Author Topic: Further investigations on hydrogen-free propellants for Mars and Venus  (Read 15555 times)

Offline Lar

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Moved to "Missions to Mars (HSF)" section, seemed the best fit.

And ixnay on the sniping, calling out sniping, saying you weren't sniping, etc.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline Lampyridae

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Station.

I find this ISRU production of polymers interesting. How easy is it to produce Zylon, since that seems to be the current favourite for space tethers? (Excluding the possibility of Venus orbital tethers for the moment, rather Mars). UHWMPE as you say seems to be the best fit, but the high temperatures that tethers might reach in sunlight seem to preclude their use.

Offline mikelepage

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By the way, why is this in the SpaceX section?

This thread should either be in Mars General, in General, or probably in Advanced Concepts.

Mods, can we please have a Venus General section of the NSF forum?

I think the revival of the Venus airship proposals in recent years has generated more than enough discussion to justify one.  It would also be good for us Venus cloud colony enthusiasts not to have to counter the "Surface of Venus is a hellhole/Temperature/Pressure "-people every time this comes up in discussion.

We're talking about a airship/station vehicle that maintains its position somewhere between 50km/75 degrees C/1 atmosphere and 55km up/27 degrees C/0.5 atmospheres.

You seem like you know your chemistry.  How easy is it to make Nylon rom atmospheric CO2 N2 and H2SO4?

There's a giant section on this in the section of the document and I don't want to have to reproduce it all here, but the short of it:

Nylons, while good for affordable yet reasonably strong envelopes on Earth, really aren't optimal for Venus, and especially not a Landis habitat (transparent).  More to the point is, if you want a good mass ratio while limiting permeation, first off you have to use a multilayer composite.

Thanks for the detailed response :) but I'm not sure you quite understood what I was driving at.

Firstly, given the amount of sunlight hitting Venus is nearly 2x that on Earth, whatever material is to be used only has to be somewhat translucent in order to be viable for a Landis habitat.

But secondly, because ISRU on Venus will be a huge barrier to entry, and because any platform big enough to launch rockets from Venus will need to be huge (!), I'm taking the opposite tack, by wondering chemically what is the simplest monolayer gas envelope we will be to make quickly and cheaply out of the components we know are in the Venusian atmosphere.  Maybe that's nylon or something else. 

The problem I see is that in an atmosphere that corrosive, there is no material that won't eventually decay,  so let's design the whole structure in such a way that the decay of the lifting gas envelopes doesn't matter.  The structure will be constantly self-renewing, and maintains altitude by always adding new lifting bodies to the old ones, and doing so faster than the old ones are decaying. 

I used Nautilus/Mollusk shells as an example because they have this segmented, spiralling structure that builds on itself in the way we want, and the body of the Mollusk (equivalent to our colony) moves over time into progressively larger chambers as they outgrow the earlier ones.

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As for shapes, you can build whatever shape you want.  But the optimal shape for reducing both material and fabric is spherical, tempered by the need to be aerodynamic if you want to go anywhere, which is what leads most to be ellipsoids.  Also note that non-rigid airships aren't just big gas bags, there's a whole network of hardware involved including ballonets, catenary curtains and cables, etc.  On Venus, ideally a phase-change envelope as well.  The fact that people live inside in a Landis habitat just makes the structure even more complex.

If your interest is about joining new segments onto an existing habitat, you may find the airworm concept for airships interesting.  :)  The whole point of them by the way isn't about expandability, it's actually a very efficient design when trying to combine low material use but good aerodynamics at large sizes.

I'm actually a bit of an airship fan, (go Aeroscraft!), so I do get what you're saying.  But I would also point out that we eventually want these things to be quite tall so they can generate energy from the temperature/pressure differential between various layers of the atmosphere.

It seems to me that the way to build big and fast, is to make the gas envelopes as chemically simple as possible, and not spend excessive effort making them durable.

Offline Rei

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Station.

I find this ISRU production of polymers interesting. How easy is it to produce Zylon, since that seems to be the current favourite for space tethers? (Excluding the possibility of Venus orbital tethers for the moment, rather Mars). UHWMPE as you say seems to be the best fit, but the high temperatures that tethers might reach in sunlight seem to preclude their use.

Zylon has dependency chains a mile long, unfortunately.  :Þ  Great stuff, though.

