Editing Project Mercury (section)

== Spacecraft ==
The Mercury spacecraft's principal designer was [[Maxime Faget]], who started research for human spaceflight during the time of the NACA.{{sfn|Catchpole|2001|p=150}} It was {{convert|10.8|ft}} long and {{convert|6.0|ft}} wide; with the launch escape system added, the overall length was {{convert|25.9|ft}}.{{sfn|Catchpole|2001|p=131}} With {{convert|100|ft3|m3}} of habitable volume, the capsule was just large enough for a single crew member.{{sfn|Alexander & al.|1966|p=47}} Inside were 120 controls: 55 electrical switches, 30 [[Fuse (electrical)|fuses]] and 35 mechanical levers.{{sfn|Alexander & al.|1966|p=245}} The heaviest spacecraft, Mercury-Atlas 9, weighed {{convert|3000|lb}} fully loaded.{{sfn|Alexander & al.|1966|p=490}} Its outer skin was made of [[René 41]], a nickel alloy able to withstand high temperatures.{{sfn|Catchpole|2001|p=136}}

The spacecraft was cone shaped, with a neck at the narrow end.{{sfn|Catchpole|2001|p=131}} It had a convex base, which carried a heat shield (Item '''2''' in the diagram below){{sfn|Catchpole|2001|pp=134–136}} consisting of an aluminum [[honeycomb structure|honeycomb]] covered with multiple layers of [[fiberglass]].{{sfn|Alexander & al.|1966|pp=140, 143}} Strapped to it was a retropack ('''1'''){{sfn|Catchpole|2001|pp=132–134}} consisting of three rockets deployed to brake the spacecraft during reentry.{{sfn|Catchpole|2001|p=132}} Between these were three posigrade rockets: minor rockets for separating the spacecraft from the launch vehicle at orbital insertion.{{sfn|Alexander & al.|1966|p=188}} The straps that held the package could be severed when it was no longer needed.{{sfn|Catchpole|2001|p=134}} Next to the heat shield was the pressurized crew compartment ('''3''').{{sfn|Catchpole|2001|pp=136–144}} Inside, an astronaut would be strapped to a form-fitting seat with instruments in front of him and with his back to the heat shield.{{sfn|Catchpole|2001|pp=136–137}} Underneath the seat was the environmental control system supplying oxygen and heat,{{sfn|Catchpole|2001|p=138}} scrubbing the air of CO<sub>2</sub>, vapor and odors, and (on orbital flights) collecting urine.{{sfn|Catchpole|2001|p=139}} The recovery compartment ('''4'''){{sfn|Catchpole|2001|pp=144–145}} at the narrow end of the spacecraft contained three parachutes: a drogue to stabilize free fall and two main chutes, a primary and reserve.{{sfn|Catchpole|2001|p=144}} Between the heat shield and inner wall of the crew compartment was a landing skirt, deployed by letting down the heat shield before landing.{{sfn|Catchpole|2001|p=135}} On top of the recovery compartment was the [[Antenna (radio)|antenna]] section ('''5'''){{sfn|Catchpole|2001|pp=145–148}} containing both antennas for communication and scanners for guiding spacecraft orientation.{{sfn|Catchpole|2001|p=147}} Attached was a flap used to ensure the spacecraft was faced heat shield first during reentry.{{sfn|Alexander & al.|1966|p=199}} A launch escape system ('''6''') was mounted to the narrow end of the spacecraft{{sfn|Catchpole|2001|pp=179–181}} containing three small solid-fueled rockets which could be fired briefly in a launch failure to separate the capsule safely from its booster. It would deploy the capsule's parachute for a landing nearby at sea.{{sfn|Catchpole|2001|p=179}} (See also [[#Mission profile|Mission profile]] for details.)

The Mercury spacecraft did not have an on-board computer, instead relying on all computation for reentry to be calculated by computers on the ground, with their results (retrofire times and firing attitude) then transmitted to the spacecraft by radio while in flight.<ref name="NASAComp1" /><ref name="DanCompSpace" /> All computer systems used in the Mercury space program were housed in [[NASA]] facilities on [[Earth]].<ref name="NASAComp1" /> (See [[#Ground control|Ground control]] for details.)

