with these various propulsion systems have been defined but more work on specific problems must be done before the best system can be selected.

Minimum structural weight is imperative to the development of a reliable, efficient supersonic transport. Because of temperature environment of the supersonic transport more efficient structural techniques and long-life materials are mandatory.

Operating problems include a myriad of areas requiring continuing work, for example: sonic boom, landing approach techniques, air traffic control, and flying and handling qualities. Past research effort on the sonic-boom problem has indicated the need and direction for further research. Landing approach techniques and piloting problems must receive increased attention in the future. A ground-based flight simulator for advanced aircraft is a necessary tool for investigating with adequate realism, critical areas of a pilot's capability. Flying and handling quality requirements must be defined and solutions found to provide adequate dynamic stability and control in all operating regimes of the supersonic transport. The results of research effort in the area of operating problems have a direct influence upon the aircraft configuration and the propulsion system.

During fiscal year 1964 we will approach the time when design choices for a U.S. supersonic transport will be made and the NASA's performance and mission studies, wind-tunnel studies, simulator studies, and associated flight studies will change from the general character they now have to more specific studies in greater depth of detail. Since the effects of even small changes in design or operation can have a significant effect on the whole machine, due to the strong mutual interdependence of the various components of the vehicle, it is necessary to carry on a carefully coordinated and detailed research program.

I have been talking of the problem of two specific examples of supersonic aircraft, one in the military field, the F-111, and one in the civil field, the commercial supersonic transport. I will now direct your attention to some examples of our general work which is applicable to all supersonic aircraft. For example, in the field of structures, because of our present lack of ability to predict accurately the fatigue life of aircraft structural members, a fail-safe philosophy has been widely adopted in the aircraft industry. This philosophy accepts the probability of fatigue cracks, and the structure is designed so that a single crack will not unduly weaken the structure. In support of this philosophy, systematic programs are underway to study the rates of progression of fatigue cracks and the reduction in residual static strengths due to such cracks.

The series of accidents which befell the Comet aircraft is an example in which a relatively small fatigue crack in the pressurized fuselage led to a catastrophic failure. As a result of structural tests of pressurized cylinders and complementary theoretical work, NASA developed principles for making fail-safe fuselages. These principles have been adopted by U.S. aircraft industry and are employed in all our current jet transports.

The fuselage of a supersonic transport cruising at 70,000 feet poses a much more severe problem. If a fuselage crack develops and pressure is lost, provision for dropping oxygen masks to the passengers, is no longer adequate. At these rarefied altitudes a passenger really needs a pressurized space suit. This being impractical, the fuselage must be 100-percent fail-proof. This is no small task in view of the heating, noise, and other loadings to which the fuselage is subjected and the requirement that the structural weight be kept to a minimum.

Despite the continued existence of all the old structural design problems and the addition of new foreign environments which the structure must withstand, notable progress is being made to overcome the new obstacles, and yet provide methods for building structures with lower weight fractions than have been attainable in the past.

A program of intense effort, and one which is of tremendous significance in planning for the supersonic transport, is the sonic-boom research carried out principally by the Langley Research Center. The intensity of the sonic-pressure wave registered on the ground, and community reaction to it, will influence supersonic transport cruise altitudes and flight plans and thus to a large extent the design of the airplane.

During the past year much information concerning the sonic boom has been obtained from a joint USAF/NASA/FAA flight program. In this program B-58 and other supersonic aircraft have been flown over instrumented ranges at various speeds and altitudes to obtain ground-pressure measurements. Extensive windtunnel studies have been made and are continuing. At the present time, techniques are being developed to permit a determination of the effects of aircaft configuration on sonic-boom intensity. The next chart, figure 177, page 2083,

illustrates the status of this work. The ordinate is a measure of sonic-boom intensity and shown here are curves that indicate the intensity of the sonic boom that will be felt on the ground with a transport-size aircraft flying over at three times the speed of sound. The two upper curves are each for a different designconcept aircraft. The dashed curve is a lower bound that can be approached and is a result of current theoretical and experimental work on the problem. The theory that has evolved from this work permits the study of the effect of aircraft configuration on the sonic boom. The important fact to be noted in this chart is the significant effect of model configuration on boom intensity as shown by the difference in level of the two solid lines. For the first time, a theory is now available to permit study of these efforts. Much work remains to be done and will continue in fiscal year 1964.

A vital part of the simulator program is the variable-stability aircraft to be operated at the Flight Research Center. Flight research is required, in order to (a) Check analytical studies, (b) validate and correlate results from ground simulator programs, and (c) investigate problem areas which cannot be studied in ground facilities.

