I was confused by a NASA pdf document about flight physics. I provide quote from NASA document instructor's flight manual, flight testing Newton's laws. Document is categorized DFRC-X41-1. Did NASA use some fast-and-loose language right there? Imprecise definition of drag using newton third law? When thrust is more than drag, then acceleration occurs. Correct or not? When thrust and drag are equal, then aircraft remains at constant speed, unchancing speed. Drag exists because aircraft pushes against airmass and airmass pushes against aircraft. Correct or not? No drag exists in outer space. Rocket in vacuum has thrust, surely rocket can accelerate in outer space. Correct or not? In outer space vacuum, rocket has thrust. Newton third law states that fuel gases push upon rocket body. Rocket body push upon fuel gases. Basically. No mention of drag and supposed relationship between thrust exists in this case of outer space vacuum rocket. In practical terms outer space vacuum contains hydrogen atoms (?) so it's not a pure vacuum. But surely there must be some sensible reason behind this NASA logic? Can you guys help me understand newton third law with respect to the normal aircraft in atmosphere, and also vacuum rocket with thrust. I was also wondering about another unrelated matter about flight physics, related to the compressibility phenomenon with certain WW2 aircraft. One such aircraft was P-38 Lightning. I was wondering what exactly prompted the flight surfaces to become so ineffective with this aircraft for example. The true cause was not understood during WW2, except that they installed a stopgap measure of an airbrake, to slow down a diving fast aircraft. I remember watching a history channel show about the compressibility phenomenon of a P-38 aircraft. The documentary noted that the propeller blades became transsonic, which caused some kind of disturbant airflow to the flight surfaces. (such as elevator tabs in the tail) Could the compressibility be explained in somehow more layman's terms? Why it happens at certain speed, as opposed to slower speeds that normally aircraft fly at ? Compressibility tended to happen at high speed dives, during air combat. This made it difficult for a P-38 pilot to pull enough elevator to pull up from a dive. This tended to cause pilot deaths to crashes. Soemtimes the thicker atmosphere at low altitudes allowed regaining of control for elevator tabs.