[webXL]

Was there anything special about the center core other than boosting longer and needing to land out at sea? Seems like they've landed plenty out at sea, but not while landing two on land.

I thought this was impressive: "With a total of 27 first-stage engines, Falcon Heavy has engine-out capability that no other launch vehicle can match—under most payload scenarios, it can sustain more than one unplanned engine shutdown at any point in flight and still successfully complete its mission." http://www.spacex.com/falcon-heavy

[phpnode]

The centre core is significantly strengthened to cope with the additional load from the side boosters. It also has slightly different aerodynamic properties because of the fixed struts that hold the boosters in place. Either of these could have affected the landing. They'll collect data and fix it.

[jpgvm]

It ran out of fuel. There was enough to relight the centre engine but not the 3 outer engines required to complete the landing.

[GraemeL]

It didn't actually run out of fuel. It didn't have enough of the hypergolic mix used to ignite the engines (triethylaluminium & triethylborane that causes the green flash sometimes seen at ignition) and only one of the three engines needed for the landing burn re-lit.

[eunoia]

Here are some really rough numbers derived from pausing the live stream.

Falcon Heavy Side Booster Separation - 61km, 6,881 km/hr

Falcon Heavy Core 1st Stage Separation - 92.5km, 9,474 km/hr

Also a link to screen caps of speeds/altitudes of previous Falcon 9 separations

http://www.planetary.org/multimedia/space-images/charts/falc...

[NamTaf]

It is probably the fastest/highest burn they've ever tried to land, and thus needs the most fuel to land, expeirences the most structural stress, etc. etc.