Mach number and the four flow regimes, how compressibility changes aerodynamic behavior, shock wave basics, and why high-speed aircraft need fundamentally different design assumptions than low-speed aircraft.
Module 2's lift and drag relationships assumed air density stays effectively constant as it flows around an aircraft — an assumption that breaks down as an aircraft goes faster. This module covers Mach number and the subsonic, transonic, supersonic, and hypersonic flow regimes it defines, how critical Mach number and drag divergence change aerodynamic behavior well before an aircraft reaches the speed of sound, and the shock wave physics — normal shocks, oblique shocks, and expansion fans — that govern flow above Mach 1.
By the end of this module you should be able to explain why a supersonic jet engine inlet uses a staged series of oblique shocks rather than one abrupt normal shock, and why hypersonic vehicles need dedicated thermal protection systems that a supersonic aircraft does not — the propulsion and materials consequences Module 6 and Module 12 each pick up from here.