Reentry: Aerodynamics to Thermodynamics

[59] The Ames study had been specifically requested by NACA Headquarters, which in its initial prospectus on the new research airplane project had identified stability and control in high-speed, high-altitude flight as one of two areas needing much additional research. The other and far more critical area was aerodynamic heating, which becomes acute as an object knifes into the atmosphere from the airless environment of space and collides with atmospheric molecules of ever-increasing density. For several years NACA researchers had been studying aerodynamic heating, which begins to be troublesome at about twice sonic speed. The X-15 program was established largely to return data on heating generated up to mach 7. But such investigations of thermal stress hardly approached the heating problem faced by the military services and the missile industry in their efforts to produce a durable warhead for an intercontinental missile. In a typical ICBM flight with a peak altitude of 900 miles and a range of 6,500 miles, the stagnation temperature in the shock wave at the front of the nose cone could reach 12,000 degrees F. This is some 2,000 degrees hotter than the surface of the Sun and 10 times the maximum surface temperature that was calculated for an X-15 trajectory.14 Of the myriad puzzles involved in designing, building, and flying the Atlas, the first American ICBM, the most difficult and most expensive to solve was reentry heating. The popular term "thermal barrier" to describe the reentry problem was coined as an analogy to the "sonic barrier" of the mid-1940s, although research in the fifties would reveal that the problem could have been described more accurately as a "thermal thicket."

During June 1952, in the same summer that NACA had decided to move toward space flight research and had proposed an advanced research aircraft, one of the scientist-engineers at Ames had made the first real breakthrough in the search for a way to surmount the thermal barrier. He was Harry Julian Allen, a senior aeronautical engineer at Ames and chief of the High-Speed Research Division since 1945. The burly Allen, who signs his technical papers "H. Julian" but who is known familiarly as "Harvey," was 42 years old in 1952 and looked more like a football coach than a scientist. Holder of a bachelor of arts degree in engineering from Stanford University, Allen in 1935 had left the Stanford Guggenheim Aeronautical Laboratory, where he had received the degree of aeronautical engineer, to join the NACA staff at the Langley laboratory. When Ames was opened in 1941, he went west with Smith DeFrance and others from Langley.15

At Ames, Allen had invented a technique of firing a gun-launched model upstream through a supersonic wind tunnel to study aerodynamic behavior at high mach numbers. This notion led to the construction of the Ames supersonic free-flight wind tunnel, opened in 1949. The tunnel had a test section 18 feet long, one foot wide, and two feet high. By forcing a draft through the tunnel at a speed of about mach 3 and by firing a model projectile upstream at a velocity [60] of 8,000 feet per second, the Ames researchers could simulate a mach number of about 15. Schlieren cameras set up at seven stations along the test section, three on the side and four on the top, made shadowgraphs to show airflow characteristics over the model and thus determine the aerodynamic forces experienced. During the 1950s the facility, constructed at an original cost of only about $20,000, was to prove one of NACA's most valuable tools for hypersonic investigation.16

As a member of one of the panels of the Department of Defense Research and Development Board, a group charged with supervising weapons research, Allen was intimately familiar with the payload protection dilemma confronting the Air Force and Convair, the prime contractor for the difficult Atlas project.17 In their designs the Convair engineers had already provided that at the peak of the Atlas' trajectory, its nose, containing a nuclear warhead, would separate from the sustainer rocket and fall freely toward its target. These exponents of the ICBM knew that without adequate thermal protection the nuclear payload would burn up during its descent through the atmosphere.

Fifty years of progress in aeronautics had produced more and more slender and streamlined aircraft shapes, the objective being to reduce aerodynamic drag and increase speed. In approaching the Atlas reentry enigma, the Convair group drew from the huge reservoir of knowledge accumulated over the years by aerodynamicists and structures experts dealing with airplanes, rockets, and air-breathing missiles. The men at Convair fed their data into a digital computer, which was supposed to help them calculate the optimum design for structural strength, resistance to heat, and free-flight stability in the separable nose section of a long-range rocket. The computer indicated that a long, needle-nosed configuration for the reentry body, similar to that of the rocket research airplanes, would be best for the ICBM. But tests of this configuration, using metal models in the supersonic wind tunnel at Ames and in rocket launches at Wallops Island, showed that so much heat would be transferred to the vehicle that the warhead would [61] shortly vaporize as it plunged through the atmosphere. No protection system known at that time could prevent its destruction by aerodynamic heating.18

This disclosure evoked another spate of predictions that an intercontinental military rocket would not be feasible for many years. And while relatively few people were thinking seriously about manned space flight in the early fifties, those who were also understood that something radical would have to be done on the problem of reentry before it would be practicable to send a man into space and recover him.

