trip is short, you don't need it and probably shouldn't use it.
Most people would not consider 3-4 months a short trip.

However the practical work on AG is so limited it would absurd to bet your plans on it working.

If space exploration were done logically it's a technique that would have been tried decades ago and its benefits and problems already found. Yet in 2016 it still has had no full scale test.  :(

It doesn't matter. It's a voluntary trip. If the people decide, with the evidence to date, that this is a non-issue, more power to them. If lack of AG kills them it's no fault but their own. If you're not convinced that it's safe (as I am far from being) you shouldn't of signed up for the ride.

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1743 on: 02/23/2016 07:51 PM »
The cold pack dumps its heat to the surrounding air. The heat is still present, it's just been moved, namely from the pack and to the surrounding air. It's why the backs of fridges are hot. Because the fridge is merely dumping the heat, not unmaking it.

"Endothermic" does not mean "laughs in the face of entropy", rather it states that a certain reaction requires heat to happen and it most certainly does not remove the heat, merely moves it around.

You are simply wrong. The heat pump on a refrigerator does move the heat, but that is not what happens in an endothermic reaction. I used to teach both chemistry and physics, as well as biology. When carbon oxidizes, potential energy is given off as heat. In an endothermic reaction, ambient heat is absorbed and changed into potential energy in the new chemical bonds that are created. The energy changes from thermal energy to potential energy. A cold pack does not dump heat; it soaks it up. You are absolutely and totally W...R...O...N...G ! ! !  Go look it up. Exo thermic means puts out heat. Endo thermic literally means that heat goes into the reaction. Quit arguing and go do the research.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1744 on: 02/23/2016 09:17 PM »
I doubt that Musk is planning to use AG, however...

a bolo made of two [MCTs BFSs] connected by tether at the nose. Spinup and spindown by synchronized use of RCS thrusters.
[my edit]

A tether is almost certainly not necessary. If the crew can tolerate/adapt to higher RPM as easily as they are expected to adapt to micro-g, and if 1/3-1/2g is sufficient to offset micro-g damage, then a single BFS spinning around its long axis should be sufficient. Put the work and exercise areas against the hull, with the sleep areas down the centre (low-g sleep is more pleasant, and gravity works less when you are reclining anyway). (Sorry Kaoru, I guess you need to redesign the ship again.)

If higher-g or lower RPM is required, spinning the BFS around its short axis (tumbling pigeon) would allow up to 1g at 6RPM or 1/2g at 4RPM. (Depending on the length of the BFS, I've arbitrarily used 50m.) Work/exercise areas at the nose, sleeping areas towards the CoM. (Engines/cargo as countermass.) And if you dock two BFSs together by their aft ends, spinning them end-over-end, the fore-end of each will be able to reach half a g from just 3RPM, and 1g from just over 4RPM.

If you need a lower RPM and higher g-load, then frankly AG is really not worth the effort. (As JS19 notes, it would be nice to have some firm figures by now. Especially given how small an experiment it would be compared to Apollo/STS/ISS/SLS development costs.)

Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1745 on: 02/23/2016 09:20 PM »
Having looked a little more into the ISS ECLSS system it looks like they have a long way to go on weight reduction.

My very rough estimate is at present we're looking about 333Kg/person of ECLSs hardware per person.

Reading the descriptions of the hardware is quite odd. All the systems seem to have been built completely separately and some of the design choices seem strange. The trace gas monitor, that monitors 8 (and no other) airborne species, weighs 58Kg and is a sort of mass spectrometer IE needs high vacuum to operate without destroying filaments and a 5000psi gas canister to regularly re-calibrate it.

This is despite the fact people have been shrinking gas chromatographs (as in systems that analyse gas) from silicon wafers down to chips since the 1970's. Likewise optical sensors have improved beyond all recognition.

And the commode weighs 50Kg. ::)  A 110lb toilet.  I get it, gas,water and solid separation is complex. But 50Kg. Really?

[EDIT An interesting special case. Hair & nail clippings. Keratin is a tough protein and can foul equipment but could make the raw material for say 3d printing objects. Or should it be broken down to its elements? ]

It's pretty clear they're going to need a lot more work toward integrating these systems, tapping heat released from some cycles to drive others, common design elements so basic units can be repurposed as necessary.

The SoA in this stuff is the use of integrated  heat exchangers and chemical reactors mfg using diffusion bonded metal foils. These units are typically 1/3 the size of more conventionally built systems. It's referred to as "process intensification" and several chemical plant mfg do this.

My instinct is somewhere between the current system and the highly integrated PI systems is a system efficient enough to get the job done and light enough to make it worthwhile.

