Module drag 

Drag: the terms of Niskanen’s zero-lift drag buildup and the angle-of-attack scaling of axial drag, as functions of their inputs. crate::AeroModel::drag sums them over a rocket.

The zero-lift drag coefficient on the reference area is (Niskanen 2009 eq. 3.75, 3.97) C_D0 = C_D,friction + Σ (A_T/A_ref)(C_D•)_T, each pressure, base and parasitic term T taken on its own area A_T:

• Skin friction (skin_friction_coefficient): a fully turbulent boundary layer (Niskanen §3.4.1) with the Reynolds number R = V L/ν on the rocket’s length, limited by roughness (eq. 3.78–3.81) and corrected for compressibility (eq. 3.82–3.84). Wetted areas are weighted by the body form factor 1 + 1/(2 f_B) and the fin thickness factor 1 + 2t/c̄ (eq. 3.85).
• Body pressure drag: noses, shoulders and steps up in radius from 0.8 sin² φ at rest (eq. 3.86) through Mach 1 to appendix B’s wave drag (crate::nose_drag); boattails by the boattail rule (eq. 3.88, boattail_factor) to Mach 0.8 and their supersonic wave drag from Mach 1 (crate::afterbody); a lip in a boattail’s wake loses a share of its own.
• Base drag (base_drag_coefficient, eq. 3.94) on the aft base, less the thrusting motors’ area, relieved behind a boattail faster than sound (crate::afterbody).
• Fin pressure drag (fin_pressure_drag_coefficient, eq. 3.89–3.93) on the fins’ frontal area N t s.
• Parasitic drag of launch lugs and rail buttons (launch_lug_drag, eq. 3.95–3.96, and Niskanen’s rail-pin rule, p. 52).
• Angle of attack (axial_drag_alpha_factor, §3.4.7): C_A = C_D0 f(α).

Interference drag and fin-tip vortices are neglected, as in Niskanen p. 41.

Every term has its transonic and supersonic branch, and the buildup covers Mach 0 to 5 (BUILDUP_MACH_LIMIT).

See docs/physics/aero.md and the decision records on subsonic drag and drag override tables, ADR-009, on drag through Mach 1, ADR-028, and on the afterbody faster than sound, ADR-030.

Structs

BaseBehindBoattail

The aft base behind a boattail: its drag coefficient is scaled by 1 − Σ w (1 − k) over its sources, each a boattail the base still takes relief from, with k that boattail’s base-pressure ratio (Boattail::base_pressure_ratio) and w its share of the flow behind the boattails; the shares add up to at most 1.

BoattailTerm

A narrowing transition’s pressure drag: as a boattail of its own, or, by merge weights, as its share of the boattails it may continue.

ComponentDrag

One component’s share of the drag buildup, at zero lift.

ComponentDragTerms

A component’s precomputed drag terms, built by crate::AeroModel::new. Areas are divided by the reference area.

Drag

A rocket’s drag, or one component’s share of it, at a flow condition. Coefficients are on the reference area.

DragConditions

What the drag buildup needs beyond the crate::Flow.

FinPressureTerms

A fin set’s pressure-drag inputs.

MergedBoattail

A narrowing transition’s share of a boattail it continues (BoattailTerm).

PressureDragTerm

A nose’s, shoulder’s or step’s pressure drag: its coefficient against Mach number, on an area.

ReliefSource

A boattail the aft base takes relief from (BaseBehindBoattail).

WakeTerm

A lip in a boattail’s wake: its step up’s pressure drag is scaled by 1 − step_fraction and its shoulder’s by 1 − shoulder_fraction.

Constants

BUILDUP_MACH_LIMIT

The top of the buildup’s range, which it doesn’t reach: Mach 5, where the hypersonic region begins (Niskanen 2009 Table 3.1, p. 19), as for the normal force. Niskanen expects the simulation “to be reasonably accurate to at least Mach 1.5” (p. 94); how far it holds against measurements is in docs/physics/aero.md.

LOW_REYNOLDS

Reynolds number below which the friction formulas no longer hold and the coefficient is held at its value there (Niskanen 2009 p. 44).

LOW_REYNOLDS_FRICTION

The skin-friction coefficient below LOW_REYNOLDS (Niskanen 2009 eq. 3.81).

MERGE_FULL_TURN_RAD

Two narrowing parts whose half-angles differ by up to this merge wholly (BoattailTerm), a judgement for a curved boattail drawn in parts: 3°.

MERGE_MIN_ANGLE_RAD

Below this half-angle a narrowing part is partly a tube: it merges with a boattail, and a later part with it, by the smaller of the two angles over the larger, the larger taken as at most this, a judgement: 1°.

MERGE_NONE_TURN_RAD

Two narrowing parts whose half-angles differ by this or more don’t merge (BoattailTerm), a corner: 10°.

