Talk:Project Mercury

Capsule volume discrepancy
Check out the following differing figures from the article, given for the Mercury capsule's volume:
 * Spacecraft section: Only 12.13 cubic meters of volume (...)
 * Data table: Volume: 60 ft³ 1.7 m³

I wonder if a typo mighirockt've crept into the Spacecraft sec's number. My guess so far is that the correct number should be, say, 1.213 m³, and that it represents the capsule's internal volume. Any comments? --Wernher 19:54, 12 Oct 2004 (UTC)
 * states that the habitable volume was 1.7m&sup3;. There is often a bit of descepancy between various sources on things like volumes, capacities etc. --enceladus 21:23, 12 Oct 2004 (UTC)

It's been 8 years since this discrepancy was pointed out and it's still live... the Mercury did NOT have an internal volume of 12 meters, can we please get it corrected to something CLOSER to reality... or at least smaller then the Apollo CSM's volume? 97.125.180.25 (talk) —Preceding undated comment added 03:22, 14 June 2012 (UTC)


 * What are you on about, then? The number has been corrected in the article (1.7 cubic meters), which is about as good as we're likely to get. JustinTime55 (talk) 21:34, 2 October 2012 (UTC)

Image on the Left
What is it? Why is it not labeled?
 * It is the Mercury program monument at pad 14. The entry in the article to include the image was missing the word "thumb". I believe that I have fixed it. Vaoverland 12:37, 13 November 2006 (UTC)

g-Forces
During reentry, the astronaut would experience about 4 g-forces. The separate mission articles say otherwise: Mercury-Redstone 3: 11.6 g, Mercury-Redstone 4: 11.1 g, Mercury-Atlas 6: 7.7 g, Mercury-Atlas 7: 7.8 g, Mercury-Atlas 7: 8.1 g, Mercury-Atlas 9: 7.6 g. So this might need to be corrected. --Proofreader 18:16, 10 January 2007 (UTC)


 * Not necessarily; the booster numbers are obviously giving the launch accelleration. Re-entry g-force is completely different, caused by the friction of the air, and is totally unrelated to the acceleration at launch, and would mainly be driven by re-entry speed. I would expect the orbital flights (M-A) to be perhaps a bit different (bigger?) than the suborbitals (M-R). JustinTime55 (talk) 22:29, 26 February 2010 (UTC)
 * Opps--I mis-spoke. The articles explicitly quote re-entry g's and are fuzzy or silent about the launch g's, so Proofreader is correct.  The sub-orbitals probably pulled much higher re-entry g's because of the steeper ballistic trajectory. JustinTime55 (talk) 22:47, 26 February 2010 (UTC)

Launch Escape System
The article says the LES generated 52,000 lbs of thrust. With a launch weight of 4265 lbs, that works out to a little over 11 Gs. Is that really possible? Can people survive 11 Gs? -- RoySmith (talk) 01:29, 5 May 2008 (UTC)

I don't have the reference in front of me, but if I recall correctly, the maximum G force experienced (and survived) was 20--in a centrifuge. MWShort (talk) 19:19, 9 May 2008 (UTC)

reading John Glenn's memoir just recently, 16 gs was the max survivable "upper limit" that they found they could subject astronauts to if they had to and still be ok (due to heart/lung issues). Yes the LES would have kicked it higher very briefly, I imagine if it had come down to that kind of emergency situation they figure better to have the astronanut alive if hurt then nothing at all. —Preceding unsigned comment added by 166.56.100.23 (talk) 19:23, 23 January 2009 (UTC)

space capsule
The space capsule stuff should be expanded, perhaps into a separate article Mercury space capsule 70.55.203.112 (talk) 08:15, 28 September 2008 (UTC)

skin
Why is the skin on mercury (and gemini) currogated rather than smooth? Strength? What about drag? Bachcell (talk) 00:53, 21 January 2009 (UTC)

