Today the air-breathing engine stands at the threshold of the greatest technical opportunity in its history.
Even the most conservative engineers, scientists, and managers in the engine industry, in the government, and among their university advisers agree that the jump in performance from today's engines to the next generation of gas turbines will be by far the biggest yet achieved. The anticipated performance jump is much bigger than the one from the piston engine to the jet, which resulted in aviation's most important revolution to date. It is greater than the jump from subsonic to supersonic turbojets, which many steadfastly said couldn't be done—until it was.
The key to the new engine technology is weight. The engines deemed to be within the state of the art today are absurdly light by any past standard. In fact, no one dared to predict them as little as five years ago.
The most conservative estimators—and the engine industry has its share of cautious operators—believe that the new engines will weigh less than half as much as today's powerplants for each pound of thrust delivered. In more technical terms, engine thrust-to-weight ratio (the number of pounds of thrust produced for each pound of engine weight) is expected to rise to from fifteen to eighteen to one for specialized lifting engines which will operate only during landing and takeoff phase of VTOL flight. On cruise engines, which operate in the conventional mode during an entire flight, the thrust-to-weight ratio is expected to go up to from twelve to fifteen to one. This compares with a thrust-to-weight ratio of less than six to one for most existing engines with some of the most advanced approaching eight to one. The first turbojets had a thrust-to-weight ratio of about 1.5 to one, under static conditions. The piston engine-propeller systems of that day had about the same ratio, which was only topped by the jet engine at high speeds where the propeller efficiency dropped off rapidly.
The dramatic reduction in engine weight has important implications in every speed range and every type of aircraft. At subsonic speeds it means that future aircraft can be built with payload capacity, range, acceleration rates, and maneuverability that will completely outclass any existing today.
VTOL aircraft finally will become operationally attractive. The new engines will be light enough to allow a transport to take off and land vertically and still carry cargo as efficiently as does the DC-3. The VTOL gap, the penalty paid in extra propulsion system weight to achieve vertical takeoff and landing capability, can possibly be closed, because the extra propulsion system weight may be no higher than that of a conventional landing gear.
The new lightweight technology also will make it possible to build the most efficient engines ever. New turboprops will have a lower specific fuel consumption than the turbocompound piston engine, the best flown so far. It may even be possible to reduce turboprop fuel consumption to the level of the ground-based diesel engine.
To improve the efficiency of turboprop and turbo-fan engines and to lower their fuel consumption it is necessary to add a "heat-saver" unit, a regenerator. Regenerative engines combined with Laminar Flow Control, the new low-drag aerodynamic development by Northrop (see AIR FORCE/SPACE DIGEST, August , page 31), will make possible endurances of three more days and nearly global ranges for aircraft carrying substantial military payloads. Aircraft of this type are the multipurpose vehicles, known as Dromedary and Maple, now being considered for development by the DoD (see illustration, page 39).
At supersonic and hypersonic speeds the ability to lighten all engine components allows air-breathing propulsion to be considered for all speeds short of 18,000 mph—orbital speeds. Elaborate cooling systems, thrust-augmentation systems—which literally transform turbojets into ramjets—and powerful efficiency-improving units such as variable-area exhaust nozzles and engine air inlets can be added to a propulsion system without making it too heavy to perform the two-stage-to-orbit or even one-stage-to-orbit missions, which are sometimes called the aerospace plane, and the recoverable booster missions.
The new technology makes even an economical supersonic transport, even the Mach 3 variety, technically feasible.
These rosy predictions are not built on a paper foundation. There is much more behind them than new theories, concepts, and computer studies. The technical literature makes it clear that basic new hardware has been created in applied research programs, both government- and industry-sponsored. Exact details are either classified or proprietary, but performance has been good enough to convince those who have access to the data that a revolution in air-breathing propulsion can be achieved.
The revolution does not involve any radical new devices. It would be built on a foundation of major improvements in propulsion-system components which have been known for years—compressors, combustion chambers, turbines, and regenerators. All of the new powerplants—whether designed for very high thrust-to-weight ratios, very low fuel consumption, or very high speeds—must be created from this lightweight component technology.
Compressors of the future will not look much different from today's, except that they will be shorter. The big technical prediction is that each stage, or row of compressor blades, in an axial-flow compressor will be able to raise the pressure of the airstream around two times higher than is possible today, without an appreciable loss of pumping efficiency. A given compressor, therefore, regardless of the total pressure rise it is required to produce, will be shorter, with relatively fewer stages, and very light in weight. For instance, most of today's large engines designed for cruising at high subsonic speeds with short supersonic dashes have compressors with around fifteen stages. These raise the pressure of the airstream more than ten times—above ten atmospheres. When current laboratory compressors are developed into complete operational units, the same compressor performance will be achieved with fewer than nine stages.
