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Atmospheric drag
Drag against an atmosphere that rotates with its body, from launch through Low Earth Orbit.
Status: Implemented.
Model
a = −½ ρ(h) A (C_pressure + C_skin) |v_rel| v_rel / m| Symbol | Meaning |
|---|---|
| ρ(h) | Atmospheric density at altitude h |
| A | Reference area of the body, in m² |
| C_pressure | Pressure (form) drag coefficient |
| C_skin | Skin-friction drag coefficient |
| v_rel | Velocity of the body relative to the atmosphere |
| m | Current mass of the body |
A rotating atmosphere
The atmosphere turns with its body, so drag is computed from velocity relative to the air and not from inertial velocity:
v_rel = v_body − (v_source + ω_source × r)This matters at launch, where the ground speed of the atmosphere is a large part of the vehicle’s inertial speed.
Density
Density follows a piecewise-exponential profile of 28 layers, from sea level (1.225 kg/m³) to 1,000 km. Each layer has its own base density and scale height, which keeps density realistic at orbital altitudes.
In the application
- The Aerodynamics section of the Inspector shows drag acceleration and force as they act.
- The drag coefficients and the reference area of a body can be edited there.
- Switch Atmosphere off and on to compare a run with and without drag.
Scope
The profile is static: it does not vary with solar or geomagnetic activity. Lift, winds and coefficients that depend on Mach number are not in this release. The model is intended for planning-level decay and launch studies, not re-entry analysis.