The problem with all these types of proposals, like the Jupiter III, was that the infrastructure couldn't support multiple SRB's beyond the pair used on the shuttle. The pair of shuttle SRB's practically maxed out the crawler and crawlerway. Adding a second pair of SRB's would totally blow the roof off the capabilities required to handle their weight. The four-SRB proposals for supplementing the thrust of Saturn V would have had the SRB's added to the stack at the pad to avoid the issues of moving all that weight. There have been other four SRB proposals I've seen in the past (somewhere-- were lost when my hard drive melted down last year) and they were equally unrealistic simply due to their infrastructure requirements impacts...
The other issue is, that IIRC the pads at SLC-39 were only designed to accommodate a maximum of 11 million pounds thrust (IIRC). Most of these proposals would blow the lid off that as well... Requiring the pads be rebuilt or refurbished and strengthened to withstand the extra liftoff thrust.
Finally, there's the issue of flight rates versus vehicle development and operations costs... how often would a vehicle like Jupiter III be needed... it could launch basically a year's cargo in a single flight, but that single vehicle would be INCREDIBLY expensive to sustain, support, and operate. At some point, it's better and more efficient to launch several smaller rockets that you already are geared up to handle and support, increasing the flight rates and better amortizing support overhead costs among more launches of an "existing" vehicle you already have, rather than engineer some rarely used mega-booster at terrific expense... That was the argument that DIRECT made, anyway.
Jupiter III would make one heck of a flying model rocket, though...
Later! OL JR