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Firstly, given the amount of sunlight hitting Venus is nearly 2x that on Earth, whatever material is to be used only has to be somewhat translucent in order to be viable for a Landis habitat.

Well... sort of.  Do recall that sunlight is not constant everywhere on Venus, and the higher latitudes give you the more earthlike combinations of altitude and pressure as well as shorter effective day lengths and less meridional winds to resist. So you want to be high latitude (perhaps not in the polar vortices, mind you!).  But poleward means reduced lighting in the cloud deck

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But secondly, because ISRU on Venus will be a huge barrier to entry, and because any platform big enough to launch rockets from Venus will need to be huge (!)

Yeah.. unfortunately  :(  It's one of those things where you can start small, and as an inherently cautious person I love to see everything done in the small scale first.  But the scaling factors on Venus all favour going big as soon as possible. Because lift is proportional to radius cubed while mass for components ranges from constant to somewhere between squared and cubed (agriculture, power, and thus crew are proportional to radius squared, because lighting is proportional to radius squared).

It's not hard to come up workable figures for a basic first-stage habitat for under 10 people that you could launch to LEO on a Falcon Heavy, both from a mass and fairing volume perspective, with existing airdocks for construction. But it gets so much easier if you go at least to CargoLifter scales, or bigger.  In a habitat scaled for 10 or so people you could easily have enough agriculture for several dozen people.... you just can't promise them a return trip home, not at least for a long time.  You also can't start accumulating large amounts of in-situ produced mass for future construction needs or anything of the sort - not without making your ascent stage even more mass limited. 

If you go bigger from the start, CargoLifter-sized or ideally even larger,  you can actually put all that space to use, and have enough people for serious construction efforts.  Small scale is especially frustrating with "you just need one" items.  For example, a combination CNC router + laser sintering or laser spraying printer like you sometimes see on the market would be immensely useful for maintaining and expanding ISRU systems.  But ones big enough to make parts of relevant size are massive.

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I'm taking the opposite tack, by wondering chemically what is the simplest monolayer gas envelope we will be to make quickly and cheaply out of the components we know are in the Venusian atmosphere.  Maybe that's nylon or something else. 

Unfortunately, it's not nylons.  For example, nylon 6 is about as simple as they come (far simpler than most), but still involves going into aromatic chemistry (several steps in, at that) to produce the caprolactam.  And the properties just don't beat easier to produce polymers.  It's not as strong, it dissolves in sulfuric acid, it's not transparent... it's just not a good choice.

And it's not, when we talk about longevity, like it's a situation of "polymer A lasts three times as long as polymer B".  It's a situation of "polymer A lasts half a dozen orders of magnitude as long as polymer B".   Here's what happens if you just expose a random household nylon product to sulfuric acid, at a far lower concentration than you find on Venus:

https://www.youtube.com/watch?v=uh5nzFoJXMU?t=118

Also, the problem with a "just build out of whatever" approach is that you need things to be as thin as possible and yet last because you want to get as much envelope fabric as you can, and not have to replace it more often than necessary, to maximize construction rates. And you're constrained by how fast you can process the sparse mists, which are your source of hydrogen - and ascent stage propellant production wants to divert as much of this as it can  ;)  In analyzing materials, I've paid close attention as well to what percentage of their mass is hydrogen, as it varies quite a bit - for example, PVDC is only 2,1% hydrogen (thank you, chlorine!), while polyethylene is 14,4%.

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The problem I see is that in an atmosphere that corrosive, there is no material that won't eventually decay,

The same can be said for everything on Earth, but you still choose "good enough".  Numerous polymers can tolerate decades of exposure to concentrated mineral acids.  More often it's organic solvents that are the problem.  Fluoropolymers in particular stand out, but even some everyday polymers like PVC, polyethylene, polypropylene have superb acid compatibility. If you made your envelope out of additive-free polyethylene, it'd likely break apart from UV degradation before it did from acid exposure  ;)

Or, to put it another way, google image search for "X acid container", replacing X with various types of mineral acids.  I'm sure you notice the common thread: the vast majority of them are made out of plastics.  That's standard for the long-term storage of acids, many quite concentrated, the majority of which will eat through metal containers, and some of which (most famously HF) will even eat through glass.  At home right now I have jugs of both hydrochloric and phosphoric acid; both are plastic, and while I haven't checked the type, it feels like polyethylene.  But Venus isn't like a vat of acid - it's just a sparse vog, a few to a couple dozen milligrams per cubic meter . I mean, OSHA even lets workers breathe up to 1mg/m³ of sulfuric acid.  Albeit Venus's H2SO4 is about double the molarity of Earth's, and there's anhydrous acidic compounds too... but it's important to keep the Venus environment in perspective.