The next chart, figure 178, page 2084, illustrates schematically the aircraft chosen for this work and some of the modifications required A test pilot will fly the aircraft and will be presented with a simulated instrument display which, coupled with the modified aircraft motion, will appear to the pilot as having the characteristics of the aircraft being simulated. A safety pilot will have a standard instrument display to indicate the real aircraft flight conditions. The safety pilot will thus be able to take over if needed. The aircraft will also contain the simulation system computers, a test director's console, and displays for observers as well as the more or less standard NASA flight sensors and recording equipment. Materials for the construction of aircraft to fly more than twice the speed of sound must be investigated to provide the necessary information to permit a selection to be made. The materials problem became very acute about 2 years ago. At that time a search of available information revealed that no information existed to permit the selection of skin materials that would withstand the temperature associated with speed greater than twice the speed of sound with the reliability and endurance capability required for commercial operation. An intensive program was immediately undertaken by the NASA to provide such information. A large number of candidate alloys were selected for a screening program to eliminate those not suitable. The next chart, figure 179, page 2085, shows a rough approximation of the relative rates at which the strengths of some of these materials deteriorate with time. Today the number of candidate materials has been reduced to about seven alloys which have passed the necessary tests. These metals must now undergo extensive tests before final selection of the materials for the supersonic transport is made.

HYPERSONIC AIRCRAFT

The X 15 research airplane program was initiated in advance of the space age as a joint effort of NASA and DOD to pioneer in the hypersonic flight regime. It has fulfilled its original purpose exceedingly well and the flight research program on this manned maneuverable hypersonic aircraft is still in progress.

The next chart, figure 180, page 2085, shows the altitude-speed flight envelope of the X-15 already explored in relation to the region of interest for continuous flight in the sensible atmosphere. The X-15 is providing valuable inflight data for the development of future suborbital and space-vehicle systems. "Follow-on" phases of the program having space technology as well as aeronautical objectives are currently underway.

With the solid success of the X-15 program, the point has been made to the Congress that we are looking ahead in aeronautics to advanced research and technology needs of the Nation in the area of airplane-like hypersonic vehicles. The place of such vehicles in the aeronautical transportation spectrum is shown in the next chart, figure 181, page 2086.

The lower end of the range-versus-speed plot is covered by subsonic aircraft of both rotary-wing (helicoptors) and fixed-wing types including the familiar subsonic jet transport. The projected supersonic transport will extend this lower end up to 2,000 miles per hour or so and perhaps 5,000 miles ultimate range. There remains to be exploited, although already spanned by the expendable rocket booster, a wide range of useful higher speeds and ranges up to halfway around the earth. There also are emerging needs for transferring personnel and their equipment to or from space stations in orbit, and there is the possible need for military control of near-space for national defense.

The cost of present rocket technology for bridging the gap in the pathway to space is high. These are reasons to believe that a hypersonic airplane with airbreathing engines using the oxygen from the atmosphere shown here for global range earth transportation or for boosting a second stage like the X-20 (DynaSoar) into orbit will be much more economical. It is likely that this area of the spectrum will eventually be filled in the manner outlined.

One attractive concept for a combined earth-to-orbit and earth-to-earth transportation system is illustrated in the next chart, figure 182, page 2087. The firststage booster is a HTOHL hypersonic-cruise airplane with turboramjet airbreathing propulsion. At the optimum hypersonic speed and altitude, it launches a second-stage rocket-propelled carrier capable of orbital rendezvous with a space station or an alternate vehicle for the simpler earth mission. Either second-stage carrier is capable of horizontal landing at a preselected point. The HTOHL feature with suitable abort capability permits the transfer or ordinary people as well as highly trained astronauts.

Now I have a model here to illustrate what such a system might look like. In this model the second stage is carried internally to improve performance on the way to the staging point; it might alternately be carried externally on top of the hypersonic airplane.

As listed on the chart, the potentialities of the system include a wide rendezvous launch window and a wide choice of operational sites. The vehicles can be fully recoverable and reusable to save costs on a long-term and high traffic density basis.

The current NASA program is outlined in the next chart, figure 183, p. 2089. It embraces conceptional, operational, and economic studies to establish the practical desirability of the concept, and research and development effort in the critical areas of propulsion, structures, materials, and aerodynamics to provide the information necessary to permit long-term development decisions. I will amplify briefly on the effort in these, technical areas.

The question of what part air-breathing propulsion should play in the systems being studied is being intensively examined. Initial results in this area are encouraging, at least up to hypersonic mach numbers of 10 to 12.

