According to the SpaceX junkies, the core ran out of fuel on 2 out of the 3 engines they planned to use for landing. I think the speed at core/stage 2 separation was quite high - 9,500 km/hr - compared to 5,000 km/hr to 8,000 km/hr for a typical Falcon launch. I am guessing that’s why SpaceX was experimenting with…