Comment by dredmorbius | Hacker News Reader

Comment by dredmorbius | Hacker News Reader

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Which means that if you're planning to drive already, flying may be a better option. Though ground-based electrified rail would be better still -- even if it has higher net energy consumption, solar-fed electrified rail beats fuel-fed aircraft.

The key is the frivolous word in the GP comment. Aircraft actually are fairly efficient, it's just that the mileage of a single flight is a substantial fraction of most people's total annual driving. That old Jevons paradox again.

As for electric aircraft: think "powered glider" rather than "emissions-free 747". Roughly half an airliner's takeoff weight is fuel, and that's burnt up over the course of the flight. Battery storage densities are on the order of 1/100th to 1/50th of petrol or kerosene (latter is essentially jet fuel). The trend's been slight on improvement, and though there've been some battery-powered aircraft, they're comparable to a very-small-engined aircraft with a tiny fuel tank -- think 20-40hp and less than a gallon of fuel (about 15-30 kW and 2-3 litres, for rational people).

MacKay talked about this in Without Hot Air (https://www.withouthotair.com/).

If I remember correctly, he said that the energy use per-person in a (full) commercial aircraft flying from A to B is half as much as the energy use per-car if you drove from A to B.

This means that if two people carpool from A to B, you're as efficient per-person as flying from A to B; if four people carpool, it's twice as efficient per-person as flying.

Please call me out if I'm wrong or if I misunderstood the passage from Without Hot Air.

EDIT: Source: See table 5.3 on https://www.withouthotair.com/c5/page_36.shtml

That's about right, though there are a few additional considerations:

1. Aircraft have heaviest fuel use during takeoff and ascent. Short flights are the least efficient. I'm not aware of a good modeling for this, but you might consider that there's a fixed takeoff/landing/taxi cycle and a variable cruise component to most flights. Also, larger aircraft (at capacity) tend to be more efficient than smaller ones, though that's a fairly varyable dynamic.

2. Automobiles offer numerous other capaibilities (and limitations). Generally, more cargo, and more direct transit. With airport delays adding 1-3 hours to a flight, that's 150 miles / 240 km of ground travel possible at highway speeds.

3. More luggage capacity / fewer restrictions. You can carry items on a car you couldn't on an airplane.

4. Point-to-point service. Aircraft fly to airports. Cars drive to specific addresses. That's another consideration in on-ground time, possibly several hours, in a flight. Autos start looking viable for trips of 200-250 mi / 320 - 350 km.

5. There are larger autos availabe. Five adults in a small compact might not be reasonable, but might consider an SUV or van. And efficient choice here would make up for lower fuel economy with higher passenger capacity.

Aircraft still generally win on safety on a pro-rated distance basis.

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