And you can't just ignore permeability (which it seems most people do - although not mission planners, it's a heavy topic of discussion in papers on balloon probes to Venus).  You have a lot of square meters of surface area.  If your envelope is high permeability, going to be losing your oxygen and water and taking in far too many acidic gases from the external environment. It's easy to think of plastics as impermeable, but they do leak.  To put it another way, think of a helium party balloon, versus how it looks a few days later after the helium has permeated out through the skin.  You don't want that happening with your habitat - in either direction  ;)  Yes, helium permeates much better than, say, oxygen, but they all permeate.  The food industry pays close attention to this when choosing packaging (EVOH is very popular there**).  If one uses just  polyethylene or similar versus a proper barrier polymer, it would have to be orders of magnitude thicker.

I could post some permeability stats in case you have an interest, I've compiled them for a range of polymers of interest.  Or I could point out graphs, like:

http://www.dsm.com/content/dam/dsm/akulon/en_Us/images/Akulon_Permeability_Large.png

Note that this is a log scale(!)  :)


** You may notice that I'm pretty fond of the concept of using EVOH.  It's not that it's the only barrier possibility that's on the shorter end of production dependency chains; far from it. But one of the neat things is that between it, its homopolymer counterpart PVOH, and their predecessors, EVA and PVA, is that there's a massive range of useful products that you can produce from them - hot glue, elmers' glue, tackifiers for tape, foam rubber, dissolvable printing scaffoldings, lubricants, thickeners, eye drops...  you name it.   Also, the monomer, vinyl alcohol, is just ethylene reacted with acetic acid and oxygen in the presence of a catalyst. 

But again, there are lots of possibilities beyond EVOH.  PVDC and PET being good examples (PVDC being much better than PET, although not as good as EVOH, except in regards to water which EVOH is very poor at).  PVDC, for example (Saran - although they don't make Saran Wrap out of it any more) - the route is ethane or ethylene or actylene + HCl, chlorinated with a catalyst, yields 1,1,2-trichloroethane, which is saponified to vinylidene chloride, PVDC's monomer (the waste NaCl + H2O needs to be recycled in the chloralkali process).  Not too complicated, and PVC is an easy side diversion from that route.  PET is popular because it's quite strong in addition to being a good barrier, although the route to produce it somewhat more complex than the above two.
« Last Edit: 01/19/2017 10:02 am by Rei »

Offline high road

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Very interesting post. Do you have a reference for this part?

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Albeit Venus's H2SO4 is about double the molarity of Earth's, ...

Offline Rei

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Very interesting post. Do you have a reference for this part?

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Albeit Venus's H2SO4 is about double the molarity of Earth's, ...

The standard molarity cited for Venus's middle cloud layer is "70-85%".  This paper goes into more precision for specific layers and latitudes:

https://mipt.ru/dppe/upload/ispavr/2015_Krasnopolsky2.pdf

I'm having trouble finding a reference on short notice on the concentration in Earth air; it's lower because sulfuric acid is highly hygroscopic and tends to self-dilute into droplets within a fixed size range, while on Venus there's little free water vapour for dilution.  To the point that many other compounds (such as HCl) that would quite readily form aerosols are often left in the gas phase because of competition for moisture with H2SO4:

http://adsabs.harvard.edu/full/1982LPSC...12.1517B - pg. 1519
« Last Edit: 01/21/2017 04:24 pm by Rei »

Offline mikelepage

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But secondly, because ISRU on Venus will be a huge barrier to entry, and because any platform big enough to launch rockets from Venus will need to be huge (!)

Yeah.. unfortunately  :(  It's one of those things where you can start small, and as an inherently cautious person I love to see everything done in the small scale first.  But the scaling factors on Venus all favour going big as soon as possible. Because lift is proportional to radius cubed while mass for components ranges from constant to somewhere between squared and cubed (agriculture, power, and thus crew are proportional to radius squared, because lighting is proportional to radius squared).

