Final Report: Angle-Extended RK4 Drag Sweep with Timestep Convergence — θ = 30°, 45°, 60°
Summary
We extended the previously validated RK4 quadratic-drag projectile pipeline to a three-angle sweep at θ ∈ {30°, 45°, 60°} and v0 = 30 m/s, across five drag strengths k/m ∈ {0, 0.0125, 0.025, 0.0375, 0.05}. Every one of the 15 (θ, k/m) points was simulated twice — at the baseline dt = 1e-3 s and at dt/2 — and the relative range difference was recorded as an explicit convergence metric. All 15 points converged with a maximum relative difference of 2.4e-8, four orders of magnitude below the 1e-4 rejection tolerance, so no point was rejected and no adaptive step refinement was needed. Validation against the analytic no-drag range passed at all three angles, and the 45° column reproduces the iteration-3 RK4/RK45 consensus to within 0.02%. The headline physics result: the range-optimal launch angle among the three candidates shifts from 45° to 30° as drag increases past k/m ≈ 0.03, while 60° is never optimal.
Key results
All numbers below come from evidence job 433f3d26-8ee0-4ed9-b2ad-b9de61353bed (published as knowledge finding 7c08b94f-f764-4e4d-8c70-92b4f912d24d).
Timestep convergence (primary new check this iteration):
- All 15 sweep points passed the dt/2 halving check; maximum relative range difference was 2.36e-8 (tolerance 1e-4), confirming the baseline dt = 1e-3 is well inside the converged regime even at the highest drag, where trajectory curvature is largest.
Analytic no-drag validation (guards against hidden hard-coded assumptions in reused code):
- k/m = 0 ranges: 79.45187 m at 30°, 91.74312 m at 45°, 79.45187 m at 60°.
- Relative error vs the analytic R = v0²sin(2θ)/g: 9.96e-9 (30°), 8.16e-9 (45°), 5.88e-9 (60°) — all far below tolerance, confirming no angle-specific bugs were imported with the reused pipeline.
Consistency with iteration 3 (45° column):
- Max relative deviation vs the iteration-3 RK4/RK45 consensus: 2.05e-4 (e.g., 91.7431 m at k/m = 0, 24.8084 m at k/m = 0.05), confirming the extension did not perturb the previously validated configuration.
Range vs drag at each angle (v0 = 30 m/s):
Optimal-angle shift:
- The range-optimal angle among {30°, 45°, 60°} shifts from 45° to 30° between k/m = 0.025 and k/m = 0.0375.
- At k/m = 0.0375, 30° beats 45° by 0.7%; at k/m = 0.05, by 2.1%.
- 60° is never optimal at any drag strength tested — drag penalizes the longer, higher flight path of steep launches.
What did not work
- run_experiment submission failures (from iteration 2) and the 'method'/'mode' parameter collision (from iteration 3) did not recur, because the whole 15-point × 2-dt grid was batched into a single job (job
433f3d26) whose parameters carried only v0 = 30 and dt_base = 0.001, avoiding any argument namedmethodormode. - No convergence-check failures occurred: the anticipated risk that dt/2 differences might exceed 1e-4 at high drag (largest curvature) did not materialize; the worst case was 2.36e-8, so the planned fallback of adaptive re-halving was never triggered.
- No failed runs: the "Failed runs" list for this iteration is empty.
Confidence and limitations
- Confidence in the numerical results is high (agent-reported confidence 0.99): a 15/15 convergence pass at dt/2, analytic agreement at the 1e-9 level in the no-drag limit, and 0.02% agreement with an independent RK45 implementation from iteration 3.
- Limitations:
- The optimal-angle "shift" is identified only among the three discrete candidate angles {30°, 45°, 60°}; the continuous optimal angle θ*(k/m) is not resolved, and the crossover is bracketed only coarsely (between k/m = 0.025 and 0.0375).
- The drag model is idealized quadratic drag (a = −g ŷ − (k/m)|v|v) with constant k/m, no wind, no spin, and a flat launch/ground height — real projectile data would deviate.
- All results are simulation-derived; no laboratory or experimental validation has been performed. Measured ranges at known drag coefficients (or field trials with a calibrated launcher) would be needed to confirm that the 30°-favorable regime at k/m ≳ 0.03 is physically realized.
- The 45°-column agreement with iteration 3, while strong, is a comparison between two numerical pipelines sharing the same drag model — it validates the numerics, not the physics.
Next steps
- Fine-grained angle sweep (e.g., 20°–60° in 2° steps) around the crossover region k/m ≈ 0.03 to locate the continuous optimal-angle curve θ*(k/m).
- Compare the numerical optimal-angle shift against the analytic small-drag perturbation prediction θ* ≈ 45° − c·(k/m)v0²/g.
- Extend to velocity-linear (Stokes) drag to compare optimal-angle behavior between drag laws.
- Consider physical/experimental validation of at least one drag regime (e.g., a launch apparatus with known projectile and measured drag coefficient) to anchor the purely numerical findings.