[Edited: See link to identical Steam Balloon, to actual balloons of this sort since the mid 1850's and to a patent about the shape of the balloon] H2O is lighter than O2 and N2 - reason even clouds (liquid water particles!) float.
So all we need to do is fill a balloon with water vapor (not steam) and keep it hot. any water condensing on the inside shell should drip (by tiny points meant for this) back down to the heater. Perhaps the heating should be all along the balloon shell and not only at one point on the bottom. Also reconsider the shape because of this requirement, so that water is quickly dropped from the walls back to the heater(s).-- pashute, May 14 2009 Steam flight in history http://www.flyingkettle.com/priorart.htm [xaviergisz, May 14 2009] More prior art (2006) Balloon_20NotionsA hot-steam balloon is proposed, including some calculations, among other things. I didn't know about [supercat]'s 2002 posting of this Idea, at that time. [Vernon, Jun 24 2011] The other Jean Paul Aerofeather [pashute, Aug 14 2011] //water vapor (not steam)// - in what way is it not?-- lurch, May 14 2009 Can't see why this wouldn't work... In fact it seems like a pretty good idea. You'd need insulation on the baloon skin, as the temperature inside would need to stay above 100 degrees at sea level. Obviously the higher up you are, the lower the temp needs to be, in terms of keeping the water gaseous.-- Custardguts, May 14 2009 lurch, pashute was probably trying to distinguish between water vapor and the fine mist condensation that often accompanies water vapor. From the wikipedia article on steam:
In common speech, steam most often refers to the white mist that condenses above boiling water as the hot vapor ("steam" in the first sense) mixes with the cooler air. This mist is made of tiny droplets of liquid water, not gaseous water, so it is no longer technically steam. Pure steam is present at the base of the spout of a steaming kettle where there is no visible vapor.-- xaviergisz, May 14 2009 I like it. Water is cheap and safe. This would be contained like a helium balloon but full of water. You could model it with a big mylar balloon and an heating element (auto cigarette lighter?) powered by a 9v battery.-- bungston, May 14 2009 I like it. density of hot air (99°C): 0.95 g/l density of water vapor: 0.8 g/l
What is the efficiency of heating a litre of air compared to heating a litre of water vapor?-- xaviergisz, May 14 2009 This is an interesting idea, I think. But first, the opening statement about clouds is a red goose chase. Clouds are only visible because they are a group of liquid water droplets hanging around together. The droplets themselves are as dense as any other water (ie, about a thousand times denser than air); the only reason they stay up is that they are tiny, and hence have a huge relative surface area and enormous drag. Therefore, in still air they will fall very slowly (just as iron filings would sink very slowly in honey), and any small updraft is enough to keep them aloft.
But that's not the point. Water vapour (ie a gas) is, as you point out, less dense than air, so this should work. I think your main problem will be keeping the air hot.
If you have a balloon full of air at 100°C, the air near the walls will be at about 20°C (or whatever the outside air is), but this is fine: that first layer of air insulates the rest of the air in the balloon (apart from convection, which disturbs it a little). The skin of the balloon stays quite cool because of the boundary layer of cool air.
However, suppose you had a balloon full of water vapour at 100°C. The vapour near the walls will quickly cool to ambient temperature but, as it does so, it will condense onto the walls, heating them up to almost 100°C. As it condenses, of course, more hot vapour must move in to take its place, and so on. For that reason, a hot-water- vapour balloon will lose heat at a ferocious rate: there will be no insulating boundary layer, and the walls will always be at 100°C.
How much energy would it take? A lot. Your household radiators have a surface area of maybe one or two square metres, and are kept at (say) 70°C (50°C above ambient); each one uses maybe a kilowatt of power. Now imagine a balloon with a surface area of several hundred square metres at 100°C (80°C above ambient)- it's going to need something on the order of a megawatt to keep it hot.-- MaxwellBuchanan, May 14 2009 Most materials used in balloon making - selected for strength/weight ratio, imperviousness (or coatability to obtain such), and resistance to elevated temperature - won't tolerate the higher temperature you'd need for water vapor.
Plus, heated water vapor is extremely active chemically - in ways that cloth generally doesn't like.
I think you'd have to use a material - say, aluminized mylar, for example - that would not typically be employed for a man-rated balloon for safety reasons. If you get a good material that'll handle it, you'll probably negate a good amount of the lift advantage with the added weight.
