The Science of Kerbal Space Program

Atmospheres and Aerodynamics

In Kerbal

Atmospheric physics are a little bit of a joke. For example, the total drag coefficient of a craft is defined by the mass based average of the drag coefficients of its parts. This leads to some unusual results. For example, position of parts is not accounted for.

The consequence of this is that while an offset drag force may cause rotation of the vehicle, the drag is going to be the same whether a part is horizontal or vertical.

This is slightly more complicated for a wing, where the drag changes by the equation

$d_{wing}=d*sinφ$

Lift is even worse. Looking through source code on the community's part eventually yielded the equation for lift

$F_L=v×k*Cos(φ)*(1-|cos(φ)|)-*Cos(θ)*C*P$

where v is the velocity, k is the cross product of the forward and vertical vectors of a wing, φ is the angle between velocity and the upward vector of a wing, θ is the angle between the velocity vector and the forward vector of the wing, P is air pressure and C is a constant defined by the part (which includes the shape and the size of a part). Because C contains various unknowns, its validity cannot be properly evaluated.

Reality

Simplification is very useful however when one wants to avoid dealing with drag that varies with speed.





The Reynolds number is given by the equation

$$R_e=vD/ν$$
where D is a required linear dimension and ν (nu) is viscosity of the fluid over the density. This means that as a body moves faster and faster, the coefficient of drag goes down, but at a Reynolds number on the order of E5 to E6, the coefficient of drag becomes unstable, dropping and increasing rapidly. This is part of the reason flight at high velocities is difficult and requires special construction.