At more modest altitudes, flying faster compresses the air at the front of the engine, and this greatly heats the air. The upper limit is usually thought to be about Mach 5–8, as above about Mach 5.5, the atmospheric nitrogen tends to react due to the high temperatures at the inlet and this consumes significant energy. The exception to this is scramjets which may be able to achieve about Mach 15 or more, as they avoid slowing the air, and rockets again have no particular speed limit.
At more modest altitudes flying faster compresses the air at the front of the engine and this greatly heats the air The upper limit is usually thought to be about Mach 5–8 as above about Mach 55 the atmospheric nitrogen tends to react due to the high temperatures at the inlet and this consumes significant energy The exception to this is scramjets which may be able to achieve about Mach 15 or more as they avoid slowing the air and rockets again have no particular speed limit