Following Yuri Gagarin's historic flight in Vostok 1, two American astronauts made sub-orbital flights to test the Mercury spacecraft. On 5-May-1961, less than four weeks after Gagarin's flight, Alan B. Shepard became the first astronaut. While he did not orbit the Earth, he reached an altitude of some 186 km. Shepard, launched from Cape Canaveral in Mercury 3 atop a modified Redstone ballistic missile, spent 15 min 22 s in flight before landing in the Atlantic. He proved that a man could manually control a spacecraft though he is weightless. Mercury's first orbital flight came on 20-Feb-1962. John Glenn was launched in Mercury 6 by a modified Atlas ICBM from Cape Canaveral. Glenn spent only 4 hr 55 min circling the Earth, completing just three orbits before landing safely. Glenn's objective was to evaluate man in Mercury. Although he had some trouble adjusting to the use of the hand control during reentry, the mission was a success. Subsequent missions were to verify the man-machine concept upon which Mercury was designed. While previous flights had proved that man could function in space with no serious difficulties owing to weightlessness, later missions were to prove that man and Mercury could work together for longer periods of time than previously demonstrated.

The Mercury spacecraft consisted of only one module, the manned capsule, and had the shape of a truncated cone. Its nickel-alloy pressure vessel had an outer shell of titanium for protection against the heat of reentry. A special heat shield on the base was made of an ablative material. The retro package consisted of solid propellant rockets and was attached to the heat shield by metal straps. The spacecraft also had small thrusters for attitude control. For reentry, the attitude control thrusters oriented Mercury to the proper angle, and the retrorockets fired and were then separated. Mercury entered the atmosphere base down. Because of its shape, a degree of aerodynamic control was available during the transit of the atmosphere because the spacecraft produced some lift. By using manual controls, the astronaut could vary his flight path and so his touchdown point. In the final stage of descent, a parachute deployed and lowered Mercury and the astronaut gently into the sea.

MERCURY SPECIFICATIONS

First flight: 9-Sep-1959; first manned flight 5-May-1961 (MR-3)
Last flight: 15-May-1963 (MA-9)
Number of flights: 16 total; 6 manned (2 suborbital, 4 orbital)
Principal uses: manned earth orbit
Unit cost: $5.50 million
Crew size: one
Endurance: 1.5 days
Orbital storage: 1 day
Overall length: 4.0 m / 7.9 m (including escape tower and aerodynamic spike)
Maximum diameter: 1.9 m
Habitable volume: 1.70 m³
Launch mass: 1,935 kg (including escape tower)
Orbital mass: 1,355 kg
Propellant mass: 219 kg total
RCS total impulse: 29.8 kNs
Power: batteries; 13.5 kWh total

(Weights are not typical for every mission)

ESCAPE TOWER
The escape tower was designed to pull the capsule clear of the launch vehicle in the event of a mishap on the launch pad or after take-off. It jettisoned from the rocket when a safe altitude had been reached, after separation of the booster engines. In a normal mission the escape tower was jettisoned by firing the escape motor; a small separation motor was used to jettison the tower in an aborted mission.
Length: 5.2 m
Total mass: 580 kg
Escape motor: 231.3 kN, 1 s burn
Separation motor: 3.56 kN, 1.5 s burn
Propellant: solid

CAPSULE MODULE
Crew size: one
Length: 3.5 m
Maximum diameter: 1.9 m
Habitable volume: 1.70 m³
Total mass: 1,118 kg
(structure 340 kg; heat shield 272 kg; reaction control system 40 kg; recovery equipment 60 kg; navigation equipment 40 kg; telemetry equipment 50 kg; electrical equipment 80 kg; communications systems 20 kg; crew seats & provisions 80 kg; crew mass 72 kg; environmental control system 50 kg; propellant 14 kg)
Reaction control system
    thrusters: coarse: 6 x 107 N + fine: 6 x 4.45 N
    propellant: hydrogen peroxide
    specific impulse: 167 s
    total impulse: 23 kNs
Power: batteries; 13.5 kWh total, 0.54 kW average
Environment: pure oxygen at 340 mbar
Landing system: when the capsule returned to Earth it was first braked by a 1.8 m diameter drogue parachute (deployed at 6.4 km slowing vehicle to 21 m/s). The 19.2 m diameter main ringsail parachute (deployed at about 3 km) opened in a reefed condition then inflated fully. A landing bag deployed ready for splashdown. This involved detaching the heat shield which dropped down 1.2 m pulling out a perforated skirt of rubberized glassfiber. This served as an air-cushion to reduce the landing shock. Immediately after landing, the main parachute was automatically disconnected and the capsule had sufficient buoyancy to float upright in the water. The astronaut waited inside until retrieved by a naval vessel. Maximum deceleration ~8 g (11 g suborbital); maximum heat shield temperature 1,650^oC (40 km, 24,000 km/h)

RETRO PACKAGE
The retro package was attached by metal straps to the capsule heat shield. It consisted of three solid propellant posigrade motors to push the spacecraft free of its launch vehicle after separation, and three retrograde motors to slow the spacecraft for its return to Earth. The package was jettisoned following retroburn.
Length: 0.5 m
Maximum diameter: 1.0 m
Total mass: 237 kg
Propellant mass: 205 kg
Posigrade motors: 3 x 1.78 kN, 1 s burn
Retrograde motors: 3 x 4.45 kN, 10 s burn
Propellant: solid
Specific impulse: 230 s