Must be some tooling I wasn't aware of:

Life-cycle cost also drove selection of the Rocketdyne RS-68 as the CaLV's main engine, rather than the RS-25e. However, this led to an increase in CaLV core structure diameter from 8.38m, the same as the Shuttle's external tank, to 10m. The wider core was needed to enlarge the fuel tanks to provide the extra propellant required to allow the RS-68 to burn long enough to deliver RS-25e-like performance.

Hecker says the agency's Michoud Assembly Facility, near New Orleans, can manufacture a 10m structure because "the tooling already exists so the increase in cost is negligible". Michoud, a NASA centre managed by Lockheed Martin, is already designated to be the manufacturing and assembly facility for the CLV upper stage and EDS.

He might be right about the new friction-stir weld tools for ET barrel sections, which have some amount of diameter flexibility as I recall, but the dome weld tools and assembly fixtures and the LH2 major weld tools are pretty well wedded to the ET's 27.6ft diameter.

If their diameter could be enlarged at all, it would certainly be a non-trivial expense, as would the alternative of replacing the dome weld tools with simpler and less-expensive multi-axis FSW fixtures. And while some of MAF's VAB and checkout facilities were built for the larger-diameter Saturn V, they would still have to be modified to some extent, as would the ET-specific work platforms, carriers and test cells.

Either Hecker is flat out wrong in thinking the diameter growth would cause a "negligible" increase in cost...or else, he knows exactly the kind of cost impact it would have, but sees it as a drop in the bucket compared to the cost of the CaLV program overall. Either of which should give one pause.

Posted by T.L. James on May 5, 2006 11:58 PM