SpaceX's daring reusable-rocket test last week came up just short because of an issue with a "throttle valve," company founder and CEO Elon Musk said.

The private spaceflight company tried to bring the first stage of its Falcon 9 rocket down for a soft landing on an unmanned "drone ship" in the Atlantic Ocean on April 14, during the successful launch of its robotic Dragon capsule toward the International Space Station on a cargo mission for NASA. The bold maneuver nearly worked: The rocket stage managed to hit the ship but ended up toppling over on the deck and exploding.

SpaceX engineers have figured out what happened, Elon Musk said.