The aliens--Tiberians--use aerogel-filled tanks to give aircraft bouyancy, so their aircraft are semi-aerodynamic-semi-dirigibles. When cruising they rely entirely on bouyancy, but during takeoff and landing they use aerodynamic surfaces. Tiber has a higher pressure gradient than Earth, so their atmosphere is more dense; this may be a factor.
The aerogel-filled tanks are the key: they weigh less than an equivalent volume of air when emptied of air, so it floats; to land the thing, air is pumped into the tanks, under pressure, making the whole works heavier than air.
You need something to keep the tank from collapsing, and it must be very light material--hence the aerogel. (Look up what happens to large tanks when their internal pressures drop beneath atmospheric. Hint: it's not good.)
But I got to thinking: could this be done with current technology on a small scale? Could you take--say--a ping pong ball, give it a partial atmosphere of helium, and see it float? (Or, at least, take more time to fall?)
The thing I always have trouble getting my head around is that vacuum is less dense than anything because by definition it's nothing. A sufficiently light container which could withstand atmospheric pressure would indeed float if you pumped the air out. Would float, that is, so long as the average weight of both vacuum and its container was less than the air it displaced.
I fell asleep this morning thinking about all this, trying to figure out a way to do it that doesn't involve unobtanium. It went from there into wondering how one manages to pull a really high-quality vacuum, where you're reasonably certain that the enclosed volume has only one or two gas molecules in it at most.
From there I started wondering how helium behaves at partial pressure. Everyone has seen how a helium balloon deflates over time, because the helium atoms migrate through the rubber. Mylar has a denser structure so the helium takes more time, but the result is the same.
That is, however, when the helium is at positive pressure with respect to the atmosphere. (Pressure gage rather than pressure absolute.) What if the helium is at negative (or partial) pressure? What if you have 10 PSI of helium inside a container? While helium can migrate through the material, in these circumstances why would it? It would have to move uphill against pressure to do that, and I don't see that happening. At most, I think, there would be a sort of very slow osmosis where you'd lose an atom here and there, once in a while, but I think it would take a long time to be noticeable.
So: take a ping pong ball and fill it with water, then turn it upside down and fill it 2/3 full of helium. Draw out the water, then plug the hole--does it float? If you try to play ping pong with it, does it act strange because of its lowered weight? Or is the difference negligible compared to the total weight of the thing?
Just before I fell asleep, then, I realized I could calculate all this. I can work out how much an ordinary ping pong ball weighs with the air inside it, how much it weighs with 10 PSI of helium in it, and how much the air weighs that is displaced by its volume. I could even calculate what average density the ping pong ball must have in order to display neutral bouyancy.
The fact that I can calculate it is sufficient answer for now, at least for me.
During that little journey I had a mental image of someone releasing thousands of spheres into the air, each of a different density, to seek its own level. (Some kind of art, I suppose.) I had another image of an aircraft colliding--without damage--with one (or several) of these spheres. I also gave a brief thought to some kind of atmospheric monitoring system that used this kind of apparatus. (Print circuitry on the inside and outside, including photovoltaics to power it. Not all that hard to do, really, since you've already got the unobtanium and it's obviously cheap enough to be disposable if you are literally throwing it to the wind.)
Thinking of things like this is not a problem for me. The problem I have is, how do I connect these neat ideas with stories? How do I take the bouyant sensor idea and use it in a story that doesn't end up being Yet Another Trite SF Parable About The Dangers of Misusing Technology?
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Yesterday, Mrs. Fungus and I made a huge bowl of ultimate tuna salad...and it's nearly gone. *sigh* Well, I don't lightly apply "the ultimate" to anything for a reason.