50 years ago Air Force Magazine - A Mach 3 transport?

bobbymike

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What About a Mach 3 Transport
By J. S. Butz, Jr.
Technical Editor

An analysis of the technical problems and requirements involved in the development of the commercial supersonic transport, with some comment on factors common to the B-70 and the civil craft as well as differences in requirement. In its early aviation judgments the Kennedy Ad­ministration decided the Air Force should not build the B-70 Mach 3 bomber in quantity. At the same time, the Administration started the ball rolling on a Mach 3 transport for commercial airlines to use around 1970. Paradoxical as these decisions might appear, they imply much more than just an opinion that the Air Force does not need a 2,000-mph bomber to fulfill its responsibilities in the last half of this decade. The Administration also has strongly implied that a Mach 3 bomber cannot be easily modified into a Mach 3 commercially attractive transport. While there are some competent people in industry who dispute this idea, it obviously is the judgment of Mr. Ken­nedy's advisers.-

The basis for this judgment can be summed up in one sentence: The supersonic transport mission is technically more difficult than the supersonic bomber requirement that the B-70 satisfies. The fact is, the B-70 project has pioneered Mach 3 technology in the US. With a redesigned fuselage, the plane could serve as a military transport for the rapid movement of troops or high-priority cargo. If a tight development cycle was considered a key factor in choosing a supersonic transport for civil use, the B-70 would be the logical choice because the airframe design is essentially complete. This B-70-type transport could be ready for service three or four years ahead of any new design. However, no design exists today, either a modified B-70 transport or a paper preliminary design, which would satisfy all of the basic requirements for a money­making 100-120 passenger, transoceanic, Mach 3 trans­port. Some of the unique commercial requirements for this airplane are: The airframe must last for 30,000 to 40,000 hours as against about 4,000 hours normally figured for a military aircraft. In addition, the structure must sur­vive heating to around 500 degrees F. and then cooling down on the average of twice a day. New-type, low-noise turbofan engines will be re­quired. Large afterburning turbojets, such as the J93 in the B-70's six-engine, 150,000-lb.-thrust-plus installa­tion, are too noisy for present civil airport regulations. Public complaints have united airport operators in resisting further increases in noise. A new type of large-airflow turbofan engine under study shows promise of doing just this. It could make a Mach 3 transport a little quieter than current jet airliners. This engine also appears to satisfy the stringent demands made on a long-life transport powerplant. Its unusual cycle is probably best described as a supercharged ramjet (see illustration, page 47).

¾ Large fuel reserves, for "stacking" or diversion to an alternate landing field in case of bad weather, are much more pressing in terms of the Mach 3 transport than in the case of a military aircraft. Weight of the necessary fuel reserves on a 2,000-mph transport would almost equal the airplane's entire payload. Studies are under way now to relax the civil rules to reflect the rapidly improving capabilities of the air traffic control system and all-weather landing equipment. By the time the supersonic transport is available, it should be possible to land at all major airports with almost com­plete disregard of weather conditions. If this proves to be true, the need for fuel reserves will be greatly reduced. But, as of now, the need is there. ¾ Landing and takeoff speeds for most Mach 3 air­plane designs will be higher than those of present air­line jets. Highly swept, short-span wings are needed for efficient cruise at Mach 3. But these wings give poor performance at subsonic speeds and during land­ing and takeoff. Most airport operators oppose the lengthening of runways to accommodate higher speeds. Few airlines want to take off passenger aircraft faster than the present high of 185 mph. The present-day 140-mph touchdown speed also is about the maximum that most airlines want to handle.

In contrast, the short-span, deltawing airplane with a canard horizontal control surface on its nose (a very efficient Mach 3 cruise configuration) would have to land at 165 to 175 and take off at more than 220 mph. Low-speed performance depends basically on the wingspan. To realize a significant improvement, the span must be increased. Additional power, slats, flaps and other high-lift devices help, but the wing must be lengthened if major reductions are to be made in landing and takeoff speeds. Two methods are available today to increase the span on a Mach 3 airplane. Actually it is the aspect ratio which must be increased. This is equal to the span divided by the wing chord.

