Choosing a Motor for Your American Rocketry Challenge Rocket

The motor is the single biggest variable in whether your rocket hits the target altitude window. Every design decision that follows — fin size, nose cone shape, recovery system timing — is downstream of how much impulse you are putting in and how fast it comes out. If you are new to the American Rocketry Challenge, the Getting Started with the American Rocketry Challenge guide covers the competition structure and team requirements before you start hardware decisions.

ARC restricts which motors teams may use. The competition specifies allowed motor classes and a total-impulse cap that can change from season to season. Do not assume last year's allowed motors are still legal — check the current official ARC rules before you order anything. What this guide covers is the underlying motor-selection logic that applies regardless of which specific motors fall within the current rules.

Reading the Motor Designation Code

Every commercial model rocket motor is labeled with a standardized NAR/TRA designation. Once you can read it, the label tells you almost everything you need to know before you open the package.

The format is a letter, followed by a number, a hyphen, and a second number. For example: F32-8.

• The letter is the total-impulse class. Each successive letter represents a doubling of the impulse range: A covers 1.26–2.5 N·s, B covers 2.5–5 N·s, C covers 5–10 N·s, D covers 10–20 N·s, E covers 20–40 N·s, F covers 40–80 N·s, and G covers 80–160 N·s. A motor at the top of the F class carries roughly twice the total impulse of one at the top of the E class, and half that of the top of G. This doubling progression means moving up one letter class is a significant jump in total push.
• The first number is the average thrust in newtons over the burn. In F32-8, the motor averages 32 N of thrust. Two motors with the same letter but different average-thrust numbers will push your rocket off the pad very differently even though their total impulse ranges overlap.
• The second number is the ejection delay in seconds — the time between motor burnout and when the ejection charge fires to deploy your recovery system. In F32-8, the charge fires 8 seconds after burnout.

ARC rockets commonly fly in the composite F and G range, though the exact permitted classes and the total-impulse ceiling are defined in the current season's rules. Confirm those limits before selecting candidates.

Total Impulse vs. Average Thrust

Total impulse — measured in newton-seconds — is the area under the thrust curve for the entire burn. It is the total mechanical work the motor delivers to the rocket. For a given rocket mass, total impulse is the primary driver of how high you go.

Average thrust is a different quantity. It tells you how hard the motor pushes on average during the burn but does not tell you how long the burn lasts. Two motors with identical letters — and therefore overlapping total-impulse ranges — can have very different average thrust values, which means they have different burn durations to deliver similar total impulse.

A practical example: an F52 and an F20 are both F-class motors, so their total impulse falls somewhere in the 40–80 N·s range. But the F52 pushes harder and burns shorter, while the F20 pushes gentler for a longer time. The F52 will accelerate your rocket faster off the rail and reach a higher velocity earlier in the flight. The F20 will accelerate more gradually. If your rocket is borderline heavy, the F52's higher initial thrust may be the safer choice for a stable liftoff. If you are trying to stretch burn duration to reach a precise altitude, the F20 might give you more predictable control over the coast phase. The letter class alone does not settle the question.

Thrust Curves

A thrust curve is a graph of thrust in newtons (vertical axis) plotted against time in seconds (horizontal axis). Published thrust curves are available for almost every commercial motor from the manufacturer and from databases like thrustcurve.org. Reading them before you simulate is worth the two minutes it takes.

Key features to check:

Peak thrust. The highest point on the curve, usually near ignition. Some motors spike sharply at ignition and drop quickly — this is called a regressive burn profile. Others ramp up gradually and peak mid-burn, called a progressive profile. Regressive motors produce a harder kick off the pad. Progressive motors build thrust more gently.

Burn time. How long the motor produces meaningful thrust. Burn time affects when your rocket transitions from powered flight to coast, which in turn affects how much velocity you carry through the coast phase to apogee.

Thrust-to-weight at liftoff. Before anything else, your rocket needs to leave the launch rail with enough velocity to be aerodynamically stable. The general rule of thumb is a thrust-to-weight ratio of at least 5:1 at liftoff — meaning the motor's average initial thrust should be at least five times the weight of the fully loaded rocket. If your liftoff thrust-to-weight is too low, the rocket exits the rail slowly, making it vulnerable to wind and more likely to weathercock badly before it has built up stabilizing airspeed. Check the thrust curve's early profile, not just the average thrust number, when evaluating this.

Matching the Motor to Your Rocket

You need two numbers before you pick a motor: your rocket's liftoff mass and your target apogee.

Liftoff mass is the rocket fully loaded — airframe, fins, nose cone, recovery system, egg payload, altimeter, and a charged motor. Weigh it. Do not estimate. A 50-gram error in liftoff mass translates to a meaningful apogee error at competition.

