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SpaceX’s huge Super Heavy-Starship rocket, by far the most powerful ever built, was primed for launch on its third test flight Thursday, an attempt to boost the unpiloted upper stage into space for a sub-orbital hop to a controlled re-entry and splashdown in the Indian Ocean.

Liftoff from SpaceX’s Boca Chica, Texas, flight test facility on the Gulf Coast was planned for 8 a.m. EDT, kicking off an eight-and-a-half-minute climb to space.

The flight follows formal clearance from the Federal Aviation Administration, issued late Wednesday, that concluded “SpaceX met all safety, environmental, policy and financial responsibility requirements.”

Problems with previous test flights

Two previous test flights ended with spectacular self-destruct conflagrations — the first, last April, after multiple Super Heavy engine shutdowns and a stage separation malfunction and the second, in November, just before the Starship would have started a planned loop around the planet for a splashdown in the Pacific Ocean north of Hawaii.

SpaceX engineers modified multiple systems in the wake of the failures, including work to beef up the rocket’s self-destruct system, to improve engine performance and to protect the pad with a high-power water deluge system that also deadens the acoustic shock of engine ignition.

The company also implemented a “hot staging” technique in which the Starship’s six Raptor engines ignite while the stage is still attached to the Super Heavy booster. Hot staging, used for decades by Russian Soyuz rockets, helps ensure a more efficient stage-separation sequence.

“Starship’s second flight test achieved a number of major milestones and provided invaluable data to continue rapidly developing Starship,” SpaceX said on its website. “This rapid iterative development approach has been the basis for all of SpaceX’s major innovative advancements.”

Goals for third Super Heavy-Starship test flight

For its third test flight, the primary goals were more ambitious, to boost the Starship into space for a sub-orbital flight featuring several first-time tests and to carry out controlled landings by both stages, the Super Heavy in the Gulf of Mexico and the Starship in the Indian Ocean.

While both stages are designed to be fully reusable, there were no recovery plans for the third test flight. Both stages were expected to attempt rocket-powered descents mimicking actual landing procedures but both were expected to break up and sink on ocean impact.

During the Starship’s coast toward entry, flight controllers planned to test of payload door that will be used on future flights to launch Starlink satellites.

More important to NASA, the rocket was to attempt moving cryogenic propellants from one tank to another in the weightless environment of space and to carry out the first restart of a Raptor engine outside the atmosphere.

The propellant transfer test and Raptor restart are critical milestones for NASA, which is paying SpaceX billions to build a Starship variant to serve as the Human Landing System, or HLS, for the agency’s Artemis moon program.

The HLS will require automated refueling in Earth orbit by multiple Super Heavy-Starship tankers before restarting its engines to head for the moon to await the arrival of astronauts who will use the spacecraft to carry them to and from the surface.

Up to 10 or so refueling flights will be necessary for one HLS flight to the moon.