Mobile Launcher Platform - Evolution - Space Shuttle

Space Shuttle

In the post-Apollo years, the umbilical towers from Mobile Launchers 2 and 3 were removed. Portions of these tower structures were erected at the two Space Shuttle (or STS, for Space Transport System) launch pads, Pads 39 A and B. These permanent structures are now known as the "Fixed Service Structure" or in NASA's language of acronyms, FSS. The umbilical tower from Mobile Launcher 1 (which was the platform used for the most significant Apollo missions) was taken apart and stored in the Kennedy Space Center's industrial area. Efforts to preserve it in the 1990s failed, however, for lack of funding and it was scrapped.

In addition to removal of the umbilical towers, each Shuttle-era MLP was extensively reconfigured with the addition of two Tail Service Masts, one on either side of the Main Engine exhaust vent. These 31-foot (9.4 m) masts contained the feed lines through which liquid hydrogen (LH₂) and liquid oxygen (LOX) were loaded into the shuttle's external fuel tank, as well as electrical hookups and flares that were used to burn off any ambient hydrogen vapors at the launch site immediately prior to Main Engine start.

The Space Shuttle Main Engine (SSME) vented its exhaust through the original opening used for the Saturn rocket exhaust. Two additional holes were added to vent exhaust from the Solid Rocket Boosters that flanked the external fuel tank.

The Space Shuttle assembly was held to the MLP through eight attach posts, also called "hold-down bolts", four on the aft skirt of each Solid Rocket Booster. Immediately before SRB ignition, frangible nuts attached to the top of these bolts were detonated, releasing the assembly from the platform.

When NASA began launching shuttle missions, it became clear that the MLP might inadvertently pose a danger to the crew or the vehicle: massive acoustic shock waves and rocket exhaust can bounce off the platform and hit the shuttle as it lifts off. This was true for the Saturn V launches as well, but there was less risk because the Apollo modules, atop the 363-foot (111 m) stack, were much farther away from the engines. Because the shuttle was about half the height of the Saturn, the crew-cabin and payload bay were much closer to the platform and much more vulnerable to the tremendous forces bouncing back off the MLP - on the first mission, STS-1, the shock waves damaged many of the protective thermal tiles.

NASA's solution to this danger was to cushion the MLP at every launch with a flood of flowing water. Starting 6.6 seconds before engine ignition, a 300,000-US-gallon (1,100 m3) water tank at the launch site began dumping water down a pipeline and into the exhaust vents of the MLP. Next, six 12-foot (3.7 m)-high towers known as "rainbirds" began to spray water over the MLP and into the flame deflector trenches below it. The water absorbed some of the bruising forces of the acoustic waves, and discouraged fires that might be caused by the rocket exhaust. This water-dumping mechanism, known as the Sound Suppression System, emptied the launch pad tank in around 41 seconds. The giant white clouds that billowed around the shuttle at each launch were not smoke, but water vapor generated as the rocket exhaust boiled away huge quantities of water. The suppression system reduced the acoustic sound level to approx 142 dB.