Saturday, 30 July 2011

Ballonts under pressure (Ballonts IV)

The previous post dealt with the physics of balloons, with an eye on what it would take to design a viable animal using a lighter than air approach. The main thing that emerged, not very surprisingly, was what makes a balloon work is the difference in density between the gas inside it and the air outside it. It was also clear that balloons below a certain size do not even get off the ground; bigger is better for balloons. And that could raise difficulties, for how do big ballonts breed if not by producing little ones?

Click to enlarge; copyright Gert van Dijk

Seeing how small ballonts cause trouble, here's one painting in the Furaha collection with small ballonts. It was destined for oblivion regardless of whether the ballonts it showed could work. It was an early painting; the hexapod (Caeruleacornu rubrum) is much too insectile and I don't like the colours or the composition anymore. The 'balloon tree' (Mollum trisiphonitum) is a mixomorph making use of sunlight to create little hot spots in which interesting thermal reactions take place. That gave me a nice excuse to paint half-transparent bubbles, always a nice thing to do. Molla (that would be the plural of 'mollum') launch their young into the air in the form of a larvae suspended from a balloon sac. The adult mollum blows gases into the sac, forcing it upwards through one of its siphons. Once the sac pops free, a valve between the sac and the larva closes, and the larva drifts off into the wild blue yonder (or hither, as the case may be). The larva is supposed to crawl around a bit before becoming sessile for the rest of its life.

As you can see, the mollum contains some of the ideas mentioned in the comments on the previous post, such as using a ballont for just one stage on a being's life cycle, or having it produced by an adult. What it also shows is the kind of ballonts I would have liked to have, i.e. fairly small ones... Oh well; what remains to do now is to play around with all the factors in the ballont equation to see how we can get as big as body mass as possible with as little a sac as possible.

A thinner membrane
In the calculations the membrane consisted of a Mylar-like substance. The Mylar party balloons you see everywhere use metal to resist gases diffusing through the Mylar. Whether animals can do that as well is uncertain, but, as fishes face a similar problem with swim bladders, and their sealing method works. I looked at spider silk to see if that would be better, but its density is about the same as that of Mylar. I did not dare to make the membrane thinner than 0.1 mm, which I thought was stretching it already (sorry about that one...).

Change the gas in the balloon
The lighter a gas is inside a balloon, the better, and hydrogen is as light as it gets. About the only way to get less mass would be to heat the hydrogen: after all, hot air balloons float because one cubic meter of hot air weighs less that one cubic meter of colder air. Does heating hydrogen make a difference? The 'ideal gas law' nicely describes the relation between pressure, volume and temperature of a gas. After expanding the ballont model a little bit the model allowed a calculation how much mass of hydrogen could be saved to fill a balloon with a 1 meter radius for a range of temperatures. This is what came out: this hypothetical balloon could lift 4.8519 kg with the inside and outside both at 15 degrees centigrade. With hydrogen heated to 25 degrees less hydrogen was needed to get the same pressure and so the balloon could lift more: an additional 12.4 grams, to be precise.

What!? A bit more reflection clarified why this was so. A hot gas requires fewer molecules to exert the same pressure as a colder gas, and the differences in the amount of molecules needed determines the difference in mass, i.e. how much it lifts. But hydrogen weighs so little that the reduction doesn't amount to anything. It does if you are dealing with a heavier gas such as air. In air, there's not much point in using a hot hydrogen balloon. By the way, those designing their own ballonts should make certain that the bladder is filled with hydrogen only. Water vapour is much heavier than hydrogen, so the bladder should not be 'contaminated' with it!

Change the composition of the atmosphere
Adding heavy gases to your atmosphere will increase how much mass a ballont can lift. Earth air largely contains nitrogen and oxygen, but there are heavier gases. The real heavyweights are noble gases such as krypton (3.7 kg per cubic meter) and xenon (5.86 kg per cubic meter). Radon is even heavier but radioactive. You can dream about replacing half of the nitrogen in the Earths air by xenon: the density of the air would increase 2.4 times, and so would the lifting power of a hydrogen-filled ballont. The snag is of course that heavy elements are very rare in the universe, so such an atmosphere would make little sense. Some other gases might help, such as chlorine, sulfur dioxide or benzene. Large amounts of those would create a nice atmosphere for ballonts. Do not ask me to design a biochemistry to make such an atmosphere probable; I would not know.

