At the Aeromedical Field Laboratory in New Mexico, Harald J. von Beckh, a physician who had immigrated from Germany by way of the Instituto Nacional de Medicina Aeronáutica in Buenos Aires, was especially concerned about the ability of a space traveler to tolerate the high deceleration forces of atmospheric entry after several hours of weightlessness. In the last few months before such  research ended at Holloman, von Beckh inquired into the relationship between zero g and the multiplication of g. He added a steep downward spiral to the level, weightless portion of the Keplerian trajectory in order to impose heavy g loads on a test subject immediately after a half minute or so of weightlessness. After a number of these parabolic-spiral flights, he reported pessimistically, "Alternation of weightlessness and acceleration results in a decrease of acceleration tolerance and of the efficiency of physiologic recovery mechanisms . . . Because there is a decreased acceleration tolerance," he warned, "every effort must be made to reduce G loads to a minimum."24
Throughout the 1950s a substantial number of aeromedical experts concerned themselves with acceleration-deceleration loads per se, not necessarily in connection with the gravity-free state. Research on g forces reached back for decades, to the primitive period of aviation medicine. The state of knowledge with regard to the physiology of acceleration-deceleration was still hazy and fluid in the early fifties, although for at least 25 years aviation physicians in Europe and the United States had been studying blackout, redout, impact forces, and other effects of high g in aircraft.25 The V-2 and Aerobee animal rocket shots also had added to research data on the problem. But until the X-15 was ready, researchers had about exhausted the airplane as a tool for studying g loads, and from 1952 to 1958 experimentation with animal-carrying rockets was suspended in the United States. Consequently American scientists had to turn to two devices on the ground - the rocket-powered impact sled, used for studying the immediate onset of g loads, and the centrifuge, where the slower buildup of g could be simulated - to enlarge what they knew about the limits of human endurance of heavy acceleration and deceleration.
On December 10, 1954, Lieutenant Colonel John P. Stapp of the Aeromedical Field Laboratory gave an amazing demonstration of a man's ability to withstand immediate impact forces. Stapp rode a rocket-driven impact sled on the 3,550-foot Holloman research track to a velocity of 937 feet per second and received an impact force of 35 to 40 g for a fraction of a second as the sled slammed to a halt in a water trough.26 In February 1957 a chimpanzee rocketed down the track, now 5,000 feet long, braked to a stop, and survived a load of some 247 g for a millisecond, with a rate of onset of 16,000 g per second. And 15 months later, on the 120-foot "daisy track" at Holloman, Captain Eli L. Beeding, seated upright and facing backward, experienced the highest deceleration peak yet recorded on a human being - 83 g for .04 of a second, with 3,826 g per second as the calculated rate of onset. Afterward Beeding, recovering from shock and various minor injuries, judged that 83 g represented about the limit of human tolerance for deceleration.27
Such studies of deceleration were not directed primarily toward space missions but rather toward the problem of survival after ejection from or crashes in high-performance aircraft. The Holloman sled runs of the fifties, however, did broaden considerably the available data on the absolute limits of man's ability  to endure multiples of g. And, perhaps more important, the New Mexico experiments in biodynamics were directly applicable to the problem of high g forces resulting from the uncushioned impact of a spacecraft on water or land. Stapp reasoned that a properly restrained, aft-facing human being could withstand a land impact of some 80 knots (135 feet per second) in a spacecraft if the g forces were applied transversely, or through the body, and if the spacecraft did not collapse on him.28
The centrifuge, the other laboratory tool used by students of acceleration-deceleration patterns, became increasingly useful in the fifties. The basic feature of the centrifuge was a large mechanical arm with a man-carrying gondola or platform mounted on the end, within which a test subject would be rotated at high angular velocities. Centrifuge experiments had more immediate pertinence to space medicine than impact sled tests, because on the "wheel" investigators could duplicate the relatively gradual buildup of g forces encountered during the launch and reentry portions of ballistic, orbital, or interplanetary flight. In the fifties, centrifuges existed at several places in the United States. The best-known and most used were at the Navy's Aviation Medical Acceleration Laboratory, Johnsville, Pennsylvania, and at the Aeromedical Laboratory at Wright-Patterson Air Force Base. During the decade, researchers at Johnsville, Wright-Patterson, and elsewhere simulated a wide variety of acceleration and deceleration profiles, using an almost equally wide variety of body positions and support systems, to compile an impressive quantity of data on the reactions of potential space pilots to heavy g forces.29
Just after the Second World War, Otto Gauer and Heinz Haber, who had conducted centrifuge experiments for the German Air Force, proposed a series of acceleration patterns, ranging from 3 g for 9˝ minutes to 10 g for 2 minutes, all of which would be tolerable for a space pilot.30 Then, in 1952, E. R. Ballinger, leader of the research program at Wright-Patterson, conducted one of the earliest series of centrifuge tests directed expressly toward the problem of g forces in space flight. Ballinger found that 3 g applied transversely would be the ideal takeoff pattern from the physiological standpoint, but he realized that the rocket burning time and velocity for such a pattern would be insufficient to propel a spacecraft out of the atmosphere. Consequently he and his associates subjected men to gradually increasing g loads, building to peaks of 10 g for something over two minutes. Chest pain, shortness of breath, and occasional loss of consciousness were the symptoms of those subjected to the higher g loads. The tests led Ballinger to the conclusion that 8 g represented the acceleration safety limit for a space passenger.31
