Technicalities: Making Aviation Sustainable

Why not just harness geese to the airplane?

JetBlue announced in January that it intended to become a carbon-neutral airline. To reduce its net carbon footprint, it would begin purchasing carbon offsets—credits generated by sponsoring activities and investments designed to reduce carbon-dioxide emissions. Its flights originating in San Francisco would use a low-emissions substitute for jet-A refined from a mess of pottage that includes used cooking oil, animal and fish fat, tall-oil pitch (a byproduct of pulping wood), and—get ready—“spent bleaching earth oil.”

The announcement was not shocking. Other airlines, and the US Air Force, are already using several million gallons of biomass fuel a year on an experimental basis, and way back in 2008, Virgin Atlantic staged a publicity event in which one of its Boeing 747s flew on such a brew, blended—as experimental fuels usually are—with a good deal of the real thing.

So, you might have expected little hostile reaction to the new policy, which the airline promised would have no effect on safety or ticket prices and was probably just intended to enhance its cred with environmentally concerned customers. Nevertheless, it ignited the highly combustible fury of commenters on a Fox News site. Sustainability in aviation, it seems, is more of a political matter than a technological one. Sustainability is connected with resource depletion and climate change, and for many people and corporations, both are hoaxes inimical to commerce and profits.

While we argue, however, energy pours down from the sun. Finding ways to capture it and convert it to usable forms is what sustainability means, and I don’t see why this should be controversial.

Two-thirds of US oil consumption takes place in pursuit of transportation. Jet fuel represents about a tenth of that; aviation turbines consumed about 100 billion gallons of fuel in 2018. Less than two percent of aviation fuel consists of the boutique cocktail known as avgas, and so practically all discussion of sustainable fuels for aviation focuses on renewable replacements for jet fuel.

Fortunately, turbine engines are not so finicky as recips, and the magic of industrial chemistry can turn all sorts of energy-containing stuff into liquids of tolerably digestible viscosity and volatility. There are collateral issues, however—land and water use, environmental impacts, cost, and (in the case of corn) diversion of a needed foodstuff from human and animal consumers—so the best avenue to a sustainable fuel remains uncertain.

Plants convert sunlight into organic-chemical structures that can in turn be transformed into so-called biomass liquid fuels. The process is inefficient and requires large amounts of arable land. Current feedstocks such as corn, sugar cane and switch grass yield only 400 to 1,100 gallons of fuel per acre and consume vast amounts of water; at one point, corn grown for ethanol in Nebraska was slurping up 780 gallons of water per gallon of fuel produced. Ethanol, currently the most commonly used biofuel, is already blended into most road-vehicle gas sold in the US. Ethanol can also be blended with jet fuel, but it has only two-thirds the energy content of petroleum-derived fuels, so you have to carry more of it for a given trip.

Bacteria, in the form of algae, yield five times more energy per acre than plants, and even higher yields are theoretically possible. Unlike plant feedstocks, they produce fuels similar in energy density to petroleum. They are also easy targets for genetic manipulation; maybe some 17-year-old with a Crispr genome-editing kit will figure out how to make them ooze flight-ready jet-A. But bacteria are fussier about their living conditions than switchgrass, and their exploitation on an industrial scale is distant.

The piston-engine situation is less hopeful. Even the relatively modest project of developing a lead-free 100LL replacement has stumbled. High-octane gasoline works in medium-performance engines, but it is not a renewable fuel; and a leadless 100-octane fuel for high-compression or turbocharged engines, fungible with avgas, has proved elusive.

This is where electricity comes in, with a brave flourish of trumpets.

Set electric airliners aside. Even if battery energy density doubled or quadrupled, it would still fall far short of that of the liquid fuels on which the entire structure of long-range air transportation is based. The most likely route to sustainability and emissions reduction for medium- and long-range jet airplanes is a new fuel, not the creation of a completely new flight technology.

Read More from Peter Garrison: Technicalities

Nevertheless, hundreds of startups are feverishly pursuing electric propulsion. Most will fail for the usual reasons: lack of financing or lack of talent—or both. Some airplanes suitable for carrying small loads on short routes will be created; speed, which is a great devourer of energy, is less important on short routes.

Electric power is particularly appropriate for the new paradigm represented by the autonomous VTOL multicopter, whose fixed-pitch (therefore, simple and cheap) fans respond rapidly to changes in power and make self-stabilization possible. A much-simpler and more-modest

application is already here: the small electric trainer or sport aircraft rechargeable from an electric outlet or, in a slightly more utopian vision, by a patch of solar cells on the airport.

Hybrid-electric power systems are getting a lot of study, even by the likes of Boeing and Airbus. They consist of one or more electric motors driving propellers or fans, a sustainer engine, and a battery whose size, and share of the work, is optional. The sustainer engine—diesel, turbine, turbo-compound rotary or whatever else you may have handy—is sized for cruise and optimized for economical operation in a narrow band. It runs on a sustainable fuel. It drives a generator to produce voltage, which in turn drives the electric motor. For takeoff and climb, and possibly for short cruise segments, the battery contributes extra power.

Hybrid systems are compromises. There are significant inefficiencies between their stages. The supreme mechanical simplicity of the battery- driven electric motor is sacrificed. Whether the versatility and small size of electric motors lead to compensating aerodynamic advantages, for instance from distributed or vectored thrust, is now being studied.

For small, medium-range airplanes, a possible alternative to the complexity of a hybrid powertrain is the fuel cell, which is a kind of battery that is continuously recharged by a chemical fuel. The fuel of choice is hydrogen. Hydrogen is three times as potent, per pound, as gasoline, and it’s clean; the main exhaust product of a hydrogen fuel cell is water. So far, so good. But unfortunately, hydrogen is a very light gas. Even compressed to 10,000 psi, it is less compact, as an energy reservoir, than gasoline. This is not an insuperable problem for road vehicles—several models of hydrogen-fuel-cell cars and buses are on the market—but it’s inconvenient for airplanes. Still, cylindrical high-pressure tanks could be integrated into airframes, some perhaps doubling as spars, and certain nanomaterials may be found to soak up and hold hydrogen like sponges.

Aviation technology has remained essentially static—or, at best, very slow-moving—since the jet engine came into use in middle of the past century. Fresh attention to new fuels, including electricity, and new configurations for using them, opens the door to interesting and exciting innovations. They will be decades in development, but we ought to welcome them, regardless of our politics.


This story appeared in the April 2020 issue of Flying Magazine

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