From what i've seen looking @ a few references, each capsule was built with some minor differences as they went along. Mainly I've seen the corrugated parts on the upper as opposed to the lower part of the capsules. For exmaple "The Recovery Section had corrugated Rene 41 shingles rather than the flat plate-type beryllium shingles used on manned flights." also "MR-2 and MR-3 were also unique in having a different main hatch. These two missions had a thick hatch with a relatively smooth surface, not corrugated like the shingles covering the capsule." so not positive but I would guess for strength mainly, as the LES was conected to that part up top. —Preceding unsigned comment added by 166.56.100.23 (talk) 19:31, 23 January 2009 (UTC)


 * Actually, those corrugations you see on the Mercury spacecraft are for keeping it cool rather than for strength. What you're looking at are the outer "shingles" on the spacecraft.  The spacecraft's structural skin, which provides the strength, lies underneath the shingles.  The shingles are corrugated to increase their surface area.  That would give the spacecraft more surface area from which to radiate heat away into space while absorbing the same amount of sunlight.


 * I think they might also provide more surface area for air to flow over during atmospheric reentry, which would help remove heat during that stage. That would explain why the shingle corrugations run across the direction of airflow, rather than parallel to it as you might expect.  That does increase drag, but only by a very small amount compared to the blunt heatshield, and the relatively slow turbulent airflow over the corrugations would efficiently carry away the heat that was conducted from the heatshield to the rest of the spacecraft.


 * During launch, the corrugations wouldn't increase the drag that much. The sharp aerospike at the tip of the escape rocket did a lot to reduce drag during the supersonic portion of the launch, when drag was at its worst, because it caused the supersonic shock wave to build up ahead of the rocket's main body.


 * --Colin Douglas Howell (talk) 08:47, 17 May 2009 (UTC)


 * In general, such corrugations wouldn't increase the effective surface area for radiation, because indented portions would have a corresponding amount of their "view" obscured by adjacent portions of hot metal. This is known as view factor.  Likewise, the corrugations would not help to cool the cladding via convection to air, as the air is the source of the heat.  The corrugations are almost certainly to allow for thermal expansion of the extremely hot cladding, to prevent buckling and excessive stress on the cladding's fastening system.


 * --Anonymous engineer. — Preceding unsigned comment added by 71.174.93.74 (talk) 14:29, 17 September 2019 (UTC)


 * I'll accept the "view factor" explanation, and also accounting for thermal expansion to prevent excessive stress and buckling, but disregarding cooling convection because "the air is the source of the heat" ignores the fact that after reentry, once the spacecraft has lost most of its speed, you now have much denser and cooler air moving more slowly past the hot shingles. Air cooling would be effective then, and the corrugations may well have aided that. Admittedly that's not quite the same as what I described initially. Also I haven't researched the details of the design, which would likely answer these questions more definitively. Just wanted to get this thought in here. --Colin Douglas Howell (talk) 01:56, 8 July 2021 (UTC)

Last American to orbit the Earth solo
The article describes Gordon Cooper as the "Last American to orbit the Earth solo", but isn't that actually David Scott, the Command Module pilot of Apollo 9? He orbited the Earth solo while the Lunar Module was being test flown. 70.153.127.71 (talk) 03:09, 27 July 2009 (UTC)
 * Cooper was the last American to orbit the Earth solo. No one else was in orbit at the time.  To be more definitive, though, he was unequivocally the last American to be launched into orbit alone.  —Preceding unsigned comment added by SEWalk (talk • contribs) 02:58, 23 October 2009 (UTC)
 * Bruce McCandless could also be considered an astronaut flying in solo orbit. I have modified the table remark. ShelbyBell (talk) 23:30, 24 June 2012 (UTC)

Mercury 13 section
I think this section should be removed. NASA never considered using women. This was all the idea of Lovelace, who had tested the Mercury Seven. Bubba73 (You talkin' to me?), 04:02, 4 July 2010 (UTC)


 * There have been no objections, so I removed that section. It is mentioned in the See also section.  Bubba73 (You talkin' to me?), 15:24, 15 July 2010 (UTC)

Merge
The removal of the lists of unmanned and manned Mercury launches diminishes the utility of this article, to no discernible purpose. Relevant information should be grouped together for easy reference and comparison; having to go to several different articles for the relevant information significantly impairs the encyclopedic usefulness of the article. RandomCritic (talk) 03:42, 28 October 2011 (UTC)