Just realised lifting off from Venus may get a whole lot easier if the launch vehicle only only needs to go suborbital.  Will start a new thread in the advanced section to describe the concept in the next day or so, but let's just say that passengers are going to want to be wearing their brown pants. :D

Offline gospacex

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No no no, a Landis habitat, not a surface habitat  :)  Dear god no, building a habitat on the surface would be a crazy idea.

Landis habitat: ~55km, ~70°n, airship filled with breathable air (which is a lifting gas on Venus), getting most of its resources straight from the atmosphere which contains most of your major industrial acids in a sparse smog (or more accurately, vog).  Earthlike gravity, earthlike air pressure, earthlike temperatures, naturally sufficient radiation protection, huge amounts of living/agricultural space

Only if you consider atmosphere to be "space".

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and easy to get to

"Landing" on a dirigible habitat, to me, looks like a more difficult task than landing on Mars. You miss, and you go down to be crushed. You miss and hit the habitat, and _habitat_ goes down to be crushed.

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It's best to think of the lower atmosphere like an ocean on Earth; you may want to live by the ocean, even sometimes go diving in it or dredge it, but you don't live in it!  On Venus, this ocean has no "shore", so if you want a habitat, it needs to be a "boat" to float on it.

Do you know of examples of Earth communities based on permanently freely-floating ocean ships? Evidently, when you need to synthesize *everything* to sustain a colony from only air and water, there are problems. Make that "only air", and it gets a bit worse. Not impossible, but far, far from being easy.
« Last Edit: 02/08/2017 10:00 pm by gospacex »

Offline high road

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No no no, a Landis habitat, not a surface habitat  :)  Dear god no, building a habitat on the surface would be a crazy idea.

Landis habitat: ~55km, ~70°n, airship filled with breathable air (which is a lifting gas on Venus), getting most of its resources straight from the atmosphere which contains most of your major industrial acids in a sparse smog (or more accurately, vog).  Earthlike gravity, earthlike air pressure, earthlike temperatures, naturally sufficient radiation protection, huge amounts of living/agricultural space

Only if you consider atmosphere to be "space".

What does this even mean? Space is not filled with an atmosphere to get resources from, does not provide gravity if you don't simulate it, and does not have earthlike air pressure, earthlike temperatures or sufficient radiation protection...

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and easy to get to

"Landing" on a dirigible habitat, to me, looks like a more difficult task than landing on Mars. You miss, and you go down to be crushed. You miss and hit the habitat, and _habitat_ goes down to be crushed.

Don't land on the dirigible habitat. Replace the parachute by a balloon, done. It doesn't even have to be neutrally boyant, as long as there's something around that can pick it up before it drops too low. SpaceX played/plays around with this idea for recovery to avoid ocean landings. (the parachute, not the balloon).

Mars landing success rate is about 50%. The other 50% disintegrated. Venus has a success rate to the surface of 75%, ignoring the first mission when they didn't know there was a pressure problem. AFAIK, Venus atmospheric missions have a 100% succesful landing rate.

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It's best to think of the lower atmosphere like an ocean on Earth; you may want to live by the ocean, even sometimes go diving in it or dredge it, but you don't live in it!  On Venus, this ocean has no "shore", so if you want a habitat, it needs to be a "boat" to float on it.

Do you know of examples of Earth communities based on permanently freely-floating ocean ships? Evidently, when you need to synthesize *everything* to sustain a colony from only air and water, there are problems. Make that "only air", and it gets a bit worse. Not impossible, but far, far from being easy.

To my knowledge, there are exactly zero communities that synthesize everything they need to sustain themselves from their local surroundings, either on land or on the ocean. It is so incredibly far cheaper to trade what you can't (easily) produce locally, we've been doing it for tens of thousands of years. Intercontinental trade, while the fastest way to transport stuff was on foot... Every human presence on any other planet will have to overcome that problem.

'Sometimes go diving in it or dredge it' includes extracting resources from the surface. Why else would you 'dredge' it? It's not like the keels of the habitats risk getting stuck on the ground.

Offline gospacex

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No no no, a Landis habitat, not a surface habitat  :)  Dear god no, building a habitat on the surface would be a crazy idea.