<gallery mode="packed">
Mercury-spacecraft-color.png|1. Retropack. 2. Heatshield. 3. Crew compartment. 4. Recovery compartment. 5. Antenna section. 6. Launch escape system.
McDonnellMercuryCapsule1.jpg|Retropack: Retrorockets with red posigrade rockets
Landing-skirt.jpg|Landing skirt (or bag) deployment: skirt is inflated; on impact the air is pressed out (like an [[airbag]])
</gallery>

===Pilot accommodations===
[[File:GPN-2000-001027.jpg|thumb|upright|[[John Glenn]] wearing his Mercury space suit]]

The astronaut lay in a sitting position with his back to the heat shield, which was found to be the position that best enabled a human to withstand the high [[g-force]]s of launch and reentry. A fiberglass seat was custom-molded from each astronaut's space-suited body for maximum support. Near his left hand was a manual abort handle to activate the launch escape system if necessary prior to or during liftoff, in case the automatic trigger failed.{{sfn|Catchpole|2001|p=142}}

To supplement the onboard environmental control system, he wore a [[pressure suit]] with its own [[oxygen]] supply, which would also cool him.{{sfn|Catchpole|2001|p=191}} A cabin atmosphere of pure oxygen at a low pressure of {{cvt|5.5|psi|disp=or}} (equivalent to an altitude of {{convert|24800|ft|disp=or}}) was chosen, rather than one with the same composition as air ([[nitrogen]]/oxygen) at sea level.{{sfn|Gatland|1976|p=264}} This was easier to control,{{sfn|Catchpole|2001|p=410}} avoided the risk of [[decompression sickness]] ("the bends"),{{sfn|Giblin|1998|p=}}{{refn|group=n|The decision to eliminate the use of any gas but oxygen was crystalized when a serious accident occurred on April 21, 1960, in which McDonnell Aircraft test pilot G.B. North passed out and was seriously injured when testing a Mercury cabin/spacesuit atmosphere system in a vacuum chamber. The problem was found to be nitrogen-rich (oxygen-poor) air leaking from the cabin into his spacesuit feed.{{sfn|Giblin|1998|p=}}}} and also saved on spacecraft weight. Fires (which never occurred during the course of Project Mercury) would have to be extinguished by emptying the cabin of oxygen.{{sfn|Catchpole|2001|p=139}} In such case, or failure of the cabin pressure for any reason, the astronaut could make an emergency return to Earth, relying on his suit for survival.{{sfn|Alexander & al.|1966|pp=48–49}}{{sfn|Catchpole|2001|p=139}} The astronauts normally flew with their [[visor]] up, which meant that the suit was not inflated.{{sfn|Catchpole|2001|p=139}} With the visor down and the suit inflated, the astronaut could only reach the side and bottom panels, where vital buttons and handles were placed.{{sfn|Alexander & al.|1966|p=246}}

The astronaut also wore [[electrode]]s on his chest to record his [[heart rhythm]], a cuff that could take his blood pressure, and a [[Medical thermometer|rectal thermometer]] to record his temperature (this was replaced by an oral thermometer on the last flight).{{sfn|Catchpole|2001|pp=191, 194}} Data from these was sent to the ground during the flight.{{sfn|Catchpole|2001|p=191}}{{refn|group=n|Pilot and spacecraft data sent automatically to the ground is called [[telemetry]].{{sfn|Alexander & al.|1966|p=313}}}} The astronaut normally drank water and ate food pellets.{{sfn|Catchpole|2001|pp=343–344}}{{refn|group=n|Moisture and urine was recycled into drinking water.{{sfn|Alexander & al.|1966|p=47}}}}

Despite the lessons learnt from the [[Lockheed U-2|U2]] program, which also utilized a pressure suit, initially no urine collection device was included for the Mercury astronauts. An inquiry on the subject was made in February 1961 by a student, but NASA responded by stating that "the first space man is not expected to have 'to go".{{sfn|Hollins|2013|p=}} The expected short flight times meant that this was overlooked, although after Alan Shepard had a launch delay of four hours, he was forced to urinate in his suit, short-circuiting some of the electrodes monitoring his vital signs. Gus Grissom wore two rubber pants on the second Mercury flight as a crude workaround. It would take until the third flight in February 1962 before a dedicated urine collection device was installed.<ref>{{cite web |url=https://abcnews.go.com/Technology/history-urinating-space/story?id=19641585 |title=The History of Urinating in Space |publisher=ABC |access-date=27 Dec 2023}}</ref>

