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.