 * Since there was no objection, I completed the merge. RandomCritic (talk) 19:06, 15 November 2011 (UTC)


 * I agree with this merge.--Craigboy (talk) 00:31, 20 November 2011 (UTC)

Interior volume of spacecraft
I had changed cubic meters to decimeters/liters since cubic meters seemed like a large metric to describe the volume and used both cubic decimeters and liters because cubic decimeters are the recognized but uncommon SI unit. I also added cubic feet in parentheses for three reasons:

1. The spacecraft was built by McDonnel in the 1950s in the US, and most likely designed to specifications described in US customary units.

2. As a convenience and courtesy to the smaller yet significant portion of the English language wikipedia readers who lack an intuitive scope of area defined by SI units.

3. A secondary metric of 60 cubic feet is also helpful to those readers familiar with both SI and US customary units.

The change was reverted, and while I concede to the reversion of the SI volume back to "cubic meters" on the proposed basis that liters are not as intuitive as cubic meters when describing volume, I object to the removal of the secondary metric given in "cubic feet". I intend to edit the secondary metric using "cubic feet" following "cubic meters" into the article in parentheses following the primary description in cubic feet. If it is removed afterwards I will accept the change and refrain from editing this detail in the future.

173.67.242.156 (talk) 11:05, 30 March 2012 (UTC)Moi

After reading the cited link, I discovered that the original interior dimensions were incorrect in the first place. According to centennialofflight.net a website designed to provide public information on the history powered human flight run by the US government who also commissioned, owned and operated every Mercury spacecraft that was produced:

"The spacecraft that was designed was cone-shaped with a cylinder on top. It was 6.8 feet (2 meters) long, 6.2 feet (2 meters) in diameter, and had a 19.2-foot, (5.8-meter) escape tower with a solid-rocket motor fastened to the cylinder. In a launch emergency, the rocket would fire and lift the capsule from an explosion and parachute it into the ocean. With a volume of only 428.5 cubic feet (12 cubic meters), there was barely enough room for its pilot, who sat in a custom-designed couch facing a panel with 120 controls, 55 electrical switches, 30 fuses, and 35 mechanical levers. The cabin's atmospheric pressure was one-third of that on Earth and contained pure oxygen." (Source: )

So while I previously stated I would no longer edit this detail, I must revise that to I will not reinsert the non SI metric if it is reverted but will ensure the proper SI metric of "12 cubic meters" is maintained if reverted back to the incorrect "1.7 cubic meters" metric.

(and just to be clear, my use of the term "metric" in these comments is not a reference to the "Metric System" but as a term describing any generic measurement irregardless* of the scale used)


 * just kidding, I know the word is "regardless" sorry for those it made cringe.

173.67.242.156 (talk) 11:32, 30 March 2012 (UTC)Moi


 * Thank you for obtaining the accurate volume number. And thanks so much for your "convenience and courtesy" extended to the "smaller yet significant portion". There is a strong national tie to Project Mercury, since it was undertaken by the US, which in 1958 did not (and to this day, still in most of the aerospace industry, does not primarily) use metric units; therefore the English units should go first. That's the most authoritative way to obtain the correct value for the historical US space programs, as that was what was originally used in the design, and the metrics were/are a conversion extended as a courtesy. (NASA didn't switch officially to metric units until the 1970's.) We also have the Template:convert to maintain accurate unit conversions. JustinTime55 (talk) 16:28, 6 June 2012 (UTC)


 * Please see: Talk:Project Mercury, is this helpful? Soerfm (talk) 14:25, 4 April 2012 (UTC)
 * Not really, as the old value was determined to be inaccurate. JustinTime55 (talk) 16:28, 6 June 2012 (UTC)

Just for everyone's information, the Mercury spacecraft's cone frustrum had a total volume of about 2.25 cubic meters. The cone frustrum is the tapered section containing the Vehicle's peripheral superstructure, the Pilot, his couch, all of the instruments, and all of the consumables.

Clarification needed
I have a problem understanding just what this sentence is trying to say:
 * "Twelve companies bid to build the Mercury spacecraft, an unusually high number for a $20 million contract, given the project's great prestige."

