Background on Tonight's Launch | SpaceX: "In the case of the Falcon 9 rocket, the boost stage is able to accelerate a payload mass of 125 metric tons to 8000 km/h and land on an ocean platform or to 5000 km/h and land back at the launch site. The second one is lower because the rocket is moving super fast away from the launch site, so it has to do a screetching U-turn with nitrogen attitude thrusters, then fire the engines to create a reversed ballistic arc, then reorient again for atmospheric entry and have the engines pointed in the right direction for the landing burn. Since the propellant is liquid, it wants to centrifuge out during these maneuvers, so there has to be a system of baffles and internal holding tanks to keep it in place. It also needs three axis control surfaces that don't melt easily and work well from hypersonic through subsonic speeds.
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