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A quick dive into the rabbit hole...

Looks like pumps and nitrogen over water.
I would like to see more on this system, only so many hours in each day.

http://www.aft.com/images/casestudies/Aerospace-I-ReynoldsSmith-FullReport.pdf

Currently,two pumps are used at 55% rated speed during launch with one additional pump on standby should a failure occur in one of the first two pumps. The sound suppression valve station consists of three 20” butterfly valves in parallel with pneumatic/hydraulic actuators. The valve actuators are capable of opening the valves in 0.5 seconds.

The Sound Suppression Deflector ring, located downstream of the valve complex,consists of an 18” diameter headerand (8) 12” diameter branches and nozzles. The ring delivers water directly into the rocket engine exhaust stream to provide sound suppression during lift off.

The existing pumps were capable of providing enough waterto satisfy the addition of two deflector ringsby increasing the pump speed to 98% of maximum, however, in order to provide enough cooling and sound suppression to the pad surface, a separate system was implemented. Shown in Figure 4,four rainbirds(nozzles J55, J25, J26, and J27)spraythe pad surface with water just followingvehicleliftoff. This water provides additional sound suppression as well as cooling to the pad surface and ground support equipment (GSE). Instead of using pumps, this system is supplied with water froma pressurized tank. This45,000-gallon tank is pressurized with gaseous nitrogen(GN2) to a pressure of 200 psi.A source of GN2 maintains the pressure nearly constant as the water is withdrawn from the tank.Redundant butterflyvalves openjust after vehicle liftoffto allow the pressurized tank to supply 100,000 gpm to the four rainbirds.Figure 4–Proposed Modificationsto SLC-9 in Impulse–Additional Tank, Valves,and RainbirdsThe pressurized tank was chosen over using additional pumps because the current pump house would require extensive modificationsto create the needed room. The pressurized tank system also has a small footprint that does not greatly impact this environmentallysensitive location.Further, this new system, independentof the current pump system,allows greater flexibility for future modifications of this now multiuser pad.