If you have to do a dual-layer envelope, then there goes even more of the advantage.
This idea has been halfbaked, baked, patented, but never really seen actual service. The impression that I get is that it's not impossible, but sufficiently difficult that it loses its appeal.
I was tempted to [m-f-d] this because of the prior art "Steam Balloon" (I'll get around to linking it later, if someone doesn't beat me to it), but decided against it because I think that there's points here not covered before.-- lurch, May 14 2009 I was thinking about a conventional hot air ballon. Suppose it were full of hot water vapor. A collection cuff on the interior could route condensed vapor into a pipe that took it back to th heat element.
For the heat element you would use the standard gas jet, which I think burns propane. Adding the water would cause this jet to emit a hot CO2 / H20 mix: greater lifting power, and so more economical with the propane.
But probably not practical. Max, that was a trenchant and excellent analysis of the problems of water as a lift gas. I likes.-- bungston, May 14 2009 I kind of like it for a couple of reasons.I think that it should be possible for at least a large portion of the lifting gas to be water vapor using nothing more than solar energy and black plastic, and it would be an efficient meathod for transporting water this way in arid climates even if the balloons had to be tethered to another means of transport.-- 2 fries shy of a happy meal, May 14 2009 Sorry, I somehow missed the "steam-balloon" I REALLY DON"T KNOW HOW!. I'm always excited when I find that (the 4 or 5) ideas (a day) I have, have already been written in the Halfbakery, discussed, patented and actually created. Anyway, because its such an interesting discussion, lets leave it please and not delete it.
And yes, I meant "not steam" - not mist, in other words gaseous water which is not seen.
I edited the entry to point to the Steam Balloon.-- pashute, May 15 2009 Or you could paste the entire discussion of this idea as an anno in steam balloon. It is not so long.-- bungston, May 15 2009 er. jutta, what do you say?-- pashute, May 17 2009 [jutta] answered in email that its better to leave the discussion because people can then delete or change their annos. I'm linking from there to here.-- pashute, May 26 2009 the math supports steam ballons, since the specific heat capacity of water is several times that of air, while it would take a long time to generate enough steam to lift off, once inbound it would take a much longer time for the envelope to cool. It would basically be a "heat battery" the boiler on board would just be enough to replace the reboil the condensation faster than it's formed
secondly due to the higher lift capacity of steam, the envelope would be smaller thus having a lower surface area exposed to ambient temperature further increasing efficiency. You could also paint the balloon black to increase the envelope temperature and get some free solar heating. The difference in lift capacity would more than allow you to attach lightweight insulation such as aerogel.
my favourite part is that you could attach a small steam engine to the balloon to power a propeller. The envelope would then act as a condenser and economizer, essentially you would have a 99% efficient system, the only waste energy would be heat that didn't make it into the envelope. Also any condensed water could be stored to act as ballast for when you want to come down or converted back into steam if you want to go up essentially letting you lift off with 0 ballast but gain some if needed.
In the modern globalized economy in order to capitalize on this invention, you could install a steam cleaning nozzle and operate the worlds first mobile steam cleaning business to impoverished villages around the world, becuase everyone wants crisp steam cleaned clothes.-- metarinka, May 14 2010 A few people here think that water vapour in the air as to condense out as soon as the temperature drops below the boiling point of water. In fact air can be kept supper saturated with water vapour, down to minus 15 C. the challenge is the elimination of condensation nuclei.
— pashute, Have you considered a thermal fogger as your boiler?-- j paul, Jun 23 2011 j paul, are you j.p.david from the Aerofeather personal blimp?-- pashute, Aug 11 2011 No I’m J P Fellows from Middlesbrough!-- j paul, Aug 11 2011 If the balloon were surfaced with one way reflective foil as is done on limosine windows, sunlight could get in but not out. This might not be enough to boil it but would help. The balloon could be used in daylight in sunny areas.-- bungston, Aug 11 2011 Sounds better than my hot hydrogen balloon. (May the pilot and crew R.I.P.)-- AusCan531, Aug 12 2011 Dont remember in which idea, but I was almost eaten to the bone about "one way mirrors". There's no such thing. But thanks bungs.-- pashute, Aug 14 2011 random, halfbakery