In the beginning, progress was slow. For many years following the historic Wright Brothers' flight, aviation remained largely a technical curiosity apart from limited military developments. Mail service and transportation were adventures and air shows and races captured public interest. The technology evolved from surface-based experiences-the bridge-like truss structures, the piston engine from the automobile, and the air-screw or propeller from ships. These derivatives, although important in providing a technological base from which aviation could develop, were also limiting shackles until a major breakthrough came in developing lightweight structures. In the early 1930's, the metal monocoque structure, in which the skin of the structure carries the primary load, reduced the structural weight of aircraft and paved the way for increased payloads and revenue.

The monocoque structure has since become familiar in other types of construction, particularly in launch vehicles and spacecraft. Structural improvements also increased the demand for aluminum and spurred development of new, high strength and lightweight alloys and led to widespread use of these materials throughout industry.

The monocoque structure also lifted a major constraint on aircraft speed as it made possible a clean aerodynamic shape with less drag than old designs. Aircraft speeds continued to increase rapidly until the propeller, coupled with the piston engine, could do no better. The stage was thus set for the development of the turbojet engine which gave aviation another major breakthrough.

Turbine engines are not new; indeed, they are one of man's oldest techniques for converting heat energy into mechanical energy, dating back almost 2000 years. Steam turbines were in wide use in the last century. The concept of a turbojet engine where a compressor is driven by a hot gas turbine-is not a new idea but earlier models were so temperature-limited by materials that virtually all the available energy delivered by the turbine was used in driving the compressor, leaving little for useful work.

The advent of high strength, high temperature materials was the greatest single contributor to the success of modern turbine engines.

The first turbojets were greedy consumers of fuel which slowed their initial acceptance as economical transports. Today, the specific fuel consumption of jet engines is nearly as low as the best piston engines, while also enabling twice the flight speed. In addition, the time between overhauls has almost tripled with attendant reductions in maintenance costs.

In 1958, the air carriers had 21 piston engine aircraft for every turbine engine aircraft. Seven years later, they had almost an equal number of turbine engine aircraft as piston engine aircraft. More significant, the passenger miles of turbine engine aircraft rose from 10 percent of the total in 1958 to about 90 percent in 1966.

The gas turbine engine is rapidly invading fields long dominated by other power generators. Ships, which gave the aircraft the propeller, are now being driven by gas turbines. Electric power is being generated by gas turbines and experimental gas turbine automobiles have been built.

The monocoque structure and turbine engine are but two of the technological advances that mark the growth of aeronautics. The applied research of the NACA and NASA, the contributions in aeronautical sciences by the universities, the spur by the military for aircraft to fly higher, faster, farther, and the development, manufacturing, and operations by industry were all essential ingredients in this growth of aviation. In 1966, aviation contributed an estimated 30 billion dollars to the national gross product and about 12 billion dollars in exports. In 1966. the airlines carried over a hundred million passengers and, in terms of passenger miles, far surpassed ships, railroads, and buses combined.

The growth of aeronautical capability that I have been describing can be illustrated by means of a quantitative assessment of aircraft productivity. This is expressed, Figure 355, in terms of passenger miles per aircraft per day and is a measure of revenue producing capability. In early years, productivity grew rapidly but was reaching a leveling period about 1930. The monocoque structure re-established the rapid growth rate until a second leveling period appeared around 1960 representing the limit of piston engines. The turbine engine caused the next jump in productivity and further increases will occur when the supersonic transports and jumbo jets get into service. In the distant future, hypersonic transports offer still further increases in productivity.

The evolution of aviation from relatively simple machines to more complex

simpler days of aviation when unit costs were low and interaction between the elements of the design were small, the manufacturer could afford a substantial uncertainty in assembling the elements of an airplane, since (if the whole aircraft was not satisfactory) it was relatively easy and inexpensive to correct any deficiency once it was discovered. The correction of deficiencies exposed in flight has persisted into recent times; for example, some of today's jet transports were retrofitted with larger vertical tails and wing-leading-edge flaps after entering commercial service.

However, airplanes now in design or expected in the future have become so complex and embody such great interactions between the elements of the system that the technological risk of achieving a satisfactory system in a first try is high; moreover, the possibility of easily correcting a first-flight-exposed deficiency is low.