Landis habitat: ~55km, ~70°n, airship filled with breathable air (which is a lifting gas on Venus), getting most of its resources straight from the atmosphere which contains most of your major industrial acids in a sparse smog (or more accurately, vog).  Earthlike gravity, earthlike air pressure, earthlike temperatures, naturally sufficient radiation protection, huge amounts of living/agricultural space

Only if you consider atmosphere to be "space".

What does this even mean?

It means that most people do not consider airspace as "living/agricultural space", no matter how huge it is.

Offline gospacex

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Mars landing success rate is about 50%. The other 50% disintegrated. Venus has a success rate to the surface of 75%, ignoring the first mission when they didn't know there was a pressure problem. AFAIK, Venus atmospheric missions have a 100% succesful landing rate.

Venus surface landings are irrelevant to your proposal of landing onto a flying habitat.

Offline gospacex

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Do you know of examples of Earth communities based on permanently freely-floating ocean ships? Evidently, when you need to synthesize *everything* to sustain a colony from only air and water, there are problems. Make that "only air", and it gets a bit worse. Not impossible, but far, far from being easy.

To my knowledge, there are exactly zero communities that synthesize everything they need to sustain themselves from their local surroundings, either on land or on the ocean.

For thousands of years almost all humans were living off the local land. Trade often was limited to exchanges with nearby tribes; sometimes circumstances were so that there was no trade at all (isolated tribes or hostile neighbors).

Offline dror

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That's a very interesting and informative thread, thanks.

1. As for the original question at the title, I would suggest you to look into  C+O2 hybrid motors of some sort.
2. As an alternative, I get to think of JP Aerospace's "airship to orbit" concept. IF it can be made to work on earth, than it may be made to work on Venus only much better since it will take the whole station in and out of the atmosphere.
« Last Edit: 02/09/2017 08:38 pm by dror »
Space is hard immensely complex and high risk !

Offline high road

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No no no, a Landis habitat, not a surface habitat  :)  Dear god no, building a habitat on the surface would be a crazy idea.

Landis habitat: ~55km, ~70°n, airship filled with breathable air (which is a lifting gas on Venus), getting most of its resources straight from the atmosphere which contains most of your major industrial acids in a sparse smog (or more accurately, vog).  Earthlike gravity, earthlike air pressure, earthlike temperatures, naturally sufficient radiation protection, huge amounts of living/agricultural space

Only if you consider atmosphere to be "space".

What does this even mean?

It means that most people do not consider airspace as "living/agricultural space", no matter how huge it is.

Ah, you forgot the 'living'. Same goes for deserts, rocky mountains, polar regions, space stations, and all planets without an existing biosphere. 'Most people' will only 'consider' something if it's already been proven to work.

Mars landing success rate is about 50%. The other 50% disintegrated. Venus has a success rate to the surface of 75%, ignoring the first mission when they didn't know there was a pressure problem. AFAIK, Venus atmospheric missions have a 100% succesful landing rate.

Venus surface landings are irrelevant to your proposal of landing onto a flying habitat.

No 'my' proposal. I see it as an unnecessary complication of the easiest planet to do EDL in the inner solar system. Launching from such a platform is a different matter, because it's also the hardest planet to launch from in the inner solar system.

Do you know of examples of Earth communities based on permanently freely-floating ocean ships? Evidently, when you need to synthesize *everything* to sustain a colony from only air and water, there are problems. Make that "only air", and it gets a bit worse. Not impossible, but far, far from being easy.

To my knowledge, there are exactly zero communities that synthesize everything they need to sustain themselves from their local surroundings, either on land or on the ocean.

 ...

Every human presence on any other planet will have to overcome that problem.

For thousands of years almost all humans were living off the local land. Trade often was limited to exchanges with nearby tribes; sometimes circumstances were so that there was no trade at all (isolated tribes or hostile neighbors).

Exactly how many of those tribes that did not trade, survived? How many rose to power by other means than plundering their neighbours? How many would be able to survive or compensate for the wearing of their infrastructure that sustains them on a different planet? Trade is essential to long term survival.

Either you grow, you disappear, or you get gradually assimilated by some other tribe whose territory grows across yours. There is no status quo.

Offline Rei

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That's a very interesting and informative thread, thanks.

1. As for the original question at the title, I would suggest you to look into  C+O2 hybrid motors of some sort.
2. As an alternative, I get to think of JP Aerospace's "airship to orbit" concept. IF it can be made to work on earth, than it may be made to work on Venus only much better since it will take the whole station in and out of the atmosphere.

