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Design


The spacecraft was shaped as a cone with the narrow end facing forward during launch to ensure low air resistance. At reentry on the other hand the base of the cone with a rounded heat shield would face forward. In this way a shochwawe would build in front of the spacecraft lead away most af the heat. Strapped to the heatshield during most of the flight was the retropacked which contained three rockets to break the spacecraft for reentry and between these, three minor rockets for separating the spacecraft from the launch vehicle at insertion into orbit. The straps that held it could be severed when it was no longer needed. Next to the heatshield came the pressurized crew compartment. Apart from the astronaut this contained the instruments for control af the spacecraft and the environmental system that supplied him with oxygen and heat. The system also cleaned the air from CO2, wapor and smell as well as (on orbital flights) collecting urine. The seat was molded from the body of each astronaut and as a safety procedure he was wearing a pressure suit with its own oxygen supply. Inside the suit, sensors were attached to the body of the astronaut to monitor his physical condition.

In orbit the astronaut would be facing backwards to make certain that the retropacked could bring the spacecraft out of orbit at any time. He could also change the orientation of the spacecraft but not the orbit. The orientation or attitude of the spacecraft could be controlled manuelly by a handle that could make the spacecraft rotate along its center axis (roll), along an axis whisch was vertical as seen by the astronaut (yaw) or along an axis seen as horizontal to the astronaut (pitch). The rotation was created by thrusters on the outside of the spacecraft that, based of the reaction principle, send out a stream of oxygen and wapor into space in the opposit direction of the desired rotation.

To compensate for the limited view through the spacecraft window the astronaut could get a bigger of the earth from a screen connected to a periscope which could be turned 360°. He entered the spacecraft through a hatch on its side where he could also leave. Alternatively he could exit through the recovery compartment at the narrow end of the spacecraft. This contained three parashutes: one drogue to stabilize free fall and two main parashutes of which only one was used, the other was a reserve. To further soften the landing, a landing skirt would be deployed by letting the heat shield hang down by it. In the side of the skirts were holes through which air would be sucked in at deployment and pressed out again at landing. On top of the recovery compartment was the antenna section containing antennas for communication with earth and scanners for finding the horizon and guide the orientation of the spacecraft compared to earth and align it with its orbit. Attached to it was a flap used to stabilize flight during reentry and ensure the right heatshield-first orientation

A rescue tower or launch escape system was mounted to the narrow end of the spacecraft. In case of failure during the first minutes of launch this was meant to bring the spacecraft free of the launch vehicle and lift it to an altitude from where it could deploye its parashute and land by itself. The launch escape system had three rockets on top of it to produce the necessary power. The spacecraft was 130 inches long and 75 inches wide; with the launch escape system added, the over all lenght was 311 inches. Because of their small size, it was said that the Mercury spacecraft were worn, not ridden. With 60 ft3 of habitable volume, the spacecraft was just large enough for the single crew member. Inside were 120 controls: 55 electrical switches, 30 fuses and 35 mechanical levers.

The spacecraft could be controlled entirely from the ground and in some phases of the flight, such as insertion into orbit and deployment of parashute by build in automatic control guided by baro-sensors gyroscopes or the horizon-scanners. In some case it could be an advantage to override the automatic control for instance by letting the spacecraft rotate freely for a while in order to save fuel for the attitude thrusters. Data of the spacecraft and astronaut was send to the ground control each time the spacecraft passed a station of the WW network

Developement


The spacecraft was designed by Max Faget and NASA's Space Task Group. Despite the astronauts' test pilot experience NASA at first envisioned them as "minor participants" during their flights, causing many conflicts between the astronauts and engineers during the spacecraft's design. Nonetheless, the project's leaders always intended for pilots to be able to control their spacecraft, as they valued humans' ability to contribute to missions' success. John Glenn's manual attitude adjustments during the first orbital flight were an example of the value of such control. The astronauts requested—and received—a larger window and manual reentry controls.

Production summary
NASA ordered 20 production spacecraft, numbered 1 through 20, from McDonnell Aircraft, St. Louis, Missouri. Five of the 20, Nos. 10, 12, 15, 17, and 19, were not flown. Spacecraft No. 3 and No. 4 were destroyed during unmanned test flights. Spacecraft No. 11 sank and was recovered from the bottom of the Atlantic Ocean after 38 years. Some spacecraft were modified after initial production (refurbished after launch abort, modified for longer missions, etc.) and received a letter designation after their number, examples 2B, 15B. Some spacecraft were modified twice; for example, spacecraft 15 became 15A and then 15B.

A number of Mercury boilerplate spacecraft (including mockup/prototype/replica spacecraft, made from non-flight materials or lacking production spacecraft systems and/or hardware) were also made by NASA and McDonnell Aircraft. They were designed and used to test spacecraft recovery systems, and escape tower and rocket motors. Formal tests were done on test pad at Langley and at Wallops Island using the Little Joe and Big Joe rockets.