SpaceX has identified why its Starship super heavy rocket exploded during a test flight last month.

On 16 January, Starship lifted off from a launchpad in Texas and all 33 of its Raptor engines started up successfully. But after several minutes of flight time the upper stage dramatically broke up and disintegrated over Caribbean airspace, forcing several planned flights to alter their course to avoid falling debris.

SpaceX said the “most probable root cause” for the failure was identified as a harmonic response several times stronger in flight than had been seen during testing, which led to increased stress on hardware in the propulsion system. 

“The subsequent propellant leaks exceeded the venting capability of the ship’s attic area and resulted in sustained fires,” it said in a blog post.

The debris resulting from the explosion fell within the pre-planned area. The firm said there were no hazardous materials present in the debris and no significant impacts were expected to occur to marine species or water quality.

“SpaceX reached out immediately to the government of Turks and Caicos and worked with them and the United Kingdom to coordinate recovery and clean-up efforts,” SpaceX said.

However, residents of the affected islands complained they had to deal with a considerable amount of debris that had landed near homes and businesses. People living on the archipelago, which comprises around 40 islands and has a population of roughly 50,000, reported finding fallen wires and tiles after the incident. 

Many locals argued that SpaceX should have been liable to clean up the debris. But in a meeting between the firm and representatives from the island, it simply said that residents should wear gloves while picking up the material.

“To address flammability potential in the attic section on Starship, additional vents and a new purge system utilising gaseous nitrogen are being added to the current generation of ships to make the area more robust to propellant leakage,” SpaceX said. “Future upgrades to Starship will introduce the Raptor 3 engine, reducing the attic volume and eliminating the majority of joints that can leak into this volume.”

The firm follows a controversial “rapid iterative development” process, which effectively sees it carrying out live tests of prototype rockets. 

“Starship’s seventh flight test was a reminder that developmental progress is not always linear, and putting flight hardware in a flight environment is the fastest way to demonstrate how thousands of distinct parts come together to reach space,” it added.

In October, SpaceX successfully caught the Starship rocket booster for the first time as it returned to Earth after launch. As the booster returned to its launch mount, a pair of mechanical arms were deployed to reach out from the launch tower and bring it to a halt.