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I was recently reading up on the Atlas rocket family, and I noticed that for some reason the numbering went from the Atlas 3 straight to the Atlas 5.

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Why did they skip number 4?

I am familiar enough with rocketry to know that there are many incremental changes making the numbering schemes complicated and unintuitive, but I can't find any reason for the bad counting.

Did they count Atlas 3B as number 4, or did they want to be one better than the Delta 4, or is it something else entirely?

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  • $\begingroup$ That's really interesting... globalsecurity.org/space/systems/atlas-iv.htm $\endgroup$
    – JSCoder says Reinstate Monica
    May 22, 2018 at 21:33
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    $\begingroup$ I suspect that at the time, they were competing (Lockheed and Boeing) and the NOT being the Atlas 4 competing against the Delta 4 was a compelling reason. Now that they merged companies and killed the Delta 4, less so. $\endgroup$
    – geoffc
    May 22, 2018 at 21:45
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    $\begingroup$ Another theory is that Atlas V was, design-wise, a successor to both the Atlas III and Lockheed's Titan IV. forum.nasaspaceflight.com/index.php?topic=30436.0 $\endgroup$
    – Russell Borogove
    May 22, 2018 at 22:56
  • $\begingroup$ Possibly worth noting is the Atlas V can support up to 5 AJ-60 SRBs. Even though the number of boosters in a launch is given by the vehicle designation (eg. an Atlas V 552 has 5 motors), using Atlas V could have helped avoid confusion here. $\endgroup$
    – Jack
    Jun 8, 2018 at 9:56

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I did some searching. Russell's comment provides the only theory I've found:

LM had Atlas III and Titan IV. Atlas V as a follow on to both and since it incorporated elements of each, so did the name.

It was called Atlas because it was an Atlas: same 10' dia Centaur, same fairing, same avionics (Centaur controls the whole vehicle), same first stage fuel and same integration processes (physical and analytical)

The Titan contributions are fewer and less visible. Structurally stable first stage, Al metal, 3 body Heavy ... and use of rail system and SLC-41

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