Personally I quite like photocatalyst systems built around flowing most of the stuff through a reactor loaded with TiO powder and exposed to UV light. IIRC this has been used to rip most molecules down to CO2, H2O and various metal salts, although I'd guess N2 is a probably end point as well.
« Last Edit: 02/24/2016 07:06 AM 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. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline RocketmanUS

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Re: MCT Speculation and Discussion Thread 4
« Reply #1746 on: 02/23/2016 11:46 PM »
Forgive my naivety.

In the novel Sundiver (David Brin), they dumped excess heat by converting to electricity,

How was the electricity produced?  Conversion of energy produces heat.
Thermocouple
https://en.wikipedia.org/wiki/Thermocouple

Thermopile
https://en.wikipedia.org/wiki/Thermopile

Radioisotope_thermoelectric_generator
https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

Stirling_engine
https://en.wikipedia.org/wiki/Stirling_engine

May not be mass and or volume efficient inside the MCT. Radiators might be better.

The electrical energy produced could then be used to help run the heat pump ( A/C ). I don't know if this could generate more or less power than what is needed to run the heat pump ( A/C ).

There would need to be a study to see if this could remove all the heat or just some of it ( still needing some amount of radiators ).

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1747 on: 02/23/2016 11:50 PM »
the commode weighs 50Kg. ::)  A 110lb toilet.  I get it, gas,water and solid separation is complex. But 50Kg. Really?

I wonder if it needs to be that size just to do the job, regardless of whether it is used by 6 people or 60 people. If more people can use some of these systems, the needed mass per person could go down

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1748 on: 02/24/2016 12:25 AM »
The cold pack dumps its heat to the surrounding air. The heat is still present, it's just been moved, namely from the pack and to the surrounding air. It's why the backs of fridges are hot. Because the fridge is merely dumping the heat, not unmaking it.

"Endothermic" does not mean "laughs in the face of entropy", rather it states that a certain reaction requires heat to happen and it most certainly does not remove the heat, merely moves it around.

You are simply wrong. The heat pump on a refrigerator does move the heat, but that is not what happens in an endothermic reaction. I used to teach both chemistry and physics, as well as biology. When carbon oxidizes, potential energy is given off as heat. In an endothermic reaction, ambient heat is absorbed and changed into potential energy in the new chemical bonds that are created. The energy changes from thermal energy to potential energy. A cold pack does not dump heat; it soaks it up. You are absolutely and totally W...R...O...N...G ! ! !  Go look it up. Exo thermic means puts out heat. Endo thermic literally means that heat goes into the reaction. Quit arguing and go do the research.
I know what you're talking about WRT endothermic reactions. These do not violate the laws of thermodynamics because entropy still increases, and you soon run out of the little reservoir of low entropy.

This other device you're talking about, a device that's able to generate useful energy by absorbing waste heat indefinitely would clearly NOT obey the laws of thermodynamics.

Please realize that this device that you keep defending is not possible.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1749 on: 02/24/2016 02:17 AM »
I know what you're talking about WRT endothermic reactions. These do not violate the laws of thermodynamics because entropy still increases, and you soon run out of the little reservoir of low entropy.

This other device you're talking about, a device that's able to generate useful energy by absorbing waste heat indefinitely would clearly NOT obey the laws of thermodynamics.

Please realize that this device that you keep defending is not possible.

I have not defended any device. I stated above that I do not know the methodology by which it supposedly works, however, such a device is theoretically possible.

Photosynthesis is classified as an endothermic reaction, even though the energy input is in the visible range of the spectrum rather than the infrared portion. While lacking in efficiency, photovoltaic cells convert the visible portion of the electromagnetic spectrum into electricity as photons elevate electrons across a barrier. The electricity can then drive a chemical reaction that builds potential energy in a battery or stores the electrons in a capacitor.

Now I am not aware of a mechanism whereby infrared energy in the spectrum can excite electrons and move them across a photovoltaic membrane, however the second law of thermodynamics would not preclude it. It would likely be inefficient. A good deal of heat still would radiate away, just as it does in a normal photovoltaic cell, but there is no theoretical reason that thermal energy could not be converted back into potential energy by mechanisms other than endothermic chemical reactions. You are simply converting some kinetic energy to potential energy and losing some of it through inefficiencies and conversion.

In endothermic chemical reactions, heat is absorbed, the temperature of the reactants drops as measured in a calorimeter, and potential energy is created in bonds and in valences which have exothermic bonding potential. The second law of thermodynamics is not violated. In like manner, there could exist other mechanisms which perform a similar function.

I don't know anything about the device that has been mentioned. I haven't looked at the purported mechanisms at all. All I am saying is that in theory, thermal energy can be converted to potential energy without violating the 2nd law of thermodynamics, and that this principle is verified in endothermic chemical reactions.

