Why Did Starship SN4 Explode?

On Monday, SpaceX's prototype Starship SN4 exploded in a spectacular fireball that destroyed both it and the test stand it was mounted on. This happened about a minute after a successful static firing of its Raptor engine.

According to Teslarati.com, the cause of the explosion was likely related to the umbilical that connects the rocket to the liquid methane supply. It may have failed to reconnect properly after the static fire, releasing a cloud of explosive methane gas at the base of the rocket. 

Shortly after a post-launch briefing celebrating and discussing SpaceX’s inaugural astronaut launch on May 30th, Reuters reporter Joey Roulette was able to ask Musk about Starship SN4’s spectacular demise the day prior. The SpaceX CEO was quoted saying that “what we thought was going to be a minor test of a quick disconnect ended up being a big problem”, confirming suspicions based on careful analysis of public views of the explosion that it was caused by issues with Starship’s ground support equipment (GSE).

 In Musk’s statement, “quick disconnect” (QD) refers to an umbilical port that connects a launch vehicle to GSE, enabling the loading and offloading of propellant and fluids, clamping down the rocket, and providing a wired telemetry and communications link for ground controllers. QDs must perform all those tasks while also being able to rapidly release and disconnect, allowing the rocket to lift off while still protecting its sensitive ports for ease of reuse.

Given that SpaceX already has one Starship prototype close to fully built and two others under construction, this likely won't hold up their test program for very long.