The single most critical piece of technology in any spacefaring civilization's vast tool kit is the rocket engine. Without rocket propulsion, most sophonts could not even get to the upper atmosphere of their homeworlds, let alone ascend to orbit and beyond. Thus, the rocket engine -modernly referred to more broadly as a "reaction engine," "drive system," or simply "drive"- is one of the unifying elements of all civilizations at or above technological level 8. There is a very broad array of rocket propulsion methods, and no two sophonts will follow the exact same techno-developmental pathway, but almost every known civilization utilizes the same fundamental principles in designing their drive systems. The exception to this is the highly advanced and reclusive Xib Zjhar, who have somehow achieved the physics-defying holy grail of space travel: reactionless thrust. All other known drives are classified as reaction engines, as discussed below.