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Declining Fossil Fuel Reserves and the Future of Air Travel
Submitted by Dan Sweeney on Fri, 2008-05-23 20:35.
Most of those concerned with our energy future, at least those who assume that business as usual cannot be maintained indefinitely, give relatively little thought to air travel in the years to come. Everyone just seems to assume that the same kinds of jets that take us hither and yon to almost anywhere in the world will continue to do so for the next fifty years, if not forever.
It is a curious fact that aircraft, the most recent form of human transport to be developed, are in many ways the most conservative in design. The first jet passenger plane, the ill fated and dangerous British Comet, began flying more than fifty years ago. If you saw one on the runway at an airport today you wouldn’t give it a second look. The basic design looks entirely modern. It is also interesting to note that the first passenger jets of the nineteen fifties were just about as fast as their modern descendents. There just hasn’t been much visible progress.
The same is true of private aircraft by and large. They look like aircraft from the thirties, forties, and fifties, and generally use thoroughly antiquated engine designs. Only in the areas of small experimental kit aircraft and private jets have there been much obvious innovation. Military aircraft have advanced to be sure, but still only in an evolutionary sense. Jet fighters and bombers look much the same as they have for decades. The main difference has been slightly improved materials technology and far more sophisticated electronics and weapons systems.
The one area where considerable improvement has occurred is perhaps the least visible. Jet engines have manifested double digit improvements in fuel efficiency over the past thirty years or so and will probably continue to improve in this respect. Which will be to their advantage in a fossil fuel constrained world, but may not ensure business as usual.
Jet passenger planes use thousands of gallons of jet aviation fuel, a type of kerosene, in their transcontinental flights. At projected mid term pricing of several dollars a gallon, that makes these birds extremely expensive to operate. A jet airplane, for all its improvements in efficiency, has the lowest energy efficiency of any common form of transport.
If, as we expect, fossil fuel prices continue to escalate, airlines will be forced to raise their prices in order to continue to operate. If, at the same time, fuel prices increases are sufficient to affect the overall economy adversely, air travel could begin to revert to what it was prior to the advent of jet aviation in the fifties, a mode of travel that was priced out of reach for most Americans. Determining the overall effects of restrictions in long distance travel is a difficult undertaking, but it is safe to say that such a change would not be welcomed. That honeymoon in Hawaii might have to be reset for Palm Springs. And that the hospitality industry would suffer enormously is virtually a certainty.
So what are the possibilities of maintaining relatively low cost air travel in the face of nearly certain increases in the real cost of jet fuel?
We see several ways in which the aircraft industry could adapt to significantly elevated fuel costs, some rather likely, and some quite far fetched. But that it will have to adapt is not in doubt. If it does not, it will atrophy terribly.
There are many new technologies in development for increasing the efficiency of turbo jet and fan jet engines used in most passenger aircraft. Schemes have been proposed for equipping the turbine or fan blades with actuators for changing pitch, which could improve efficiency by over ten percent. More efficient combustion cycles are certainly possible, and Pratt Whitney is doing a lot of work on perfecting a pulse detonation turbojet which promises to be vastly more efficient than the current art.
In the area of smaller aircraft we see two stroke diesels, just now being introduced, prevailing by virtue of their high power to weight ratios and excellent fuel economy. The fact that they don’t required leaded aviation gas also works in their favor.
NASA has sponsored a lot of research on what is known as boundary layer control, that is the control of air flow near the surface of the lifting surfaces. Normally the boundary layer, an air mass which adheres to the wing and thereby lessens drag, begins to separate well before the back of the wing thereby creating drag inducing turbulence. Suction holes, forced air directed over the rear flaps, and rotating cylinders placed at the front of the wings are all capable of delaying boundary layer separation and improving aerodynamics. Such schemes are not new, dating back to the fifties and even earlier, but in the past they were difficult to implement successfully using the cut and try aircraft design techniques prevalent before the age of computer aided design. Now they are possible.
Even more radical techniques have been proposed involving actuators and deformable surfaces on wings that optimize their shape according to the speed at which the aircraft is traveling. These are probably further out but could begin to appear in another decade.
Another approach, one that seems to be easier to implement, is the blended wing or lifting body design, originally developed by Vincent Burnelli, an American aeronautical engineer, in the nineteen thirties. Lifting bodies are related to flying wings where the wings and fuselage form a single lifting surface, but lifting bodies tend to have elongated forms and do not suffer from the longitudinal instability that plagued the old flying wings. Unquestionably lifting bodies can achieve mush lower drag than conventional aircraft and they are up to 50% lighter for an equivalent payload. They are also structurally strong and scale to larger sizes than conventional aircraft.
