SpaceX is counting down to liftoff Saturday of a Falcon 9 rocket with a commercial cargo craft heading for the International Space Station after engineers replaced a component in an upper stage steering system that grounded the launcher Tuesday.
Launch of the 208-foot-tall Falcon 9 rocket is scheduled for 4:47:07 a.m. EST (0947:07 GMT). The mission has just one second to blast off Saturday or else wait for another opportunity to fly Tuesday.
The tight launch windows are required for SpaceX’s Dragon supply ship to reach the International Space Station with more than 2.5 tons of supplies and experiments.
A launch attempt Tuesday was scrubbed less than 90 seconds before liftoff after SpaceX engineers detected a problem with an actuator in the Falcon 9 rocket’s upper stage steering system.
The actuator was one of two components that gimbals the launcher’s second stage Merlin 1D vacuum engine to steer the rocket on the correct trajectory into orbit.
A NASA official said Friday that the actuator was replaced by SpaceX, which did not provide any information on the fix implemented to clear the Falcon 9 rocket for liftoff Saturday.
SpaceX’s launch team plans to begin fueling the Falcon 9 rocket with kerosene and liquid oxygen propellants about three hours before launch, followed by a series of tests, communications checks and a 10-minute computer-controlled countdown sequence leading to liftoff.
The rocket’s nine Merlin 1D first stage engines will fire in the final seconds of the countdown and build up to 1.3 million pounds of thrust. If the engines pass an automated health check, restraints will open to release the Falcon 9 rocket for liftoff.
The nine main engines, arranged in an “octaweb” pattern on the rocket’s first stage, will power the Falcon 9 on a trajectory northeast from Cape Canaveral. After a nearly three-minute firing, the first stage engines will switch off and the booster will fall away as a single-engine upper stage ignites to propel the Dragon cargo carrier into orbit on a two-day pursuit of the space station.
Moments after stage separation, the Falcon 9 booster will restart a subset of its Merlin engines for a series of three maneuvers to guide the rocket back to Earth.
Burning leftover propellant after its main job is complete, the first stage will descend toward a ship stationed 200 miles northeast of Cape Canaveral.
Fitted with thrusters to maintain its position within a narrow corridor, the landing vessel measures 300 feet long with foldable wings to reach a maximum width of 170 feet.
The rocket stage’s final landing burn, using one of the nine Merlin 1D booster engines, will occur as four landing legs extend for a soft vertical touchdown.
Saturday’s launch will be the first time such a daring maneuver has ever been tried, and SpaceX founder and CEO Elon Musk put the probability of achieving the feat at about 50 percent. SpaceX says it is a big step in making the Falcon 9 rocket reusable, an objective long-pursued by Musk, who says it would dramatically reduce the costs of space launches.
Around the same time as the rocket’s planned barge landing — about nine minutes after liftoff — the Falcon 9’s upper stage will shut down after reaching Earth orbit. Deployment of the Dragon capsule is scheduled around 10 minutes into the mission.
The Dragon spaceship will fine-tune its approach to the space station over the weekend, with capture by the space station’s robotic arm set for Monday.
Follow Stephen Clark on Twitter: @StephenClark1.