Physical Constants

Physical Constants

Source of truth is source/constants.hpp and source/eng_units.hpp (convert::). These values are constexpr and feed directly into the builder, atmosphere, unit helpers, and solver. They are documented here rather than scattered through the source so a reader can audit the physical model without reading every file.

ISA / atmosphere

source/constants.hpp:

kPi                               = 3.14159265358979323846
kStandardGravityFtPerSecSq        = 32.17405  ft/s²  (also kLbsPerSlug)
kDegreesPerTurn                   = 360
kSeaLevelDegF                     = 59.0 °F
kSeaLevelHumidityPercent          = 0.0 %
kSeaLevelPressureInHg             = 29.92 inHg
kSeaLevelAirDensityLbsPerCuFt     = 0.0764742 lb/ft³
kSeaLevelSpeedOfSoundFps          = 1116.45 fps
kLapseDegFPerFt                   = 0.00356616 °F/ft
kTropopauseAltitudeFt             = 36 090 ft
kStratosphereAltitudeFt           = 65 617 ft
kMinimumTempDegF                  = -69.7 °F
kGasConstantAir                   = 1716.46 ft·lbf/(slug·°R)
kHydrostaticExponent              = g/(R·L)−1 ≈ 4.2562
kBarometricExponent               = g/(R·L)   ≈ 5.2562
kArmySeaLevelAirDensityLbsPerCuFt = 0.0751265 lb/ft³
kArmyToIcaoBcConversionFactor     = 0.0751265/0.0764742 ≈ 0.982
kAngularVelocityOfEarthRadPerSec  = 7.292115e-5 rad/s

kArmyToIcaoBcConversionFactor scales G* splines to ICAO; the BC setters respect BCAtmosphere (source/lob_builder.cpp). BarometricFormula and Lapse T(h)=max(T0−L·h, T_min) are in source/calc.hpp (McCoy p. 166); tropopause then isothermal exponential above 36 090 ft.

Units

source/eng_units.hpp convert:::

Angle: kRadiansPerDegree=π/180, kMoaPerDegree=60, kMoaPerRadian=60/kRadiansPerDegree,
       kMilPerRadian=1000, kMoaPerMil, kIphyPerMoa=1.047
Length: kInchPerFoot=12, kFeetPerYard=3, kMeterPerFoot=0.3048,
        kMmPerFoot=304.8, kCmPerFoot=30.48
Pressure: kInHgPerPa=0.000295…, kInHgPerPsi=2.036…, kInHgPerMillibar=0.02953…
Mass: kGrainsPerLb=7000, kLbsPerSlug=32.17405, kLbsPerKg=2.204623
Sectional density: kLbsmPerSqInPerKgPerSqM=703.069…
Speed: kFpsPerMph=1.4666…, kFpsPerKph=0.9113…, kFpsPerKn=1.6878…
Time: kMsecPerSec=1e3, kUsecPerSec=1e6
Temp: kDegFPerDegC=1.8, kFreezePointDegF=32, kAbsoluteZeroDegF=-459.67

All StrongT cross-category conversions are explicit operator Other via these constants; helpers like LobMoaToInch(moa, range_ft) multiply by them exactly (source/lob_convert.cpp, include/lob/lob.h).

Formulas

CalculateCdCoefficient(ρ, BC) = ρ·π / (8·BC·144)   (144 = in²/ft²)  `source/calc.hpp`
CalculateKineticEnergy(v,m)   = m·v²/2            `source/calc.hpp`
CalculateProjectileReferenceArea(D) = π·D²/4
Miller sg = ftp· 30·mass / (twistRatio²·D³·L/D·(1+(L/D)²)) · cbrt(v/2800) · sign(twist)
  with ftp = ρ0/ρ  (`source/calc.hpp` `CalculateMillerTwistRuleCorrectionFactor`)
SpeedOfSound(T) = 49.0223·√T_R  + humidity `1+0.0014·h·p_sat/p0` (`source/calc.hpp` `CalculateSpeedOfSoundInAir`)
Air density ratio = (p/p0)·(T0_R/T_R)  and `ρ_corr = 1−0.00378·h·p_sat/p0` (McCoy p. 167)

Coriolis terms are 2Ω·cos(lat)·sin(az) etc. in BuildCoriolis (source/lob_builder.cpp), DsDx in source/solve_step.cpp.