The "pads" weren't converted for Saturn IB.
Originally the Saturn I/IB's launched from over at Space Launch Complex 34, which was there long before the VAB and pads 39A and B. The rockets were stacked on the pad-- the stages were trucked out to the pad on transporters and then lifted upright by cranes and installed on the pad. All the work to complete and check out the rocket was done on the pad, more or less in the open. There were a lot of drawbacks to this method, especially for the HUGE Saturn V, which is why it was designed with the new paradigm of integration and checkout performed inside a massive permanent building and then transport the rocket to the pad via crawler. This required that the rockets be assembled on massive "Main Launch Platforms" (MLP's) that, in the case of the Saturn vehicles, also had a large steel tower with all the requisite vehicle umbilicals and service arms necessary to serve the vehicle. The "pad" is merely a concrete hardstand with the service connections necessary to connect the vehicle with its propellant supply storage tanks, with the firing room, etc. The MLP's were all built with Saturn V in mind, with the towers having the umbilical and service arms at specific locations and heights for that vehicle alone.
Now, when the moon landings were pared back, and the Apollo Applications Skylab space station approved (along with ASTP), it was realized that there would be a lot of benefits to launching the Saturn IB from the existing MLP's and pads at SLC 39, even though they were never designed for that. After looking at the problem, the easiest (and cheapest) was convert the Saturn V MLP's for use by Saturn IB by building a steel tower (a "milkstool") to raise the Saturn IB up the same height as the S-IVB would be at on a Saturn V, enabling the use of the existing white room and umbilicals to the S-IVB and spacecraft, and adding connections for the S-IB first stage. The "pad" itself wasn't changed much if any-- it simply wasn't necessary.
Now, when the shuttle was approved, the original idea was to abandon the KSC Saturn infrastructure and build an entirely new setup to support shuttle, and not even necessarily at KSC. They even looked at building a new shuttle launch center in Matagorda County, Texas, to be called the Spiro T. Agnew Space Center, which would have been about 90 miles west from Johnson Space Center. The shuttles would have flown dogleg trajectories out over the Gulf and dropped their boosters into the Gulf of Mexico on ascent, probably off the west coast of Florida for recovery and return to the launch site. Of course this would have been TREMENDOUSLY expensive to start 'from scratch', so this plan basically didn't last til the ink was dry (much like the Orion's methane SPS engine or the Prometheus nuclear engine). The shuttle program was directed to re-use the existing Saturn infrastructure at KSC's SLC 39, which meant reusing the VAB, MLP's, pads, and crawlers.
The SRB's are extremely heavy (due to having to be moved from the VAB fully fueled, versus the liquid fueled Saturn V that was moved empty) and the Saturn tower was designed and built quite specifically for the Saturn V (and as mentioned modified for Saturn IB) so the tower served no further purpose, and they were scrapped. The MLP's were modified to support the shuttle, with the offset exhaust holes necessary for the twin SRB's and the orbiter. The requirements of integrating the payloads into the shuttle with it standing erect, often fairly quickly before launch drove the design towards the Fixed Service Structure (FSS) built at the pad to support the shuttle's umbilicals and white room, and the Rotating Service Structure (RSS) being rolled into place against/over the orbiter to load the payload bay with whatever was to be carried, and checkout of the payload with the vehicle. The payload bay doors would then be closed and the RSS moved back to its "parking" position for launch.
Now that shuttle is retired, the FSS/RSS towers at 39A and B serve no purpose, and have already been demolished at 39B. In fact the white room from the FSS at 39B just arrived a few weeks ago at JSC to eventually be installed as an exhibit. The paradigm has switched back to a Saturn-like "clean pad" design where the umbilical tower will again be installed on the MLP.
As for specific use or non-use of the pads at KSC, originally SLC 39 was laid out for up to FIVE pads... with 3-4 actually slated to be built. (The remaining pads were to be built north of the existing pads at A and B, which would have been C and D, with "E" reserved for any 'follow on' vehicles that might require larger or stronger pads beyond the 11 million pound thrust the original Saturn pads at 39A and B were designed to handle, and of course being further from the VAB as well. Of course even with Apollo having a "blank check", money was an issue and so pads C and D were deferred, and subsequently never built. Pads A and B were deemed sufficient to handle the anticipated traffic of Saturn V's for the foreseeable future, even for a Mars program had one ever been approved... of course by the time of the first moon landing, the Saturn V program had already been canceled, Saturn IB's were stockpiled and their production halted as well, and later entirely scrapped. The remaining vehicles would be sufficient to fly out the pared back Apollo lunar landings and the follow on AAP/Skylab program, and the final Saturn IB flying ASTP.
Had the shuttle program ever even REMOTELY lived up to the wildly optimistic flight rates made to sell the program, then its quite likely that pads C and D would have been necessary to keep up with the flight rates. (Remember shuttle was ORIGINALLY justified by the low costs and economies of scale capable from "airliner" type operations with high flight rates, approaching 50 flights PER YEAR! Think about it... at that flight rate, the entire 134 flights of the shuttle program, over 30 years, would have flown in ONLY ABOUT THREE YEARS!!! This was a fantasy, but it's one that served everyone's purposes, so they went with it).
Now, supposedly NASA is going to 'adapt' the expensive and very vehicle-specific infrastructure at SLC 39 to support "commercial rockets"... the idea is, anybody who wants to RENT the KSC facilities can launch their rocket from KSC. Of course, this "rent" won't come cheap, and most of the vehicles are designed around specific ground handling and integration "paradigms" that aren't easily "plug-n-play" with other pads or facilities. Each vehicle has specific requirements, different propellants, different data requirements, different support requirements such as electrical, pneumatic, hydraulic, air conditioning, service connections, payload integration, checkout, etc... making this "plug-n-play" is going to be both expensive and difficult. The only way *I* can see it working is to build 'generic' MLP's that can be fitted with "kits" much like the milkstool, to adapt specific vehicles to the MLP and re-route the connections to the necessary points... (for instance, when launching an Atlas the LH2 connections would be sealed off, and the propellant lines re-routed to the proper connections with the vehicle umbilicals themselves, while when launching a Delta, the RP-1 lines would be sealed off and the lines routed on that kit for the proper connection locations to the Delta IV... differing vehicle heights and connection/umbilical/service arm requirements would have to be addressed by either having *all* or most of the different service arms on the umbilical tower on the MLP, or using various ones in combination with kits outfitted for each specific vehicle, perhaps in combinations with "mini-milkstools" or something similar to adapt the vehicle heights to the service tower heights, probably as part of the "kit" adapting the MLP for each specific rocket... Either way, I personally see the whole "21st Century Launch Complex" thing as basically make-work to keep KSC open for the next decade while nothing is flying from there... I don't see any "commercial" companies *choosing* to spend big bucks to modify their equipment to fly from SLC 39 when they have their OWN pads/infrastructure tailored to their specific vehicles just down the road... of course, if the flight rates ever get high enough, yeah, additional capability at 39 would be nice to have... heck with a high enough flightrate, even having a specific MLP for each different type/kind of rocket becomes cost effective, but from everything I see we're a LONG, LONG way from EVER seeing those kinds of flight rates, if at all.
Interesting topic! Later! OL JR