Exactly why would one expect fewer than 12 contractors to bid? The $20 million is in 1958 U.S. dollars, which doesn't give modern readers a good handle on it (would be equivalent to much more today.) And what is the assumed relationship of price to attraction? (One would assume it's positive.) And then, "given the project's great prestige" is thrown in at the end, which should attract more bidders. The use of "given" is confusing, and probably not what you intended? The phrase "given [the fact that...]" is usually used to reinforce an expectation when used in such a sentence. Is this the essence of what you were trying to say?
 * "It was surprising that as many as 12 contractors bid, since [here's where "given" would be more appropriate], it was only worth $20 million; yet it had a great deal of prestige."

JustinTime55 (talk) 15:55, 6 June 2012 (UTC)

Cabin volume: source needed
The US Centennial of Flight Commission has a mistake on its Mercury page; the volume of the capsule could not possibly be 428.5 cubic feet, when the Apollo Command Module had only 218 cubic feet. (This is why we need verification!) I believe the 60. figure, because I tabulated a comparison with the others (Vostok for reference, is 63 cubic feet) but unfortunately I can't remember where I got that. Can someone help, please? JustinTime55 (talk) 21:27, 13 July 2012 (UTC)

Mission Profile
In the mission profile page it talks about how the orbital version needed a bigger rocket as compared to the sub-orbital version so that it can achieve a "higher altitude". This is misleading, the biggest reason why an orbital mission needs a more powerful rocket is the substantially higher speed needed to achieve orbit. 166.250.33.101 (talk) 04:44, 19 July 2012 (UTC)

Article insufficiently covers the Mercury spacecraft
This was the article a year ago, this is it now. A lot of information was removed for reasons not entirely clear to me, if it was removed because someone believed it was beyond the scope of the article than a page should have been created specifically for the Mercury spacecraft. Can we do anything to rectify this?--Craigboy (talk) 02:29, 29 September 2012 (UTC)

Revision July 2013
I have made a revision mostly based on

I have deleted some information:
 * claim in lead about planet Mercury (original research and not verified in body)
 * claim about spacecraft being worn not ridden (no citation)
 * claim about minor participants (no citation)
 * claim that MJ-1 was proposed as a heat shield test (no citation)
 * claim that MJ-2 was deemed unnecessary (no citation, contradicts budget constraint claim)

I reduced the information about T.J. O'Malley pushing the button to a footnote. I found a citation for the volume of the spacecraft, on the other hand, but it was 100 cubic feet and not 60 cubic feet. As requested, I have expanded the spacecraft section. I have also maximized the use of pictures. Finaly, I have deleted dead links and links overlapping Bibliography from External links section.

By the way, have you noticed that the kindle version of This New Ocean on Amazon includes an old version of the Wikipedia article? Soerfm (talk) 17:30, 9 July 2013 (UTC)

JFK "invited" Glenn to explain delays?
This dubious footnote was just added:
 * "...a frustrated politician compared the spacecraft-Atlas combination to "a Rube Goldberg device on top of a plumber's nightmare" and John Glenn was invited to the White House to explain the delays."

This New Ocean verifies the senator's remark, but makes it clear that JFK, while disappointed like everyone else, believed NASA knew best what was required to make the flight with reasonable safety, and describes Glenn's February visit (incidental to his visiting family in Virginia) as "a brief visit with President Kennedy, who asked him many semitechnical questions about plans and systems for the orbital flight" (citing several newspaper reports), but says nothing at all about Kennedy demanding Glenn "explain delays". This would seem totally out of character for JFK; I removed it. I have no idea who this Brit John Catchpole is, but I would suggest being cautious about regarding his Project Mercury - NASA's First Manned Space Programme as a reliable source about certain things. JustinTime55 (talk) 17:32, 18 September 2013 (UTC)
 * I think you are right, putting information together like that tends to be OR. Maybe it should be stressed that the two are not related: On the one hand people made jokes about the project on the other John Glenn And JFK had an informal meeting to fill out the wait before the launch of MA-6. Soerfm (talk) 18:24, 18 September 2013 (UTC)

