Suppose you have an aircraft flying at extremely high altitude, such that air is very thin. The aircraft’s air intake geometry can be adjusted to handle any arbitrary intake velocity. How fast could it fly?
Obviously, the speed limit is reached when the drag experienced by the aircraft equals to the thrust supplied by the engine. However, if you are drag limited because there is too much air, you send the aircraft even higher. Where does it end?
Let’s assume the aircraft has unlimited jet fuel or uses a nuclear engine project pluto style. Would the ultimate speed limit be near orbital velocity?
IANA propulsion engineer. But from what I’ve read …
For something far-fetched like a Project Pluto engine, as long as the engine adds heat and creates expansion in the working fluid (atmosphere passing through the engine) it’ll create thrust. When the thrust is no longer enough to offset drag, you’ve reached your speed / altitude limit. Naturally the shorter the dwell time of the air passing through the heater section or the les dense it is, the less heat can be imparted. So your engine gets longer & hence heavier to produce a given thrust at higher speed and/or higher altitude.
I don’t know of any physics-imposed “speed limit” or “attitude limit” beyond that. Since we can’t really build one with present day tech, we don’t have hard engineering answers to where the limit of the state of the art is. Somebody closer to the industry could hazard a better guess than I could though.
As to scramjets combusting a hydrocarbon fuel, I’ve read about speeds up to about Mach 8 and altitudes of about 250,000 feet as theoretical limits given foreseeable, but not yet available, tech. Sorry no cite.