Once in orbit, the spacecraft could be rotated in [[yaw, pitch, and roll]]: along its longitudinal axis (roll), left to right from the astronaut's point of view (yaw), and up or down (pitch).{{sfn|Alexander & al.|1966|p=98}} Movement was created by [[Rocket engine|rocket-propelled]] thrusters which used [[hydrogen peroxide]] as a fuel.{{sfn|Alexander & al.|1966|p=499}}{{sfn|Catchpole|2001|p=143}} For orientation, the pilot could look through the window in front of him or he could look at a screen connected to a [[periscope]] with a camera which could be turned 360°.{{sfn|Catchpole|2001|p=141}}

The Mercury astronauts had taken part in the development of their spacecraft, and insisted that manual control, and a window, be elements of its design.{{sfn|Catchpole|2001|pp=98–99}} As a result, spacecraft movement and other functions could be controlled three ways: remotely from the ground when passing over a ground station, automatically guided by onboard instruments, or manually by the astronaut, who could replace or override the two other methods. Experience validated the astronauts' insistence on manual controls. Without them, [[Gordon Cooper]]'s manual reentry during the last flight would not have been possible.{{sfn|Alexander & al.|1966|p=501}}

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;Spacecraft cutaway
<gallery mode="packed">
Mercury Spacecraft.png|Interior of spacecraft
Mercury-spacecraft-control.png|The three axes of rotation for the spacecraft: yaw, pitch and roll
Mercury-spacecraft-temperature-profile.png|Temperature profile for spacecraft in [[Fahrenheit]]
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;Control panels and handle
<gallery mode="packed">
Control panels mercury atlas 6.png|The control panels of ''Friendship 7''.{{sfn|Unknown|1962|p=8}} The panels changed between flights, among others the periscope screen that dominates the center of these panels was dropped for the final flight together with the periscope itself.
Three-axis hand controller mercury project.jpg|3-axis handle for attitude control
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===Development and production===
[[File:Technicians working in the McDonnell White Room on the Mercury-crop.jpg|thumb|upright|Spacecraft production in clean room at [[McDonnell Aircraft]], St. Louis, 1960]]
The Mercury spacecraft design was modified three times by NASA between 1958 and 1959.{{sfn|Catchpole|2001|p=152}} After bidding by potential contractors had been completed, NASA selected the design submitted as "C" in November 1958.{{sfn|Catchpole|2001|p=153}} After it failed a test flight in July 1959, a final configuration, "D", emerged.{{sfn|Catchpole|2001|p=159}} The heat shield shape had been developed earlier in the 1950s through experiments with ballistic missiles, which had shown a blunt profile would create a shock wave that would lead most of the heat around the spacecraft.{{sfn|Catchpole|2001|p=149}} To further protect against heat, either a [[heat sink]], or an ablative material, could be added to the shield.{{sfn| Alexander & al.|1966|p=63}} The heat sink would remove heat by the flow of the air inside the shock wave, whereas the ablative heat shield would remove heat by a controlled evaporation of the ablative material.{{sfn| Alexander & al.|1966|p=64}} After uncrewed tests, the latter was chosen for crewed flights.{{sfn| Alexander & al.|1966|p=206}} Apart from the capsule design, a [[rocket plane]] similar to the existing [[X-15]] was considered.{{sfn| Alexander & al.|1966|pp=78–80}} This approach was still too far from being able to make a spaceflight, and was consequently dropped.{{sfn| Alexander & al.|1966|p=72}}{{refn|group=n|The rocket plane approach to human space flight was pursued by the Air Force with their [[Boeing X-20 Dyna-Soar|Dyna-Soar]] project, which was canceled in 1963.{{sfn|Catchpole|2001|pp=425, 428}} Toward the end of the 1960s, NASA began the development of a reusable space plane, which was ultimately developed into the [[Space Shuttle]] program.<ref>{{cite web|title=Introduction to future launch vehicle plans [1963–2001]. 3.The Space Shuttle (1968–72)|url=http://www.pmview.com/spaceodysseytwo/spacelvs/sld001.htm|access-date=3 February 2014}}</ref> The first rocket plane to enter space was an X-15 in 1963.<ref>{{cite web|last1=Garber|first1=Steve|title=X – 15 Hypersonic Research at the Edge of Space|url=https://history.nasa.gov/x15/cover.html|website=NASA History Homepage|publisher=NASA|access-date=18 July 2015}}</ref>}} The heat shield and the stability of the spacecraft were tested in [[supersonic wind tunnel|wind tunnels]],{{sfn|Alexander & al.|1966|p=88}} and later in flight.{{sfn|Catchpole|2001|p=229}} The launch escape system was developed through uncrewed flights.{{sfn|Catchpole|2001|p=196}} During a period of problems with development of the landing parachutes, alternative landing systems such as the [[Rogallo wing|Rogallo glider wing]] were considered, but ultimately scrapped.{{sfn| Alexander & al.|1966|p=198}}