Cost
"It is estimated to have cost $1.71 billion and have involved the work of 2 million people." - this should be clarified to specify either the cost in 1950s/60s dollars, or the cost in ~2013 dollars. Apparently this is the cost accounting for inflation? — Preceding unsigned comment added by 199.212.215.11 (talk) 13:49, 11 October 2013 (UTC)


 * Look further: in the infobox and down in the article body, it says that's today's cost. JustinTime55 (talk) 16:46, 11 October 2013 (UTC)

Reentry: pressure vs friction
Both pressure and friction seem to create heat during reentry... "The Ames researcher determined that the amount of heat absorbed by an object descending into the atmosphere depended on the ratio between pressure drag and viscous or frictional drag. The designer of a reentry body, by shaping the body bluntly, could alter pressure drag and thus throw off much of the heat into the surrounding air."

- This New Ocean, p 61. I must admit I didn't think of the pressure drag when I wrote the paragraph. Can anyone suggest a better explanation? - Soerfm (talk) 22:04, 7 November 2013 (UTC)


 * You are probably referring to my revert of the anon's change of the page text on this issue. As the issue is more complex than either friction or a pressure wave and the change was not ref'd, I reverted it assuming I'd come back to it later (which I didn't). That being said, the sentence probably needs to be changed to reflect the more complex description of what is actually going on. The returning capsule causes the atmosphere to be compressed and heated while the bow shock causes frictional drag - both of which contribute to the capsules heating. Ckruschke (talk) 13:53, 8 November 2013 (UTC)Ckruschke

which would have been Slayton's flight?
Does anyone know which of the cancelled flights would have been Deke Slayton's before he was grounded? Slayton's Wiki article says he was supposed to fly on the second orbital flight - named Delta 7 - but his place was taken by Scott Carpenter, on Aurora 7. My main question is, which of the cancelled flights would have been flown by Slayton? And did Carpenter simply replace Slayton, or was Carpenter's flight moved up and Slayton's cancelled outright? Elsquared (talk) 00:02, 30 November 2013 (UTC)


 * I remember recently coming across Carpenter replacing Slayton in one of the Mercury sources (This New Ocean?) so I believe that (but it should be cited.) That can easily be cross-verified by finding the date of his grounding. I think you're taking the term "cancelled flights" wrong; these were planned near the end of the program (and would have been longer flights, but cancelled because they wouldn't have been long enough to overcome the Soviet lead at that point) and had nothing to do with Slayton's grounding, which certainly happened by then.
 * BTW: I don't know if everyone gets this, but the astronauts themselves named the flights, so Delta 7 would have been Slayton's name for Mercury-Atlas 7; Carpenter named it Aurora 7. JustinTime55 (talk) 14:00, 18 February 2015 (UTC)

Incorrect Habitable Volume
The article incorrectly states that "With 100 cubic feet (2.8 m3) of habitable volume, the capsule was just large enough for the single crew member." The source, however, says that it was a space cabin simulator that had about 100 cubic feet of habitable space. Mercury actually had 36 cubic feet of habitable space according to NASA. Ajaxfiore (talk) 04:05, 18 February 2015 (UTC)


 * Thanks for the info. There has been much confusion about this. Check the previous threads; the error was even bigger in the past. I think part of the problem stems from the vagueness of the term "habitable volume"; is this all empty space in the conical capsule interior, or only the immediate space around the astronaut's seat and control panel? The cited source (This New Ocean) says: "The cabin provided about 100 cubic feet of living space (emphasis added), room enough for an ordinary aircraft seat and a panel of lights, switches, and displays to test the psychological reactions of the subject."
 * I tend to be a bit leery of modern NASA pages about space-race era programs. The person who wrote this was obviously interested in emphasizing the difference between the Mercury capsule and the Shuttle. Also, keep in mind 36 cubic feet is only two by three feet around a six-foot man (though the Mercury astronauts were limited to 5'10".)
 * Also: information on Gemini states its 90 cubic feet was "50% larger than Mercury"; this wold have made the figure for Mercury about 60 cubic feet. JustinTime55 (talk) 14:35, 18 February 2015 (UTC)