Above the speed of the supersonic transport the ramjet engine appears to be the most promising solution. Two types of ramjet are shown in the next chart, figure 184, p. 2089, the subsonic-burning type for operation up to perhaps mach 8 and the more advanced supersonic-burning type for operation up to mach 25 or so. The upper range of usefulness of subsonic-burning ramjets is limited by excessive pressures, temperatures, and exhaust nozzle losses. With the supersonicburning principle, the temperatures and pressures are less severe and there is theoretical indication that nozzle losses due to recombination of the products of combustion will be less severe. However, experimental research over a wide range of pressures, temperatures, and fuel-air ratios is required to determine the practical potentials of hydrogen-fueled supersonic-burning ramjets.

You are aware that flight vehicles are subjected to aerodynamic heating at high speeds. For this reason the materials and structures problems become extremely critical at hypersonic speeds as shown in the next chart, figure 185, p. 2090. Here is shown the structural weight percentage of total aircraft weight as a function of speed. (The structural weight percentage is in effect an inverse indication of the structural efficiency.) At the lower speeds the familiar aluminum alloy airframes weigh something like 20 percent of the total; the present-day effectiveness of civil and military aircraft is in a large measure attributable to this low weight fraction. As the speed and heating increase, aluminum and steel are no longer suitable materials because of deteroriation of their strength, and heat-resistant refractory metals or ceramics must be used. As can be seen, these are inherently heavier and less efficient so that at hypersonic speeds of 10,000 miles per hour the structural weight will constitute something like 50 percent of the weight of the aircraft unless marked improvements can be achieved.

Our goal in structures research is to offset this adverse trend by means of basic improvements in structural design. What we seek are more efficient methods of utilizing less efficient materials. It is imperative that ways be found to decrease what we have here labeled the "heating penalty."

One example of a structural concept that is evolving from NASA research effort and may provide the needed improvement is shown in the next chart, figure 122, p. 1975. On the left is shown a construction typical of present-day technology. On the right is shown a type of structure where the reflectivity of the material is combined with a vacuum to insulate the very cold inner (hydrogen tank) surface from the very hot outer skin. The structure is composed of very thin dimpled

metal sheets welded together at matching dimples thereby restricting the area available for heat flow from sheet to sheet. This concept also provides a means for inert gas purging to prevent any leaking hydrogen coming in contact with the hot external surface and causing a destructive explosion.

The aerodynamics problems for the hypersonic vehicle systems are very much like they have always been except for their complexity due to the wide range of subsonic, supersonic, and hypersonic mach numbers encountered. In order to have low-enough drag and high-enough lift-drag ratio while maintaining reasonable stability and control characteristics over this wide range of speeds, advanced variable-geometry features may be necessary. The fundamental problem of the hypersonic-cruise vehicle is one of configuration synthesis, of determining how the large-volume body associated with hydrogen fuel tanks, abnormally large air inlets, and other components can be put together in an acceptable arrange

ment.

We are planning for the near future an investigation of mutual interference and component arrangement effects on wing-body-stabilizing surface combinations. We are also planning at a later date to initiate a flight-test program to evaluate skin friction and heat transfer for complex lifting configurations. These items cannot be adequately studied except in the actual high mach number, Reynolds number, and enthalpy environment of the space corridor.

V/STOL AIRCRAFT

Most of our research effort on subsonic aircraft is concentrated on vertical or short takeoff and landing (V/STOL) types. For military use in limited war, the requirement for an aircraft capable of rapidly transporting troops, support equipment, and supplies to and from areas close to the combat zone has emphasized the need for VSTOL aircraft. This chart, figure 187, p. 2091, illustrates some of these needs—the tilt-wing transport being used to connect conventional jet transports operating to conventional airports to helicopters and VTOL fighters operated from relatively unprepared landing strips in the combat zone not able to handle conventional aircraft.

For commercial aircraft, practical use of the VTOL or STOL transport is foreseen, as indicated on this chart, figure 188, p. 2091, in providing faster service in regular intercity shuttle flights of medium-range vehicles from relatively small, close-in airports where steep climbout and approaches are desired to avoid noise nuisance and building interference. Such aircraft could also be utilized in transporting passengers to terminals servicing supersonic-transport aircraft; this feeder-line transport would provide more rapid transit to these terminals (which will be few in number and remotely located), with little interference to other transport operation. There is a tremendous potential market if noise, traffic control, and cost problems can be solved.

