A shock wave is a nearly discontinuous change in pressure, density, and temperature that forms when a flow exceeds the local speed of sound (Mach number > 1). Unlike sound waves, which propagate as small, reversible pressure perturbations, shocks involve large, irreversible compressions — the flow loses energy to heat across the shock, and this energy loss manifests as wave drag.
Types
Normal shock — perpendicular to the flow direction. The flow downstream of a normal shock is always subsonic. Normal shocks produce the strongest compression and the greatest energy loss for a given upstream Mach number.
Oblique shock — angled to the flow. The downstream flow may remain supersonic. Oblique shocks are weaker than normal shocks and produce less drag. Pointed noses and sharp leading edges generate oblique shocks; blunt bodies generate detached bow shocks that are locally normal at the stagnation point.
Bow shock — the curved, detached shock that stands ahead of a blunt body in supersonic flow. The shock is normal at the centerline and becomes oblique further from the axis. Bow shocks produce intense heating at the stagnation point — the primary thermal design driver for reentry vehicles.
Expansion fan — not a shock, but the complementary phenomenon: a smooth, isentropic acceleration of supersonic flow around a convex corner. No energy is lost.
Why shocks matter
In aeronautics, shocks cause wave drag — a drag component that does not exist below M = 1 and that roughly doubles total drag in the transonic regime. Swept wings, area ruling (the Sears-Haack body), and supercritical airfoils are all strategies for managing shock formation on aircraft.
In rocketry, shocks define the thermal environment during ascent and reentry. A reentering capsule at M = 25 produces a bow shock that heats the surrounding air to 7,000–10,000 K — hot enough to ionize nitrogen and oxygen. The thermal protection system must absorb or reject this heat without structural failure. Ablative heat shields (used by Apollo, Stardust, and Orion) work by allowing the surface material to char and sublimate, carrying heat away from the structure.
Related terms
- Mach Number — the parameter governing whether shocks can form
- Dynamic Pressure — the aerodynamic loading that increases through the transonic regime
- Drag — wave drag from shocks is the dominant drag source above M = 1