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.

4. Check Thrust-to-Weight

Minimum thrust-to-weight ratio off the rod: 5:1 recommended.

thrust_to_weight = avg_thrust_n / (rocket_mass_kg × 9.81)

If below 5:1, the rocket will leave the rod slowly and be vulnerable to weathercocking. Choose a higher-thrust variant or longer launch rod.

5. Check the Reference Collection for Known Issues

Query rag_reference(action="search", collection="motor_reviews", query=f"{motor_designation} {manufacturer}", n_results=3) for real-world reports (delay reliability, ignition problems, CATO reports, observed vs. predicted apogee). Weigh any hits against the theoretical numbers. Proceed if no results or on errors.

6. Fly Candidates

For each candidate motor:

1. openrocket_flight(action="create", motor_designation=<designation>) — assign motor
2. openrocket_flight(action="run") — run flight
3. Note max_altitude_m and min_stability_cal
4. openrocket_flight(action="delete") before trying the next candidate

Pick the motor that hits the target apogee while keeping stability >= 1.0 cal throughout flight.

7. Confirm Fit

After selecting a motor, verify:

• Motor diameter ≤ motor mount inner tube ID
• Motor length ≤ motor mount tube length
• If using a reloadable case, the reload length matches the mount

Red Flags — Stop and Check

• Assigning a motor designation not returned by openrocket_database — it won't load
• Skipping the flight run and trusting the rule-of-thumb estimate alone
• Thrust-to-weight < 2:1 — unsafe launch, not just sub-optimal
• Changing motor class without re-checking stability (heavier motors shift CG aft)

Quick Reference

Impulse needed ≈ target_apogee / 40
Motor must fit: diameter ≤ mount ID, length ≤ mount length
Thrust-to-weight ≥ 5:1 recommended off the rod
Always run the flight — rule of thumb is a starting point only