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In NASA spacecraft development, subsequent versions of a design are referred to as "Block 1", "Block 2", etc.

What is the origin of this naming convention? Why not just call them "Version 1", "Version 2" (or "Model 1", "Model 2")?

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I am not sure, but I think it comes from US military procurement, where it refers to the actual items (vehicles, weapons systems, etc.) being produced and delivered. For example, if you want 10 ships of a single class built over 4 years, but the latest and greatest radar system won't be available right away, the contract might refer to two "blocks" of ships, 4 ships in Block I with the older radar, and 6 ships in Block II with the new radar.

To me, it connotes something more concrete than "model" or "version" -- the block is the actual set of items going to a customer, as opposed to the picture in the catalog or the set of design drawings.

I don't know how long it's been in use, or where it started.

The earliest military use I've seen is in reference to the Redeye surface-to-air missile, which had a "Block I" as early as 1963. The first set of GPS satellites contracted in 1974 and launched in 1978 was referred to as "Block I", and the first version of the Phalanx CIWS that went into production in 1978 is "Block 0", but that name may well have been retroactively applied when the Block 1 upgrade became a thing.

Ranger's three development blocks predate Redeye, though:

Ranger was originally designed, beginning in 1959, in three distinct phases, called "blocks". Each block had different mission objectives and progressively more advanced system design. The JPL mission designers planned multiple launches in each block, to maximize the engineering experience and scientific value of the mission and to assure at least one successful flight.

USN also uses "flight" in exactly the same sense as "block".

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    $\begingroup$ see merriam-webster.com/dictionary/block 'a quantity, number, or section of things dealt with as a unit'. In engineering, a 'block buy' is where you buy all the parts to produce x machines at once, making sure all of them are the same. $\endgroup$ – Hobbes Aug 4 '15 at 16:18
  • $\begingroup$ "Block 2" usage sighting in regards to Apollo command modules: youtu.be/QUHyDnMS5oo?t=313 $\endgroup$ – uhoh Oct 8 '18 at 19:02
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According to The Interplanetary Podcast's "space word of the week" 10 minutes in to episode 97, it comes from the US military aircraft "technical data block" (usually painted on the port side near the cockpit) The model code takes the form $$\begin{matrix} Type&number&variant&version number\\ B&29&B&55\\ F&101&B&40\\ \end{matrix} $$ The version number is conventionally increased by 5 for each manufacturers revision, allowing single increments for field upgrades. B-29B-55 Superfortress F101 Voodoo

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