Target apogee for ARC changes each season and is specified in the rules. Your goal is to select and tune a motor that puts your specific rocket within the altitude window reliably, not just on the best day of the year but across reasonable variations in temperature, humidity, and launch-day conditions.

The practical workflow: build your rocket in OpenRocket and simulate candidate motors against your actual measured liftoff mass. Compare the predicted apogee for each candidate to the target. OpenRocket uses published thrust curve data and models drag, so it gives you a much more accurate prediction than any hand calculation.

Most teams find they cannot just pick a motor and call it done. The simulation will tell you whether you are hitting high or low. If you are consistently simulating too high, your options are to add nose weight, choose a lower-impulse motor in the same class, or select a motor with a lower average thrust. If you are simulating too low, you can reduce mass, choose a higher-impulse motor, or move up to the next letter class if the rules permit it. Work through these adjustments in simulation before you commit to hardware.

One thing to watch: if you add mass to bring altitude down, that mass also changes your liftoff thrust-to-weight ratio. Check the liftoff number again after any significant mass change.

Ejection Delay and Timing

The trailing number in the motor designation — the 8 in F32-8 — is the ejection delay in seconds. This is how long after burnout the motor waits before firing the ejection charge that deploys your recovery system.

You want ejection to happen at or just after apogee, when the rocket is moving as slowly as possible. Deploying the parachute well before apogee means it opens at higher velocity, which increases shock on the airframe and parachute and can cause the rocket to drift farther from the pad. Deploying it well after apogee means the rocket is moving fast again during the descent phase before the chute opens — harder on everything, and potentially outside the landing zone.

In your OpenRocket simulation, look at the time from burnout to apogee. That is approximately the delay you want. If the simulation shows burnout-to-apogee is 6 seconds, a motor with an 8-second delay fires the ejection charge 2 seconds after apogee — slightly late but usually acceptable. A 4-second delay fires it 2 seconds before apogee — too early for most ARC-class flights.

Some commercial motors come in multiple delay variants (F32-4, F32-6, F32-8). Match the delay to your simulation result. If no off-the-shelf delay matches your burn-to-apogee time exactly, most composite motor systems allow delay trimming with the manufacturer's provided tools — follow the manufacturer's instructions and practice before competition. Cross-referencing ejection timing with your altimeter data from test flights lets you verify whether your predicted delay is actually working in the field.

Safe Handling and Storage

Model rocket motors are pyrotechnic devices. Handle them accordingly.

Store motors in a cool, dry location away from flammable materials. Do not leave composite motors in a hot car — high temperatures can degrade the propellant and affect the burn profile. Check the manufacturer's storage recommendations and expiration guidance.

At the range, follow the NAR Model Rocket Safety Code and all range officer instructions. ARC rules require adult supervision during motor preparation and loading. Do not load a motor until you are at the pad and ready to launch. Never lean over a loaded rocket or point it at people while handling the igniter.

If you are under 18, you will be working within the ARC team and adult-mentor structure for any motor-related steps at the launch site. Know the procedure, follow your mentor's lead, and do not improvise.

Common Mistakes to Avoid

• Picking a motor based on the letter class alone, without checking average thrust or the thrust curve shape. Two F motors can fly very differently.
• Ignoring thrust-to-weight at liftoff. A motor that produces plenty of total impulse but has low initial thrust can produce an unstable, slow exit from the rail.
• Using last season's motor list without confirming it against the current ARC rules. Allowed motors and total-impulse caps are updated annually.
• Choosing a delay by guessing rather than reading it from the simulation. A two-second error in ejection timing is the difference between a clean deployment and a streamer landing.
• Skipping simulation and buying a motor because it "worked for another team." Their rocket has a different mass and drag profile. Run your own numbers.
• Not accounting for mass changes late in the build. If you add mass after running your motor simulation, re-run the simulation. Apogee predictions shift meaningfully with even moderate mass additions.
• Testing the recovery system without matching the ejection charge to the motor you will actually use. Ground test with the correct motor's ejection charge, not a substitute.

Where to Go From Here

Motor selection sits at the center of the altitude-tuning loop: simulate, evaluate, adjust mass or motor choice, simulate again. The teams that hit the target window consistently are the ones who run that loop methodically before competition day, not the ones who pick a motor that worked last year and hope the weather cooperates.

If you want structured support working through motor selection, simulation, and competition-day tuning, our coaches work through exactly this process with students during the season. Explore our ARC coaching classes to see how SEALS Academy supports teams competing in Orange County and beyond.