On the official webcast of SpaceX's Iridium-4 mission a "throttle down and throttle back up of the Falcon 9 first stage engines" is mentioned. Why would one throttle down at a crucial time during steep ascent, way before MECO (Main Engine Cut-Off)? Is it to weaken the effect of max-Q?
Why would one throttle down at a crucial time during steep ascent, way before MECO? Is it to weaken the effect of max Q?
Yes, exactly that. The engines are throttled up to the highest extent possible during all phases of flight to reduce gravity losses; but must be throttled down during at least two points during flight:
When Falcon passes through Maximum Aerodynamic Pressure (Max-Q) which is a combination of atmospheric density and velocity that puts the greatest strain on the vehicle. This effect is more prominent during Dragon launches due to the frontal shape of Dragon.
As Falcon nears MECO to limit acceleration on the payload, if necessary.
The graph on the right hand side shows a reduction in net acceleration for Dragon missions around Max-Q, along with a tailoff in net acceleration just before booster cutoff.