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.

To increase stability (move CP aft or CG forward):

Fix	Tool	Notes
Increase fin span	openrocket_component update fin-set span	Most effective single change
Increase fin root chord	openrocket_component update fin-set root_chord	Moves CP aft
Move fins aft	openrocket_component update fin-set axial_offset_method="bottom", axial_offset_m=0	Fins flush with aft end
Add nose weight	openrocket_component create mass component in NoseCone	Moves CG forward

To reduce stability (move CP forward or CG aft):

Fix	Tool	Notes
Reduce fin span	openrocket_component update fin-set span	Start with 10–15% reduction
Reduce fin root chord	openrocket_component update fin-set root_chord
Add aft mass	openrocket_component create mass component in lower airframe	Moves CG aft

4. Iterate

After each change:

1. openrocket_flight(action="run") — check new min_stability_cal
2. Also check max_stability_cal — stability varies over the flight as propellant burns
3. Stop when 1.0–1.5 cal across the full flight

5. Verify Final Design

Run openrocket_component (action="read") one more time. Confirm:

• cp_x > cg_x (CP aft of CG)
• stability_cal in range
• No component geometry was accidentally broken

Red Flags — Stop and Investigate

• Accepting null stability without manual computation
• Making two geometry changes at once (can't isolate which worked)
• Stability in range on launch but diverging later in flight — check max_stability_cal too
• Fin span larger than 1.5× body diameter (drag penalty outweighs stability gain — find another fix)

Quick Reference

Stability	Problem	Primary Fix
< 0.5 cal	Severely unstable	Increase fin span significantly
0.5–1.0 cal	Unstable	Increase fin span or add nose weight
1.0–1.5 cal	✓ Target range	No change needed
1.5–2.0 cal	Over-stable	Reduce fin area
> 2.0 cal	Very over-stable	Reduce fin area + check mass distribution