With regard to concepts for obtaining VTOL-the conventional helicopter will continue to be a practical aircraft for military or civil missions of relatively short range in which vertical takeoff and landing and hovering are required. Substantial improvements in the performance of the helicopter are considered desirable and possible and will receive continued study during the next year. For example, several research studies at Langley and Ames have recently been initiated on models using nonarticulated rotors, with the objectives of reducing complexity and maintenance problems and improving performance and flying and handling qualities. The problem of all-weather terminal-area operation (for the helicopter as well as other V/STOL types) will be studied with a variablestability research helicopter in which combinations of flying qualities, paneldisplay information, signal sources, and approach techniques permitting transition from steep approaches to vertical touchdown under instrument conditions, will be investigated.

The helicopter is, at present, the only operational VTOL aircraft, at least in this country. As stated previously it is likely to remain a very practical vehicle for VTOL missions that are of very short range or require extensive hovering capability. When this is not the case, other _V/STOL concepts appear more promising because of their greater operating efficiency, speed, and range capability. The NASA has, for several years, undertaken extensive research programs on models of a variety of such V/STOL aircraft in its low-speed wind tunnels and with five flying "test beds" provided by the military services. It is believed that, although additional research is still required, most of the concepts utilized on the test beds could be incorporated into the design of a successful operational aircraft. Research is continuing on these concepts in such areas as ground interference and recirculation effects, wing stalling, and control requirements, to

provide information enabling the design of V/STOL aircraft having the greatest utility and productivity for a given mission. In the immediate future, emphasis will be placed on investigations of three specific configurations of current interest to the military services, shown in this chart, figure 125, p. 1979, the Vought XC-142A triservice assault-transport which utilizes a tilt wing; the Bell X-22 tilt duct; and the G. E. Ryan XV-5A fan in wing. Particular attention will be given to studies of the XC-142A, scheduled for flight in 1964. Large-scale model studies have already been initiated to determine the aerodynamic characteristics of this vehicle and, where required, to investigate methods of alleviating undesirable or marginal characteristics, such as wing stalling. Similar studies are being made or planned for the other two vehicles. In the near future, more emphasis will be placed on studies of higher speed concepts-such as those using lifting and vectored-thrust engines-considered feasible for supersonic V/STOL application.

Although only a small part of our total V/STOL research effort is directed specifically to study of purely short takeoff and landing (STOL) aircraft, many of the results of the VTOL studies have direct STOL application. It is expected, for example, that the tilt-wing type will be an excellent STOL aircraft and will be used that way for economy and safety reasons unless the operating site demands VTOL. Particularly valuable information on required handling qualities for a large STOL aircraft has recently been obtained from NASA flight studies of the C-130C airplane which utilizes boundary-layer control to obtain increased lift and steeper takeoffs and landings. Further flight studies will be made with this aircraft in the next fiscal year to investigate the effects of a modification to the control system, which NASA simulator studies have indicated to be desirable to improve handling characteristics during the steep descent.

With regard to powerplants for V/STOL application, there are a number of propulsion concepts under study and development by the military and the industry which can be categorized as follows:

(1) Lift engines: These are engines which provide lift thrust only for VTOL aircraft.

(2) Lift-cruise engines: These engines provide both lift and cruise thrust for VTOL aircraft and incorporate a system for vectoring thrust from the vertical to the horizontal direction.

(3) Augmentation systems for obtaining increased VTOL thrust from convenventional and advanced powerplants with a minimum added weight penalty: These systems include rotors, free and ducted propellers, ejectors, and lift fans. In the area of augmentation systems, NASA will conduct testing of lift fan and ducted propeller systems at the Ames and Langley Research Centers. These programs will be carried out in conjunction with the evaluations of the various VTOL aircraft being conducted by the military.

Additional work is planned in areas of research applicable to lift and lift-cruise engines as well as in the areas of augmentation systems applicable to VTOL propulsion. This work will include research, evaluation and testing on basic engine components as well as on fans, propellers, ducted propellers; etc.

It is our strong conviction that the use of air vehicles for purposes of civil and military transportation will continue to grow and expand. We believe that the welfare of the country will be best served by maintaining a strong program of research and development of advanced technology in this field. The program I have just described represents our best judgment as to what that program should be.

Mr. ZIMMERMAN. Mr. Chairman and members of the subcommittee, we are continuing our aeronautical research program very much in the vein in which it has been carried on for many years by the NACA and now NASA. However, we have had to make a change in emphasis. We are no longer trying to achieve higher speeds or higher altitudes.

The X-15 has flown practically out of the atmosphere and we have achieved orbital speeds in missile and manned spaceflight. We are, however, interested in two goals.

One is to bring about the technology necessary to permit efficient, economical and available air transportation for the civilian population and the other the advanced technology necessary for the most effective possible air transportation for our military services.