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During the last ten years all 100 or so launches by Atlas V and Ariane V together have been successful. (One Atlas V payload entered too low orbit, but that would hardly have risked the life of a crew). Why is that not enough for them to be man-rated and carry crewed capsules?

Are there requirements on technical details which overrule actually demonstrated performance? Or is there something else than reliability which plays a role, like higher G-forces than a human can take?

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I see on Wiki that ESA planned to launch the manned HERMES shuttle on Ariane 5, back in the 1990s. So Ariane 5 seems to be "man-rateble". The Atlas V and Delta IV don't seem to be far from it either: en.wikipedia.org/wiki/Human-rating_certification Maybe it is mostly just demand that is lacking? –  LocalFluff Jun 12 at 9:58

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Man rating seems like an obvious thing. Safe enough for manned flight.

But in reality, there really was/is no standard for it. Some things are generic. Sufficient (Usually triple) redundancy in flight computers/controls.

SpaceX has cited building to 1.4X structural margins expected, instead of 1.25. (This is a hard one to retrofit in after the fact).

NASA actually came up with an attempt at a standard. Thing is, Soyuz does not meet it, yet has flown how many hundreds of manned flights (And 1700 total flights for the booster).

Per the Wikipedia entry on Human Rating Certification NASA in 2008 came out with a standard for the new entrants.

(NASA) has published NASA Procedural Requirement NPR 8705.2B - Human Rating Requirements for Space Systems, defining the certification process and a set of technical requirements to be applied to its crewed space systems in addition to the standards and requirements that are mandatory for all of NASA's space flight programs

The Shuttle would not meet the standards either. Not sure about Apollo/Saturn V (and NASA would probably very much like people NOT to ask that question, I suspect.)

So asking why were Atlas V/Ariane V are not human rated is hard to answer. Mostly because until recently, human rated did not have a very hard target to hit.

Obviously no one plans to build an unreliable rocket. But obviously as well, this is still rocket science and it is hard. Additionally, if you believe in a man rated standard, a large number of successful flights alone does not prove it is man rated. But if you are NASA you get to ignore the rules (Shuttle/Soyuz) at will, if it is expedient. Which leads to the conclusion that man-rating is sort of tenuous at best.

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What was this standard you mention that NASA came up with that isn't met by any of the rockets? –  TylerH Jun 12 at 13:16
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@TylerH NASA never had a solid, 100% defined standard. (Mostly because Shuttle/Soyuz could not meet it, so better not to make themselves look bad, if you are being cynical). For the CCiCAP/CCtCAP/COTS-D style programs, they are trying to apply some standard. That has become 'the standard' as applied to DragonRider(v2?), DreamChaser, and CST-100. Part of the current contract for CCtCAP is to design a certification program, to prove they can meet this standard. So not very hard and fast, as you might have expected it to be. –  geoffc Jun 12 at 13:29
    
no cynicism here; just curiosity since you mentioned it in your answer but didn't go into any detail. –  TylerH Jun 12 at 13:45
    
@TylerH I meant that I was being cynical. If it is not clear, I am cynical on this topic. :) NASA is playing games. But it is their ball, so they take responsibility for their decisions. –  geoffc Jun 12 at 13:48
    
I believe the safety standards are a device to exclude the new aerospace players and keep lucrative good ol' boy contracts within certain congressional districts. Will SLS and Orion comply? Congressmen like Shelby or Mo Brooks probably don't know or care -- their main goal is to keep the pork. The best way to achieve safe rockets is through experience. To get experience we need to launch a lot of rockets, a high flight rate. The SLS is expected to have a low flight rate. –  HopDavid Jun 13 at 13:48

Space.com answered this very question:

The emergency detection system needs to be finished, for example. And a human-rated Atlas 5 will use a two-engine Centaur upper stage rather than the single-engine version currently in use, Patton said, so some more tweaks will be needed to accommodate the change.

There's a good list of other things, but the bottom line is, it's reliable enough, it just needs a few additional tweaks that a man rated spacecraft requires.

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But are such "tweaks" just irrelevant rating formalilites? The actual reliability seems to have been demonstrated as a matter of fact, with or without gadget X or procedure Y. –  LocalFluff Jun 12 at 17:43

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