This is a section of a post Geoff Forden wrote for ArmsControlWonk.com

Iran’s aerospace program has been so active in the last few years it should be possible to say something about their development philosophy; the technological arc or trajectory they are following.  For instance, why did they “jump” from SCUD-type missiles to the Shahab-3-type?  Why didn’t they put a higher priority on clustering engines in order to achieve greater ranges before moving on to the Shahab-3?  Many analysts believe they should have.  A large portion of their argument is centered on the fact that they believe Iran would have established a missile capable of hitting Israel much sooner if they had done that, perhaps as early as the mid-1990s.  Of course, such arguments place an extraordinary amount of emphasis on such a military objective, especially when Iran’s nuclear program was much, much less advanced.

I’ve always thought, however, that Iran did make a strategic decision about the direction its missile development program was going in.  But it was not a military-strategic decision but an industrial-strategic decision even if there were military advantages to be had further down the road.  It appears that Iran decided they needed to assimilate the technology for producing large engines indigenously and that this was a much higher priority for them than early production of a larger range missile.  New images released on Iran’s Space Day do indeed show a cluster of engines but they are the more powerful Nodong engines.

What trajectory did Iran’s missile development program actually follow?  First, it appears that Iran purchased a production line for Nodong engines (and the other components of the Shaha-3 missile) from North Korea.  However, through the years of producing Shahabs, flight testing them, and modifying them--with the design and production of the Safir and other rockets--Iran has fully assimilated this technology and are moving on to the next stage of development—clustering large engines (they obviously gained some highly important experience with the cluster of two engines on the Safir’s second stage)—and they are probably doing this largely on their own.