Data gained from the first Soviet and American instrumented satellites of late 1957 and early 1958 showed that the atmosphere reached considerably farther out than scientists previously had realized. Until these disclosures aeromedical experts had assumed that the deceleration, or backward acceleration, forces of reentry, producing what was graphically described as an "eyeballs out" sensation,  would be much greater than the acceleration during the ascent, or "eyeballs in," phase of the mission. Proceeding on this assumption, a team of physiologists from the Army, Navy, and Air Force had used the 50-foot centrifuge at the Navy's Johnsville installation to study the anticipated high reentry g buildup, exposing five chimpanzees to a peak of 40 g for one minute. Post-run examinations of the primates showed internal injuries, including heart malfunctions. It appeared that prolonged subjection to high g might be severely injurious or perhaps even fatal to a man.32
The tests conducted by Ballinger at Wright-Patterson and the interservice experiments with the chimpanzees on the Navy centrifuge featured frontward (eyeballs-in) application of g loads during the launch profile, backward application (eyeballs-out) during the reentry simulation, and the use of rather elaborate restraint straps and basic aircraft bucket seats as a support system. The problem of determining optimum body position and support was vigorously attacked by biodynamicists during 1957 and 1958. A series of especially careful studies on the Wright Air Development Center centrifuge indicated that when the subject was positioned so that the g forces were applied transversely and backward to the center of rotation, breathing became easier. Acceleration-deceleration patterns of 12 g for 4 seconds, 8 g for 41 seconds, and 5 g for 2 minutes were endured without great difficulty by practically all the volunteer subjects, some having even higher tolerance limits. Results of runs on the Johnsville centrifuge with the subjects in an aft-facing position for both acceleration and deceleration patterns also appeared favorable.33
The students of g forces tried various support devices in the late fifties in their search for ways to increase human tolerance to acceleration and deceleration loads. One specialist in the Wright-Patterson centrifuge group came up with a suit of interwoven nylon and cotton material, reinforced by nylon belting, and attached to the pilot seat at six places to absorb the g loads and distribute them more evenly over the entire body. Later, Wright-Patterson scientists using a nylon netting arrangement in conjunction with a contour couch were able to expose several men to a peak of 16.5 g for several seconds without any discoverable adverse effects. Other Air Force specialists experimented with subjects partially enclosed in a "rigid envelope," actually a plaster cast, as protection against both g-load buildup and impact forces. And von Beckh, whose concern with the weightlessness-deceleration puzzle led him to experiment with anti-g techniques, developed a device called "multi-directional g protection," a compartment that turned automatically to ensure that the g forces were always applied transversely on its occupant. Von Beckh's invention was used to protect a rat that went along on Beeding's record sled run in 1958, and a modified compartment carried three mice on a Thor-Able rocket launch the same year. Results in both experiments were encouraging.34
Navy scientists were especially interested in water immersion as a means of minimizing g loads. Researchers in Germany, Canada, and the United States  had experimented with water-lined flying suits and submersion in water tanks, beginning in the 1930s. Specialists had carried out sporadic biodynamic tests with immersed rabbits and mice in the late forties at the Navy School of Aviation Medicine and, after the giant centrifuge began operation in 1952, in Johnsville.35
In 1956, R. Flanagan Gray, a physician at the Johnsville laboratory, designed an aluminum centrifuge capsule that could be filled with water and was large enough to hold a man. After some initial troubles installing the contraption on the centrifuge and perfecting an emergency automatic flushing mechanism, the "Iron Maiden," as it was rather inaccurately nicknamed, went into use. In March 1958, Gray, immersed to his ribs in a bathtub-like device developed at the Mayo Clinic during the Second World War, had endured 16 g of headward (head to feet) acceleration. Then, the next year, Gray enclosed himself in the Iron Maiden and, positioned backward to the center of rotation and immersed in water above the top of his head, held his breath during the 25-second pattern to withstand a peak of 31 g transverse acceleration for five seconds. This performance with the water-filled aluminum capsule established a new record for tolerance of centrifuge g loads.36
Nylon netting, multidirectional positioning, and water immersion were all promising methods for combating g forces and expanding human endurance limits. But netting had a troublesome tendency to bounce the subject forward as the g forces diminished, while directional positioning and water-immersion apparatus required more space and weight than would be available in a small, relatively light spacecraft.37 And considering the thrust limitations of the Thor, the Atlas, or the somewhat larger Titan ICBM, a small spacecraft was the only feasible design for an American manned satellite in 1958.