Offline Lampyridae

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Re: MCT Speculation and Discussion Thread 4
« Reply #1750 on: 02/24/2016 08:47 AM »
I agree that AG should not be in the critical path.  With that said...

Zero-g certainly allows a much more compact ship than would otherwise be the case.

Please convince me of this. 

Essentially, the difference between playing Tetris and playing Tetris without being able to rotate the bricks.
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Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1751 on: 02/24/2016 01:34 PM »
[Put this in a single post to make it easier for mods to remove, when they finally get sick of this off-topic argument. Sure, I'm a jerk, but I'm a considerate jerk.]

Thermocouple
Thermopile
Radioisotope_thermoelectric_generator
Stirling_engine
May not be mass and or volume efficient inside the MCT. Radiators might be better.

Both¹ require radiators. They operate by exploiting the temperature difference between a hot-side and a cold-side. Without a radiator, the two sides will equalise in temperature. In order to do work, you have to constantly pump concentrated heat in and pump/radiate waste heat out. You need a heat source and a heat sink, and the heat source must be a higher grade of energy (more concentrated) than the heat sink. To do work, you turn the high-grade source heat into lower-grade waste heat.

¹ Both thermopiles and sterling engines. The other two are redundant. Thermocouples use the same principle as thermopiles (or vice versa). And RTGs use thermopiles to actually produce electricity (research RTGs have also used sterling engines).

The electrical energy produced could then be used to help run the heat pump ( A/C ). I don't know if this could generate more or less power than what is needed to run the heat pump ( A/C ).

Always less. Laws of physics.

Please realize that this device that you keep defending is not possible.
I have not defended any device. I stated above that I do not know the methodology by which it supposedly works, however, such a device is theoretically possible.

Not really. It must violate thermodynamics.

It's possible to develop a refrigerator (obviously) which concentrates heat energy to allow it to radiate away. And perhaps it's possible to develop a refrigeration laser (Brin was obviously riffing on the then newly invented laser doppler cooling), which concentrates its waste heat as a beam. But thermodynamics requires that the heat output is in a lower grade form than the total energy input.

In a refrigerator, you can "cheat" to turn low grade room-termperature heat into higher-grade hot-radiator and cold-interior by stealing very-high-grade electricity and turning it into more low grade waste heat. The waste-heat created at the power-plant to generate electricity is not being pumped into the fridge interior along with the electricity. It's the decoupling that allows the system to work.

[Think about it. The compressor (or other heat-pump) of a standard fridge uses a certain amount of electricity (or other power source) in order to cool the interior from room-temp to 3-4℃. If you pumped in the waste-heat from the power-production as well, the compressor would clearly need to work harder to get rid of that heat too, which means using more electricity, which means more waste heat, which means yet more electricity, which means... You can't close the loop unless you have a method of creating high-grade energy which the heat-pump doesn't have to deal with.]

In Brin's ship, where does the high-grade energy coming from to power the laser? A reactor of some kind inside the ship? The waste heat from that reactor must also be disposed of by the same refrigeration-laser that is cooling the ship.

Forget the internals. You've got a closed box that is not only cooler than the environment, but outputs a concentrated beam of high-intensity energy. Clearly you could use that super-high-grade energy to generate power to run the refrigeration-laser. Hence you've invented a free energy machine, and Maxwell's Demon is sadly shaking his head no.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1752 on: 02/24/2016 02:31 PM »
This is dumb. Can we get back on topic instead of giving a platform to people (who ought to understand thermodynamics) to demonstrate that they don't understand thermodynamics?

You can't make your radiator smaller by trying to get free energy. Argue about it in the New Physics section if you like.
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Offline JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1753 on: 02/24/2016 03:14 PM »
This is dumb. Can we get back on topic instead of giving a platform to people (who ought to understand thermodynamics) to demonstrate that they don't understand thermodynamics?

You can't make your radiator smaller by trying to get free energy. Argue about it in the New Physics section if you like.

No, it's not dumb. Because most people don't understand the laws of thermodynamics. But should be elsewhere, certainly.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1754 on: 02/24/2016 04:25 PM »
I agree that AG should not be in the critical path.  With that said...

Zero-g certainly allows a much more compact ship than would otherwise be the case.

Please convince me of this. 

Essentially, the difference between playing Tetris and playing Tetris without being able to rotate the bricks.

So you're talking about increased packing efficiencies from being able to store things in the ceiling/floor racks?

You know we get to build lower ceilings (and more floors) if we remove those, right?

Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1755 on: 02/24/2016 04:30 PM »
I'd like to say a few words about laundry.

Assuming people wear their clothes for a week each that's a minimum of 1500 sets of clothes you'll be taking to Mars unless something gets done about reusing them.

