Drag Functions

Standard curves

lob ships six standard drag curves G1, G2, G5, G6, G7, G8 sampled at 87 Mach points from 0 to 5.0 (source/tables.hpp). The tables are BRL data as published by JBM Ballistics. Each curve is stored as kG*Drags alongside the shared Mach grid kMachs and is projected once at compile time onto a compact spline (Cubic Hermite Splines).

The ballistic coefficient scales the standard curve. With atmosphere reference kArmyStandardMetro the BC is converted by kArmyToIcaoBcConversionFactor = 0.0751265/0.0764742 ≈ 0.982 (source/constants.hpp) so the spline always represents the ICAO standard. The context stores drags[60] = coefs / (BC·conversion) (source/lob_builder.cpp).

A BC ≤ 0 or missing BC is rejected at build time.

Custom tables

LobBuilderSplineFitTable / Builder::MachVsDragTable accepts a user-supplied Mach vs Cd table. Requirements (source/lob_builder.cpp):

size ≥ 2, Machs strictly increasing, Cd finite and ≥ 0.

The table is spline-fit onto the production knots (see Cubic Hermite Splines). If the table does not span Mach 0–5 the spline is cubically extrapolated (PCHIP Hermite using the outer interval's tangents); extrapolation that would yield negative Cd at 0 or 5 is rejected with kLobErrorMachDragTableInvalid — pad with explicit entries at 0 and/or 5 to define the edge behavior. Custom tables set the effective BC to 1 in the context (source/lob_builder.cpp).

BC/velocity bands

LobBuilderBCVelocityBands / Builder::BCVelocityBands tailors the standard curve with BCs measured at several velocities (fps). See BC/Velocity-Band Transformation for the transformation.

The drag curve in the solver

At each step SolveStep evaluates Cd = curve.Eval(Mach) · drag_coeff (source/solve_step.cpp) where Mach = |v| / speed_of_sound and curve is a CurveView over the context's 60 coefficients. CurveView::Eval clamps Mach outside 0–5 to the edge value; the integration never evaluates beyond the checked domain.

The precomputed G* coefficients are constexpr (source/splines.hpp) so they cost no runtime construction. Custom/BC-band curves are built at Build() time via the same spline machinery.