The first method is simply to increase the aspect ratio by making the wing longer and slimmer. A weight penalty and an increase in drag at cruise speed work against this system. The second method is to use variable-sweep wings. During subsonic flight, the outer portions of the wing would be moved forward so that their leading edge had thirty degrees sweep or less (see illustration page 43). Landing performance then would be close to that of current subsonic jets. In flight, as the aircraft accelerated to its cruise speed the wings would be swept back until the angle reached seventy degrees or a little more (as shown in the illustration above). There would then be no penalty in cruise drag because the leading edges would be behind the strong shock wave created by the nose of the aircraft. Disadvantages of the variable-sweep approach are increased structural weight and mechanical complexity. The variable-sweep wing had a disappointing development history in the early and middle '50s. Recently, however, the situation has changed radically. Manufacturers who wouldn't have considered a variable-sweep wing under any circumstances two years ago are now creating bomber, transport, fighter, and recon­naissance aircraft designs around this wing principle. The change of attitude stems from research studies performed at the Langley Research Center of the Na­tional Aeronautics and Space Administration. These have spurred US variable-sweep research. The NASA researchers kept after the idea even though their early designs lacked appeal for both industry and the mili­tary because of control problems and excessive weight. Once the Langley group began to report solutions to their former troubles, industry started its own inves­tigations. Some manufacturers now have more hours of wind-tunnel time on variable-sweep wing studies than does NASA. The results have been extremely encour­aging. Today it is safe to predict that a portion of the next generation of US aircraft will have variable-sweep wings.

Variable sweep is figuring prominently in the com­petition for a new Tactical Air Command short-take­off-and-landing airplane. Theoretically, variable-wing sweep offers great performance benefits for any high-­speed airplane. These are being investigated by many preliminary design teams. In the case of large aircraft, such as a Mach 3 trans­port, some companies believe that the aerodynamic benefits of variable sweep can be enjoyed at only a slight cost in increased structural weight. Several sim­ple mechanical schemes have been devised to move the wing forward and back and carry the loads on the outer panels into the wing roots.

Najeeb E. Halaby, the new Administrator of the Federal Aviation Agency who is coordinating the Ad­ministration's effort on development of the Mach 3 transport, has given four compelling reasons why the United States should proceed at once with the project. First, Mr. Halaby believes that it is important to advance the technology of manned flight. Secondly, he regards the supersonic transport as a valuable tool for our armed services. In special situa­tions, it would allow troops to be moved to almost any of our outposts in three or four hours. His third reason concerns national prestige. He be­lieves that the US is close to losing the undisputed lead it has held in aviation for the last two decades. Cer­tainly a supersonic transport will be built by someone. If we don't, someone else will. Fourth, Mr. Halaby's studies show that a fleet of supersonic transports will add to the gross national product by generating new air traffic, allowing goods to be moved more quickly, and saving travel time of executives. Halaby hasn't openly said that he favors a Mach 3 transport over one that cruises at Mach 2, probably out of deference to the British government, which is still interested in a joint US-British Mach 2 develop­ment venture. Under this plan, maximum use would be made of the existing state of the art. Both nations would be able to contribute substantially on the basis of their recent military aviation experience.

A major arguing point for this plan is that a Mach 2 airliner could be ready two or three years ahead of the Mach 3 airplane. This could make the difference between being first or second in development of the supersonic transport—with a consequent effect on Western prestige. The vast majority of US experts have been opposed to the Mach 2 airplane. They would rather take a chance on being second with a solid Mach 3 money­maker. It is reasoned that the airlines would not be able to buy both a Mach 2 and a Mach 3 transport. Probably no nation would want to subsidize the de­velopment of both airliners. Therefore, if the US and Britain went ahead with the Mach 2 airplane, they would be forced to live with it no matter what hap­pened elsewhere in the world. Any nation that went ahead with the Mach 3 airplane would be in a com­manding position over the US-British combine. Several technical factors have been cited to show just how commanding that lead would be. First recent tests have shown that aluminum structure does not have a long fatigue life at Mach 2 temperatures of 200 degree F. or so. Heating and reheating through a few hundred cycles quickly lowers the strength of aluminum, so a supersonic airliner constructed of alu­minum probably would have to cruise around Mach 1.5 or a little better. In turn, the Mach 1.5-plus airliner would have to have an arrow-wing configuration to cruise efficiently. Unfortunately, the arrow wing performs poorly from Mach 2.5 on up. For Mach 3 or 4, therefore, a whole new aerodynamic configuration and wing shape would be needed as well as a complete change of structural material from aluminum to steel.

