Aviation industry tries to go green

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... )

The question is not "how can we keep our air travel habits?" after all.

Air travel has been transformational, fast, reasonably convenient, and cheap. It's also been an accident of geohistory, in many regards. The coming / current fuels transition will raise the costs of many things, transport included, and air transport most especially. One of the lessons of history is that the consequences of costs falling, as many have through the industrial revolutions, have subtle and profound influences, mostly unanticipated by those who first started down specific technical pathways.

My sense is that rising costs will operate similarly. We'll have much of the knowledge and technical understanding achieved over the past 200 years, but we'll see costs rising or capabilities falling in many domains. And that will have profound and subtle ripple effects.

Small aircraft may be possible, and perhaps slow airships, but the era of mass bulk human transcontinental transport, quite probably not so much. Or possible, but at prices at some multiple of today: 2x, 10x, 100x, 1,000x. Or transoceanic travel of 1-3 days, rather than 6-18 hours.

Getting back to airships: there are only so many materials available. An airship requires some sort of frame, some sort of lifting gas containment, motors, an energy source, and passenger or cargo facilities. Even excluding hydrogen as a lifting gas, they've proved expensive and dangerous. Vehicles such as LEMV / Airlander 10 are not strictly rigid, but is in part supported by pneumatic pressure. The craft itself was written off after coming loose from moorings in high winds.

Top speeds will be on the order of 160 - 240 kph, rather than 1,000 kph for jet aircraft. Flight levels will likely be I've watched and read breathless accounts of an airship revival for decades. To date they've failed to meet promised capabilities.

Yeah, it happens with every carburetor car I've ever had, less so with fuel injected ones. It takes about a half mile of driving to where it won't happen.

It has higher viscosity, meaning it takes more energy to move it out of the way. As it warms up, its lubricating power decreases, but not too much, and its viscosity decreases, enabling better engine efficency. A cold engine is less efficient, but only for a short time, until it heats up.

Oil too thick to pump rarely happens in normal life. In arctic and subarctic conditions people use electric heaters to keep the oil thin enough to pump.