What goes up.. is apparently breaking the laws of physics.
March 16, 2012 9:04 PM   Subscribe

What kind of gas would you need to keep a steampunk/fantasy airship aloft?

Your basic fantasy-style airship is typically a balloon of some kind with a regular wooden boat (anything from a canoe to a galleon) slung underneath it and some kind of rotary propellers at least on the rear of the ship, to provide thrust, and often some along side the sides aligned vertically to - ostensibly - provide some extra lift but mostly there to just look nifty. The balloon is usually the length of the ship, and occasionally slightly - slightly! - larger, overhanging by less than 10% of the length of the ship.

Last time I looked at this what I came up with was that, even using hydrogen, at that kind of ratio - of a balloon of roughly the same dimensions as the ship - and using modern light materials and a moderate ship size (mostly-fiberglass yacht of around 60 feet length and 20 feet wide) that the amount of lift provided by the balloon would be minimal, enough that you might as well ignore it entirely and rely entirely on propellers/rotors.

1) Does that sound about right? I understand that modern blimps use helium for safety reasons, but the balloons are VASTLY larger than the cabins.
2) If a ~60'x20' elliptical balloon of hydrogen is not enough to lift a similarly-sized fiberglass yacht, what kind of gas WOULD you need?
posted by curious nu to Science & Nature (19 answers total) 7 users marked this as a favorite
I believe hydrogen is the lightest element, so I don't know what could give you more lift by chemistry alone. (I am not a chemist.)
posted by elizeh at 9:10 PM on March 16, 2012

For an ideal gas, density scales with molecular weight (at a given temperature and pressure). Hydrogen is as light as it gets. You could heat the gas to lower the mass in a given volume, or hold it a lower-than-ambient pressure.
posted by mr_roboto at 9:11 PM on March 16, 2012

Vacuum would be the lightest thing you could fill your balloon with. Provided a sufficiently strong balloon.
posted by ook at 9:16 PM on March 16, 2012 [1 favorite]

I think even vacuum isn't light enough for most steampunk airships— you'd have to fly them on a planet with a thicker atmosphere, or use cavorite, or something. I haven't done the calculation, though.
posted by hattifattener at 9:23 PM on March 16, 2012

Hattifattener is right. What you need is a thicker atmosphere, and a planet with lower gravity.

If it weren't so hot, Venus would be perfect.
posted by Chocolate Pickle at 9:30 PM on March 16, 2012

Best answer: Can't build a vacuum balloon. There's no material strong enough, not by five orders or magnitude.
posted by bonehead at 9:32 PM on March 16, 2012 [2 favorites]

I've "designed" a few of these. I figure that the difference is held by the vertical thrust propellers and/or ornithoptic wings.

Also, steampunk fiction has traditionally involved some kind of "unobtanium" to explain the technological splinter-path. Often this is something that allows heavy things to fly, or otherwise provides great power in a world still technologically in a steam age.

Also, hydrogen is much cooler than helium. Not only does it have more lift, not only is it a renewable resource, it can be manufactured aboard ship from rainwater and [powerful energy-source], plus of course, it burns better :-)
posted by -harlequin- at 9:57 PM on March 16, 2012 [1 favorite]

Even if you could build a vacuum balloon, the difference it would make from hydrogen would be negligible next to a few superfluous brass dohickies.
posted by Blasdelb at 9:59 PM on March 16, 2012 [1 favorite]

Can the gondola be made of balsa or something similarly light and strong?
posted by mattoxic at 12:49 AM on March 17, 2012

Speaking of Venus, I believe there is a statum in the Venusian atmosphere that might be temperate, like 20000m above the surface or something like that, and isnt there an elevated massif at the north pole of Venus that rises several kilometers above the rest of the surface, where the weather might be somewhat less hellish...food for a steampunk airship thought.
posted by Abinadab at 1:25 AM on March 17, 2012

Wouldn't they be using lead molecules with the gravity dipoles reversed? Not steamy enough for ya?
posted by biffa at 1:51 AM on March 17, 2012

I too would expect there to be some complicated "steam" engine with lots of levers ("leevers") and gauges that generates hyper-hydrogen or something like that.
posted by gjc at 4:26 AM on March 17, 2012

Phlogiston, my good sir, extracted from the starry aether itself! A wondrous substance, capable of supporting Her Majesty's entire fleet of airships while maintaining dirigible-to-hull proportions desirable to both the draftsman and the wordsmith!

- Cap't Ramius, Royal Aeronautical Corps.
posted by Spacelegoman at 4:43 AM on March 17, 2012 [5 favorites]

I think you'd need Charles A. A. Dellschau's “supe”.
posted by scruss at 5:46 AM on March 17, 2012

The turbines whistle reflectively. From the low-arched expansion-tanks on either side the valves descend pillarwise to the turbine-chests, and thence the obedient gas whirls through the spirals of blades with a force that would whip the teeth out of a power-saw. Behind, is its own pressure held in leash or spurred on by the lift-shunts; before it, the vacuum where Fleury's Ray dances in violet-green bands and whirled turbillions of flame. The jointed U-tubes of the vacuum-chamber are pressure-tempered colloid (no glass would endure the strain for an instant) and a junior engineer with tinted spectacles watches the Ray intently. It is the very heart of the machine—a mystery to this day. Even Fleury who begat it and, unlike Magniac, died a multi-millionaire, could not explain how the restless little imp shuddering in the U-tube can, in the fractional fraction of a second, strike the furious blast of gas into a chill grayish-green liquid that drains (you can hear it trickle) from the far end of the vacuum through the eduction-pipes and the mains back to the bilges. Here it returns to its gaseous, one had almost written sagacious, state and climbs to work afresh. Bilge-tank, upper tank, dorsal-tank, expansion-chamber, vacuum, main-return (as a liquid), and bilge-tank once more is the ordained cycle. Fleury's Ray sees to that; and the engineer with the tinted spectacles sees to Fleury's Ray. If a speck of oil, if even the natural grease of the human finger touch the hooded terminals Fleury's Ray will wink and disappear and must be laboriously built up again. This means half a day's work for all hands and an expense of one hundred and seventy-odd pounds to the G. P. O. for radium-salts and such trifles.

---R Kipling, With the Night Mail
posted by bonehead at 7:15 AM on March 17, 2012 [1 favorite]

Best answer: Because I have nothing better to do this morning I went ahead and did the math. Lifting force (at sea level, on earth) of:
helium: 1.1 g/L
hydrogen: 1.19 g/L
vacuum: 1.28 g/L

Your 60'x20' balloon would contain roughly 475,000 liters (can't remember the formula for ellipses so I cheated a bit and capped a cylinder with hemispheres), so could lift
helium: 522 kg or 1151 lb
hydrogen: 565 kg or 1246 lb
vacuum: 608kg or 1340 lb

A 60' boat weighs about 50,000 pounds.

posted by ook at 7:50 AM on March 17, 2012 [2 favorites]

Kenneth Oppel used a magic made-up gas in Airborne. There's a reason no one really uses zeppelins.
posted by GuyZero at 12:27 PM on March 17, 2012

And in steampunk-ish airship adventure novel The Court of the Air I think they pretty much hand-wave it all away. There are airships! yay! No explanation needed.
posted by GuyZero at 3:23 PM on March 17, 2012

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