Rather than to review such Divisional projects in detail the intent here is to give an overall picture of the shuttle technology effort. For this purpose the work can be described more comprehensively in terms of the areas for which the Technology Working Groups are assigned technical responsibility as previously listed in Figure 2.

Figure 4 (RS 71-3604) provides an overview assessment of four of the technologies considered at the outset to hold the key to a successful shuttle system. The Thermal Protection System/Structure (airframe) weight is the major uncertainty in the reusable shuttle system. Our technology programs in FY 1971 and 1972 are allocating over one-third of the total shuttle technology R&D funds to that area.

A second pacing technology is the total Auxiliary Propulsion System (APS) which encompasses the attitude control system (ACS) and the orbital maneuvering system (OMS), which is used in the orbiter only.

These and other elements of the program are described in the following paragraphs.

AEROTHERMODYNAMICS, CONFIGURATION SELECTION AND DEFINITION

A large early effort was required to validate the aerodynamic characteristics of the candidate orbiter and booster configurations and to provide entry thermal environment data. OART met this requirement by providing expert manpower and test time in several wind tunnel facilities at Ames and Langley Research Centers.

Our first tasks, carried out in conjunction with the initial OMSF Phase A studies, were an extensive set of screening tests to select the most promising approaches for more concentrated attention in the OMSF contracted Phase B preliminary design studies. Two types of orbiter configuration, one a straight wing design and the other a delta, and three booster configurations were selected for preliminary design study.

OART's next task was to provide aerodynamic data over a wide range of speeds, from hypersonic to subsonic, along with entry heating data, on the candidate configurations. As noted in this year's testimony by the Entry Technology Office, approximately 10,000 hours of wind tunnel time were devoted to these investigations during CY 1970.

This second-step effort, which was completed in December 1970, together with industrial studies, provided a firm basis for selecting a delta-wing configuration as the preferred orbiter vehicle and narrowed the candidate booster types.

A great deal of further analyses and test will be required before final design selections can be made. Aerothermodynamically, fairly large differences still remain among the candidate design approaches, and even obscure differences can be important. This was dramatically borne out in OART's Lifting Body Program where three somewhat similar configurations designed with the same basic objectives by three different teams proved to have widely different flight characteristics.

During the remainder of FY 1971 we will provide more detailed data covering a wider range of test conditions on the remaining configurations. During FY 1972 our objectives will be to provide detailed aerodynamic and heating data for the selected orbiter and booster vehicles (see Figures 5 RS71-3609) and to determine the characteristics of the combined vehicles during launch. Attention will be given to the problems of separation of the two vehic e components at normal high-altitude staging and under abort conditions at high dynamic pressures Consideration is being given jointly by OMSF and OART to the construction of approximately 1%-scale, rocket-powered, B-52-launched flight test vehicles to confirm the flight characteristics of the selected shuttle design in the regime characterizing terminal approach from about 100,000 feet down to landing. Such a vehicle will demonstrate the validity and flightworthiness of the configuration, and it may disclose problems that have not been revealed in wind-tunnel tests.

STRUCTURES, THERMAL PROTECTION SYSTEMS, AND MATERIALS

As pointed out earlier, it was recognized from the outset that the shuttle airframe system, made up of the load-carrying structure, cryogenic propellant tanks, and thermal protection system, constituted the major technological uncertainly of the shuttle. The performance of the shuttle system is particularly sensitive to its inert weight, of which the airframe con titutes about 60 percent. Once the overall size of the shuttle has been set any change in the inert weight will have a pronounced effect on the actual payload weight. A change of 20 percent of the estimated structural weight can eliminate the payload or double it. Thus there is a high premium on achieving the lowest possible structural and thermal protection system weight.

To develop such a vehicle it is necessary to predict the vehicle weight with high accuracy; the weight must be prevented from increasing significantly over the course of developing the vehicle; and structural reliability cannot be compromised to achieve lightness. Furthermore reusability for many flights with a minimum of maintenance and refurbishment is the key to low-cost operations, and this consideration applies especially to the thermal protection system which is exposed cyclically to an entry environment in which the air temperature exceeds 10,000° F.

These requirements of the shuttle have accelerated our technology effort in structural and thermal protection systems and in the materials associated with these systems. Five NASA Centers are involved, each in areas reflecting its particular specialities. Lewis Research Center, for example, is engaged in hightemperature metallic materials technology and in the technology of lightweight propellant tanks utilizing fiber-reinforcement. Ames Research Center is utilizing its excellent arc-jet facilities to determine the performance and reuse potential of a variety of advanced metallic and non-metallic material specimens supplied by Lewis and other sources. Langley Research Center is engaged in the design, fabrication, and test of full-scale advanced heat shield panels. Various aspects of structural design are also being worked at Langley, including the development of structural design criteria, the further development of advanced computerized methods for structural analysis and design, and design techniques for advanced load-bearing structural components such as paneis and truss eiements. The Manned Spacecraft Center, with its extensive experience with non-metallic heat shields, is pursuing the development of a new class of non-metallic refractory materials that offer reuse potential and potentially greater simplicity of application with lighter installation weight.

Marshall Space Flight Center is investigating the application of composite (fiber-reinforced) materials in the fabrication of large structural segments that support the main rocket engines and their thrust loads. Marshall, with assistance from other Centers and industrial groups, is also designing and constructing large-scale structural test elements that represent segments of the shuttle airframe. These test specimens will typify liquid hydrogen tanks with cryogenic insulation, the load-carrying structure, and the outer heat shield with its hightemperature insulation, all structurally integrated into a test assembly. The test specimen will be subjected to cyclic mechanical and thermal loadings to simulate repeated missions.

