Quote:
Originally Posted by Raellus
Helium-filled observation balloons were also used during WWI.
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Hydrogen-filled. In 1917, helium sold for $2,500 per cubic foot, and filling a small blimp would have cost $1,000,000 in 1917 dollars (a little over $25,000,000 today). The first flight of a helium airship was in December 1921, when the Navy blimp C-7 flew from Norfolk. The first helium dirigible was 1923's Shenandoah.
For hot air balloons, the most common fuel currently is propane. Typical fuel on board a "sport" balloon is 75 to 170 liters, and fuel consumption for a 4-hour period is roughly 450 liters. This is for an average-sized balloon of modern construction (so something like polyurethane sealed nylon for most of the envelope and Nomex for the section closest to the burner). Heavier or more permeable materials will need more fuel, as will sustained high-altitude (~10,000 foot) flight as opposed to the 2,000 or 3,000 foot flight of "entertainment" HABs.
I am curious how much a Kevlar basket would weigh, since they're typically wicker to reduce weight and remain flexible enough to land safely. A 5-person Sport basket from Cameron Balloons weighs 140 pounds empty while an 8-person Aristocrat basket is 210 pounds.
Replacing passengers with fuel tanks could extend flight. Using Cameron's tank information, steel tanks are 37.2 kilograms for 41.6 liters, 49.9 kilos for 56.8 liters, or 61.2 kilos for 72.3 liters. Aluminum tanks are 32.7 kilograms for 37.7 liters, and titanium tanks are 41.3 kilos for 56.8 liters or 55.8 kilos for 75.7 liters.
And while observation might be possible outside of small arms range, a balloon would still be vulnerable, either to small cannon (25-30mm) or ultralights or sport planes equipped with improvised weaponry. Shooting from an ultralight would be difficult, but something like Ranken Darts (a 1-pound black powder and phosphorous bomb with spring-loaded arms to snag Zeppelin fabric) would be rather unpleasant.