stability-analysis

Stability Analysis

Overview

Stability margin = (CP − CG) / reference diameter, measured in calibers. The target is 1.0–1.5 cal. Outside this range the rocket is either dangerous or excessively weathercock-sensitive.

Core principle: Always fix the root cause — geometry or mass distribution — not the symptom. Never accept a null stability result; compute it manually.

When to Use

• min_stability_cal < 1.0 (unstable)
• min_stability_cal > 1.5 (over-stable)
• min_stability_cal is null after simulation
• User reports unexpected flight path or weathercocking
• Stability was in range before rocketsmith:mass-calibration but fell out after applying real printed part weights — this is the expected failure mode when printed PLA/PETG is significantly heavier than OpenRocket's material defaults

Steps

1. Read the Numbers

Run openrocket_component (action="read") and note:

• cg_x — center of gravity from nose tip (m)
• cp_x — center of pressure from nose tip (m)
• max_diameter_m — reference diameter

If min_stability_cal returned null, compute manually:

stability_cal = (cp_x - cg_x) / max_diameter_m

A positive result means CP is aft of CG — stable. A negative or zero result means unstable.

1.5. Consult the Reference Collection

Query rag_reference(action="search", collection="stability_notes", query=f"stability {round(current_cal, 2)} cal {symptom}", n_results=3) for prior cases with known fixes. Cite close matches and weigh against the heuristic below. Proceed if no results or on errors — the collection is an enrichment, not a prerequisite.

2. Diagnose the Direction

Under-stable (< 1.0 cal): CP is too close to CG, or forward of it.

• Fins too small or too far forward
• Nose cone too heavy
• Body tube too short relative to fin span

Over-stable (> 1.5 cal): CP is too far aft of CG.

• Fins too large
• Nose cone too light
• Aft mass (motor, motor mount) dominating

3. Apply Fixes

Work one change at a time. Re-simulate after each.