Lifting body designs have been built, albeit on a largely experimental basis, and there are couple of kit aircraft out there you can buy that embody the concept, but to date no commercial design has made it to market.
Boeing currently has a program for developing a 700 passenger blended wing design, a project specifically prompted by concerns about future fuel prices. The plane would make extensive use of advanced composites in place of sheet metal and would represent a considerable advance over the current art.
A final possibility, which we consider somewhat remote, is the emergence of WIG (wing in ground effect) aircraft. WIGs are used over bodies of water, and, depending on their size, will fly at elevations of a few feet or a few yards over the surface of the sea. WIGs easily scale to enormous sizes which makes for reduced operational cost; they are more fuel efficient than conventional aircraft by a factor of four or five. They are somewhat slower than today’s passenger jets, but can probably be operated at speeds as high as 400 knots. No WIGs are currently in full production, although a number of nations, including China, South Korea, and Japan have ambitious government sponsored program for commercialization.
Fossil Fuel Forever?
The possibility that fossil fuels will be superseded as a power source in aircraft is pretty remote at this time. Fuel cells in an airplane are almost out of the question, and hydrogen jets, while possible, present real problems in terms of practicality. The most likely change in fuel in future aircraft is the phasing out of leaded high octane aviation gasoline in favor of kerosene in propeller driven planes.
Boeing had fuel cell program, which now appears to be pretty much moribund, and a kit builder in France fabricated an all electric airplane that was to be fitted with a fuel cell. The project has never been completed. Aerovironment actually constructed an airplane with reversible fuel cells which used solar panels to generate hydrogen via electrolysis. That airplane, incidentally, established the altitude record for non rocket powered aircraft.
So why not fuel cell power plants? Fuel cells are just too heavy to be practical, not to mention the problems in storing hydrogen on board, problems that grow worse when one attempts to take high pressure tanks to very high altitudes.
Hydrogen powered jets have been built, and the possibility of reviving them has been studied by a European aircraft consortium. But even with liquid hydrogen storage, the only really practical storage method in an airplane, the amount of space required for fuel storage cuts down on payload severely and can effect aerodynamic efficiency. Couple those disadvantages to the high cost of hydrogen and the inefficiency of freezing it and you have a nonstarter.
There is, however, one area of aviation where hydrogen has more than a fighting chance, provided national governments and aircraft manufacturers ever get serious about it, and that is suborbital flight.
Boeing launched a program called HyperSoar for constructing a suborbital airplane which would store liquid hydrogen in its delta wings. The hydrogen would serve not only to fuel the aircraft but to cool its skin at the 7,000mph speeds at which the plane would travel.
Other suborbitals have been designed by various visionaries and aircraft manufacturers, but HyperSoar represents some really fresh thinking that could lead to actual production.
HyperSoar would fly between 100,000 and 130,000 feet. One hundred thousand feet can in aeronautical terms be considered the edge of the atmosphere. Above that altitude there is not enough air to provide lift for an airplane’s wings. So how does HyperSoar climb to 130,000ft.? Actually the craft executes a series of bounds as it flies. It launches into the region beyond the atmosphere in an arc and then re-enters the atmosphere where its scramjet engines resume operation and accelerate the plane to the point where it can commence another bound. While this sounds kind of like a roller coaster ride, Boeing officials claim that the accelerations and decelerations would be gentle. Five leaps would be required to cross the Pacific with a transit time of about twenty minutes.
Crazy? The U.S. Air Force and Federal Express don’t think so. They’re both pouring money into the project. Boeing says that if the design proves feasible only one platform will be constructed, and that the military and civilian versions will have the same flight capabilities. The military version will have surveillance and/or weapons systems, however.
HyperSoar is projected to achieve reasonable fuel economy as well as attaining unprecedented high speeds. But if it happens at all, it’s probably a good two decades off. Nor would it have much of a payload. Preliminary analyses by Boeing engineers indicate that suborbitals do not scale to large size. That means that they will only be used to carry very high value cargoes and very wealthy passengers.
We also see the possibility that supersonic rather than hypersonic airplanes will emerge in the private jet space. Several aircraft manufacturers are developing designs including Gulfstream, and fuselage configurations have been devised that greatly minimize sonic booms. Unfortunately, the benefits of these fuselage designs are only realized in relatively small craft—in other words SSTs that can fly over populated land will probably never be developed.
