An unmanned Falcon 9 rocket blasted off from Cape Canaveral Air Force Station in Florida on Friday to deliver a cargo capsule to the International Space Station for NASA.

The 63-metre tall rocket, built and operated by privately owned Space Exploration Technologies, bolted off its seaside launch pad at 3:25 p.m. ET, darting through overcast skies as it headed toward orbit.

The Dragon cargo ship, which is loaded with about 2,268 kilograms of equipment, science experiments and supplies, is due to reach the station on Sunday. 

SpaceX planned to use Friday's launch to test technology it has been developing to recover and reuse its rockets.

The Falcon 9's first stage holds extra fuel and four landing legs. After it separated from the upper stage and Dragon capsule, the rocket was expected to reignite its engines to slow its descent and position itself for a vertical touchdown on the ocean before toppling over on its side.

"This is a really difficult manoeuvre," SpaceX Vice President Hans Koenigsmann told reporters during a pre-launch press conference.

40 per cent chance of success

Overall, the company considers the test has less than a 40 percent chance of success. There was no immediate word on whether the test was successful.

Eventually, SpaceX hopes to fly its Falcon rockets back to land for refurbishment and reuse.

The cargo run is the third by Space Exploration Technologies, or SpaceX as the company is known, under a 12-flight, $1.6 billion contract with NASA. The U.S. space agency relies on SpaceX and a second firm, Orbital Sciences Corp. to fly supplies to the orbital outpost since the space shuttles were retired in 2011.

NASA also is planning turn over crew transport from Russia to private industry by 2017.

SpaceX had planned to fly last month, but delayed the mission to review a potential contamination issue with its rocket. The issue was resolved, but then an Air Force radar system, needed to track the vehicle during flight, was damaged, sidelining all launches from Cape Canaveral for two weeks.

Another launch attempt on Monday was called off after a valve leak was found in a part of the system that separates the Falcon 9's first and second stages. The rocket was removed from the launch pad and repaired.

On Friday, the only issue was the weather, but the rain and thunderstorms that clobbered Central Florida on Friday cleared in time for the Falcon 9 to lift off at the precise moment when Earth's rotation aligned its launch pad into the plane of the space station's orbit.

"Mother Nature is providing a window of opportunity today," NASA mission commentator Michael Curie said shortly before launch.

Originally, astronauts on the ISS were waiting for supplies from Dragon to use for an urgent repair to the space station's backup computer. But NASA decided late this week to use a gasket-like material already on board the space station for the repair instead. The critical backup computer failed outside the space station last Friday. Astronauts will undertake a spacewalk next Wednesday to fix it. The primary computer is working fine, but numerous systems would be seriously compromised if it broke, too. 

With files from the Associated Press