One of the factors is that the whole landing operation is a very tricky balancing act.
The 9 Merlin engines are designed to life the fueled and loaded rocket into space, and it's hard to design these engines for a wide range of thrusts - meaning big penalties in weight, performance, efficiency, etc. Thus, when you're dealing with just the first stage, nearly empty of fuel, even minimum thrust from one engine will accelerate it up pretty hard. It can't hover because of this, so they have to take the rocket falling down with considerable velocity and fire the engine at exactly the right time so that when it passes through zero vertical velocity, it's on top of the barge with no lateral velocity either, and then shut it off at exactly that moment.
Then fuel is a whole 'nother factor. There isn't much of it at landing time, and it probably still makes up a lot of the weight of the stage. So you also have to handle the weight constantly decreasing and any motion of the fuel, and not run out before your landing burn.
You need a huge amount of precise control, and your only tools for the job are a vastly over-powered rocket engine, a stage body not exactly designed for stable aerodynamic flight, and some teeny little hydraulic fins. It's gotta be a minor miracle that they can hit the barge at all, and not with some massive destroy-everything velocity.
The 9 Merlin engines are designed to life the fueled and loaded rocket into space, and it's hard to design these engines for a wide range of thrusts - meaning big penalties in weight, performance, efficiency, etc. Thus, when you're dealing with just the first stage, nearly empty of fuel, even minimum thrust from one engine will accelerate it up pretty hard. It can't hover because of this, so they have to take the rocket falling down with considerable velocity and fire the engine at exactly the right time so that when it passes through zero vertical velocity, it's on top of the barge with no lateral velocity either, and then shut it off at exactly that moment.
Then fuel is a whole 'nother factor. There isn't much of it at landing time, and it probably still makes up a lot of the weight of the stage. So you also have to handle the weight constantly decreasing and any motion of the fuel, and not run out before your landing burn.
You need a huge amount of precise control, and your only tools for the job are a vastly over-powered rocket engine, a stage body not exactly designed for stable aerodynamic flight, and some teeny little hydraulic fins. It's gotta be a minor miracle that they can hit the barge at all, and not with some massive destroy-everything velocity.