A big advantage of starting with the Mach 3 con­figuration is that its cruise efficiency does not fall off as the speed is increased to Mach 4 and better. The only necessary major change would be to strengthen the steel Mach 3 airframe and probably add new engines. Another selling point for Mach 3 cruise is recent research which indicates that the sonic boom heard on the ground will be greater from a Mach 2 than from a Mach 3 airliner. The reason is that the slower air­plane must cruise 10,000 feet below the faster one. And altitude is a great attenuator of pressure waves. There is one important question that can be ap­proached with a little information, a lot of intuition, and possibly a touch of the soothsayer. This is the puzzle of what the Russians intend to do. What kind of national prestige and economic competition do they intend to create in the supersonic transport field? In the past the statements of Soviet political leaders and prominent technical figures in the Soviet aircraft industry have been the best indicators of what they plan to do and are doing. Such statements gave ample advance notice of the sizable group of Russian turbojet and turboprop transports currently in service. This notice of forthcoming projects included the Tupelov TU-104 which beat US turbojet transports into service by a couple of years and gave the Russians a tempo­rary leg up prestige wise. Recently the same kind of statements have indicated that the Soviets have a supersonic transport program. The political leaders have talked at cocktail parties. Propaganda outlets have made typically convoluted references to the program. More important, leaders of the Soviet aviation industry have said they are working on a supersonic airliner. There has been no clear indication, however, of "how supersonic" the plane would be. They could take the easy road and develop the Mach 1.5 to 2 transport. They would want us to know it, counting on us to settle on the "easier" plane, also, if they did. They might, on the other hand, deceive us into believing they were at work on a Mach 2 plane—when actually they had set their sights higher.

Since the Russians operate as they do, most informed parties in the US believe the Mach 3 transport is our most sensible investment, especially if we want to ensure against being outflanked by a vastly superior airplane. The new Administration has accepted this view and decided to make full use of the currently strong technical position of the US aircraft industry. Its reserve of unused technology is higher today than it ever has been. A firm base for Mach 3 development has been built through extensive research and the abortive F-108 and B-70 programs. A new method of putting this knowledge to use must be formulated, however. Developing large supersonic airplanes for commercial use is too big a job for private industry to handle alone on a timely basis. Development costs on the Mach 3 transport will run somewhere between half a billion and a billion dollars What airframe manufacturer can carry that kind of financial burden alone for several years in the hope that someone will buy his airplane? US airlines are just beginning to pay for their fleet of subsonic jets. Few like to think about putting any money into a supersonic airliner until 1968 and 1970. By that time some other nation is bound to have taken an overwhelming lead in this field if the US has not acted. It should take about ten years to design, develop, and flight test a Mach 3 transport before it is ready for scheduled service.

The new Administration has recognized these fiscal facts of life. A request for $12 million is included in the new Federal Aviation Agency budget for the studies and experimental work necessary before a sensible supersonic-transport development program could be planned. These studies will provide specific answers on a number of aerodynamic configurations engine cycles, and the performance of various sheet-metal structures which operate at high temperature most of their life. Enough basic work has been done in the past to point out what problems should be attacked. Most supporters of the project feel that this money plus the $2 million NASA has allocated for the same purpose will provide the information, necessary to pick the best engine and airframe design approaches by the summer of 1962. The major portion of the studies will be handled by two airframe and two engine manufacturers on a competitive basis. NASA's Langley Research Center will also participate in the program.