This in-house activity is augmented by contracted efforts, primarily with aerospace firms who have especially strong competence in structural design and materials technology.

Figure 6 (RS71-3608) depicts the estimated design temperatures at various points on the surface of one candidate orbiter vehicle. The planform view shown at the top of the figure is divided along the centerline such that vehicle upper surface temperatures are depicted on the top half of the planiorm sketch and lower surface temperatures on the bottom half. The top side of the vehicle operates largely in the lee of the high-energy air, and the surface temperatures are therefore considerably lower than they are on the bottom of the vehicle which is exposed directly to the hot airstream. As shown on both planform and side views, large areas of the upper surface experience temperatures no greater than 600° F. with higher temperatures being reached under the low angle of attack launch conditions rather than during entry. In these regions of the vehicle a "warm" structure made of titanium without a separate thermal protection system may be the most efficient design approach.

At the nose, wing and fin leading edges, and over the lower surface of the vehicle the temperatures are higher. ranging from 1500° to over 3500° F in localized spots. In these regions, the main load-carrying structure is kept from reaching excessive temperatures by an overlying thermal protection system or heat shield. We are concentrating our technology efforts largely on materials that can be used at temperatures of 2000° and higher and on the fabrication of these materials into complete heat shield panels. In this work the primary emphasis is on thermal protection systems that can endure repeated exposures to these high temperatures over many simulated flights without damage or loss of effectiveness.

Figure 7 (RS71-3606) lists the objectives for FY 1972 for the primary or loadcarrying structure and the materials appropriate to the structural application. We will continue the program to achieve a completely reliable structure with the lightest possible weight. We will continue to develop structural design criteria appropriate to the shuttle and its operating conditions and will continue to upgrade

the major tools for optimizing structural design, namely the large-scale programs for computerized structural analysis, such as NASTRAN. Work on improved compression panel geometries, as illustrated at the upper right of Figure 7, and on the application of composite materials to particular structural components, as illustrated at the lower right, will also continue. The lower sketch illustrates selective reinforcement. The example shown involves the stiffening of the compression element of a truss by overwrapping a titanium tube with a composite material of boron fibers embedded in an epoxy matrix. Other related work in the shuttle structures efforts is described in the testimony of the Materials and Structures Division.

Figure 8 (RS71-3607) ties together the work of structures, thermal protection systems, and materials for FY 1972. As indicated at the left of Figure 8, we will continue our arc-jet testing of material samples to confirm the basic material performance at high temperatures. We are already progressing to the fabrication of full-scale (approximately 2 feet by 2 feet) experimental heat shield panels such as shown in Figure 9 (RS71-3601). These panels will be tested in larger facilities, such as the Langley 8-foot high-temperature structures tunnel as illustrated in the center of Figure 8. This program of design, fabrication, and test of large-scale panels will become fully implemented during FY 1972. The technology work will then proceed to the integration of thermal protection systems with the primary structure and tankage in large-scale technology demonstration models of the kind illustrated on the right side of Figure 8. During FY 1972 the first of four different large-scale integrated structural models will be completed. Behavior of the system under mechanical and thermal loadings simulating those encountered during a mission from launch to landing will be studied.

During FY 1971, special attention was given to further development of an oxidation resistant nickel-chromium alloy strengthened with dispersed fine particles of thorium oxide, commonly referred to as TD Nickel-Chrome. Work on this metal which has already been produced in sheets of useful size for the shuttle, is under the direction of the Lewis Research Center. It has been shown in tests of small samples in Ames and Langley arc-jets to be reusable at a temperature of 2000° over a period of 50 heating cycles each of 30-minute duration, a total exposure of 25 hours.

Work also progressed on other heat shield materials such as coated columbium and a new class of non-metallics composed of compared ceramic fibers. Results of the high-temperature materials work are discussed in more detail in the testimony of the OART Materials and Structures Division.

DYNAMICS AND AEROELASTICITY

This complex discipline is concerned with the transient and oscillatory behavior of the large flexible vehicle system in response to loads and aerodynamic disturbances. Since dynamic behavior involves the entire vehicle system, it can cause limiting structural stresses and hence affect structural weight. In the final analysis the solution of dynamic problems must be comprehensive and complete.

In order to conduct a comprehensive dynamic analysis and test program, more details of the system must be known than for other key technology areas of the shuttle.

Thus the dynamics and aeroelasticity program began with a modest effort to explore and identify potentially troublesome modes of motion and instabilities that might be encountered by various new types of configurations proposed for the shuttle. Aeroelastic torsional oscillations of winged vehicles, or "stop-sign" flutter, and rocking about the base support, or "galloping" instability, for certain body cross-sectional shapes were encountered in tests simulating ground winds blowing around the erected vehicles on the launch pad. Criteria for avoiding these motions have now been developed.

Also investigated were the stall flutter characteristics of straight and delta wings at high angles of attack, low angle flutter and buffet of straight wings, and the flutter characteristics of biplane wing arrangements representing the coupled orbiter and booster vehicles in the launch configuration.

Since it is essential to bring dynamics analysis into the shuttle program as early as practicable to avoid design problems that would be difficult to correct later, a detailed program of investigation has been planned to be increased progressively as the shuttle system becomes better defined. The FY 1971 and FY 1972 programs reflect this program growth.