MOTOR_POD_SETS

The most pod sets holding motor mounts that a rocket’s drag tells apart (DragConditions::thrusting_pod_motor_areas_m2); a layout with more is refused. A fixed number keeps the conditions a plain value, built at every step of a flight without allocating.

SUBSONIC_MACH_LIMIT

The top of the subsonic region, Mach 0.8 (Niskanen 2009 Table 3.1, p. 19), where Niskanen’s semi-empirical transonic method starts (p. 47): the lower bound M_L of a step’s and a blunt face’s transonic method (crate::nose_drag::PressureDragCurve::step).

WAKE_FULL_RISE

A lip behind a boattail rising up to this share of the boattail’s drop in diameter is wholly in its wake (WakeTerm).

WAKE_NONE_RISE

A lip behind a boattail rising this share of the boattail’s drop in diameter or more is not in its wake (WakeTerm).

Functions

axial_drag_alpha_factor

The scaling of axial drag with angle of attack, C_A(α) = C_D0 f(α) (Niskanen 2009 §3.4.7), with C_A positive along −z_B (toward the tail).

base_drag_coefficient

Base drag (C_D•)_base on the base area: 0.12 + 0.13 M² below Mach 1 and 0.25/M from Mach 1, continuous at 0.25 (Niskanen 2009 eq. 3.94, after Fleeman).

boattail_factor

The boattail rule’s share of base drag (Niskanen 2009 eq. 3.88), from the length ratio γ = l/(d₁ − d₂): 1 for γ ≤ 1, (3 − γ)/2 between 1 and 3, and 0 from 3. A boattail’s pressure drag is this factor times the base drag coefficient on the boattail’s decrease in area, so a zero-length boattail drags like the base it uncovers.

body_friction_form_factor

Body friction form factor 1 + 1/(2 f_B) for a body of fineness ratio f_B = body length over maximum body diameter (Niskanen 2009 eq. 3.85; Barrowman 1967 eq. 4-16).

critical_reynolds

The roughness-limited critical Reynolds number R_crit = 51 (R_s/L)^−1.039 (Niskanen 2009 eq. 3.79; Barrowman 1967 eq. 4-7), infinite for a perfectly smooth surface.

fin_friction_thickness_factor

Fin friction thickness factor 1 + 2t/c̄, with t the fin thickness and c̄ the mean aerodynamic chord (Niskanen 2009 eq. 3.85).

fin_pressure_drag_coefficient

Pressure drag of a fin set on its frontal area N t s (Niskanen 2009 eq. 3.89–3.93): the leading edge’s (C_D•)_LE⊥ cos² Γ_L plus the trailing edge’s share of base drag.

incompressible_skin_friction

Incompressible skin-friction coefficient of a fully turbulent boundary layer at Reynolds number reynolds on a surface of relative roughness R_s/L (Niskanen 2009 eq. 3.81):

joint_pressure_drag_coefficient

Pressure drag at rest of a nose cone or shoulder, (C_D•)_p,0 = 0.8 sin² φ on its frontal area (a nose’s base area, or a shoulder’s increase in area), with φ the joint angle between the surface and the body axis at the aft joint (Niskanen 2009 eq. 3.86, after NAVWEPS 1488 p. 237). A smooth joint (φ = 0) has none; a bare step (φ = π/2) has 0.8.

launch_lug_drag

Parasitic drag of a launch lug (Niskanen 2009 eq. 3.95–3.96): the coefficient max{1.3 − 0.3 l/d, 1} (C_D•)_stag on the area π r_ext² − π r_int² max{1 − l/d, 0}, returned as (coefficient, area_m2).

rail_button_drag_coefficient

Parasitic drag coefficient of a rail button on its frontal area (the side profile of its base, waist and flange): Niskanen’s rail-pin rule (2009 p. 52), a pin drags like a solid launch lug as long as its diameter, (C_D•)_stag (Hoerner p. 5-8 gives 0.80 for a pin on a wall).

skin_friction_coefficient

Skin-friction coefficient with compressibility (Niskanen 2009 eq. 3.82–3.84; Barrowman 1967 eq. 4-12, 4-13):

stagnation_drag_coefficient

Pressure drag of a blunt circular cylinder face, (C_D•)_stag = 0.85 q_stag/q on its frontal area (Niskanen 2009 eq. B.2).

stagnation_pressure_ratio

Stagnation-pressure ratio q_stag/q (Niskanen 2009 eq. B.1, after Hoerner pp. 15-2, 16-3): 1 + M²/4 + M⁴/40 below Mach 1 and 1.84 − 0.76/M² + 0.166/M⁴ + 0.035/M⁶ from Mach 1 (1.275 and 1.281 at M = 1).