SpaceX loaded propellants into the Falcon 9 rocket again Friday, one day after President Obama briefly toured the booster's launch pad during his visit to the Kennedy Space Center.

Officials say the second tanking test, also called a wet dress rehearsal, was planned to verify new cork insulation remains adhered to the 15-story rocket during tanking. It also gives the SpaceX launch team more practice before the Falcon 9's first flight, which is scheduled for no earlier than May 8.

SpaceX rolled the Falcon 9 from an assembly hangar and lifted it vertically at Complex 40 earlier this week. President Obama received a personal tour of the launch pad Thursday before delivering a speech defending his vision for NASA, which includes more reliance on commercial companies like SpaceX.

The company's Falcon 9 rocket and Dragon spacecraft will fly cargo missions to the International Space Stations beginning next year. SpaceX claims it can design a crewed version of the Dragon within three years, if NASA awards a contract for the work.

President Obama and Elon Musk visited at Complex 40 Thursday afternoon. Credit: NASA/Bill Ingalls

Elon Musk, the founder and CEO of SpaceX, hosted Obama on a 15-minute tour of the complex.

Obama did not visit other launch pads at the Cape during a visit that lasted more than two hours, according to NASA officials.

"The president spent 15 minutes walking around the pad and rocket asking me questions about it," Musk said. "For a non-engineer, he displayed an impressive understanding."

Musk told Spaceflight Now that Obama specifically asked what modifications are required for the Falcon 9 rocket and Dragon capsule to be ready for human occupants.

President Obama mentioned the tour at the opening of his remarks to a crowd of NASA and space industry brass later Thursday afternoon.

Friday's countdown dress rehearsal was the fourth time the 30-person launch team fueled the two-stage rocket.

During a fueling test in late February, chunks of cork fell from the Falcon 9 after liquid oxygen was removed from the rocket. Chilled to about -300 degrees Fahrenheit, liquid oxygen causes the rocket's structure to contract and expand as it flows in and out of the booster.

The super-cold oxidizer is used by both stages of the Falcon 9. The rocket's Merlin engines consume kerosene as fuel.

The cork is a thermal shield to protect the Falcon 9 first stage during its re-entry into the atmosphere after separating from the second stage three minutes after liftoff.

Once the nine-engine first stage is jettisoned, it is designed to fall more than 50 miles back to Earth, deploy parachutes and splash down in the Atlantic Ocean for recovery.

SpaceX fueled the Falcon 9 twice in early March for a static fire test that checked the rocket's nine Merlin first stage engines.

The Falcon 9 rocket fires its nine Merlin 1C first stage engines on March 13. Credit: Chris Thompson/SpaceX

Workers added new cork after the engine test, and Musk said managers ordered the new practice countdown to make sure the cork sticks to the rocket.

Engineers also planned electromagnetic compatibility, or EMI, tests with the Air Force Eastern Range while the Falcon 9 is on the pad.

Testing of the rocket's flight termination system is continuing, according to Musk.

"If we get through the wet dress and EMI tests, we could be ready to launch next week," Musk said in an interview Thursday. "The qualification of the FTS is what's driving us to May."

The Air Force would use the destruct system to terminate the launch if the Falcon 9 flew off course and threatened the public.

Before the Air Force approves the launch, SpaceX and the supplier of its flight termination system must complete thorough testing of the components and provide the results to the military.

Musk said the schedule looks "promising" for launching in the first half of May.