Michael Everett wrote:In the HP fanfic Make A Wish, a couple of the secondary characters (the Doctor and Henchgirl) build a zeppelin that uses a vacuum instead of helium for lift.
Not only does it work, it works well enough to become their roving base of operations and from a purely physics-based viewpoint, it is technically feasible.
Uuuhhh… no. It's not.
I've done some calculations for a zeppelin in one of my stories. Total surface area for a 485,000 cubic meter zeppelin comes out to over 48,000 square meters, making total atmospheric pressure at sea level just over 500,000 metric tons. What is the lift chamber to be made out of? Bear in mind that gross lift available at sea level is less than 600 metric tons.
To contain a vacuum, the lift chamber has to be proportionally much stronger than a 55 gallon steel barrel. I've seen atmospheric pressure crush several of those barrels with only a partial vacuum inside on Mythbusters.
Those barrels weigh over 18 kilograms, and displace about 266 grams of air at sea level. Even if the barrel could hold a perfect vacuum, that's less than 1.5% of the lift needed to make it float.
There is no way to make a container that is strong enough to withstand that much pressure, light enough to float in air at Standard Temperature and Pressure, and large enough to provide a useful amount of additional lift. The forces involved would require construction materials at least three orders of magnitude stronger than anything we know of.
There are online calculators that can provide most of those answers. If only more writers could take the trouble to look a few things up, do a little math, and not put insanely stupid things in their stories.
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Count Vordarian: "You're a Betan! You can't do—"