SpaceX has been developing its next-generation Starship rocket for some time now, but the large-scale prototypes it’s building in Boca Chica, Texas, have thus far always encountered a fatal error during an important part of testing called “cryo” – or filling the fuel tank to full pressure in conditions that simulate the vacuum of space. The latest prototype, called ‘SN4’ for ‘serial number 4,’ has finally passed this test however, and that clears the way for an engine fire test followed by a short flight.

SpaceX’s SN4 prototype resembles what its final rocket will look like, unlike the Starhopper sub-scale demonstrator that the company originally flew just to show off what its new Raptor engine could accomplish. The SN4, like the Starhopper, is equipped with a single Raptor engine, which will make it possible for the vehicle to make short flights for testing purposes. The next version, SN5, will have three raptor engines according to SpaceX CEO and founder Elon Musk, which is still less than the six that the full, functional version of Starship is intended to have, but that will allow it to perform longer test flights in preparation for an orbital launch demonstration.

Testing and developing a new rocket and launch system is always going to have hiccups, since all the simulation in the world can’t replicate real-world use conditions and physics. But Starship’s prior failures at the cry testing phase were beginning to look like they could be a more fundamental problem – it’s what laid low SN1 through SN3, after all.

SpaceX will now perform a static test fire of the Raptor engine installed on the prototype, which could happen as soon as sometime later this week, and then the development craft will look to do a flight of around 150 meters (around 500 feet), which is the same height as the Starhopper performed. That’s nowhere near as high as it’ll need to go to fly orbital missions, of course, but it’s a test that will show how a full-scale vehicle performs at low-altitude, which is key info that SpaceX needs before developing its high-altitude and orbital prototypes.