Goodbye Avgas, Hello Switchgrass

Nearly all aircraft engines operate by burning a liquid fuel. The resulting release of heat expands a gas, and the ensuing pressure rise does some kind of work, either turning a propeller shaft or driving the heated gases and combustion products out the tailpipe at high speed. Some engines are fussy about their fuels, others aren't. Rockets are notably omnivorous: Small rocket motors have been made that consume wadded paper, animal fat and even, in the case of motors made of acrylic plastic, themselves. Burt Rutan's SpaceShipOne flew into space three times, and collected a $10 million prize, on the energy released by burning common rubber in a torrent of laughing gas.

An essential requirement for an aviation fuel is that it possess a lot of latent heat per unit of volume - gallon or liter or cubic foot or whatever you like. In addition, there are many practical constraints. A fuel must be able to be stored, pumped, metered to the engine, and so on. It must remain liquid at low temperature. It must not fizz or boil at low ambient pressure, or leave harmful deposits after evaporation or combustion. It must be compatible with alloys and compounds used in engines, seals and pumps. In the near term, our concern will not be with what entirely new technologies may replace present ones, but with how present fleets of aircraft will continue to operate, without major modifications, for several decades into the future. For current types of engines in commercial use - as opposed to SpaceShipOne's hybrid rocket - there is no alternative to liquid fuels similar, in as many ways as possible, to petroleum-based ones.

Wikipedia, the use-it-at-your-own-risk online encyclopedia, provides a table of energy densities for various substances and storage methods. They are arranged in decreasing order of energy per unit of mass. Setting aside the immense energy of nuclear reactions, we arrive at hydrogen and then, after some impractical and toxic metals, at our old friend gasoline. The units of measurement don't matter; the essential point is that hydrogen scores 120, gasoline 47, jet-A 43, ethanol 30, methanol 20, camel dung 15 and TNT 4. Batteries are way down at the bottom, represented by numbers like 0.1 (in other words, they are very heavy in relation to the energy they contain).

Moving over to the per-volume column, however, we find a different state of affairs, with diesel oil leading off the combustible liquids with a 38, gasoline a close second at 35 and cooking oil a 30. Ethanol scores 24. Hydrogen, even in its comparatively dense liquid form, which requires storage at a rather inconvenient -423º F, trails with an 8. These numbers represent the relative amounts of energy, and therefore the available range, that can be stored in an existing fuel tank; the first set of numbers - energy density per unit of weight - represent the amount of energy that can be carried within a given weight allowance, so that a "heavier" fuel is one that weighs more for a given trip. Both constraints are important; but since fuel is usually carried in wings, where the lift is, it might be easier to rationalize gross weight increases for a heavier fuel than to increase fuel volume for a lighter one. Tip tanks remain a simple option for increasing fuel capacity to compensate for a less energetic fuel (like ethanol) without requiring major modifications of primary structure.

Mercedes turbo diesels are being inexpensively modified to run on discarded french fry oil, which will be free at the local hamburger joint until the converted Mercedeses outnumber the hamburger joints - which, if my Los Angeles neighborhood is any indication, will be pretty soon. But that would not work in an airplane, because the oil congeals at low temperature. (Global warming will help with this.) Turbine fuel can be synthesized from coal or natural gas. South Africa already operates airliners on synthetic fuel, and the U.S. Air Force has successfully tested a B-52 on a 50-50 blend of JP-8 and a fuel derived from natural gas by a German process predating World War II. Boeing is looking at fuels derived from various kinds of plants and algae, and also at fuel cells and electric motors. (For that matter, GM has been "looking at" electric cars for a couple of decades.)

Turbines are relatively fuel-tolerant. Piston engines, particularly those with high compression ratios, are a different story, having experienced, since the 1920s, a convergent evolution with a highly specific type of fuel. Nevertheless, ethanol derived from corn has been used, with some success, to fuel airplanes. Unfortunately, there is a problem with these so-called biofuels (that is, ones derived from plants) as well. They are renewable, but immense amounts of cropland are required to produce them. For example, it has been estimated that the energy needs of the U.S. airline industry alone would require 10 percent of the nation's cropland. None other than Fidel Castro has complained that dedicating arable land to energy rather than food production is robbing the world's poor to transport the rich, and he has a point; but so far politics and distribution, rather than supply, have been the barrier to charitable use of our surpluses.

