[JshWright]

The engine can only throttle down to about 70%. For the final landing, the first stage will only use the center engine (leaving the other 8 shut down).

Even with only one engine at 70%, the thrust to weight ratio is greater than 1. That means the rocket can't hover, they just have to time the burn perfectly so the speed reaches 0 at the moment it reaches the ground.

[grinich]

Why can they only throttle to 70%? Is that related to old-school turbopumps?

[Mvandenbergh]

Most rocket engines don't throttle down well. The main reason isn't the turbopumps (although that can also be an issue) but combustion instabilities in the chamber and flow separation in the nozzle.

Nasa overview of liquid engine throttling:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2010003...

[grinich]

Exactly the answer I was looking for-- thanks!

[Serow225]

That's a very readable reference, thanks.