 * I researched this a bit further. The 100 cubic feet figure does not indeed refer to Mercury but rather to a simulator. If you read the paragraphs before you will find that Fritz Haber developed a "Space Cabin Simulator" that provided about 100 cubic feet of living space. If you read the following paragraph you will find that Airman D.F. Smith spent a day in the simulator "performing a number of tasks for psychological monitoring and wearing instrumentation to record his heart action, temperature, and respiration rate." Mercury actually had 36 or 60 cubic feet of habitable volume depending on how you define habitable. The volume of the average male is between 2.5 and 3 cubic ft. This source states that it contained 55 cubic ft of "habitable space, much of which was taken up by equipment, making freedom of movement impossible." NASA gives a similar figure of 1.7 cubic meters (60 cubic ft) for the capsule module, and 36 cubic feet for net habitable volume. The net habitable volume, "after subsystems, stowage, outfitting, etc. have been accommodated and design inefficiencies are considered," traditionally equals about 60% of the pressurized volume. 60% of 60 cubic ft is exactly 36 cubic ft. Ajaxfiore (talk) 01:44, 19 April 2015 (UTC)

Cabin pressure and air composition
I like to expand the paragraph about cabin pressure and air composition based on Giblin.

Suggestion:


 * The choise of cabin air composition and pressure was a matter of controversy. Pure oxygen presented a risk of fire in the cabin at least at launch. In space, without gravity, it was believed that combustion products would gather around the fire and smother it and; in any case, the pilot could vent the oxygen into empty space to the same effect. In addition, the long-termed health effects of breathing pure oxygen were not known. On the other hand, a cabin air of oxygen and nitrogen was difficult to control. On the launch pad, the pressure and composition would be like the surrounding air (20% oxygen and 80% nitrogen at the pressure of one atmosphere (15 psi)). During launch the pressure inside the cabin would be gradually reduced and nitrogen replaced with oxygen; the fragile spacecraft would not allow too big af pressure difference between its inside and outside. In orbit, the craft would contain about 67% and 33% nitrogen at a pressure 5 psi, although the astronaut would be able to breathe pure oxygen from his space suit. I all, the oxygen/nitrogen-solution was complicated and added weight to venting system. Further, it gave a health risk since a reduction in nitrogen level of inhaled air could led to a potentially lethal formation of nitrogen bubbles in the blood. Eventhough the atmospheric air-solution was complicated and added weight to the venting system, NASA originally favored it, but an accident during a ground test in 1960 changed their mind as a test pilot nearly died when the pure oxygen in his space suit was somehow diluted by nitrogen from the cabin. Unable to eliminate the problem, NASA decided to use pure oxygen for Mercury—not only in space but on the ground as well. [Note: the Soviet spacecraft used oxygen and nitrogen at atmospheric pressure, but their spacecraft was heavier and more strongly build than the American. The controversy in USA was not over and the risk of ground fire was proven by the Apollo 1 accident.]

From The pure oxygen atmosphere and the existence of combustibles in the cabin had long been controversial among aerospace engineers, but until Apollo 1 NASA was firm in its resolve. The earth’s atmosphere is about 21 percent oxygen and 78 percent nitrogen. In space flight, to avoid putting a strain on the craft’s thin shell, the internal pressure is reduced to about one-third of that on earth. The partial pressure of oxygen in normal air under such conditions would be much too low to support life. In fact it takes only a moderate drop in the partial pressure of oxygen to affect brain activity.

In addition, the presence of nitrogen in the spacecraft presents a danger of its own. Astronauts exposed to a sudden change in pressure—whether due to an accident or malfunction or after donning space suits filled with low-pressure oxygen—could develop the condition known as the bends, in which nitrogen escapes from body tissues and forms gas bubbles in the bloodstream, blocking circulation. (Oxygen presents no such threat.) This illness, which was first noticed among sandhogs and is most often associated today with scuba-diving accidents, can cause neuralgic pain, difficulty in breathing, paralysis, or even death.