The man who did something radical was Allen. As Allen put it, the Convair engineers "cut off their computer too soon." He took the sharp-nosed Atlas reentry shape and began making mathematical calculations, using only a pad and pencil. Eventually he reached a conclusion that seemingly contradicted all the years of aeronautical research and streamlined aircraft design. For Allen's analysis showed that the best way to cut down reentry heating was to discard a great deal of one's thinking about orthodox aerodynamics and deliberately design a vehicle that was the opposite of streamlined. "Half the heat generated by friction was going into the missiles," recalled Allen. "I reasoned we had to deflect the heat into the air and let it dissipate. Therefore streamlined shapes were the worst possible; they had to be blunt." The Ames researcher determined that the amount of heat absorbed by an object descending into the atmosphere depended on the ratio between pressure drag and viscous or frictional drag. The designer of a reentry body, by shaping the body bluntly, could alter pressure drag and thus throw off much of the heat into the surrounding air. When the bluff body collided with stratospheric pressures at reentry speeds, it would produce a "strong bow shockwave" in front of, and thus detached from, the nose. The shock wave, the air itself, would absorb much of the kinetic energy transformed into heat as the object entered the atmosphere.19

Allen personally submitted his findings to select persons in the missile industry in September 1952. A secret NACA report memorandum embodying his conclusions on the blunt-nose design, coauthored by Alfred J. Eggers of Ames, went out to industrial firms and the military the next spring. The report bore the date April 28, 1953, but six years passed before the paper was declassified and published in the annual report of NACA.20

For his conception of the blunt-body configuration, Allen received the NACA Distinguished Service Medal in 1957. The award brought sharp criticism from H. H. Nininger, director of the American Meteorite Museum at Sedona, Arizona, who asserted that he had first proposed the blunt nose for reentry vehicles. In August 1952, Nininger, a recognized authority on meteorites, had suggested to the Ames laboratory that a blunt shape appeared promising for missile warheads. Nininger based his conclusion on his studies of tektites and meteorites, contending that the melting process experienced by a meteorite during its descent through the aerodynamic atmosphere furnished a lubricant enabling the object to overcome air resistance. Nininger's letter evidently came to Ames some weeks after Allen, [62] assisted by Eggers, had completed his calculations on the relationship between warhead shape and heat convection. At any rate, what Allen wanted to do was exactly the reverse of Nininger's suggestion: deliberately to shape a reentry body bluntly in order to increase air resistance and dissipate a greater amount of the heat produced by the object into the atmosphere.21

Allen's high-drag, blunt-nose principle was of enormous interest and benefit to the missile designers. It led directly to the Mark I and Mark II nose cones developed by the General Electric Company for the Atlas and later for the Thor. [63] Years after the discovery, James H. Doolittle, chairman of NACA's Main Committee, pointed out that "every U.S. ballistic missile warhead is designed in accordance with his once radical precept."22 In 1952 the problems of the missilemen were not of immediate concern to designers of manned flight systems, not even to those drawing up plans for the X-15, which would encounter a greater heating load than any previous airplane. Yet Allen's presentation of a new way to minimize the aerodynamic heating of reentry not only made possible an ICBM within a few years but "marked the potential beginning of manned space flight, with all of its attendant new structures and materials problems."23

The blunt-nose concept was just that - a concept. Succeeding years would see much experimentation with spheres, cylinders, blunted ogives, and even concave shapes at the supersonic free-flight tunnel, ballistic ranges, and various other facilities at Ames, at the 11-inch hypersonic tunnel at Langley, and at the Pilotless Aircraft Research Station on Wallops Island.24 As aerodynamicists began thinking about space flight they would propose a variety of configurations for potential manned space vehicles, although all of the designs would feature some degree of bluntness. Finally, blunting a reentry body furnished only part of the solution to the heating problem. Allen's calculations presupposed that some kind of new thermal protection material would be used for the structure of a high-drag body. In 1952, aircraft designers and structures engineers were working mainly with aluminum, magnesium, and titanium, and were giving some attention to such heat-resistant alloys as Monel K, a nickel-and-steel metal used in the X-2, and Inconel-X, the basic alloy for the X-15.25 But it would take much "hotter" materials to protect the payloads of the intercontinental and intermediate-range ballistic missiles - the Atlas, the Thor, the Jupiter, and later the Titan. Far more materials research was needed before the recovery of a manned spacecraft would be practicable.