Interesting - what form of carbon are you thinking of - graphitic, amorphous, diamond or lonsdaleite?  Do you know of any existing work on this concept?  At the very least, amorphous carbon is easy enough to make (carbon black).  Would still need a binder.   Maybe EVA. The binder could function as a hydrogen source to improve performance.

Might run some calcs when I get the chance.  Of the top of my head I expect it to perform worse than cyanogen (which can also be polymerized and thus used in a hybrid), since cyanogen can be induced to spontaneously reduce into carbon and nitrogen with the release of energy.  And the difference in exhaust molecular weights is too small to help..
« Last Edit: 02/10/2017 11:49 am by Rei »

Offline Rei

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It means that most people do not consider airspace as "living/agricultural space", no matter how huge it is.

They would consider the air "living /  agricultural space" if harmless lifting gases were as easy to come by on Earth as they are on Venus.  Heck, even before humans, lighter-than-air life might have taken flight.  Species that already make use of air bladders, like aquatic plants or portuguese man o'war, would surely have long since evolved to make use of them to fly (at least to spread offspring).  Aquatic plants usually float theirs with oxygen from photosynthesis; men o'war, mainly with carbon monoxide.  Both are lifting gases on Venus with about half the lift of helium.  Also, some plants also already lighten their pollen with air sacs of respiratory byproducts, so that the lower density pollen flies further.  Those same pollen grains would provide lift on Venus, lofting them further, and encouraging the evolution of ever-larger air sacs..  I don't know how heavy, say, a man o´war pneumatophore is, but it certainly looks paper thin.

On Earth technically you could fly with a hydrogen-filled bladder, but I'm not aware of any bladder-growing species that have the ability to produce hydrogen; that's usually a trick of bacteria or algae in exotic environments.  Hydrogen is generally a valuable resource to plants, not a waste product to be discarded.

Offline gospacex

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It means that most people do not consider airspace as "living/agricultural space", no matter how huge it is.

They would consider the air "living /  agricultural space" if harmless lifting gases were as easy to come by on Earth as they are on Venus.

What is your basis of this statement? It is possible to fly with a lifting gas, yes. Even on Earth: CH4, H2.

Is it giving a bit enough advantage to actually go for it and switch to constant flight lifestyle?

Offline gospacex

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To my knowledge, there are exactly zero communities that synthesize everything they need to sustain themselves from their local surroundings, either on land or on the ocean.

 ...

Every human presence on any other planet will have to overcome that problem.

For thousands of years almost all humans were living off the local land. Trade often was limited to exchanges with nearby tribes; sometimes circumstances were so that there was no trade at all (isolated tribes or hostile neighbors).

Exactly how many of those tribes that did not trade, survived?

You missed my point. I'm not saying trade is not useful. I'm saying that is someone "knows exactly zero communities that synthesize everything they need to sustain themselves from their local surroundings", it's only because of lack of knowledge of history.

Offline gospacex

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No no no, a Landis habitat, not a surface habitat  :)  Dear god no, building a habitat on the surface would be a crazy idea.

Landis habitat: ~55km, ~70°n, airship filled with breathable air (which is a lifting gas on Venus), getting most of its resources straight from the atmosphere which contains most of your major industrial acids in a sparse smog (or more accurately, vog).  Earthlike gravity, earthlike air pressure, earthlike temperatures, naturally sufficient radiation protection, huge amounts of living/agricultural space

Only if you consider atmosphere to be "space".

What does this even mean?

It means that most people do not consider airspace as "living/agricultural space", no matter how huge it is.

Ah, you forgot the 'living'. Same goes for deserts, rocky mountains, polar regions, space stations, and all planets without an existing biosphere.

There are human communities, entire ethnic groups in fact, living in deserts, rocky mountains, polar regions. There are no human communities living on free-floating ships in the ocean, much less free-flying habitats. For a simple reason: resource constraints are much more severe for that situation than for living on a solid land.

Offline guckyfan

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There are human communities, entire ethnic groups in fact, living in deserts, rocky mountains, polar regions. There are no human communities living on free-floating ships in the ocean, much less free-flying habitats. For a simple reason: resource constraints are much more severe for that situation than for living on a solid land.

Compared to a venusian cloud city any ship on earth's oceans would have access to an abundance of resources. Fish and the salts of the sea.

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