A pure oxygen atmosphere would provide sufficient oxygen even at the reduced pressure used in space and would purge nitrogen from the bloodstream to prevent the bends. But there was no consensus about the possible health effects of long-term exposure to pure oxygen. Soviet spacecraft had equipment that duplicated the fullpressure, two-gas environment found on earth, but they needed a thicker shell. Complex air locks were also required, and the cosmonauts had to purge the nitrogen from their bodies for hours before leaving the cabin for a space walk to protect against the bends. NASA thought that the complicated sensing and regulating system that such a scheme would require was too unreliable, and the additional tanks, piping, and controls as well as the heavier shell needed for a two-gas system would add too much weight to the spacecraft. Also, because a pure oxygen system could operate at one-third the pressure, leaks through any joints in the shell into the vacuum of space were less likely.

From NASA’s point of view, then, the case for using pure oxygen in space was overwhelming. For ground operations, the argument was less clear-cut. Indeed, the original design for the Mercury spacecraft had called for the cabin to be filled with normal air while on the launch pad. As it ascended into orbit, the air would be bled from the cabin and replaced with pure oxygen as the external atmosphere thinned. By the time it reached space, the craft would contain about two-thirds oxygen and only one-third nitrogen at a total pressure of approximately 5 psi, although the astronaut would breathe pure oxygen at the same pressure through his space suit at all times (unlike Apollo, whose astronauts would mostly breathe the cabin atmosphere). The primary reason for this arrangement was precisely to reduce the risk of an oxygen-rich fire on the ground. Fire in space was not considered to be as serious a problem, since scientists believed that with no gravity to make hot gases rise, the flames would smother themselves in their own combustion products. In any case, an astronaut could always vent the cabin into the vacuum of space to extinguish the flames.

AN ACCIDENT DURING A ground test in April 1960 changed NASA’s thinking. A McDonnell test pilot was about one hour into a test of the environmental control system when he fell unconscious and nearly died from oxygen depletion. Because of a difference in pressure, nitrogen had leaked into his space suit from the cabin, diluting the oxygen he was breathing. Unable to eliminate the problem reliably, NASA decided that a pure oxygen environment was the best option for Mercury—not only in space but on the ground as well.

This policy was questioned within the space-flight community. In 1964, for example, two separate scientists working for NASA warned of the hazards posed by pure oxygen, which can cause fires that are virtually inextinguishable. In February 1966 an editor of Science Journal, reviewing the proceedings of a conference held the previous fall, noted a general lack of attention to launch-pad safety and predicted, “The odds are that the first casualty in space will occur on the ground.” Yet a conversion to oxygennitrogen for ground use would have introduced complications of its own at a time when NASA already had plenty to worry about. Pure oxygen at all times remained official policy in the Gemini and Apollo programs.

The question of combustible materials was raised again when CSM 012 was delivered to NASA in August 1966. Joe Shea had reiterated the firesafety requirements for the spacecraft and asked North American to investigate the problem. Within six weeks North American documented the results of a “walk-through” inspection of CSM 012 and requested specific direction from NASA on addressing them. NASA responded in turn, but given the volume of concurrent design revisions in the works, compliance was never verified.

Comment:
 * Giblin has: Because of a difference in pressure, nitrogen had leaked into his space suit from the cabin, diluting the oxygen he was breathing. This seems unlikly to me since the spacesuit is under internal pressure, meaning that any leak will result in oxygen leaving the space suit and not nitrogen entering it.
 * It is still not clear to me how the internal pressure was maintained at ground level. Did they use pure oxygen at a pressure of one atmosphere or did they mix oxygen with helium (there is a helium tank on the spacecraft cutaway - what was it for)?

- Soerfm (talk) 15:53, 28 May 2015 (UTC)

Split of flight tables
I have split the flight tables which means that the remarks per default are hidden. They could also be shown if the majority thinks so, it is up to you. - Soerfm (talk) 09:56, 10 July 2016 (UTC)

External links modified
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Perigee and apogee
It is true that the sources don't support the claim that perigee is the insertion point, however, the fact that the orbital flights all have the same perigee seems to suggest this. The difference in apogee then comes from a difference in speed at insertion. Soerfm (talk) 22:18, 2 October 2018 (UTC)

Featured picture scheduled for POTD
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