SpaceX called off a test flight of its powerful Starship rocket and Super Heavy booster as the countdown clock reached zero Thursday at the company’s spaceport in South Texas.

The launch team at Starbase, Texas, just north of the US-Mexico border, aimed to launch the more than 400-foot-tall rocket at 5:45 pm local time (6:45 pm EDT; 22:45 UTC). The countdown proceeded smoothly throughout the day, culminating in the loading of more than 11.5 million pounds of liquid methane and liquid oxygen into the two-stage rocket.

But the computers controlling the countdown called an abort during the Super Heavy booster’s engine startup sequence. SpaceX scrubbed the launch attempt, and engineers began preparations to drain the rocket’s propellant tanks. Officials did not immediately announce when they plan to try to launch again.

Elon Musk, SpaceX’s founder and CEO, posted on his social media platform X that the company might not be able to launch during the next available opportunity on Friday evening. “Some of the engines didn’t start, triggering an automatic launch abort,” Musk wrote. “Now offloading propellant. Next launch attempt hopefully in a few days.”

Later Thursday evening, Musk added that ground teams at Starbase will replace two of the Raptor engines on the Super Heavy booster. “Most probable launch timing is early next week.”

Tuning the engine

The Super Heavy booster has 33 methane-fueled Raptor engines, each capable of generating more than half a million pounds of thrust. The engines are supposed to ignite in a staggered sequence after activation of the launch pad’s water-cooled flame diverter, designed to protect the launch facility from the intense heat and vibrations during liftoff of the world’s most powerful rocket.

SpaceX officials did not say how many engines failed to start during the ignition sequence, but a graphic of engine status on SpaceX’s live video stream indicated four of the 33 engines never ignited. The engines on this Starship and Super Heavy come from SpaceX’s third-generation Raptor design. This test flight—the 13th full-scale Starship launch—is the second to use the Raptor 3 engine flying on SpaceX’s upgraded Starship Version 3 rocket.