At the inception of the NASA manned satellite project, in the fall of 1958, the apparent solution to the problem of body support was an anti-g contrivance developed not by biodynamicists but by a group of practicing aerodynamicists in NACA's Pilotless Aircraft Research Division, part of the Langley Aeronautical Laboratory in Virginia. Maxime A. Faget, William M. Bland, Jr., Jack C. Heberlig, and a few other NACA engineers had designed an extremely strong and lightweight couch, made of fiber glass, which could be contoured to fit the body dimensions of a particular man. In the spring of 1958, technicians and shopmen at Langley molded the first of a series of test-model contour couches. The following July a group from Langley went to the Aviation Medical Acceleration Laboratory at Johnsville to try out their couch on the Navy's big centrifuge.38
The Navy biodynamicists and the NACA engineers experimented with the couch and various body positions in an effort to amplify a g-load tolerance. The couch made at Langley had been molded to fit the physical dimensions of Robert A. Champine, one of the foremost NACA test pilots. Champine rode the Johnsville centrifuge to a peak of 12 g on July 29, then departed for a conference on the Pacific Coast. The next day Navy Lieutenant Carter C. Collins volunteered to test the couch. Since his frame was smaller than Champine's, the Johnsville  experts had to pack foam-rubber padding into the recesses of the fiber-glass bed. Collins then climbed into the centrifuge gondola and seated himself in the couch, the back angle of which was set forward 10 degrees. The 4,000-horsepower centrifuge motor whirled the gondola progressively faster. On the first run the loads reached a peak of 12 g. Five more runs pushed the peak to 18 g. Then, on the sixth try, using a grunting technique to avoid blackout and chest pains, Collins withstood a peak of 20.7 g, applied transversely for a duration of six seconds. Later that day, Gray, inventor of the Iron Maiden, rode the centrifuge with the contour couch and also endured a 20-g peak. The acceleration patterns to which Collins and Gray were exposed corresponded to a reentry angle of 7.5 degrees. At that time the optimum reentry angle being considered for a manned satellite, 1.5 degrees, theoretically would expose the spacecraft passenger to only 9 g.39
The NACA engineers, already working overtime on designs for a manned orbital capsule, were elated. It seemed that they finally had an effective anti-g device that was small enough and light enough to fit into a one-ton ballistic capsule they had in mind for the initial manned space venture.40 They had, in fact, made a major contribution to the protection of a space rider from sustained high g forces, although they did not fully realize as yet that body angles were more significant features of the couch than its contoured support.
The procedure ultimately used for protecting the Mercury astronauts from the g loads of acceleration to orbital velocity and deceleration during reentry represented a combination of the advantages gained from many experiments by military and other specialists in flight physiology, as well as from the ingenuity of the aeronautical engineers in NACA and NASA. Although the idea of using a hammock either for the basic support or in combination with the contour couch was perennially attractive to the human-factors experts in Project Mercury, all Mercury astronauts sat in essentially the same couch designed by Faget and his coworkers in the spring of 1958. But added to this basic technique were restraining straps, a semi-supine posture, frontward application of acceleration loads, and the reversal of the spacecraft attitude during orbit to permit frontward imposition of reentry loads as well. The final elements in the NACA-NASA campaign to minimize the effects of insertion-reentry g buildups was the use as astronauts of experienced test pilots provided by the military services. During the centrifuge experiments of the fifties such men had consistently proved capable of withstanding higher g forces than nonpilots.