Early in 1956, the Army Ballistic Missile Agency at Huntsville, Alabama, modified some of its medium-range Redstones in order to extend the studies of reentry thermodynamics that the Army had pursued at Redstone Arsenal since 1953. As modified, the Redstone became a multistage vehicle, which Wernher von Braun and his colleagues called the "Jupiter C" (for Composite Reentry Test Vehicle). Meanwhile the Air Force conducted its own investigations of reentry in conjunction with its nose-cone contractors, General Electric and the Avco Manufacturing Corporation, using a special multistage test rocket called the X-17, manufactured by the Lockheed Aircraft Corporation.26

Two principal techniques for protecting the interior of the nose cone offered themselves - "heat sink" and "ablation." The heat sink approach involved using a highly conductive metal such as copper or beryllium to absorb the reentry heat, thus storing it and providing a mass sufficient to keep the metal from melting. The major drawback of a heat sink was its heaviness, especially one made of copper. In the ablation method the nose cone was covered with some ceramic material, such as fiber glass, which vaporized or "ablated" during the period of reentry heating. [64] The vaporizing of the material, the conversion of a solid into a gas, dissipated or carried away the heat. Thus the essence of the ablation technique was deliberately burning part of the exterior surface of the reentry body, but designing the body so that the surface would not burn through completely.27

Apparently no consensus existed among students of the reentry problem by late 1957. The "first generation" ICBM nose cones produced by General Electric, the Mark I and Mark II, were blunt, heavy copper heat sinks, and the Air Force had decided to use the Mark II on its Thor intermediate-range missile. But the Air Force's full-scale tests of the lighter, more sophisticated, but more difficult and less tidy ablation process had not begun yet. Meanwhile, the Army and the Vitro Corporation, using the exhaust of liquid rocket motors as a heat source and the hybrid Redstone in reentry simulations, demonstrated to their own satisfaction the practicability of consuming part of the structural material during its use, the principle of ablation. The Army's Jupiter-C shot of August 8, 1957, carrying a scale model Jupiter nose cone to an altitude of 600 miles and a range of 1,200 miles, supposedly "proved the feasibility of the ablative-type nose cone" and "fulfilled the mission of the reentry test program."28 Yet the Ballistic Missile Agency engineers at Redstone Arsenal were working only on the intermediate-range Jupiter, not on an ICBM. The question of whether an Atlas warhead or a manned reentry vehicle could best be protected by the heat-sink or ablation method, or by either, remained undetermined. Much time and effort would be expended before the Army's claims for ablation would be fully verified and accepted.

NACA's official role in this accelerated program of materials research was that of tester and verifier. Even so, the NACA experimenters greatly enlarged their knowledge of thermodynamics, became well grounded in the new technology of thermal protection, and prepared themselves to cope with the heating loads to be encountered in manned space flight.

At the request of the Air Force, the Army, and also the Navy (which was involved with the Polaris after 1956), NACA devoted an increasing portion of its facilities and technical staff to tests of such metals as copper, tungsten, molybdenum, and later beryllium for heat sinks, and of ablating materials like teflon, nylon, and fiber glass. During 1955-1956 the installation of several kinds of high-temperature jets at the Langley and Lewis laboratories greatly aided NACA thermodynamics research. These included, at Langley, an acid-ammonia rocket jet providing a maximum temperature of 4,100°F and a gas stream velocity of 7,000 feet per second, an ethylene-air jet yielding temperatures up to 3,500°F, and a pebble-bed heater, wherein a stream of hot air was passed through a bed of incandescent ceramic spheres. Both Langley and Lewis had electric arc jet facilities, in which a high-intensity arc was used to give energy to compressed air and raise air pressure and temperatures. The hot, high-pressure air then shot through a nozzle to produce a stream temperature of about 12,000°F. NACA investigators used these high-temperature jets and other research tools, including the 11-inch hypersonic tunnel at Langley, to gather data eventually [65] reinforcing the Army's contention that ablation was the most effective thermal protection method.29

