Grid fins and landing gear responsible for Falcon-9 booster’s unique sonic boom
Grid fins and landing gear responsible for Falcon-9 booster’s unique sonic boom lead image
When most flight vehicles become supersonic, the resulting sonic boom typically consists of a characteristic N-shaped wave with two shocks. When the SpaceX Falcon-9 rocket booster falls back to Earth after launch, however, its far-field sonic boom typically includes three shocks. Anderson and Gee combine different observations to point to why.
Researchers have combined sonic boom theory with nonlinear propagation modeling, computational fluid dynamics, and photographic evidence to describe the mechanism by which the Falcon-9 rocket boosters produce their unusual three-part sonic boom. Combining these modalities allowed them to find the central role of the booster’s fins and landing legs.
Owing to their unique boom signature while showing little to no evolution in the far field, Falcon-9 boosters provide a way to test the limits of current sonic boom models.
“Rocket booster flyback events are a relatively new phenomenon and are going to become increasingly common,” said author Mark Anderson. “Most sonic boom models have been validated for air-breathing aircraft in horizontal flight, whereas these booster flyback booms are different in almost every way.”
The researchers found that the extra middle shock is caused by a forward-migrating compression wave, related to the booster’s grid fins, merging with rearward-migrating rarefaction wave caused by lower elements of the booster, including its landing legs. The first shock comes from the base of the rocket and the final shock comes from the top of the booster.
Next, they plan to simulate Falcon-9 booster geometries passing through three-dimensional turbulence and to extend their findings to SpaceX’s Starship Super Heavy booster, which also has been observed to have a triple-shock boom.
Source: “Why does the Falcon-9 booster make a triple sonic boom during flyback? An initial analysis,” by Mark C. Anderson and Kent L. Gee, Journal of the Acoustical Society of America (2025). The article can be accessed at https://doi.org/10.1063/10.0035649 .
