motor-selection

Motor Selection

Overview

Motor selection balances impulse (altitude), diameter (fit), and thrust curve (stability off the rod). A motor that fits the mount and hits the target apogee is not enough — it must also produce enough thrust to clear the launch rod stably.

Core principle: Always verify the motor exists in the database before assigning it. Never guess a designation.

When to Use

• Initial motor selection for a new design
• User wants to reach a specific apogee
• Current motor undershoots or overshoots the target
• Motor diameter doesn't match the motor mount

Steps

1. Gather Constraints

Before querying, establish:

• Rocket mass — from openrocket_component (action="read") component weights, or user-provided
• Target apogee — what the user wants to achieve
• Motor mount ID — inner diameter of the motor mount inner-tube
• Motor class ceiling — any certification or field limits (e.g. Estes field = max D)

2. Estimate Required Impulse

Rule of thumb: 30–50 m of apogee per newton-second, depending on mass and drag.

required_Ns ≈ target_apogee_m / 40    (midpoint estimate)

Map to impulse class:

Class	Total Impulse	Rough apogee (500g rocket)
C	10 Ns	~100 m
D	20 Ns	~200 m
E	40 Ns	~350 m
F	80 Ns	~600 m
G	160 Ns	~1000 m
H	320 Ns	~1500 m

This is a starting point only — simulate to confirm.

3. Query the Database

openrocket_database(action="motors", impulse_class="D", diameter_mm=18)

Useful filter combinations:

• impulse_class + diameter_mm — most common starting point
• name="H100" — when you know the approximate designation (matches H100W-DMS, H100T, etc.)
• manufacturer="Estes" — when field or certification requires a specific brand

Review avg_thrust_n and burn_time_s in the results. A high average thrust with short burn (spike profile) vs. a lower average with longer burn (progressive) affects flight character.