The spacecraft were produced at [[McDonnell Aircraft]], [[St. Louis, Missouri|St. Louis]], Missouri, in clean rooms and tested in vacuum chambers at the McDonnell plant.{{sfn|Catchpole|2001|pp=132, 159}} The spacecraft had close to 600 subcontractors, such as [[Garrett AiResearch]] which built the spacecraft's environmental control system.{{sfn| Alexander & al.|1966|p=137}}{{sfn|Catchpole|2001|p=138}} Final quality control and preparations of the spacecraft were made at Hangar S at Cape Canaveral.{{sfn|Catchpole|2001|pp=184–188}}{{refn|Test and rework of Mercury-Redstone 2 at the Hangar required 110 days.{{sfn|Alexander & al.|1966|p=310}} Hangar S was also the place where the chimpanzees were trained.{{sfn|Alexander & al.|1966|p=312}}|group=n}} NASA ordered 20 production spacecraft, numbered 1 through 20.{{sfn|Alexander & al.|1966|p=137}} Five of the 20, Nos. 10, 12, 15, 17, and 19, were not flown.{{sfn|Grimwood|1963|pp=235–238}} Spacecraft No. 3 and No. 4 were destroyed during uncrewed test flights.{{sfn|Grimwood|1963|pp=235–238}} Spacecraft No. 11 sank and was recovered from the bottom of the Atlantic Ocean after 38 years.{{sfn|Grimwood|1963|pp=235–238}}{{sfn|Catchpole|2001|pp=402–405}} Some spacecraft were modified after initial production (refurbished after launch abort, modified for longer missions, etc.).{{refn|They received a letter designation after their number, ''e.g.'', 2B, 15B.{{sfn|Grimwood|1963|pp=216–218}} Some were modified twice: for example, spacecraft 15 became 15A and then 15B.{{sfn|Grimwood|1963|p=149}}|group=n}} A number of Mercury [[Boilerplate (spaceflight)|boilerplate spacecraft]] (made from non-flight materials or lacking production spacecraft systems) were also made by NASA and McDonnell.{{sfn|Alexander & al.|1966|pp=126 & 138}} They were designed and used to test spacecraft recovery systems and the escape tower.{{sfn|Alexander & al.|1966|pp=96, 105}} McDonnell also built the spacecraft simulators used by the astronauts during training,{{sfn|Catchpole|2001|p=107}} and adopted the motto "First Free Man in Space".<ref name="logo_history">{{citation |url= http://www.mdc.com/version2/history/logo.htm |publisher= McDonnell Douglas |title= McDonnell Douglas Logo History |access-date= 29 November 2020 |archive-url= https://web.archive.org/web/19970605044807/http://www.mdc.com/version2/history/logo.htm |archive-date= 5 June 1997 |url-status= dead }}</ref>

<gallery mode="packed">
Heatshield-test3.jpg|[[Shadowgraph]] of the reentry [[shock wave]] simulated in a [[supersonic wind tunnel|wind tunnel]], 1957
Mercury-design.png|Evolution of capsule design, 1958–59
Mercury Space Capsule-wind-tunnel.jpg|Experiment with boilerplate spacecraft, 1959
</gallery>
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{{hidden begin|title={{center|Development of Earth landing system}}|style=border:solid 1px #aaa;background:#F9F9F9;padding-left:10px;}}
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Mercury-project-earth-landing-system-test.png|Drop of boilerplate spacecraft in training of landing and recovery. 56 such qualification tests were made together with tests of individual steps of the system.{{sfn|Catchpole|2001|pp=172-173}}
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