One major improvement, already operational in small engines, is compressor stages which operate with acceptable efficiency at high rpm and at transonic speeds. Further improvement can be made in transonic compressors, and faster-turning supersonic compressors also can be made to pay their way. That is, the number of stages can be reduced by a factor of three or four, because even though the addition of shock-wave drag through supersonic compression would lower the pumping efficiency, the over-all engine weight would drop enough to make an engine with a supersonic compressor attractive for many applications.
The main objective in combustion chamber, or burner, development is to shorten them. Shorter burners mean shorter, lighter engines. The purpose of a burner is to release heat, and the more it releases in a given volume of air in a given time, the more desirable it is. Good progress is reported in improving heat release. No slackening of this progress is predicted, and the heat release possible with hydrocarbon fuels in a given size burner will probably double in the next ten years. Improvements of three or four times today's values are estimated for advanced fuels such as hydrogen.
Some new engines, operating at about today's temperature levels, will have extremely short combustion chambers. Other new high-temperature engines, such as those described below, probably will have burner sections about as long as those of current engines. But they will release much more heat energy.
Improvements possible in turbine wheel performance verge on the miraculous.
Turbine inlet temperature—the most important measure of turbine performance—can be boosted from about 1,700 degrees Fahrenheit on today's engines into the 3,000-to-3,500-degree range in the next decade.
There is no way to overestimate the importance of this large jump in maximum engine operating temperature. A temperature rise of more than 1,000 degrees will increase the thrust delivered by an engine of any given size (air flow) by more than two and a half times. This major thrust improvement apparently can be achieved with only a modest increase in engine weight, so that the very important thrust-to-weight ratio will be at least doubled.
Another major advantage of the higher temperature is that the thrust will be more than doubled without doubling the fuel flow. The specific fuel consumption would be lowered by twenty percent or more. An even greater improvement in fuel consumption would be realized at part throttle. This would be of particular benefit to Maple-type airplanes which could loiter for long periods at either high or low altitude.
Great effort has been expended in the past to increase turbine inlet temperature, but progress has been slow. On the best of the first jets the turbine inlet temperatures were little more than 1,500 degrees. By 1960 they had been boosted to about 1,700 degrees, through improvements in the useful strength of high-temperature alloys. Today, some of the more advanced engines use some form of internal blade cooling. Most publicly discussed internal cooling procedures involve hollow turbine blades through which large quantities of air are pumped. This form of cooling allows the main engine gas stream temperature to be raised more than 300 degrees to a little more than 2,000 degrees, while the actual blade temperature is kept down around 1,700 degrees.
More sophisticated forms of cooling have been successful in the laboratory, and they provide a concrete basis for hopes that operational engines can be developed which will operate at unprecedented temperatures. The new cooling techniques borrow heavily from rocket engine technology and they fall into four general groups:
• Film Cooling—In this method, streams of air or a liquid are released through the hub of the turbine wheel at the base of the blades. When the streams are released at the proper places and under the proper pressures and flow conditions, the centrifugal force generated by the whirling wheel and the forces present in the boundary layer on the blades combine to create a film which continuously covers the blades during all engine flow conditions. This cool film on the blades allows the engine gas stream to be raised at least several hundred degrees without increasing the blade temperature beyond 1,700 degrees.
• Transpiration Cooling—This technique employs a protective layer of gas over the blade. Through the use of special material or construction techniques the blades are made "porous" so they will pass a high-pressure stream of air or some other gas pumped into a cavity inside the blades. This gaseous stream mixes with the boundary-layer flow on the outside of the blade and forms a cool layer.
• Hydrogen—Liquid hydrogen, which would be carried aboard very-high-speed aerospace-plane-type vehicles at a temperature near absolute zero, is such a powerful cooling agent that most engine people discuss it in a separate category from other techniques. Hydrogen's cooling power is illustrated by experience with the RL-10, the nation's first hydrogen-fueled rocket, being developed by Pratt & Whitney. The RL-10 is cooled by pumping cold hydrogen through the hollow walls of the nozzle and thrust chamber in which the hydrogen and oxygen propellants are burned. Overheating is a problem on all other rockets, but the RL-10 structure runs so cold that a heavy layer of ice actually forms on its outer surfaces during ground tests. As a consequence, it appears that the RL-10 could run for many hundreds of minutes, which is not of particular interest for rockets but is of great importance for aerospace-plane-type propulsion systems.