Remember this is not the ISS, where stocks can be substantial and resupply is (relatively) easy.  :(

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.

Personally I think it could be quite theraputic.  :)

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?

Naturally this all has to be plumbed into the ECLSS to to work.
« Last Edit: 02/24/2016 04:35 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. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1756 on: 02/24/2016 04:34 PM »
Here's another quick grab of improvements to my passenger model.  The biggest change is the diameter which is now 10.1 meters, the same as the Saturn V first stage.  With this diameter, each deck holds 24 sleep stations and 24 seats (fold-away).  That means only 4 decks for 96 passengers.  The deck features an airlock door.  Being a work in progress, still got fill out the sleep stations and put in a fold-away ladder for up and down on surface access.

Kaoru

This is great, however I don't think your floor will be able to stand up to the differential pressure if you close the door. The floor will become a pressure bulkhead and need to be rounded. At 1 atmosphere there will be about 750 tones of load on the floor. 101 kpa per m2, or 10 tonnes.  Even at 1/2 atmosphere you get a lot of load.

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1757 on: 02/24/2016 04:41 PM »
I'd like to say a few words about laundry.

Assuming people wear their clothes for a week each that's a minimum of 1500 sets of clothes you'll be taking to Mars unless something gets done about reusing them.

Remember this is not the ISS, where stocks can be substantial and resupply is (relatively) easy.  :(

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.

Personally I think it could be quite theraputic.  :)

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?

Naturally this all has to be plumbed into the ECLSS to to work.
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.

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1758 on: 02/24/2016 04:42 PM »
This other device you're talking about, a device that's able to generate useful energy by absorbing waste heat indefinitely would clearly NOT obey the laws of thermodynamics.

Please realize that this device that you keep defending is not possible.

No, it's not dumb. Because most people don't understand the laws of thermodynamics. But should be elsewhere, certainly.

I never said anything about absorbing waste heat endlessly. I in fact stated that any such mechanism would likely be very inefficient. I am not the person who proposed such a device. All I have ever posited is that some heat can be captured and converted to potential energy through endothermic means which differ from things like heat pumps (which require more total heat to be added to a system and just move heat from one place to another).

As far as people not understanding thermodynamics, give me a break. There are people here who seem to think that heat and energy are limited to the simplistic closed systems that are theoretically examined in Physics 102. There are people other than thermodynamicists who examine and calculate the flow of heat and energy. The canopy of a tropical rain forest lowers the ambient temperature as much as 20-30 degrees F as solar energy is converted to chemical potential energy in sugars. Ecologists calculate the flow of energy from producers through primary, secondary, and tertiary consumers in an ecosystem. At each level, only about 10% of the energy is transferred to the next level up the food chain.

Some heat can be captured and converted to potential energy through endothermic chemical reactions and endothermic mechanical processes. Most of these are rather inefficient and impractical. I am not saying you do not lose energy. I am not saying you can recapture and reuse the same energy endlessly. I know you cannot. I am simply saying there is more to thermodynamics than the simplistic calculations that are done with simple closed systems that involve nothing more than conduction, convection, radiation, and evaporation.

When astronauts eat, metabolism of glucose going through the Krebs Citric Acid Cycle changes potential energy in the glucose into thermal energy that is added to the system. Batteries release thermal energy into the system as electricity experiences resistance. Heat can be added to a system, and small amounts of heat can be removed from a system through endothermic chemical reactions as well as endothermic physical mechanisms which change some of the energy back to potential energy. Is there anything that I know of that has enough efficiency to be of practical usefulness on a spacecraft in the near future? Frankly, no; I do not know of anything that would be practical. OTOH, those who say that no thermal energy whatsoever can be converted into potential energy, that is incorrect. You are limiting you overall system to subsystems that are not endothermic.  These mechanisms, though inefficient, do not violate the second law of thermodynamics. The heat simply changes from thermal energy to potential energy.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1759 on: 02/24/2016 04:56 PM »
Zero-g certainly allows a much more compact ship than would otherwise be the case.
Please convince me of this.
Essentially, the difference between playing Tetris and playing Tetris without being able to rotate the bricks.
So you're talking about increased packing efficiencies from being able to store things in the ceiling/floor racks?

It's mainly the efficiency inherent in being able to change your own orientation.

How much more "benchtop" would you have in your kitchen if you could use the cupboard doors, the walls and the ceiling and floor as a work surface. And hence how many more people could work comfortably in that space without crowding each other.

[edit: Although, that said, the "not going crazy" volume requirements eliminate any packing-efficiency from micro-g anyway. So I don't think that's a vote against AG.]

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".

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)
« Last Edit: 02/24/2016 05:03 PM by Paul451 »

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