The oil industry naturally has an interest in discouraging the exploitation of alternative fuels, and has already taken the position that government support for alternative fuels - specifically, for corn-based ethanol - discourages investment in additional refinery capacity. Thus, the petro-giants can blame ever-shrinking supplies and ever-rising prices on the very people and policies that seek to alleviate those problems.

It remains to be seen how the nonturbine segment of general aviation will be affected by fuel prices, whose influence takes years to be felt. If you suddenly find yourself spending, say, $10 a gallon for avgas, you may not turn around and sell your airplane; but when the time comes, will you buy another one? Inevitably, there will be a revolution in fuels for aviation as well as for other types of transportation. We are at or near its threshold. Expect moderate chop ahead.

Stone Clouds In May my friend Russ Hardwick and I flew out to Page, Arizona, from Los Angeles. It was about a two-and-a-half-hour trip. Vance Swarz, a competitive off-road biker and human museum of bone-replacement surgeries, met us at the airport with his yellow Hummer, and we were off to visit a slot canyon.

The town of Page came into being, along with the Glen Canyon Dam, in the late 1950s. It is located within the Navajo lands that occupy most of northern Arizona, and it serves as the staging area for tours of several mysterious and magical cracks in the earth's surface. These so-called "slot canyons" are cuts, eroded over millions of years, through sandstone laid down when what is now a mile-high plateau was the floor of a plesiosaur-infested sea. You would guess, perhaps, that flowing water would simply cut a straight gouge, like a saw, or a V-shaped notch, like a chisel; or, at the most, that it might indulge in a few graceful meanders. Luckily, you would be wrong. Instead, it twists and turns, shuttling side to side and up and down or spiraling dizzyingly, its own cargo of abrasive particles scouring away the rock in fantastic shapes. Strata, stria, humps and hollows, tongues and hips, billows and swirls - the whole vocabulary of cloud formations, familiar to pilots, and countless other undreamed-of things, are here set forth in stone. Exposed to oxygen, the sandstone surface colors; intermittent light filters in from above - slots may be 50 yards deep and only a yard wide - in pencil-like beams and meandering glows that suffuse the eerily sculpted passages, while your feet tread orange sand as fine as flour.

This is worth seeing. Conveniently, there's an excellent airport at Page, and it's just four blocks from the center of town.

The most famous of the slot canyons is Antelope, also known as Corkscrew. If you've seen photos of slot canyons, there's a 50-50 chance they came from Antelope. And if they came from Antelope, there's a 50-50 chance they were shot over the heads of a milling crowd of camera-wielding tourists from all corners of the globe. Amateur photography in Antelope Canyon has risen to the level of the grotesque. Guides, when they are not insistently calling your attention to the supposed resemblance of some rock or rock-framed hole to a bear or a teapot, are instructing you to stand exactly here, and point your camera exactly there, to get just the right shot - a shot that you have already seen on a dozen postcards, but will now have the pleasure of taking home in your very own camera. "Please stand aside, group coming!" A parade of tripods and eight-megapixel micro-cams tramps by.

This is not the way to have an uplifting aesthetic experience. On the other hand, Antelope Canyon is, like Naples, something you should see before you die. And, to give the devil his due, photography can be a useful part of the experience; the camera heightens colors that the eye for some reason mutes.

So here is my suggestion. Fly into Page and take one of Swarz's private tours (vance@lakesidebikes.net) of Waterhole Canyon. It'll be just you and he, and if you want to linger for a while in the canyon and meditate upon its anfractuosities in solitude and silence, he'll be glad to leave you alone. The 40-minute drive to and from the canyon includes some entertaining off-road tricks by the Hummer. Then, and only then - or the next morning, if you arrived, as we did, in the afternoon - take one of the cattle cars to Antelope to experience its sheer grandiosity, which, despite the Disneyland atmosphere, is well worth seeing.

Then stroll back to the airport, pay your bill, and be back home in time for dinner.