According to many news articles, the Falcon 9 landing on 1/10/2015 was due to running out of hydraulic fluid for the steerable hypersonic grid fins. This was seemingly confirmed the Musk’s tweet “Upcoming flight already has 50% more hydraulic fluid, so should have plenty of margin for landing attempt next month.”
Why, oh why, do the fins consume hydraulic fluid?? Hydraulic fluid is normally used in a closed system that only consumes the fluid via leaks, right? Are the leaks due to temperature changes so bad, and so unsolvable, that the only solution is to just have more in reserve?
Click through for a lesson on rocket science, and why an open hydraulic system makes sense here, and is actually a fairly elegant engineering solution.