Mercury capsule
description & specifications

Interior & control:

• Because of their small size, it was said that the Mercury spacecraft were worn, not ridden. With 1.7 m³ 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 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, contrary to other reports, 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
  
  
  
• 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.

Mission profile:

• 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), 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.
• 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, 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 was deployed at 21,000 ft (6.4km) for first lowering of speed. The main parachute 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.

Production summary:

• NASA ordered 20 production spacecraft, numbered 1 through 20, from McDonnell Aircraft Company, 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.

Launch vehicles:

• Little Joe (height: 55 ft) for launch escape system tests at altitude. Eight unmanned flights were made, two of which carried live monkeys. It was a solid-fuel rocket designed specially for the Mercury program. Together with a Mercury boilerplate it was used to test the escape tower and abort procedures.
• Redstone (height 83 ft) 1-stage rocket for suborbital (ballistic) flights. Four unmanned flights were made, one of which carried a chimpanzee. After that it was used for the first two manned flights. The Jupiter rocket was originally considered for the suborbital launch vehicle, but were replaced with the Redstone in July 1959 due to budget constraints.
• Atlas D (height 94 ft) 2-stage rocket for orbital manned flight. Six unmanned test flights were made, four suborbital and two orbital one of which carried a chimpanzee. After that it was used for four manned orbital flights. The Atlas D rocket required extra strengthening in order to handle the increased weight of the Mercury spacecraft beyond that of the nuclear warheads they were designed to carry.
• The Titan missile was also considered for use for later Mercury missions; however, the Mercury program was terminated before these missions were flown. Instead, the Titan was used for the Gemini program which followed Mercury. The Mercury program also used a Scout rocket for a single flight, Mercury-Scout 1, which was intended to launch a small satellite designed to evaluate the worldwide Mercury Tracking Network. Launched on November 1, 1961, however, the rocket was destroyed by the Range Safety Officer after 44 seconds of flight

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; 13.0 kW 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.
Total mass: 580 kg

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: 220 s
    total impulse: 29.8 kNs
Power: batteries; 13.0 kW 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 separation rockets to push the spacecraft free of its launch vehicle after separation, and three retrorockets 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
Separation rockets: 3 x ? kN
Retro-rockets: 3 x 5.16 kN
Propellant: solid
Specific impulse: 230 s