Hydrogen apparently could be used to cool all internal parts of such engines, including turbine wheels for use below Mach 4. Hydrogen cooling would allow extremely high gas stream temperatures, in at least the 3,500-degree range. Consequently, air-breathing engines remain attractive propulsion devices at very high speeds.
• Proprietary—The engine business is more competitive today than ever. There are countless ways in which the new gas turbine technology can be applied and countless questions that require expert judgment. All major companies are working on at least one major cooling technique that has not been reported in principle, and a substantial number of basic patent applications are expected.
Regenerators are being considered for addition to the next generation of turboprop and turbofan engines. The regenerator reduces fuel consumption by using the hot exhaust stream to preheat the air from the compressor before it reaches the combustor section (see illustration, above). Preheating reduces the amount of fuel which must be burned to raise the airstream temperature up to the level required to produce the desired level of thrust or horsepower output
Regenerators are most effective during part-throttle operation, and it is not likely they will be applied to any gas turbine powerplant not designed primarily for fuel economy and for use on very-long-endurance aircraft designed to operate at reduced power for a large percentage of their mission.
The principle of the regenerator is old. It long has been recognized that it is uneconomical to dump a hot airstream out of an engine into the atmosphere. Consequently, regenerators and other types of "heat savers" have been used for years to make permanent ground-based powerplants more efficient.
During the 1940s attempts were made in the US and abroad to adapt regenerators to turboprops. Stationary-type regenerators were used in these designs. They resemble automobile radiators, which in proper technical language are called heat exchangers. The exhaust from an engine's turbine section was passed through a series of tubes. Large numbers of fins were attached to the outer surface of the tubes. Air from the compressor was preheated by passing it over these hot tubes and fins.
The early attempts at regeneration were not successful because the stationary heat exchangers were heavy and it proved impossible to duct air through them without large pressure drops and loss of efficiency.
The unanimous technical verdict then was that regenerators did not pay their way. Today the picture is completely reversed. The change has been brought about by intensive heat-exchanger development work conducted to a great extent in the now-defunct nuclear-airplane program and in the automotive industry. Stationary tube and fin devices have been lightened considerably. Their flow efficiency has been brought to a high level, and new, relatively inexpensive methods have been devised for their manufacture.
Another type, the rotary regenerator, also is attracting great interest. It is especially favored for use in automotive gas turbines, some of which conserve heat so well that you can hold your hand in their exhaust. The basic part of the rotary regenerator is a wide hoop made of thick, screenlike material. This hoop is mounted so that it passes through both the air duct from the compressor and the hot exhaust duct. The hoop is turned slowly (thirty to 100 rpm in most cases) so that it picks up heat in the exhaust duct and transfers it to the compressor airstream.
The rotary regenerator is more complex than the stationary, but it is credited by most authorities as being the smallest and lightest device available to transfer a given amount of heat at a given rate. Leakage losses, which are no problem with the stationary regenerator, are the primary source of trouble with the rotary units. Special seals to stop air leakage must be placed around the openings in the compressor and exhaust air ducts through which the regenerator hoop rotates. Considerable progress with the seals has been reported, but most companies say that leakage losses can be reduced further.
Regardless of which basic type is chosen, stationary or rotary, a regenerative engine is going to be considerably larger than a conventional gas turbine of the same power rating. To start with, the heat exchangers are bulky. And very large ducts must be used to turn the engine airstream through them or else the internal flow velocities will be high, the pressure losses large, and the efficiency low, thereby counteracting the purpose of the regenerators. Ingenuity is at a premium in designing and packaging regenerators, and this job is one of the most challenging mechanical and aerodynamic problems that the engine business has to face today.
The logical question is: "Why aren't the new air-breathing engines being pursued as rapidly as possible?" Certainly the technical justification has never been stronger. There is no question that vastly superior aircraft and engines can be developed within the next five to ten years.
In one sense, the problem is too much opportunity —in space and with missiles, as well as with aircraft and air-breathing propulsion. The competition among technically strong programs is intense. All worthwhile developments cannot be financed, and the government is caught in a web of indecision in selecting a technical route for the future. Key questions, such as the relative effectiveness of aircraft and missiles for many missions, and the desirability of mixed forces after the current aircraft fleets wear out, have not yet been resolved. Until they are, aviation and the air-breathing engine business will be caught in a truly frustrating limbo, within sight of the brightest possible future but unable to move out of the laboratory —and probably forced to watch the foreign competition seize the initiative.
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