Comment by RagnarD
2 years ago
This is an unusually rare, worthwhile piece. It's rare for somebody to be both very grounded, with a personal history of hands-on practical competence, yet simultaneously abstracting his experience to a high philosophic level.
A nitpick is this statement: "The massive difference in weight between a rocket full of fuel and an empty one means that a reusable rocket can’t hover if it can’t throttle down to a very small fraction of its original thrust, which in turn means it must plan its trajectory very precisely to achieve 0 velocity at exactly the moment it reaches the ground."
I don't think this is how SpaceX does it. When you have closed loop feedback control using velocity and distance measurements from radar and vectoring thrusters, it's no longer an impossibly difficult ballistic problem. I suppose that highlights the necessity of not making assumptions about solution methods.
The original statement is correct: the rocket cannot throttle down to a small fraction, to an equivalent thrust to the mass of the rocket. SpaceX rockets indeed do not "hover" for any significant length of time. The trajectory is planned to achieve 0 velocity at just above ground level. The feedback control comes in because it's impossible to know the exact performance and atmospheric characteristics, so if you tried to do it by dead reckoning it would be off vertically by tens of meters, so the trajectory is continuously adjusted.