Total impulse and the letter classification
Every rocket motor is rated by its total impulse — the total "push" it delivers over its burn, measured in newton-seconds (N·s). Rather than quoting raw numbers, the hobby uses a letter classification where each letter represents a doubling of the previous letter's impulse range. An A motor delivers up to 2.50 N·s; a B motor delivers between 2.51 and 5.00 N·s; a C motor between 5.01 and 10.00 N·s, and so on.
| Class | Total impulse (N·s) | Total impulse (lbf·s) | Notes |
|---|---|---|---|
| A | 1.26 – 2.50 N·s | 0.28 – 0.56 lbf·s | Very small, light starter rockets |
| B | 2.51 – 5.00 N·s | 0.56 – 1.12 lbf·s | Common beginner motors |
| C | 5.01 – 10.00 N·s | 1.13 – 2.25 lbf·s | Common beginner/intermediate motors |
| D | 10.01 – 20.00 N·s | 2.25 – 4.50 lbf·s | Larger or heavier models |
| E | 20.01 – 40.00 N·s | 4.50 – 8.99 lbf·s | Mid-power, bridges beginner and advanced |
| F | 40.01 – 80.00 N·s | 9.00 – 17.99 lbf·s | Mid-power |
| G | 80.01 – 160.00 N·s | 17.99 – 35.97 lbf·s | Mid-power, upper end |
| H | 160.01 – 320.00 N·s | 35.97 – 71.94 lbf·s | Level 1 certification begins |
| I | 320.01 – 640.00 N·s | 71.94 – 143.88 lbf·s | Level 1 certification range |
| J | 640.01 – 1,280 N·s | 143.89 – 287.77 lbf·s | Level 2 certification begins |
| K | 1,280 – 2,560 N·s | 287.77 – 575.54 lbf·s | Level 2 certification range |
| L | 2,560 – 5,120 N·s | 575.54 – 1,151 lbf·s | Level 2 certification range (upper) |
| M | 5,120 – 10,240 N·s | 1,151 – 2,302 lbf·s | Level 3 certification begins |
| N | 10,240 – 20,480 N·s | 2,302 – 4,604 lbf·s | Level 3 certification range |
| O | 20,480 – 40,960 N·s | 4,604 – 9,209 lbf·s | Level 3 range — FAA Class 2 limit |
| P | 40,960 – 81,920 N·s | 9,209 – 18,417 lbf·s | Amateur/Class 3, individual FAA approval required |
| Q | 81,920 – 163,840 N·s | 18,417 – 36,835 lbf·s | Amateur/Class 3 rocketry |
| R | 163,840 – 327,680 N·s | 36,835 – 73,669 lbf·s | Amateur/Class 3, advanced experimental |
| S | 327,680 – 655,360 N·s | 73,669 – 147,338 lbf·s | Amateur/Class 3, rarely flown |
| T | 655,360 – 1,310,720 N·s | 147,338 – 294,676 lbf·s | Amateur/Class 3, largest amateur flights on record |
Note: the lbf·s column is total impulse converted to imperial units, not instantaneous thrust force — a motor's actual thrust varies within a class depending on its burn profile.
Total impulse vs. average thrust, a common point of confusion
New fliers often assume a "bigger letter" motor always pushes harder. It doesn't necessarily — total impulse measures the entire push over the full burn, while thrust is the instantaneous force at any moment.
Worked example: a D12 motor delivers 12 N of average thrust. A D3 from a different manufacturer might also be Class D but deliver only 6 N over twice the burn time — same total impulse, same letter, half the punch off the rod. Always check the number after the letter.
This classification tells you how much total push a motor gives, but nothing about how quickly it delivers it — which is where thrust curves come in.
Reading a thrust curve
A thrust curve plots thrust (force, in newtons) against time for a motor's full burn. Two motors in the same letter class can have very different curves: one might spike hard for a fraction of a second (a "fast" motor, good for windy days or heavy rockets that need to clear the pad quickly), while another spreads similar total impulse over several seconds at lower peak thrust (a gentler ride, less structural stress, often preferred for larger or fragile airframes).
The area under a thrust curve — thrust integrated over time — is exactly the total impulse used for the letter classification. So the letter tells you the area under the curve; the curve's actual shape tells you everything the letter can't.
Motor designation: reading the full code
A motor is labeled with its class letter, its average thrust in newtons, and its delay time in seconds — for example, a common beginner motor might read like the example below.
That delay figure is what Module 01 flagged as safety-critical: too short and the ejection charge fires while the rocket is still under power or climbing hard; too long and it fires well after apogee, when the rocket is already falling and gaining speed nose-first — increasing the risk of a structural failure ("zipper") at deployment.
Single-use vs. reloadable motors
Below roughly the F–G range, almost every motor is single-use: a sealed casing you buy, fly once, and throw away. Reloadable motors — a reusable metal casing you pack with a fresh propellant grain, delay element, and ejection charge — become the norm from about G/H upward, mainly because it's cheaper to reload a casing repeatedly than buy a new single-use motor every flight at that size.
Field note: reloadable motor kits require careful assembly — O-ring seating, correct closure torque, and clean casing threads all matter. A poorly assembled reload is a common cause of a partial ignition or a motor venting gas past a seal instead of through the nozzle.
Matching a motor to your rocket
Motor selection comes down to three checks, roughly in this order:
- Weight and thrust — the motor must produce enough thrust to accelerate the rocket's liftoff weight to a stable speed before it leaves the rod or rail.
- Total impulse and desired altitude — more total impulse (within a rocket's structural limits) generally means a higher flight, but drag scales with velocity, so gains diminish at the high end.
- Delay time and altitude — the delay should roughly match the time from burnout to apogee for that motor/rocket combination, so recovery deploys close to peak altitude rather than well before or after it.
Most manufacturers publish a recommended motor list for each kit, precisely because getting all three of these right by hand requires either flight data or a simulation — which is exactly what Open Rocket and RockSim exist for, and worth using once you're choosing motors outside a kit's tested recommendations.
Worked example: a rocket weighs 450 g fully loaded. Using the 5x liftoff weight rule, the motor needs at least 5 × 0.45 kg × 9.81 m/s² ≈ 22 N of thrust to leave the rod safely. A C6 motor (6 N) falls well short; a D12 (12 N) is still marginal; an E15 or larger is the safer choice.
Field note: the most common beginner mistake isn't picking too small a motor for altitude — it's picking one that "feels right" on total impulse while quietly failing the thrust-to-weight check. A rocket that leaves the rod too slowly can weathercock sharply even in light wind.