The Mercury spacecraft was designed to carry one man alone into outer space and return him safely. It was cone-shaped -- 6.8 feet (2 m) long, 6.2 feet (1.9 m) in diameter at the base -- with a small cylinder on top. The spacecraft weighed between 2,300 and 3,000 pounds (1,043 to 1,360 kg), depending on the mission, and sat on top of a booster rocket. Atop the spacecraft, there was an emergency escape tower that could lift the capsule off the booster, raising it to an altitude where the parachutes could open and return the capsule and astronaut to the ground at a safe distance from the launch pad. On the blunt end of the spacecraft there was a heat shield, which protected the capsule from the heat of re-entry, and a retro-pack, which were small rocket thrusters that slowed the spacecraft down for re-entry.
Like any spacecraft, Mercury had to provide the following systems:
- Life support - T hree, spherical oxygen bottles provided a pure oxygen atmosphere in the capsule. Lithium-hydroxide canisters removed the exhaled carbon dioxide. Blowers and fans were used to circulate the air and cool the astronaut. Insulation and double walls helped keep the cabin at a liveable temperature. Drinking water was supplied on-board. A urine-collection system was integrated into the capsule. The astronaut wore a pressure suit and helmet for insulation and protection against cabin-depressurization.
- Power - Electrical power was supplied by various combinations of 1,500-watt and 3,000-watt batteries. Water was used to cool the various electrical systems.
- Radio/tracking communications - High-frequency (HF) and ultra-high-frequency (UHF) transmitters and receivers were used to relay voices, telemetry data and navigational data to and from the ground. Antennae and recovery beacons were located in the cone of the spacecraft. A camera was located in the cabin to monitor the astronaut.
- Maneuvering - Maneuvering in the pitch, roll and yaw directions (x, y, z axes) was accomplished by reaction-control thrusters that used pressurized hydrogen-peroxide fuel. The reaction-control thrusters could be controlled either automatically or manually. Three solid rocket engines (posigrade motors) were used to separate the spacecraft from the booster, and three more solid rocket engines (retrograde motors) were used to slow the spacecraft for re-entry into Earth's atmosphere. Photo courtesy NASA Landing-bag deployment
- Re-entry - The spacecraft had an ablative heat shield to protect it from the temperature during re-entry -- about 2,000 degrees Fahrenheit (1,093 degrees Celsius). The structure of the shield was honeycombed aluminum with many layers of glass-fiber material. As the spacecraft descended, the material of the heat shield boiled away, taking the heat away with it. Double walls and insulating material kept the cabin temperature liveable (but still hot).
- Landing - After re-entry, a small, drogue parachute was deployed at 21,000 ft (6,405 m) to begin to slow the spacecraft for landing. The main parachute was deployed at 10,000 ft (3,050 m), further slowing the spacecraft in preparation for its water landing. Just prior to hitting the water, a landing bag inflated from behind the heat shield to reduce the force of impact. Upon landing, additional bags inflated around the nose of the craft to keep the capsule upright in the water, and the parachutes were released. Once the recovery helicopter hooked onto the spacecraft, the astronaut blew the escape hatch to exit the capsule. It was also possible to exit the capsule through the nose by crawling behind the instrument panel and out through the nose cone.
The Mercury spacecraft had two boosters, the Redstone and Atlas rockets. Both rockets were ballistic missiles that were modified to carry spacecraft payloads instead of warheads.
NASA encountered several failures of the early Redstone and Atlas launches, with the launch vehicles exploding at various stages. When the problems were fixed, unmanned and animal launch tests were conducted. The Redstone was used on the first two manned suborbital flights, but it was not as powerful as the Atlas. The Atlas was used to place men into orbit on the remaining Mercury flights.