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> which heavier than air aircraft really ought to be able to reduce It's not clear. Dirigibles travel at very low altitude, where the air is dense and they hav

> which heavier than air aircraft really ought to be able to reduce It's not clear. Dirigibles travel at very low altitude, where the air is dense and they have a very high cross-section area. Very high as in absurdly high (A Goodyear-Zeppelin blimp has a diameter of 14m and a gross weight of 10 tons. So the cross-section area is about 600 m2 ) Jet airplanes travel at 10 km high, where it's almost vacuum. They are also very slender. They have a very high lift-to-drag ratio. A modern B-787 (Dreamliner) has an L/D ratio of 20, most other aircraft capable of crossing the Atlantic have an L/D ratio above 15.

rich_sasha 4y ago
Air drag increases with speed too. Air pressure at Mount Everest is, IIRC, 30% of sea level value, for a roughly 70% reduction in drag i guess, but if you're moving 5-10x faster, that's a 25-100x penalty. Airline jets fly at transonic speeds which complicates the calculation a bit but i think not in the favour of the aeroplane.
notahacker 4y ago
On the other hand, dirigibles don't need to worry about lift to drag ratio at all on account of not needing to generate enough thrust to produce lift. Which is why the litres of fuel per passenger km for the Hindenburg, with 1920s diesel engines mounted on platforms where fuel economy was barely a consideration compares favourably with the same economy metric for modern transatlantic flight after half a century of innovation in jet fuel economy (and cramming passengers into smaller spaces). Needless to say lighter than air platforms without comparable worries about the impact of weight on fuel consumption and with large surface areas are also theoretically superior platforms for electric engines and [partial] use of solar power than heavier than air platforms. Obviously they're not a pure emissions silver bullet, and plenty of other practical problems would need to be overcome for airships to fill more than tiny niches, but the evidence favours my statement.
- credit_guy 4y ago
  I checked your numbers, and you are right. The Hindenburg had a total mass of about 240 tons. In its last voyage it burned 42 tons of fuel over 77 hours. It was an unusually long voyage due to high winds, but presumably the engineers who built it took these type of conditions into account, so the takeoff fuel was probably 50 tons or so. A modern airplane of the same size (mass) is A-330. It has a (fully loaded takoff) mass of 240 tons, and a takeoff fuel mass of about 110 tons, so more than double the Hindenburg's. Hindenburg's speed was about 125 km/h, while A-330's speed is about 1000 km/h, so about 8 times faster. Indeed, a Frankfurt-New York flight takes about 8-9 hours in an Airbus, it used to take about 60 to 70 hours in a Hindenburg. Of course, if it takes you 2 days to cross the Atlantic, you need a bed. So, Hindenburg was able to accommodate only about 70 passengers, while an A-330 can easily fly 250. Finally, regarding solar power. Hindenburg's area as seen from above was about 10000 m2. Current solar panels can produce up to 220 w/m2, so if we cover a Hindenburg with such panels we'd get 2.2 MW peak power, or about 3000 horse-power. Hindenburg's engines were producing about 3300 HP in cruise mode. Today, with better propellers and CFD modeling, you may not need more than 3000 HP to get the same thrust, so at peak solar radiance (noon), you would not need to burn any gas at all. Of course, during the night, the solar panels' output will be zero, but the modern engines may be more efficient, so all in all it's likely that instead of 50 tons you may only need 25 tons.

rich_sasha 4y ago

Air drag increases with speed too. Air pressure at Mount Everest is, IIRC, 30% of sea level value, for a roughly 70% reduction in drag i guess, but if you're moving 5-10x faster, that's a 25-100x penalty. Airline jets fly at transonic speeds which complicates the calculation a bit but i think not in the favour of the aeroplane.

notahacker 4y ago

On the other hand, dirigibles don't need to worry about lift to drag ratio at all on account of not needing to generate enough thrust to produce lift. Which is why the litres of fuel per passenger km for the Hindenburg, with 1920s diesel engines mounted on platforms where fuel economy was barely a consideration compares favourably with the same economy metric for modern transatlantic flight after half a century of innovation in jet fuel economy (and cramming passengers into smaller spaces). Needless to say lighter than air platforms without comparable worries about the impact of weight on fuel consumption and with large surface areas are also theoretically superior platforms for electric engines and [partial] use of solar power than heavier than air platforms. Obviously they're not a pure emissions silver bullet, and plenty of other practical problems would need to be overcome for airships to fill more than tiny niches, but the evidence favours my statement.

credit_guy 4y ago
I checked your numbers, and you are right. The Hindenburg had a total mass of about 240 tons. In its last voyage it burned 42 tons of fuel over 77 hours. It was an unusually long voyage due to high winds, but presumably the engineers who built it took these type of conditions into account, so the takeoff fuel was probably 50 tons or so. A modern airplane of the same size (mass) is A-330. It has a (fully loaded takoff) mass of 240 tons, and a takeoff fuel mass of about 110 tons, so more than double the Hindenburg's. Hindenburg's speed was about 125 km/h, while A-330's speed is about 1000 km/h, so about 8 times faster. Indeed, a Frankfurt-New York flight takes about 8-9 hours in an Airbus, it used to take about 60 to 70 hours in a Hindenburg. Of course, if it takes you 2 days to cross the Atlantic, you need a bed. So, Hindenburg was able to accommodate only about 70 passengers, while an A-330 can easily fly 250. Finally, regarding solar power. Hindenburg's area as seen from above was about 10000 m2. Current solar panels can produce up to 220 w/m2, so if we cover a Hindenburg with such panels we'd get 2.2 MW peak power, or about 3000 horse-power. Hindenburg's engines were producing about 3300 HP in cruise mode. Today, with better propellers and CFD modeling, you may not need more than 3000 HP to get the same thrust, so at peak solar radiance (noon), you would not need to burn any gas at all. Of course, during the night, the solar panels' output will be zero, but the modern engines may be more efficient, so all in all it's likely that instead of 50 tons you may only need 25 tons.