Change atmospheric pressure
Another way is to increase atmospheric pressure. Gases can be squeezed, and the physics aren't complicated. Say a given volume of air on a planet X would have a mass of 1 kg; the same volume of hydrogen might have a mass of 0.1 kg. That leaves 0.9 kg to lift something with. Now we increase the pressure twofold. The same volume of air now masses 2 x 1 = 2 kg, and that volume of hydrogen masses 2 x 0.1 = 0.2 kg. The difference now is 1.8 kg, also doubled. So atmospheric density has a linear effect on liftable mass.

Click to enlarge; copyright Gert van Dijk

The graph above shows liftable mass; see the previous post for how that was arrived at. Start at the line for 1 atmosphere (that is Earth itself). If you increase the radius of your balloon, the liftable mass rises, and more so for as the radius increases. We knew that. Go to the next line, one for two atmospheres of pressure, and you get a similar curve. It is just higher.

Click to enlarge; copyright Gert van Dijk

The image above does something similar. It builds on the balloons in the previous post. Under '1 atm.' (that would be Earth) there are two balloons, one with a 0.5 meter radius and one with a 1 meter radius. Underneath are slung the bodies they can just lift. Now let's see what happens if we decide that we want balloons to lift these same bodies, but under a higher atmospheric pressure. The balloons get smaller, but not as much as you might think or wish. For instance, the balloon that had a one meter radius under one atmosphere of pressure can have a radius of 79 cm under two atmospheres of pressure (that radius defines a sphere with half the volume of the with a one meter radius - with twice the density, the mass is the same; see?).

No matter what you do, that third power effect of radius conspires against having small ballonts. I think that I will delve into the possibilities of atmospheres with hundreds of times the pressure of Earth in a later post. That should do justice to 'Jovian floaters'; in the New Hades bookshop you will find that they were supposed to be so common in every gas giant as to be boring. We'll see.

You can of course keep on increasing atmospheric pressures even on a terrestrial planet, but there will be consequences; there always are. Think of wind forces, think of hothouse effects; there are probably lots of other effects. One is 'drag', or the force that resists moving through fluids or gases. If you want a ballont to move against the wind, you will want as small a bladder as possible to reduce drag. With an enormous bladder all a ballont can do is float with the wind, against which resistance would be futile. In a dense atmosphere the bladder would be smaller, making a self-propelled ballont more feasible. But drag also increases with density; as I said, there are always complications, even in a simple Newtonian universe.

In the past I had worked on the physics of ballonts a bit but not in detail. Those earlier efforts had made me settle on a pressure of about two earth atmospheres for Furaha. Two atmospheres is about what you get with a depth of 10 meters of water on Earth. Human bodies can adapt to that, as evidenced by underwater habitats. I did not dare, then or now, to go higher for fear of the consequences. What the current more detailed analysis yields is that smaller ballonts are, how to put it, exempt from existence.

But large ballonts will stay, at least for now. How Furahan ballonts breed and what their evolutionary history is are things that need quite a bit of reflection. I would not be surprised if regular commenters solve these issues long before I ever get round to them...

17 comments:

Evan Black said...

I had never even considered the temperature of the lifting gas, but it seems that it has little effect on the density of hydrogen. But what if we turn around the equation? Hydrogen may not expand much with heat, but what if the atmosphere around the creature is cooler? Is it possible that ballonts can thrive in high-latitude areas with generally colder climates?

I have a planet idea with a surface atmospheric pressure of 9 atm (humans live at the highest point of the planet, where the density is less than 3 atm). I'm definitely going to explore ballont life there, but I'd also like to include some lighter-than-air life on Nereus as well. It seems like the square-cube law is the worst enemy of ballonts, though; I'll have to fight hard for my floaty nereids. :(

You bring up a good point with the drag coefficient of denser atmospheres. One thing that I don't think has been mentioned in this series of blogs yet is the aerodynamics of the gas bladder. Spherical objects have been analyzed up to this point (most likely because of the easier math involved) but a more oblong, bullet-shaped form would probably be advantageous in dense environments, cutting down on drag without shrinking the size of the bladder as much.