Let me illustrate the importance of interaction between elements with a consideration of aircraft drag, shown by Figure 356. In the period prior to World War II, the total drag of the aircraft was essentially the sum of the drag of the individual components. There was some advantage to be gained from optimum location of engines with respect to fuselage and wings, but the effect was not large at subsonic speeds. Thus, research on the drag of individual components provided the designer with the information he required to predict the drag of any combination that met his needs. As speed has increased, however, the total drag of an airplane is no longer equal to the sum of the drag of individual components. Indeed, at supersonic speeds, the aircraft drag could be considerably higher than the sum of the component drags. Most importantly, it has been found further that the components can be so located with respect to each other that the total drag can be greatly reduced from that obtained by the sum of the individual component drags. This is termed "favorable interference" and a wellknown early example of it is the famous "Coke bottle" shape evolved by NACA research. The first major contribution was to the success of the F-106 aircraft and it is now basic in the design of all supersonic aircraft. Favorable interference will be an important consideration to supersonic transports represented by the 1970 and later period and is illustrative of the requirement for a systems approach to obtain best solutions to these complex processes. We find the same type of systems approach is necessarily needed in considering aircraft stability, control, and handling problems; for launch vehicle stability and control; and for spacecraft entry and landing research.

Due to the increasing complexity of aircraft, the cost of correcting deficiencies found in flight tests could mean financial disaster for a commercial venture. Thus, of necessity, the designer must take an increasingly conservative approach in accepting new and advanced concepts revealed through research by NASA or others. As a consequence, an increasing gap is appearing between the technology that research indicates is possible and the technology being used in practical application. We therefore face a dilemma. On one hand, there is a conservative approach to design because of the high risk while on the other hand, advanced technology must be incorporated in new designs to assure continued growth in the performance and productivity of aircraft. To resolve this dilemma, new technological concepts must be proven through experimental hardware verification and, in most cases, by carrying out flight demonstrations. Experimental verification and flight demonstration of new structural, propulsive, or control system concepts, is required to minimize the technical and economic risks of incorporating the new concepts in an operational aircraft.

In the past, most of the new concepts incorporated in commercial aircraft were carried through flight demonstrations in the course of military aircraft developments. As aircraft become more complex, there are appearing specialized requirements for commercial aircraft which are distinct from military requirements. A major example is the commercial supersonic jet transport; another is a quieter jet engine. Thus, we see the problem of establishing civil aircraft technology at a level of confidence which is acceptable for the investment of private capital. Let us look ahead at some of the advances in air transportation that will be possible through advances in technology.

Subsonic jet transports are rapidly becoming the workhorse of the air carriers and will continue this role into the foreseeable future. For near-term application, we are working on engine noise reduction and high-lift techniques to allow operation on comparatively short runways-3500 feet or less. These two improvements would allow turbine-engined transports to use many existing airports that cannot now have this service without relocation to less populated areas or without exten

sion of runways. The economic gains from reduced noise and reduced runway capability would be great.

A very important trend in aeronautics, by far, is the prospect of bringing air transportation to a larger part of the population through aircraft capable of vertical or short runway operation (V/STOL). In urban areas, the special capabilities of the helicopter will remain unmatched for many years to come and every effort should be made to overcome present deficiencies of vibration, low efficiency, and high operating cost. New rotor concepts hold high promise for providing solutions-the jet flap rotor and the tilt rotor. If these types of rotors are successful, then the helicopter could expand its role in serving intra-city, city to airport, and other transportation needs up to distances of approximately 100 miles. Studies indicate the STOL commercial transports with stage lengths from 50 to 500 miles and runway requirements of no more than 1000 feet offer great promise of becoming an economically viable mode of transportation. The primary advantage of STOL aircraft is the reduced size of the airport facility requirements giving greatly increased utility and service. In an economic sense, successful aircraft of this type should compete favorably with present-day shorthaul airplanes with the added advantage of permitting terminals within urban areas. Two near-term concepts-one, a propeller-driven type, the other a turbofan type-appear worthy of increased investigation.

A great deal has been written about the advantages and problems of supersonic flight. The NACA/NASA has been engaged in research on supersonic flight since 1945 in support of military and civil aircraft development. We are supporting the FAA in the national supersonic transport program and, in addition, we are conducting research leading to second generation supersonic transports. The field of supersonic flight research is so important and it so well illustrates the long lead times needed for research that I would like to digress in order to recall the evolution of one aspect of supersonic flight-variable wing sweep. An interesting history of variable sweep was presented last November by R. L. Perry of the RAND Corporation at the Third Annual Meeting of the American Institute of Aeronautics and Astronautics. .

Variable sweep was patented less than a year after the first flight of the Wright Brothers, but it was unnecessary in early aircraft design and was relegated to obscurity. Busemann presented the first theoretical consideration of the advantages of wing sweep in 1935 and Lippisch designed a variable sweep aircraft in 1941-1942, but it was not built. The Germans did design and build a prototype aircraft during World War II with the capability of pre-setting the wing angle at three settings. This work was interrupted by the Allied advance and the aircraft fell into American hands. In 1945, a large amount of German research data on swept wings became available but R. T. Jones of the NACA Langley Laboratory had already worked out independently the same principles and had published his work as NACA Report No. 863.

The NACA continued research on variable sweep aircraft. Bell Aircraft had become interested after seeing the German prototype in 1945. The government research and commercial interest led to the X-5 experimental aircraft with variable sweep which flew in 1951.

