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Probably going to get lots of downvotes for that name... Anyway, assume that you have a zeppelin, made entirely out of lead. What gas would be required to hold this in the air? Would this contraption even be able to stay together, or would it fall apart in the air?

Assumptions–The gas in the zeppelin does not have to be naturally occuring, or radioactively stable. It just needs to have a very low density. The zeppelin is approximately the size of modern-day zeppelins, or, seeing as those aren't too common, 200,000 $m^3$. The envelope is the only thing made of lead. This is also at standard temperature(think between 60 ºF and 90ºF, 1 atm pressure, earth gravity.

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  • $\begingroup$ Can you specify the size of the zeppelin, as well as how much of it (i.e. what parts) are made of lead? $\endgroup$ – HDE 226868 Mar 2 '16 at 22:13
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    $\begingroup$ Note that Mythbusters already created a lead balloon. $\endgroup$ – Frostfyre Mar 2 '16 at 22:22
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    $\begingroup$ The Mythbusters made a lead balloon float on one of their shows. Hydrogen or Helium would be the best lift gas, as anything else would be denser. It wasn't particularly practical, as the lead is very weak and tears. $\endgroup$ – Oldcat Mar 2 '16 at 22:23
  • $\begingroup$ What other gas would you even consider? Is that something to do with the lead, or do you not know about the density of gasses? Gas required to hold in the air? I don't follow. $\endgroup$ – JDługosz Mar 3 '16 at 0:14
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    $\begingroup$ Important to note here is the distinction between zeppelins and blimps: Blimps are big balloons. Zeppelins have a rigid internal structure. Not too great a point, but very worth considering! $\endgroup$ – Joe Bloggs Mar 3 '16 at 10:44
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The airship has to have a lower density than air (at the surface) to float. The density depends on the total volume and the total weight of structure, engines, fuel, payload, and lifting gas. If your structure gets heavier, you need more of the low-weight lifting gas to compensate. The larger gasbags require more structure, and so on.

  • The best gas is hydrogen. Radioactive or artificial isotopes, or molecules of different elements, are all going to be heavier. Helium is the next best option, and less flammable.
  • A theoretical alternative would be a vacuum airship.
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A zeppelin entirely made of lead?

No.

  • There is no gas lighter than hydrogen, and using it in air ships is a really bad idea. So you're stuck with Helium, which is what all modern zeppelins/blimps use. (Vacuum airships are not likely to work on Earth ever, due to material constraints.)
  • Lead is not a very good metal for any kind of load bearing structure. It is way to soft to be able to build the light-weight frame which you need for the zeppelin, let alone speaking of the actual air-bag. This means you'll need to build sturdier, which makes the entire thing really heavy. Even more so, lead has more than 4 times the density of aluminium which is normally/often used for aircraft structures, so you'd need even more gas, thus a bigger structure, thus more gas, ... ad infinitem.
  • Furthermore you'll need an engine/props to actually get some propulsion and maneuverability (you can't make an engine entirely from lead).
  • Last but not least, lead is not a very healthy material to be around. You crew would be in danger of chronic lead poisoning (quite a serious issue).
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  • $\begingroup$ Do you have evidence for your assertions that making an engine or a gas bag out of lead are impossible? Difficult, sure, but impossible requires a bit more proof... $\endgroup$ – ckersch Mar 3 '16 at 23:58
  • $\begingroup$ @ckersch I don't know if it counts as proof or not, but the melting point of lead is a mere 327 degrees C. You definitely could not have a serious engine based around fire (such as the internal combustion engine) without dramatic heat considerations. Maybe a steam engine with a lot of liquid cooling below the boiler might work. $\endgroup$ – Cort Ammon Mar 4 '16 at 7:59
  • $\begingroup$ @CortAmmon For a low temperature engine, you could use a sterling temperature and control the fuel flow to keep the hot cylinder below 250 degrees C or so. $\endgroup$ – ckersch Mar 4 '16 at 14:22
  • $\begingroup$ @ckersch: Making a gas bag from lead is possible, the Mythbusters did it (mythresults.com/episode96). Note that his is only the gas bag though, not any kind of load-bearing structure, nor a cabin of any sorts. $\endgroup$ – fgysin reinstate Monica Apr 20 '16 at 13:44
  • $\begingroup$ @ckersch: Modern engines generally rely on electricity, which is dependent on being able to isolate your electrical conductors. I can't imagine how you would create conductors and their isolation from one material alone. Admittedly, some really primitive form of thermal engine like a Stirling Engine might be built just from lead. But why on Earth would you want to? :) $\endgroup$ – fgysin reinstate Monica Apr 20 '16 at 13:48
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Yes: If the lead is thin enough. See answers above. Lead has a density of about 11, or around 5 times that of aluminum.

Most zeppelins had internal gas bags, and a skin of aluminum attached to an aluminum frame. Note that when the Hindenberg burned, the bulk of the flame (and damage) was caused by the burning aluminum rubber paint, and the aluminum frame itself. See also the demise of HMS Sheffield during the Falklands war. Aluminum is flamable.

That said, the difference between helium and hydrogen for lift is small. Air runs 30 grams per mole. Helium 4, and hydrogen 1. It's the differences you care about. So it's 28 g/mole for helium and 31 for Hydrogen.

More to the point, it depends on the atmosphere of your world. If your world had twice the air pressure, then the lift per volume of gas doubles.

If you can come up with a plausible reason in terms of geochemistry, put basis of SF6 on your world. This is about 5 times the density of air at STP. Chemically inert, but a deadly asphixiant -- Fill a basement with it, and your victim goes downstairs, and passes out unaware of anything wrong at all.

Also a very potent green house gas, some 23,000 times as effective as CO2. Since it's harmless aside from it's smother potentile it may be the easiest way to warm up Mars -- if you can find planetary quantities of it.

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This is more a question of the ratio of gas to lead. With helium, you need roughly 1000 cubic feet per 65.82 lbs of lead/materials holding the lead together, and for hydrogen you need 71.05 lbs for the same number of cubic feet.

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No matter how dense is the gas within, the lifting force is limited by the mass of displaced air. Air density is around 1.3 kg/m^3, giving a theoretical upper limit of 260 tonnes. Hydrogen (both light and abundant) is 14.5 times lighter than air, and thus would eat about 7% of the total mass.

Now assuming the shell is spherical (best surface/volume ratio), its radius and surface can be deduced from volume:

V = 4/3 * pi * r^3
r = cubic root( V * 3/4 / pi ) =~ 36.2m 
S = 4 * pi * r^2 =~ 16459.1264 m^2

The shell mass (given thinkness h and density d = 11340 kg/m^3) is

M = S * h * d

Therefore the theoretical upper limit on shell thinkness is

h = M / S / d =~ 0.0013 m = 1.3 mm

For other forms, the shell has to be even thinner. Too lazy to calculate how much...

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