Why rocket ascent sonic booms are not N-waves

Rockets launched to orbit generate sonic booms downrange as the Mach cone, influenced by vehicle pitch and increasing speed, intersects the ground. Unlike the classic N-wave signature of supersonic aircraft, rocket ascent sonic booms in the focus region often resemble blast waves—a phenomenon observed during Apollo, Space Shuttle, and more recently, Falcon 9 launches from Vandenberg Space Force Base. At the altitudes where these booms originate, the near-field shock structure is dominated by the large, underexpanded exhaust plume. The shock ahead of this plume overtakes other body-generated shocks, but the plume itself lacks a distinct termination. Instead, it expands to a maximum diameter, then gradually contracts and dissipates. This elongated, curved plume creates a rarefaction wave that does not form a trailing shock, even after 100 km of propagation. The result is a boom with a sharp leading shock followed by a rarefaction and slow return to ambient pressure—similar to a Friedlander-type waveform. Several examples are presented. [Work supported by Vandenberg Space Force Base SLD-30.]