Meanwhile Maxime A. Faget, Paul E. Purser, and other members of the Langley Pilotless Aircraft Research Division, working under the supervision of Robert R. Gilruth, used multistage, solid-propellant rockets for studying heat transfer on variations of Allen's basic blunt heatshield configuration. Robert O. Piland, for example, put together the first multistage vehicle to attain mach 10. Faget served as a regular NACA member and Purser was an alternate member of a Department of Defense panel called the Polaris Task Group, set up to give advice on the development of the Navy's intermediate-range, solid-fueled Polaris, which was to be launched from submerged submarines. NACA worked with the Atomic Energy Commission and the Lockheed Aircraft Corporation, prime contractor for the Polaris, in developing the heat-sink nose cone used on the early versions of the sea-based missile.30

Although there were some 30 different wind tunnels at Langley, the members of the Pilotless Aircraft Research Division (PARD) firmly believed in the superiority of their rocket-launch methods for acquiring information on heating loads and heat transfer, heat-resistant materials, and the aerodynamic behavior of bodies entering the atmosphere. As Faget said, "The PARD story shows how engineering experimentalists may triumph over theoreticians with preconceptions. Our rockets measured heat transfer that the tunnels couldn't touch at that time." Joseph A. Shortal, chief of PARD since 1951, recalled, "PARD made us more than aeronautical engineers and aerodynamicists. We became truly an astronautically oriented research and development team out at Wallops."31

The Ames experimenters, on the other hand, were just as firmly convinced that their wind tunnels and ballistic ranges represented the simplest, most economical, and most reliable tools for hypersonic research. To the Ames group, rocket shots were troublesome and expensive, and rocket telemetry was unreliable. As one Ames engineer put it, "You might get a lot of data but since you didn't control the experiment you didn't know exactly what it meant."32

The Ames devotion to laboratory techniques, the determination to do more and more in heating and materials research without resorting to rockets, furnished the impetus for a new test instrument devised by Alfred J. Eggers, Jr., in the mid-fifties. Eggers, born in 1922 in Omaha, had joined the research staff at Ames in the fall of 1944, after completing his bachelor of arts degree at the University of Omaha. He pursued graduate studies at Stanford University in nearby Palo Alto, where he received a Master of Science degree in aeronautical engineering in 1949 and a Ph.D. in 1956.33 For years Eggers had worked with Allen and others at Ames on the aerodynamic and thermodynamic problems of hypervelocity flight, and as a conceptualizer at the California center he came to be regarded as second only to the originator of the blunt-nose reentry principle.

Eggers assumed that the major heating loads of reentry would be encountered within an altitude interval of 100,000 feet. So he designed a straight, trumpet-shaped [66] supersonic nozzle with a maximum diameter of 20 inches and a length of 20 feet, which in terms of the model scale used was equivalent to 100,000 feet of thickening atmosphere. A hypervelocity gas gun launched a scale model upstream through the nozzle to a settling chamber. While in free flight through the nozzle to the chamber, the model passed through ever-denser air, thus closely approximating the flight history of a long-range ballistic missile. Since the apparatus simulated both motion and heating experiences, Eggers called the combination of hypervelocity gun and supersonic nozzle "an atmosphere entry simulator."34

Eggers calculated that using a model only .36 inch in diameter and weighing .005 pound, he could simulate the aerodynamic heating generated by an object three feet in diameter, weighing 5,000 pounds, and having a range of 4,000 miles. "In the simplest test," he said, "the simulator could provide with one photograph of a model rather substantial evidence as to whether or not the corresponding missile would remain essentially intact while traversing the atmosphere." The reentry research technique, proposed in 1955, went into operation during the next year. Construction of a larger version began in 1958. Eggers' atmosphere entry simulator proved especially useful in materials research at Ames. Like the high-temperature jets at Langley and Lewis, the rocket tests at Wallops Island, the Army's Jupiter-C shots from Cape Canaveral, and other experimental methods, it yielded data that later pointed toward ablation as the best method for protecting the interior of reentry bodies.35

Although the official focus of the NACA materials test program remained on missile warhead development, such activity was an obvious prerequisite to manned space flight. And the experience of men like Gilruth, Faget, Purser, and Shortal in the years before the Sputniks had a direct influence on their plans for shielding a human rider from the heat of atmospheric friction. Meanwhile other NACA engineers, especially at Langley and at the High Speed Flight Station, were working closely with the Navy, the Air Force, and North American Aviation on the X-15 project. At Cleveland, Lewis propulsion specialists were studying rocket powerplants and fuels as well as cooperating with Langley and Flight Station representatives in designing, operating and studying reaction control systems for hypersonic aircraft and reentry vehicles.