24 Von Beckh, "Human Reactions during Flight to Acceleration Preceded by or Followed by Weightlessness," 391-406. After the termination of zero-g flights at Holloman and Randolph, weightless experiments in aircraft continued at Wright-Patterson and at the Air Force Flight Test Center, Edwards Air Force Base, Calif.
25 See Mae M. Link and Hubert A. Coleman, Medical Support of the Army Air Forces in World War II (Washington, 1955).
26 Hanrahan and Bushnell, Space Biology, 86-88; David Bushnell, "Major Achievements in Biodynamics: Escape Physiology at the Air Force Missile Development Center, 1953-1958," Air Force Missile Development Center, 1959, 10-13; Martin and Grace Caidin, Aviation and Space Medicine (New York, 1962), 199-203; Mallan, Men, Rockets, and Space Rats, 99-116. Stapp had taken his first rocket sled ride in 1947 at Edwards Air Force Base, Calif. He became Chief of the Aeromedical Field Laboratory in 1953 and made several more rides before his record run on Dec. 10, 1954. See John P. Stapp, "Tolerance to Abrupt Deceleration," in Collected Papers on Aviation Medicine, Presented at Aeromedical Panel Meetings of the Advisory Group for Aeronautical Research and Development, North American Treaty Organization (London, 1955), 122-169.
27 Eli L. Beeding, Jr., and John D. Mosely, "Human Deceleration Tests," Air Force Missile Development Center, Jan. 1960; Hanrahan and Bushnell, Space Biology, 93-94. The "daisy track" was named for a popular make of air rifle, because it was originally designed as a compressed-air catapult system. From 1955 to 1959 it used a cartridge system.
28 John P. Stapp, "Biodynamics of Space Flight," in Gantz, ed., Man in Space, 68.
29 See William J. White, A History of the Centrifuge in Aerospace Medicine (Santa Monica, Calif., 1964).
30 Gauer and Haber, "Man under Gravity-Free Conditions," 641-643.
31 Hanrahan and Bushnell, Space Biology, 72.
32 Ibid., 75-76. The largest centrifuge in operation in the United States during the 1950s was the Navy's mechanical arm at Johnsonville, with a radius of 50 feet and a capability of 40 g. The Johnsville facility had a device allowing the gondola to be gimbaled to simulate buffetings and cross-currents. The Wright Air Development Center centrifuge, on the other hand, had a radius of 20 feet and a capability of 20 g. Instead of a gondola it featured an open platform, which could be whirled in one plane only.
33 Ibid., 77; John P. Stapp, "Acceleration: How Great a Problem?" Astronautics, IV (Feb. 1959), 38-39, 98-100.
34 Ibid., 77, 105-109; memo for files, Gerard J. Pesman, Human Factors Br., Space Task Group, "Present Status - Major Systems, Pilot Support and Restraint" [about Feb. 1959]; Harald J. von Beckh, "Multidirectional G-Protection in Space Vehicles," Journal of the British Interplanetary Society, XVI (Sept.-Oct. 1958), 531.
35 Hanrahan and Bushnell, Space Biology, 96-98.
36 Carl C. Clark and James D. Hardy, "Preparing Man for Space Flight," Astronautics, IV (Feb. 1959), 18-21, 88; Clark and R. Flanagan Gray, "A Discussion of Restraint and Protection of the Human Experiencing the Smooth and Oscillating Accelerations of Proposed Space Vehicles," U.S. Naval Air Development Center, Dec. 29, 1959, 26-46. During 1957-1958 scientists at the Wright Air Development Center also carried out water-immersion studies, using a coffin-like container. Peak accelerations on the limited Wright centrifuge were only about 16 g, but the durations of the acceleration patterns were longer.
37 John P. Stapp, interview, San Antonio, April 24, 1964.
38 Maxime A. Faget, interview, Houston, Jan. 3, 1964, and Aug. 24, 1964; Gerard J. Pesman, interview, Houston, March 17, 1964; James M. Grimwood, Project Mercury: A Chronology, NASA SP-4001 (Washington, 1963), 20. For USAF concept of rotating couch, see p. 96.
39 Faget interview; Clark and Gray, "A Discussion of Restraint and Protection," 26; Pesman memo; John Dille, ed., We Seven, by the Astronauts Themselves (New York, 1962), 110-112.
40 Clark and Gray, "A Discussion of Restraint and Protection," 26; Pesman memo. Faget and his men became even happier in December 1958, during the early days of Project Mercury, when a second Langley couch, its back angle raised 8 degrees, supported Lt. Carter C. Collins on the Navy centrifuge during a peak of 25 g for approximately 10 seconds.