Experience with X-5 and wind tunnel research indicated that for satisfactory stability from low to high speeds, the wing needed to be translated fore and aft as well as varied in sweep. Since weight is at a premium, the additional mechanical system required to translate the wing appeared to compromise the design. Interest lagged but a few investigators continued work. In 1957, the NACA stepped up its research and, in 1958, came up with a practical answer to the stability problem. By shifting the pivot point outward and aft, and reshaping the wing in-board of the pivot, they demonstrated that stability could be maintained without wing translation. In 1959, the research had clearly demonstrated the feasibility of developing a sweep mechanism giving satisfactory aerodynamic and structural characteristics.

This technical breakthrough renewed the interest of the military and initiated the F-111 program now being carried to completion. Last month, the Air Force delivered an F-111A to the Flight Research Center for a series of flight experiments that will provide important feedback to the investigators doing theoretical and ground-based experiments.

Perry points out that variable sweep technology underwent five discreet phases over a fifty-year period. I think this is a good case history of how some ideas and concepts go through many research phases before reaching a practical application. The selection of the supersonic transport design with variable sweep

Supersonic flight is generally considered to be in the 1800-2500 miles per hour range; the hypersonic regime extends from about 3000 miles per hour to orbital velocities, or about 17,000 miles per hour. This flight region is a reseacher's challenge for it will be achieved successfully only when the present limits of materials, structures, and propulsion can be exceeded. Numerous theoretical studies indicate that hypersonic flight is not only attractive from a long-range and speed consideration, but is potentially economical for global transportation. In the extreme velocity region, hypersonic vehicles can become recoverable space boosters.

So far, I have concentrated on trends in commercial aircraft transport technology. In leaving this phase of my discussion, I would like to note again that we are aware that there are important gains to be made in light, private aircraft to increase their utility, to make them easier to fly, and so to bring them within reach of a greater segment of our population. Because we expect to see rapid growth in general aviation in the next decade, a rapid growth in research effort is also taking place.

Space technology provides the basis for the design and construction of space vehicles for the conduct of scientific and operational missions. I will not attempt to cover all facets of this complex field but will limit myself to four areas: energy conversion, materials and structures, electronics, and biotechnology. Energy Conversion.—As far back as human records can trace, man has sought methods for controlling energy and converting it to practical use. He was using horses about 4000 B.C., wheeled vehicles about 3500 B.C., and watermills, steam jets, and other machines in 27 B.C. The development of chemical and mechanical energy converters awaited the first and second laws of thermodynamics that came in the latter half of the nineteenth century. Progress in developing efficient chemical energy conversion cycles came rapidly. An important chemical energy conversion process is for propulsion.

Advances in chemical rocket propulsion technology are familiar to us all— from the shock of the German V-2 through the accelerated missile program of the fifties to the dramatic announcement of Sputnik.

The ballistic missile development provided the capability to conduct space research but our lack of payload capability placed early emphasis on building a new generation of boosters. A bold step to develop an engine of 10 times the thrust of the largest military engine marked the start of the F-1 engine. Missile technology was built on the technology of propellants of moderate energy. In 1945, the NACA formed a small group to do research on high pressure combustion processes and this group, as well as others, began to explore the potential of propellants of considerably higher energy than those considered of military value. Hydrogen and oxygen, which were recognized as desirable early in the century and by Robert Goddard, were among the propellants investigated. The work proceeded slowly until the early 1950's when surplus hydrogen liquification equipment became available from nuclear weapons development and a Collins cryostat was purchased. By 1957, the NACA had operated a lightweight hydrogen cooled thrust chamber producing 5000 lbs. thrust at high efficiency. A few months later, chambers of 20,000 lbs. thrust were being operated. This work led to the decision in 1958 to develop a liquid hydrogen-liquid oxygen rocket and later for the decision to use hydrogen-oxygen in the upper stages of the Saturn booster series. This is another example of research with long-range objectives that paid off in having technology ready when it was needed.

The growth of national booster capability has been impressive. This growth is shown by Figure 357 where Earth orbit capability is shown at the dates of first mission use. From 1958 to late 1961, the maximum payload capability rose from 31 pounds for Jupiter C to 300 pounds with Thor Able and Thor Delta. The Atlas Agena provided an Earth orbit payload capability of 5000 lbs. in 1961. In 1962, the Atlas boosted John Glenn and his 3000 lb. Friendship 7 into orbit. The Atlas Agena provided the largest payload capability until Saturn I in early 1965, which had a 25,000 pounds orbital capability. Manned flights to date have used rockets developed for military use, including the Titan II which boosted the 7000 pound Gemini spacecraft into orbit. In the near future, uprated Saturn I will increase the payload capability to 40,000 pounds and Saturn V will raise the weight to 240,000 pounds. In the more distant future, there is a possibility of launch vehicles with payloads from a half million to four million pounds and today we are working on the technology to make these very large launch