14 Mark Morton, "Progress in Reentry-Recovery Vehicle Development," pamphlet, Missile and Space Vehicle Dept., General Electric Co., Philadelphia, Jan. 2, 1961.

15 Jacques Cattell, ed., American Men of Science: A Biographical Directory: The Physical and Biological Sciences (10 ed., Tempe, Ariz., 1960), 42; H. Julian Allen, biography sheet, NASA/Ames Research Center, Aug. 1963. Besides supporting the aeronautical laboratory at the California Institute of Technology, Guggenheim philanthropies also made possible the establishment of research institutions for aeronautics at Stanford and elsewhere.

16 Ibid., Alvin Seiff, "A Free-Flight Wind Tunnel for Aerodynamic Testing at Hypersonic Speeds," NACA Tech. Report 1222, Forty-First Annual Report of the NACA - 1955 (Washington, 1957), 381-398; Alvin Seiff and Thomas N. Canning, interviews, Moffett Field, Calif., April 22, 1964. The schlieren method was invented in the early 20th century by the Viennese physics professor and philosopher Ernst Mach, who also devised the unit of measurement representing the ratio of the speed of a body to the speed of sound in the surrounding air, i.e., mach 1. The schlieren technique involves training a beam of light perpendicular to the direction of the airflow to be investigated. A camera is placed behind the light. The camera then photographs a stationary or moving object in the light beam and the surrounding air-streaks, which have varying densities and refractive indices resulting from aerodynamic pressures. See Theodore von Kármán, Aerodynamics: Selected Topics in the Light of Their Historical Development (Ithaca, N.Y., 1954), 106-108; and Dr. W. Holder and R. S. North, "Optical Methods of Examining the Flow in High-Speed Wind Tunnels," Part I: "The Schlieren Method," North Atlantic Treaty Organization Advisory Group for Aeronautical Research and Development, Nov. 1956.

17 H. Julian Allen, interview, Moffett Field, Calif., April 22, 1964.

18 Ibid.; Science News Letter, LXXII (Dec. 21, 1957), 389.

19 Ibid.; Allen C. Fisher, Jr., "Exploring Tomorrow with the Space Agency," National Geographic, CXVII (July 1960), 85; Forty-Third Annual Report of the NACA - 1957 (Washington, 1957), 5.

20 H. Julian Allen and Alfred J. Eggers, Jr., "A Study of the Motion and Aerodynamic Heating of Ballistic Missiles Entering the Earth's Atmosphere at High Supersonic Speeds," NACA Tech. Report1381, Forty-Fourth Annual Report of the NACA - 1958 (Washington, 1959), 1125-1140. Allen and Eggers pointed out that while the blunt shape was optimum for relatively lightweight reentry bodies, as warheads became heavier the total heat absorbed and the rate of heating would probably dictate longer, more slender shapes. Some blunting at the tip of the body, however, would continue to be desirable. This is precisely the evolution that has occurred over the years as rocket thrust has increased and warheads have grown heavier. See Herman H. Kurzweg, "Basic Research," in Proceedings of the Second NASA-Industry Program Plans Conference, NASA SP-29 (Washington, 1963), 127-130.

21 Letters, H. H. Nininger to Ames Aeronautical Laboratory, Aug. 23, 1952; Daniel F. Wentz, Aeronautical Information Specialist, Ames, to Nininger, Sept. 18, 1952; Nininger to Wentz, Sept. 23, 1952; Nininger to Robert Nininger, July 5, 1957; Crowley to Nininger, July 12, 1957, in selected papers of H. H. Nininger 1935-1957, NASA Hist. Archives. Regarding Nininger's claims, Allen has commented: "It is rather ironical that Nininger's 'proof' that blunt bodies are optimum was based on observations of meteorites. All meteorites . . . enter the atmosphere in a speed range for which one can demonstrate that a body which is pointed at the stagnation point is the optimum and not the blunted body as proposed by Dr. Nininger." Letter, Allen to C.C.A., Aug. 17, 1964.

22 Forty-Fourth Annual Report, 30. Doolittle, leader of the famous carrier-based raid of B-25s on Tokyo, succeeded Hunsaker as chairman of the NACA Main Committee in 1956.

