Approximately 20−30 seconds into ascent and an altitude of 9000 feet , the RS-25 engines were throttled down to 65−72% to reduce the maximum aerodynamic forces at Max Q. III–8-9 Additionally, the shape of the SRB propellant was designed to cause thrust to decrease at the time of Max Q. 427 The GPCs could dynamically control the throttle of the RS-25 engines based upon the performance of the SRBs.