Where do the weight savings come from? Hydraulic fluid is heavy; it seems like you could save quite a bit of weight by carrying less of it if it wasn't expendable. Not that I don't believe it, just curious.
I'm guessing you'd need more pistons, pumps, piping, etc. in a closed system. Probably need to repressurize the fluid.
Pumps and pipes. With an open system, you don't need to recompress the fluid and move it back, and from what I read, that hardware weights a lot.
Not to mention the power supply - what's going to run the pumps when the engines are shut off?
Correct! Instead of using compressor(s) and recirculation plumbing, you're using an accumulator [1]. Weight and complexity savings.
Seems like they actually use RP-1 (rocket fuel) as the hydraulic fluid so that it is burned as fuel after it has been used. This means that the hydraulic fluid isnt "dead mass"