Most such small SSTs would probably be purchased by charter jet companies and not by private individuals. Certainly not at projected prices in excess of $50 million apiece.
Personal Aviation – Hope of a Resurrection
The ownership of certified personal aircraft is at an all time low in the U.S. Per capita ownership was higher in the nineteen thirties, and many of the thousands of airfields built back then in anticipation of the mass produced airplane go unused today.
The reason for the unpopularity of airplanes is in no way mysterious. A new certified airplane runs about a quarter million dollars minimum, the price of large yacht, and lessons to operate it can run into the five figures. In constant dollars airplanes are more expensive than they’ve ever been, and they’re not much improved over what they were fifty years ago.
The main reason private airplanes are so costly because they’re hand made. It's pure chicken and egg, really. No sense building a mass production facility if you’ve defined your market as the top one tenth of one percent, and no way to get a bigger market unless you drastically reduce the cost of production. There is no reason why aircraft couldn’t sell for the price of a luxury automobile if mass produced, and in fact they were mass produced in World War II. But those techniques never migrated into the civilian sector.
Of course there’s the problem of operating aircraft and dealing with crowded airspaces in the event of a mass market. Most people who’ve thought seriously on the subject assume that some kind of massive automated air traffic control system would have to be initiated and that the operation of aircraft would have to be greatly simplified from what it is now. In addition, one would prefer designs that had short take off and landing characteristics and that were highly resistant to stall and instabilities in flight. A fairly foolproof semi-automated fly-by-wire system would be a must.
NASA has published a number of monographs on the subject of expanded personal aviation. It could make for a far more efficient transportation system for travel over distances of less than 500 miles because it would eliminate the delays of using large airports, but the expense of launching such a system would be immense, and the receptiveness of the public would be difficult to determine. Currently, the most popular small aircraft in the U.S. are noncertified ultralights, largely on the basis of price, but since most are home built, they’re hardly a bridge to the future. Experimental designs such as canard type aircraft abound in the ultralight category, so it could give some indication where personal aviation is ultimately going.
Any expansion of personal aviation would come up against the rising fuel prices that afflict all forms of transport, and, since aircraft are the least efficient of vehicles, the cost of fuel might ultimately prevent such a development.
End Run – Evacuated Tube MagLev
Magnetic levitation transport utilizes opposing magnetic fields to elevate the vehicle, and most schemes generally also use magnetic forces to propel the vehicle forward. Several variants exist. Pure maglev systems eliminate friction from the drive train and the rails and thus achieve high energy efficiency and higher speed, available power being held fairly constant rather than being increasingly lost to mechanical friction with increased speed.
Several magnetic levitation techniques are well proven—this isn’t blue sky stuff—but the cost of constructing maglev rail systems exceeds that of conventional railroads, at least in the case of the proven designs. Only one commercial system is currently in operating, a single line connecting the port and city of Shanghai constructed by the German TransRapid firm. The system is the pride of the Chinese government, but there are no plans to extend it. Right now it is purely a showcase.
Conventional maglev trains are capable of speeds in the 500mph range but are really only practical up to about 300mph due to noise arising from air turbulence. However, if the train is operated in a tunnel and that tunnel is pumped to a low vacuum, speeds in the thousands of miles per hour become possible. The train becomes like a suborbital only it’s traveling underground.
An evacuated tube maglev system would be enormously expensive to build, but inexpensive to operate. There is no wear out mechanism in either the propulsion or levitation devices and no friction on the skin of the cars. And the energy expenditure required to accelerate them would be far less than that needed to thrust a suborbital 100,000 feet above the earth. In fact it would be considerably less than that required for a conventional jet traveling at subsonic speeds.
What about crossing oceans? A vehicle traveling at 7,000mph is a whole different proposition than one traveling 600mph. You want to go from New York to Paris? You cross the North American continent, then cross the Bering Straits, and proceed across Siberia. In less than two hours you’re there. And if you want to go from New York to Los Angeles, that will take you about fifteen minutes. Distance doesn’t matter anymore. Not when you’re going that fast.
Could this ever happen? Most transportation authorities assume that the world seventy years from now will be just like today but with more Internet and handheld music and video devices. Real outside the box thinking. What would such individuals be saying if they could be transported back to 1800? They’d say that ubiquitous railroads or indeed any railroads would be quite impossible. Too capital intensive. Canals and stage coach lines would be the future.