How much pressure does it take to blow water out of a fire sprinkler in a 4-6 foot arc?
According to this Wikipedia article,


  a typical Early Suppression Fast Response (ESFR) sprinkler at a pressure of 50 psi (340 kPa) will discharge approximately 100 US gallons per minute (0.0063 m3/s)


With a magic 100% efficient motor and pump, that'd require almost 3 hp, so it's no surprise 1.5 hp didn't cut it for two sprinklers.