Robust DDP Controller — Thrust Curve Matching

Config: CH4/LOX (methane/LOX)

Yes — the LUT uses CH4/LOX (methane/LOX) via engine_sim/configs/default.yaml:

• fuel_name: CH4
• ox_name: LOX
• cea_cache_LOX_CH4_3D.npz

How the Controller Works

1. Inputs (LUT lookup): P_u_fuel, P_u_ox, thrust_desired, MR_ref
2. Outputs: u_safe_F, u_safe_O (duty 0–1) → sent as PWM to Fuel Press and LOX Press actuators
3. Thrust command: thrust_desired must be set over time to follow your thrust curve

Why Your Plot Shows 0–100% Duty

Your actuation plot shows Fuel/Ox duty reaching 70–100%. The current LUT has u_safe mostly in 0–10% because:

1. Engine model: At 2–8 MPa tank pressure, the model predicts high thrust with small valve opening (pressurization only needs to balance consumption).
2. Hardware mismatch: Your hardware may need higher duty to reach the same thrust (different valve Cv, line losses, or injector behavior).

Making the Controller Match Your Thrust Curve

1. Feed the Thrust Curve

thrust_desired must follow your mission profile. Right now it defaults to 1000 N.

• controller_service: --thrust N sets a constant value.
• Web backend: Can forward thrust from a mission timeline.
• TCP control: ControllerService can receive setCommand() with time-varying thrust.

Add a thrust-curve source (e.g. piecewise table or trajectory) and call setCommand() with thrust_desired(t).

2. Align Engine Config With Hardware

If your hardware needs higher duty for the same thrust:

• Tune engine_sim/configs/default.yaml (injector areas, pressure drops, etc.).
• Or scale LUT outputs in FSW (e.g. duty_F = scale * u_safe_F) — only if you have a known mapping.

3. Use duty_F / duty_O Instead of u_safe

The policy LUT outputs both u_safe_F/O and duty_F/O. The FSW prefers u_safe_F/O and falls back to duty_F/O. If the DDP/safety filter is too conservative, duty_F/O may be more representative. Check whether duty_F/O in the LUT better matches your hardware.

4. Regenerate LUT With Your Thrust Profile

Include your actual thrust curve in the LUT grid:

# policy_lut_fsw.yaml — add thrust_desired points from your curve
axes:
  - name: thrust_desired
    values: [0, 500, 1000, 2000, 3000, 4000, 5000, 6000, 7000]  # Your curve points

Then run tools/controller_lut/generate_engine_and_policy_lut.sh full.

See ../tools/controller_lut/README.md for the full LUT generation pipeline.

5. Run Without LUT (Online DDP)

For debugging, run the controller without the LUT so it uses online DDP:

./build/bin/controller_service  # No --lut-path

This uses the full DDP solver and may behave differently from the LUT.

Summary

Item	Status
Propellants	CH4/LOX (methane/LOX) ✓
LUT axes	P_u_fuel, P_u_ox, thrust_desired, MR_ref ✓
Duty range	0–1 (FSW clamps u_safe to [0,1]) ✓
Thrust command	Must be supplied; default 1000 N
Hardware match	May need engine config or output scaling