Original impetus for a government-sponsored supersonic transport program was provided by Elwood R Quesada, Halaby's predecessor as FAA Administrator, Quesada had drawn up an organizational plan and set a tentative development schedule, but his request for $17.5 million to start work on the project was turned down flatly by the Eisenhower Administration. Mr. Halaby likes his predecessor's plan and apparently will fight for it with only minor changes. Under this plan the FAA will have over-all authority over e supersonic transport development effort. A board of airline advisers will help in getting an airplane that will please all of the carriers. The Air Force will be the contract manager. NASA will provide technical counsel for the FAA. It is estimated that about $750 million will be needed to take a Mach 3 transport through its develop­ment and get it certificated. First flight would be around 1966. Certification would take three or four more years if no serious trouble occurred. The governm­ent's aim probably would be simply to see that certificated airframe and engine are available for a manufacturer to produce if he can get enough orders make it worthwhile. Most predictions of the sales market for these airplanes run from 200 to 250 airplanes. On the basis of present information, they would cost about $15 million each so that the gross business would be worth between $3 and $4 billion. Most of the economists who have reviewed the problem believe that the airlines can pay for the Mach 3 transports in less than ten years if they do not cost much more than $15 million each. The tremendous earning power of these aircraft can more than make up for the high purchase and maintenance costs. If the airlines get eight-hour-per-day utilization out of each Mach 3 120-passenger airliner, it would be possi­ble to log over 1.5 million passenger miles per day from each airplane with a load factor of less than eighty-five percent.

Final cost of a Mach 3 airliner will depend on its construction style. Under the present state of the art, steel-sandwich is lighter than steel-skin-stringer structur­e, especially for thin sections. However, skin-stringer design costs less than one-fifth as much in some cases. Energetic efforts will be made to reduce skin-stringer design weight. The outer surface of the Mach 3 transport will have to be several times smoother than the skin of any produ­ction aircraft built to date. If skin mismatches are not eliminated, the skin friction drag can go up high enough to endanger the aircraft's range. Detailed design of the Mach 3 airliner will require more new data than that to be provided by high-temperature structure tests. There is still considerable uncertainty regarding the air loads the aircraft will experience. Big questions exist on the loadings due to gusts, maneuvers, high-speed ground runs, flutter, and buffet. Complete data is still not available on air turbu­lence at 50,000 to 100,000 feet. The amount of new knowledge that must be gath­ered for the Mach 3 transport is so long that even the most ardent aviation enthusiast cannot escape the conclusion that its development will be one of the most difficult steps forward ever taken in the history of flight.

In even the most cursory discussion of the promise and problems of the Mach 3 civil transport, it is im­possible to ignore the airplane's military potential. Two of its characteristics would make it attractive to the Air Force as a bomber. First, the airliner airframe with a 30,000- to 40,000-hour life would make an airborne alert possible with­out the structural fatigue problems facing strategic bombers today. Second, the variable-sweep wings that would give the transport good subsonic and supersonic perform­ance would also allow a bomber to attack over long ranges using any combination of high and low altitude and supersonic and subsonic speeds. By many interpretations, the Administration's interest in Mach 3-class aircraft died with the B-70 weapon system. It has been predicted that Dyna-Soar-type boost-glide weapons would be given increased empha­sis as a result. But, in reality, we may still have a Mach 3 bomber in our future.

President Kennedy mentioned a "successor bomber" to the B-52 in his budget message. He also told Con­gress that, "We should explore the possibility of devel­oping a manned-bomber system specifically designed to operate in an environment in which both sides have large ICBM forces." This statement seems to indicate that the President agrees with the fundamental Air Force doctrine that manned aircraft able to penetrate over all targets would be necessary in any war in the years immedi­ately ahead. And, in the view of many officers, single-pass, very-high-speed aircraft like the Dyna-Soar cannot be the sole answer to the manned-penetration requirement for quite some time. Therefore, it seems highly likely in view of the per­formance predictions for large, Mach 3, variable-sweep aircraft, that they would receive strong consideration for the penetration mission
 

OM

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...Excellent find! And a reminder of the days when $2-$4 Million USD was a lot of money!
 
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