23 Richard V. Rhode, "Structures and Materials Aspect of Manned Flight Systems - Past and Present," NASA/MSC fact sheet, July 12, 1962.

24 See, for example, David H. Crawford and William D. McCauley, "Investigation of the Laminar Aerodynamic Heat-Transfer Characteristics of the Hemisphere-Cylinder in the Langley 11-Inch Hypersonic Tunnel at a Mach Number of 6.8," NACA Tech. Report 1323, Forty-Third Annual Report, 1001-1021; and Jackson R. Stalder, "A Survey of Heat Transfer Problems Encountered by Hypersonic Aircraft," Jet Propulsion, XXVII (Nov. 1957), 1178-1184. Throughout the mid- and late-fifties other laboratories also carried on research in reentry body configurations, especially the Jet Propulsion Laboratory at the California Institute of Technology, which did contract research for the Army. See Lester Lees, "Laminar Heat Transfer over Blunt-Nosed Bodies at Hypersonic Flight Speeds," Jet Propulsion, XXVI (April 1956), 259-269, and "Recent Developments in Hypersonic Flow," Jet Propulsion, XXVII (Nov. 1957), 1162-1178.

25 Rhode, "Structures and Materials Aspects of Manned Flight Systems."

26 Wernher von Braun, "The Redstone, Jupiter, and Juno," in Emme, ed., History of Rocket Technology, 110-111; John W. Bullard, "History of the Redstone Missile System," Hist. Div., Army Missile Command, Oct. 1965, 141-142; Frederick I. Ordway III and Ronald C. Wakeford, International Missile and Spacecraft Guide (New York, 1960), 44-45, 53-54; House Committee on Government Operations, 86 Cong., 2 sess. (1959), House Report No. 1121, Organization and Management of Missile Programs, 108.

27 General Electric Co., Missile and Space Vehicle Dept., Reentry Vehicles - Man Made Meteors (Philadelphia, undated).

28 Von Braun, "Redstone, Jupiter, and Juno," 111; Reentry Studies, 2 vols., Vitro Corp. report No. 2331-25, Nov. 25, 1958. On the significance of the differing approaches to the reentry problem of the Air Force and the Army see Organization and Management of Missile Programs, 108-109. See also W. R. Lucas and J. E. Kingsbury, "The ABMA Reinforced Plastics Ablation Program," reprint from Modern Plastics (Oct. 1960).

29 Message, John A. Powers, Public Affairs Officer, Space Task Group, to Eugene M. Emme, NASA Historian, July 5, 1960; Forty-Third Annual Report, 7; Leonard Roberts, "A Theoretical Study of Nose Ablation," and Aleck C. Bond, Bernard Rashis, and L. Ross Levin, "Experimental Nose Ablation," in "NACA Conference on High-Speed Aerodynamics, Ames Aeronautical Laboratory, Moffett Field, Calif., March 18, 19, and 20, 1958, A Compilation of the Papers Presented," 253-284.

30 Paul E. Purser, log of administrative activities related to space and missile research, Jan. 4, 1956, to April 25, 1958. See also John R. Dawson, "Hydro-dynamic Characteristics of Missiles Launched Under Water," in "NACA Conference on High-Speed Aerodynamics," 177-184.

31 Maxime A. Faget, interview, Houston, Jan. 9, 1964; Joseph A. Shortal, interview, Langley Field, Va., Jan. 7, 1964.

32 Seiff and Canning interviews.

33 Dr. Alfred J. Eggers, Jr., biography sheet, NASA/Ames Research Center, March 1963.

34 Alfred J. Eggers, Jr., "A Method for Simulating the Atmospheric Entry of Long-Range Ballistic Missiles," NACA Tech. Report 1378, Forty-Fourth Annual Report, 1009-1015; Forty-Third Annual Report, 5.

35 Eggers, "Method for Simulating the Atmospheric Entry of Long-Range Missiles," 1014; Forty-Third Annual Report, 6-7; Clarence V. Syvertson, interview, Moffett Field, Calif., April 22, 1964; letter, Eggers to C.C.A., June 24, 1964. For the kind of research done in the simulator, see Stanford E. Neice, "Preliminary Experimental Study of Entry Heating Using the Atmospheric Entry Simulator," in "NACA Conference on High-Speed Aerodynamics," 285-312.

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