Anonymous said...

i, rodlox (on lj) say,

*C.rubrum* reminds me (conceptually) of a giraffe wevil/beetle)

your puns are fine.

lots of food for thought about ballonts. thank you for sharing this.

rodlox said...

a thought - perhaps ballonts have an evolutionary intermediate stage along the lines of, not a jellyfish, but a Nereid Cloudrocket...and they gain the ability over generations to store more and more gasses in their bodies. (this would let them fine-tune their altitude/direction with little puffs of air - and emergency jetting away from serious danger)

Sigmund Nastrazzurro said...

Evan: interesting idea! I'll have a look at low temperatures. The limit will be zero centigrade though, assuming you don't want to freeze your animals. bear in mind that there is a third power law there.
As for drag, well, I thought about it, and reasoned that as long as volumes are so large that there is no change of self-propelled ballonts anyway, I might as well keep them spherical to keep membrane weight down. I suppose drag comes into play as soon as any semblance of control comes into play.

Anonymous/Rodlox: that must be what I was thinking of at the time (I do not remember, sadly). Thanks for the pun appreciation; this not being my native language it's appreciated.

Rodlox proper: might be...

JohnnyYesterday said...

Perhaps ballonts reproduce by budding. The large parent supports its growing offspring until their gas envelopes are large enough that they can fly on their own.

That still leaves the problem of the very first ballont, how did it reach adulthood if it couldn't fly below a certain volume?

Perhaps the first ballonts couldn't fly, but used their gas sacs to lessen their weight so they could jump farther and farther.

Maybe they wanted to catch the wind, so they'd jump up when it was blowing and sail on it.
For this, perhaps they could inflate their envelopes fairly rapidly by reacting concentrated hydrogen peroxide with a reducing agent.

Originally, the generation of concentrated hydrogen peroxide was part of their defense mechanism. Spill rocket-grade peroxide on something that can burn, and it'll spontaneously ignite.

Sigmund Nastrazzurro said...

From Metalraptor:

Hello, this is Metalraptor. I recently read your posts on ballonts on the Furaha blog, and was going to post a comment but my computer locked me out...again. I am going to try and have that fixed in the future. Anyway, I had a possible solution for your problem of ballont reproduction. If its fairly easy for a fifteen foot/five meter ballont to lift a mass of hundreds of kilograms or more (as shown in your graphs) it should be a simple matter for a grown ballont to simply carry its offspring on its body until they grow large enough to take...um, flight...on their own.

Alternately, a juvenile ballont could make use of a partial lifting effect in order to achieve effective gliding or powered flight abilities until it could float on its own. Gliding animals like flying squirrels and colugos can be very effective in dense forests if such animals can keep their speed of descent as low as possible, and smaller ballonts would be masters at this. A small ballont with powered flight would also waste much less energy in staying aloft than a creature that lacks any floating ability whatsoever (like a bird). Juvenile creatures taking flight as soon as they are born is actually not that radical of an idea. Pterosaur embryos suggest that these creatures could fly soon after birth.

I was also wondering if you were aware of Peter Ward's ballont experiment, a speculative lineage of flying toads he termed the "zeppelinoids". Strangely, these were the only actual speculative biology lifeforms in the whole book of Future Evolution. Ward does point out one Achilles Heel of the ballont system, hydrogen gas is extremely flammable, and with ballonts being high-flying organisms this makes them rather likely to get struck by lightning. And explode. Spectacularly. Still, he does point out that lightning strikes are so rare as to not invalidate the concept of a ballont.

mithril said...

so far all the examinations of ballonts have assumed a blimp like approach, where the gas balloon provides all the lift. it seems to me that a 'hybrid airship' approach would be more likely in light of the limitations nature puts on such schemes. hybrid airships, in real life, have shapess that provide additional lift in forward flight. the creature would have to keep moving, but with the reduction in weight caused by the gas bladder's lift, could fly longer using less energy.

http://en.wikipedia.org/wiki/Hybrid_airship

Sigmund Nastrazzurro said...

