Aviation more than any earlier transport mode is fundamentally based on high power-to-output engines and fuels with both high volumetric and weight energy density.

It's little surprise that within years of practicable petrol-fueled automobile engines, powered flight became a reality. Within a quarter century, propeller-driven craft had all but achieved their design zenith (the DC-3), and jet propulsion was being deployed within four decades.

Aerodynamics, controls, navigation, traffic control, and business systems around passenger, freight, and parcel delivery have accounted for most advances since.

Notable among failures have been both supersonic and lighter-than-air craft. Both see limited (mostly military) use. Commercial viability has been lacking.

The alternatives to the present subsonic, large-scale, heavier-than-air, fuel-based transports are ... limited.

Electric propulsion solves the power-to-weight problem (electric motors are remarkably poweful and efficient), but not the energy-storage problem. Batteries have at best about 1/10th the energy storage by weight of hydrocarbon fuels.

Supersonic flight simply requires too much fuel, on top of other environmental concerns: noise, high-altitude pollution. Unscheduled subsonic jets achieve better flexibility than supersonic scheduled flights for those needing (and able to affort) it.

Lighter-than-air craft seem to fragile, finicky, dangerous, and expensive to operate. Changed air travel economics might address that last, but the fact that ultra-lightweight, tremendously voluminous structures capable of lifting only modest cargos and subject to extreme peril from high winds and inclement weather ... does not seem promising.

(Though Germany did operate international Zeppelin service for much of the 1930s. I may be overly pessimistic.)

There are alternatives to fossil hydrocarbon fuels. Most have extreme shortcomings.

Hydrogen as an energy carrier is possible but has remarkably low volumetric energy density, as well as poor storage and handling capabilities. It's unlikely to be utilised.

Biofuels can be effectively precise analogues of present avgas (petrol) and jet (kerosene) fuels. The problem is that even the small fraction of energy demand represented by aviation fuel use is beyond the capabilities of agriculture to produce. In 2014, Boeing touted "the biggest breakthrough that there is out there" in biofuels.

At 75 gal/acre-yr (mean of reported production) and 16 billion gallons of aviation fuel consumption in the US (2013 BTS RITA estimate), 21.3 million acres would have to be under cultivation. That's about 330,000 mile2, or a region 577 miles on a side.

On the map, you could start in Shreveport, LA, head west to Hobbs, NM, north to Denver, CO, east to St. Joseph, MO, and south to Shreveport again, traversing seven states and completely bounding two (Kansas and Oklahoma).

https://old.reddit.com/r/dredmorbius/comments/1wo2hl/boeings...

The one possible alternative I've seen, and one which isn't obviously* impractical, is fuel synthesis: creating the liquid hydrocarbon fuels powered flight is dependent on from non-fossil sources.

The notion's not new -- it strongly resembles the coal-to-liquid-fuels process employed in Germany during WWII, and in South Africa since the 1950s. The notion was suggested by M. King Hubbert in 1964 (http://www.hubbertpeak.com/hubbert/EnergyResources.pdf (PDF) p. 139), and has been explored at the Brookhaven National Laboratory, M.I.T., US Naval Research Laboratory, and a Google X Projects startup (since folded), over the past five decades.

The chemistry is sound. Scaling to military or commercial capacities, and economics, have been found wanting.

There's not much in the energy situation which gives me much hope. This remains among the very few bright spots, despite 50+ years of less-than-overwhelming progress.

(I'd looked at a fair bit of the history and research a few years ago on Reddit, most represented in this search: https://old.reddit.com/r/dredmorbius/search?q=fischer+tropsc... )