Mercury Spacecraft - Mission Profile

Mission Profile

There were two kinds of missions: suborbital and orbital. In the suborbital flight, the space craft went directly from launch to re-entry at the highest point; in the orbital mission the craft went into an orbit around the Earth. To achieve the latter a higher speed and altitude and thereby bigger rocket was needed. The altitude for suborbital missions were 117 miles (188 km) and for orbital missions 156 miles (251 km).

During the launch phase of the mission, the Mercury spacecraft and astronaut were protected from launch vehicle failures by the Launch Escape System. The LES consisted of a solid fuel, 52,000 lbf (231 kN) thrust rocket with three engine bells mounted on a tower above the spacecraft. In the event of a launch abort, the LES would fire for one second, pulling the spacecraft and astronaut away from the launch vehicle and a possible explosion. The spacecraft would then descend on its parachute recovery system. After booster engine cutoff (BECO, point B), the LES was no longer needed and was separated from the spacecraft by a solid fuel, 800 lbf (3.6 kN) thrust jettison rocket that fired for 1.5 seconds (point C).

After a successful liftoff, the spacecraft fired three small clustered solid-fuel, 400 lbf (1.8 kN) thrust rockets for 1 second to separate the spacecraft from the launch vehicle. These rockets were called the posigrade rockets (point D on illustration).

The spacecraft were only equipped with attitude control thrusters; after orbit insertion but before retrofire they could not change their orbit. There were three sets of high and low powered automatic control jets and separate manual jets, one for each axis (roll, pitch, and yaw), and supplied from two separate fuel tanks, one automatic and one manual. The pilot could use any one of the three thruster systems and fuel them from either of the two fuel tanks to provide spacecraft attitude control. The Mercury spacecraft was designed to be completely controllable from the ground in the event that something impaired the pilot's ability to function.

The spacecraft had three solid-fuel, 1000 lbf (4.5 kN) thrust retrorockets that fired for 10 seconds each (point F on illustration). One was sufficient to return the spacecraft to Earth if the other two failed. The firing sequence (known as ripple firing) required firing the first retro, followed by the second retro five seconds later (while the first was still firing). Five seconds after that, the third retro fired (while the second retro was still firing).

There was a small hinged metal flap at the nose of the spacecraft called the spoiler. If the spacecraft started to reenter nose first (another stable reentry attitude for the spacecraft), airflow over the spoiler would flip the spacecraft around to the proper, heatshield-first reentry attitude, a technique called shuttlecocking. During reentry (point G), the astronaut would experience about 8 g-forces on an orbital mission, and 11–12 gs on a suborbital mission.

Initial designs for the spacecraft suggested the use of either beryllium heat-sink heat shields or an ablative shield. Extensive testing settled the issue – ablative shields proved to be reliable (so much so that the initial shield thickness was safely reduced, allowing a lower total spacecraft weight), and were easier to produce — at that time, beryllium was only produced in sufficient quantities by a single company in the U.S. — and cheaper. The surface of the heat shield had a coating of aluminum with glassfiber in many layers. As the temperature rose to 2,000 °F (1,100 °C) the layers would evaporate and take the heat with it. The spacecraft would become hot but not harmfully so.

After re-entry, a small, drogue parachute (point H) was deployed at 21,000 ft (6.4km) for first lowering of speed. The main parachute (point I) was deployed at 10,000 ft (3 km), further slowing the spacecraft in preparation for landing. Just before 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 cone.