JohnnyYesterday:
About a bladder helping with running around. A gasbag that would lift half a body is still very voluminous, and is probably very vulnerable. You solved the latter problem nicely because the the bag is not continually inflated. On a light planet that might work (not because balloons lift more -they do not- but because you only need very lightly built legs to jump).

Metalraptor:
I agree that, once balont evolution is under way, it makes sense that adults help keep young ones aloft until they are large enough to float with easy on their own.
I have my doubt about a hydrid system, at least for medium sized ballonts. If you look at the images in the ballont posts, you will see that the liftable mass for even a 1-m radius ballont is mall. That is all there is to build a body out of, and if that includes wings, these will be small. I cannot see them overcoming drag easily, at least not if there is a bit of wind.

In fact, so far I have not equiped any ballont with powered flight for that reason. Perhaps I should reconsider for large ones.

I have never read Peter ward's book. I did not worry much about ballonts bursting into flames, as there isn't that much to generate a spark. As for lightning, would ballonts with little mass attract it? Perhaps very occasionally a ballonts does explode on a dark and stormy night...

Mithril:
You and Metalraptor have a point there. I am still afraid that propeller motors as used in zeppelins provide so much more thrust than animal wings that the ballont would be severely powered. Then again, I have no idea how much thrust is needed and how much could be generated. Perhaps that is something for a future post, if I manage to find out, that is.

j. w. bjerk said...

Hot air balloons are "steered" by changing elevations. Generally at different levels of the sky their are breezes blowing in different directions, and updrafts and downdrafts.

A Ballont that had some control over it's buoyancy and good vision, and a moderate brain could go where it wanted to go, if not necessarily quickly or in a straight line.

Since it has to be able to generate hydrogen, even if slowly to fill the bladder, it could also vent hydrogen (in moderation) to descend.


I recommend Frederik Pohl's Jem for pretty well thought out Ballonts.

mithril said...

in regards to the "hybrid ballont" idea..mainly i was thinking about creatures that use lifting gas to offset most of their mass. the Festo 'jellyfish' and 'manta' balloons fits this definition pretty well. perhaps like the Festo manta, the ballont's 'wings' would actually be specially shaped lift gas bladders with lightweight tendons lining it, so that small muscles could make it flex in the right motions.

alternately, really small ballont types might have insect like wings or even batlike membrane wings stiffened with reinforced gas cells instead of bones.

Selden said...

Perhaps ballonts might have an internal bladder into which they compress (swallow) gas from their lifting bubble? This would allow them to control their lift (and thus altitude) without having to lose hydrogen, which presumably could take an appreciable time to regenerate from water.

Evan Black said...

Wouldn't that "swallowed" gas still lift the creature?

kopout said...

I think compressed gas would be denser and maybe not lift as much.Just checked, I am right, you just have to make sure you compress enough of the gas to make a significant difference but given the narrow tolerances we are dealing with that may not be a problem.

Evan Black said...

I didn't even think of that, kopout. Compression would increase the weight of a given parcel of gas.

Xenobiology is awesome!!!

Sigmund Nastrazzurro said...

I like Selden's idea about having a 'tank' with compressed gas. To get gas into the high-pressure container would probably require a few steps with compression chambers separated by valves. Kopout is right about how compression increases density. The wall of the 'tank' should be extremely strong though. Let's hope that it, and the muscles taking care of the compression, don't weigh too much.

Evan Black said...

Well, if the creatures are capable of strong-but-thin mylar-like membranes, perhaps they can handle lithe and efficient muscle fibers.

Uncephalized said...

There's another option besides compressing the lift gas or venting it--compression is a lot of work, and venting wastes both the chemical energy in the hydrogen as well as the energy needed to replace it. I would suggest a better option is to evolve a two-way biochemical pathway that stores hydrogen in a lipid or carbohydrate (I'm thinking lipids are the way to go, given higher energy:weight ratio and higher ratio of useful hydrogen to dead-weight carbon and oxygen). As far as I know this is biochemically feasible.

This way the animal's energy storage organs double as lift-gas reserves. Liberate some H2 to rise; bind it to fat to sink.

(Sorry for commenting on very old posts! I'm still working my way to the present...)