Vostok and Mercury were first steps, designed to explore the concept of manned space flight. Maxime A. Faget, chief designer of the Mercury spacecraft, summarized their importance:
Since these flights were initial efforts, the purpose of the flights was limited to the basic experience of launching the spacecraft and crew into orbit, having them remain there for a period of time, and then having them return safely to earth. These flights were made at low altitude with the spacecraft barely high enough to avoid appreciable drag from the upper fringes of the atmosphere. . . . the amount of energy required for launching was minimized, and the flight was made safer, since the difficulty of making a reentry maneuver was also minimized. . . . these flights . . . proved that it was practical for man to fly in space.4
While providing valuable lessons in the design and operation of spacecraft, Vostok and Mercury also demonstrated two different approaches to accomplishing the same tasks.
The rapid onset of multi-gravity forces accompanying the rocket launch was one of the primary concerns that faced the two technical teams. During the powered ascent from earth, crewmembers had to be protected from the increased "g-loads," vibration, and noise. It was known, from aircraft and centrifuge experiments, that human tolerance to increased gravity forces varied with the duration of exposure and the attitude of the body with reference to the force. Soviets and Americans agreed that the reclining position permitted a pilot to absorb heavy acceleration loads more comfortably than in any other posture.* In the U.S., Faget, William M. Bland, Jr., Jack C. Heberlig, and their engineering colleagues decided in favor of a couch contoured to the form of each individual astronaut to protect him from g-loads. Soviet designers also used the form fitting couch, and all Mercury and Vostok pilots rode semi-supine in their own tailor-made seats.5
Once a pilot overcame the initial acceleration forces of flight, he would encounter the phenomenon of gravity balanced by centrifugal force, generally called weightlessness or zero g. Flight physicians contended that the absence of gravity might affect man's physical and mental performance, but in the  face of limited information, the effect of zero g was mainly a topic of speculation. Some medical doctors wondered if the human organism, tailored to earth's gravity, would continue to function normally when suddenly deprived of that force. Other physicians worried about the reaction of particular internal organs to the succession of changes imposed by acceleration, weightlessness, and deceleration. Heinz Haber and Otto Gauer, who had studied the question of weightlessness in Germany, had concluded that more experimental data were needed to permit a better analysis of the role of zero g in manned space flight.6
Since it was impossible to duplicate weightlessness on earth, scientists conducted tests with animals borne aloft by rockets. In the U.S. in 1947, experimenters began launching live organisms with V-2 rockets. On 20 September 1951, the monkey Yorick and 11 mice were recovered after an Aerobee flight to 72 kilometers. From this and two subsequent Aerobee monkey launches, James P. Henry and David G. Simon concluded that weightlessness and acceleration forces did not adversely affect the animals.** 7
Soviet rocket engineers and physicians also sent animals to high altitudes, and their canine experiments led them to the same conclusions that the Americans had reached with primates and rodents. At first, the Soviet tests were conducted using pressurized capsules; then they experimented with dogs wearing special space suits and traveling in unpressurized cabins. In one case, Albina and Tsyganka were ejected from the descending launch vehicle at an altitude of 85 kilometers; both dogs rode safely back to earth in their space suits and ejection seats. These experiments convinced the Soviets that acceleration and weightlessness did not pose impossible barriers to manned flight. The significance of this conclusion was made clear to the rest of the world when the Soviets sent Laika into orbit with Sputnik II on 23 November 1957. Although she was not returned to earth, Laika ate, barked, and moved about in her space cabin for seven days without apparent ill effects.8
* Experiments with rocket sleds and the centrifuge indicated that pilots could endure forces 20 times that of the earth's gravity, a load well in excess of those anticipated in normal flights and above those expected under emergency conditions.
** Both physicians played subsequent roles in aerospace medicine. Henry became director of the animal program in Project Mercury. Simon went on to pilot a Project Man High balloon to 31 kilometers for a 32-hour study of man's performance in near space in Aug. 1957.
*** Albina and Tsyganka were veteran travelers and members of the first group of nine canine cosmonauts. Subsequently, the Soviet scientists trained eight more dogs for experimental flight and landing by parachute,
4. Maxime A. Faget, Manned Space Flight (New York, 1965), p. 8.
5. Loyd S. Swenson, Jr., James M. Grimwood, and Charles C. Alexander, This New Ocean: A History of Project Mercury, NASA SP-4201 (Washington, 1966), pp. 39-46, summarize the studies of multiple g loads as they relate to Project Mercury; and S. P. Umansky, Chelovek na kosmieheskoy orbite (Moscow, 1974), pp. 31-32 (available in translation as Man in Space Orbit, NASA Technical Translation F-15973).
6. Swenson, Grimwood, and Alexander, This New Ocean, pp. 36-39, summarize the American studies relating to weightlessness; "Problemy nevesomosti" [The problems of weightlessness], Nauka i Zhizn 22 (Dec. 1955): 17-20; translated in F. J. Krieger, Behind the Sputniks: A Survey of Soviet Space Science (Washington, 1958), pp. 127-133.
7. David G. Simons, "Use of V-2 Rocket to Convey Primate to Upper Atmosphere," Air Force Technical Report 5821, Air Material Command, Wright-Patterson AFB, Ohio, May 1949; J. P. Henry , E. R. Ballinger, P. J. Maher, and D. G. Simons, "Animal Studies of the Subgravity State during Rocket Flight," The Journal of Aviation Medicine 23 (Oct. 1952): 421-432; D. G. Simons, "Review of Biological Effects of Subgravity and Weightlessness," Jet Propulsion 25 (May 1955): 209-211; and Don Bailer, "Alas, Poor Yorick," MSC Space News Roundup [reprint, courtesy of Aerojet-General], 22 Aug. 1959, p. 6.
8. Evgeny Riabchikov,
Russians in Space, Nikolai P. Kamanin, ed., Guy Daniels, trans. (Garden
City, N.Y., 1971), pp. 140-142 and 149-151; A. V. Pokrovskii,
"Comment se comportent les animaux a 100 km d'altitude" [How animals
behave at an altitude of 100 km], Etudes Sovietiques, no.
106 (Jan. 1957): 65-70; translated in Krieger, Behind the Sputniks, pp.
156-163. Umansky, Chelovek na
kosmicheskoy orbite, pp. 33-40, cites
six articles on weightlessness that appeared in Soviet journals
between 1970 and 1972. Based on Riabchikov's comments (p. 141),
Pokrovskii appears to have been in charge of canine experimentation
in the Soviet space program. A general view of the Soviet use of
animals is presented in the Novosti Press Agency pamphlet,
Animals, Pioneers of Outer Space;
Physiological Experiments with Animals Flying in Geophysical Rockets;
Biological Studies during Space Flights (Moscow, n.d.).