HL-20 Lifting Body — Architecture Reference

Subsonic flight simulation of the NASA HL-20 lifting body: 6-DOF rigid-body dynamics, tabulated aerodynamics, 1976 Standard Atmosphere, control surface mixer, and SAS flight control laws. The closed-loop system reproduces the aft pitch-stick pulse from NASA TM-107580 Appendix F.

Reference documents

File	Content
hl20_spec.md	Condensed engineering spec. Equations, parameters, control laws, mixer logic. Primary reference.
prompt.md	Build-task statement (objective, files to create, constraints).
scanned_pages/	TM-107580 block diagrams (CSMixer, pitch/yaw control laws) and Appendix F check-case plots.
nasa_appendixF_case0_man1.csv	Digitized NASA reference time history for scenario 5 (informational).

Architecture decisions

Scalar ports

All inter-component signals are individual RealInput() / RealOutput() ports. Vectors are separate named scalars (e.g. vel_1, vel_2, vel_3). No custom connectors or array ports.

Frame convention

NED inertial frame (1=North, 2=East, 3=Down). Altitude = −pos_3. Quaternion is scalar-first (qw, qx, qy, qz), body→inertial. SI units for dynamics (m, kg, N, rad/s); aero interfaces use degrees and feet. All internal computations are SI; imperial conversions (psf, fps, degrees) happen only at aero-model boundaries and controller output interfaces.

Pre-provided aero infrastructure

The HL-20 aerodynamic database (~72 lookup tables) is encoded in HL20_aero.dml. The project ships:

• src/shared_utils.jl — DAVE-ML loader plus three registered symbolic functions (hl20_poly1d, hl20_poly2dr, hl20_poly2de) and three index accessors (idx1d, grp2dr, grp2de). Both the golden and the agent workspaces include this file, so the registered functions are available as ChallengeComponent.<name> from any .dyad file.

• dyad/shared/HL20Aero.dyad — body-axis coefficient buildup that calls those registered functions. Already wired with the correct varID groups; the agent only instantiates it as ChallengeComponent.shared.HL20Aero(...).

Atmosphere model

Atmosphere1976 uses DyadData.DyadTimeseries + BlockComponents.Tables.Interpolation (linear interpolation, constant extrapolation) over assets/shared/atmos_sigma.csv and assets/shared/atmos_sos.csv, referenced as dyad://ChallengeComponent/shared/atmos_sigma.csv and …/atmos_sos.csv.

Component hierarchy

HL20Vehicle
├── RigidBody6DOF     Newton–Euler 6-DOF, quaternion orientation
├── Atmosphere1976    CSV table lookup for σ and speed of sound
├── FlightState       Inverse quaternion rotation → aero angles, Euler, airspeed
├── HL20Aero          (pre-provided) TM-4302 coefficient buildup
└── AeroForceBuilder  Coefficients → inertial forces + body torques (CG offset)

HL20Mixer             TM-107580 Appendix C, BlockComponents-based
├── Switch            ifelse(selector > threshold, A, B)
├── Abs               |u| (extends SISO)
└── AdjustableUpperLimit  max(min(u1, u2), u_min) (extends SI2SO)

PitchControl   TM-107580 §4.1 NZQ pitch SAS with auto-trim
RollControl    TM-107580 §4.1 roll-rate damper
YawControl     TM-107580 §4.1 yaw damper with washout
SpeedControl   TM-107580 §4.1 speedbrake shaping

Interface contracts

Component	Forces / velocities	Torques / rates
RigidBody6DOF	inertial NED	body frame
AeroForceBuilder	force out: inertial NED	torque out: body
FlightState	input: inertial NED velocity	output: degrees
HL20Aero	—	body rates input (rad/s)

RigidBody6DOF adds gravity internally on der(v3). External forces do not include gravity. AeroForceBuilder also exposes the body-Z aero force component to the vehicle for normal load factor (NZ) computation.

HL20Vehicle outputs

The vehicle exposes feedback signals consumed by the controllers: ALPHA_DEG, BETADEG, BETADOT, MACH_out, PDEG, QDEG, RDEG, SINPHI, COSPHI, COSTHE, QBAR, EAS, NZ, V_FPS. See hl20_spec.md §8 for definitions.

Mixer actuator chain

Each of the 7 surfaces: FirstOrder(τ = 0.05 s) → SlewRateLimiter(±20°/s) → Limiter(per-surface range).

Validation scenarios

Scenarios, exact initial conditions, and constant input values for every test are in hl20_spec.md §11. Summary:

#	Analysis	Stop	Description
1	TestGravityOnlySim	5 s	RigidBody6DOF alone, gravity-only freefall
2	AeroSweepTransient	35 s	HL20Aero, α ramp −5° → +30° at Mach 0.2
3	TestHL20SubsonicSim	10 s	Open-loop vehicle plant at NASA trim
4	TestControllersSim	15 s	Four controllers standalone with constant inputs
5	TestHL20FullSystemSim	10 s	Closed-loop pitch pulse, matches NASA Appendix F

Debugging guide

• Gravity-only fails: check gravity direction (+pos_3 = NED down), quaternion kinematics signs, all six force/torque inputs wired.

• Aero sweep discontinuities: confirm the project module exposes the registered functions as ChallengeComponent.hl20_poly1d, etc.

• Integrated tests fail after gravity-only passes: check frame contracts — forces in inertial NED, torques in body, altitude = −pos_3 × ft_per_m.

• NaN / divergence: check division safety (v_scalar, vrw_fps denominators) and that all differential states have initial equations.

• Closed-loop transient at t = 0: ensure mixer FirstOrder actuator states are initialized at their equilibrium values (spec §11), not zero.