Every motor has a code like C6-5. The letter is the total push. Each letter is about twice as strong as the one before, so a C pushes about twice as much as a B. The 6 is how hard it pushes, and the 5 is the seconds before the parachute pops out.
Letter
How high it goes*
Field you need
¼A, ½A
15–60 m (50–200 ft)
15 m (50 ft): a big backyard
A
60–150 m (200–500 ft)
30 m (100 ft): a basketball court
B
120–200 m (400–650 ft)
60 m (200 ft): a soccer field's width
C
150–300 m (500–1,000 ft)
120 m (400 ft): longer than a soccer field
D
250–350 m (800–1,150 ft)
150 m (500 ft)
E
300–550 m (1,000–1,800 ft)
300 m (1,000 ft): a big farm field
F
500–750 m (1,600–2,500 ft)
300 m (1,000 ft)
*Rough heights for rockets made here, from Rocket Builder's flight check. Lighter rockets go higher. The field is the safety code's smallest size (its shortest side). On a windy day the parachute drifts further, so bigger is better: Watch it launch shows how far yours drifts.
Motor sizes: how wide?
Size
Motors
Good for
M13
¼A, ½A, A
Small, light rockets and short flights. A school field or park.
M18
½A, A, B, C
The most common motors in stores. Best for a first rocket.
M24
C, D
Bigger rockets and higher flights. A big field.
M24L
E
Long 24 mm motors: very high flights. A very big field.
M29
E, F
Big rockets for experienced flyers.
Picking a size
Start with your field. Find the biggest letter whose field fits where you'll launch.
Pick the motor size that uses that letter. Not sure? Choose M18.
First flight: use a smaller letter than the biggest that fits, and watch how high it goes.
The other scores in your report
Power: the motor's push ÷ the rocket's weight. 5× or more passes, so it leaves the pad fast.
Launch speed: how fast it leaves the launch rod. 15 m/s or more passes, so the fins can steer.
Parachute: the size that brings it down gently (about 4.5 m/s).
The stability score (cal): will it fly straight?
Every rocket has a balance point (blue dot), where it would balance on your finger, and a push point (red dot), where the air pushes on it. The fins pull the push point toward the back.
For a straight flight, the blue dot must be in front of the red dot. The gap between them is measured in cal (short for caliber): 1 cal is one body width.
Under 1 cal: wobbly. It could tumble, so you can't download it yet. Make the rocket taller.
1 to 2 cal: just right!
2 to 3 cal: very stable. Still fine.
Over 3 cal: too stable. It turns into the wind like a weathervane.
The score is checked with the heaviest motor of your size loaded, because the motor is heavy and sits at the back.
Try it for real: balance your finished rocket (with a motor in) on one finger. That's the blue dot!
Need supplies? Shop my list: launch pads, motors, parachutes and filament.
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How to use Rocket Builder
Design a 3D-printable model rocket for a standard motor, check that it's stable and flies well, then download STL files ready to print. New to model rockets? Start with Key terms. Where the numbers come from: Sources and references.
Choose a mode and the motor size you'll fly (on the left).
Set the size and shapes, then press Build. The first build of a new design takes 10–30 s; a bar along the top shows progress.
The rocket appears here. Drag to rotate, scroll to zoom. Exploded pulls the parts apart; Section cuts it in half to show the inside. ▶ Watch it launch plays its flight right here: liftoff, parachute and drift in the wind you set, and the field it needs; ◀ Back to 3D model returns to the same rocket.
Read the report under the view (explained below), pick a flight motor, then Download STLs (ZIP).
New rocket / Save keep the design on this site (listed under My rockets in this browser); Copy link shares a read-only copy; Download file saves the settings to your computer and Open file loads them back.
Changing any setting shows Press Build: the model and downloads only update when you build again.
Sizing a rocket: motor classes, sizes and fields
Start from the field you can use, then pick the motor class it allows, then a motor size that offers that class. The altitudes below are typical for rockets made here (printed parts are heavier than cardboard kits), from the app's flight check at usual printed masses; your report gives the number for your design. Field sizes are the NAR Model Rocket Safety Code minimums (the shortest side of the field); the app also works out the parachute drift in a 10 mph breeze and recommends the larger of the two.
Class
Total impulse
Sizes here
Typical apogee
Min. field
¼A, ½A
up to 1.25 N·s
M13, M18
15–60 m (50–200 ft)
15 m (50 ft)
A
1.26–2.5 N·s
M13, M18
60–150 m (200–500 ft)
30 m (100 ft)
B
2.51–5 N·s
M18
120–200 m (400–650 ft)
60 m (200 ft)
C
5.01–10 N·s
M18, M24
150–300 m (500–1,000 ft)
120 m (400 ft)
D
10.01–20 N·s
M24
250–350 m (800–1,150 ft)
150 m (500 ft)
E
20.01–40 N·s
M24L, M29
300–550 m (1,000–1,800 ft)
300 m (1,000 ft)
F
40.01–80 N·s
M29
500–750 m (1,600–2,500 ft)
300 m (1,000 ft)
M13 (13 mm, "mini")
¼A3, ½A3, A3, A10. The smallest and lightest rockets; low flights that suit a school field or park. The light motors leave little margin for a heavy print, so keep these rockets short and thin-walled.
M18 (18 mm, "standard")
½A6, A8, B4, B6, C5, C6: the motors most hobby and craft stores stock. The best size for a first rocket: an A or B for a small field, a C once you have room.
M24 (24 mm)
C11, D12. Heavier rockets and higher flights. The C11 suits a heavy rocket on a smaller field; the D12 needs at least 150 m.
M24L (24 mm, long)
E9, E12. Long motors, so the motor bay and the bottom section are longer. Very high flights: plan on a 300 m field and a club launch.
M29 (29 mm)
E16, F15. Big, heavy rockets for experienced flyers; a 300 m field, and check local rules (some fields and states treat F motors differently).
Rules of thumb
Thrust-to-weight at least 5:1 and at least 15 m/s off the rod (the report checks both). Fly the first flight on a smaller motor than the biggest your field allows. Heavier rockets need more push: if no motor passes, shorten the rocket, thin the wall or step up a size.
Modes
Beginner
Made for young builders (about grades 3–7): numbered steps, each explaining what it does in plain words. Motor size, your printer's max height, then the overall height you want (nose tip to fin tips; the body length is worked out to fit; slider or typed; heights that can't be split into printable pieces move to the closest one that can), then fin shape (swept, delta or trapezoid) and nose shape picked from pictures. Everything else uses proven defaults, and the fins size themselves so the rocket is stable. The report shows a stability score meter, the best motor and how high it should fly, a power bar (motor push ÷ rocket weight, 5× or more passes) and a launch-speed bar (15 m/s or more off the rod), and next steps. When no motor passes it shows the closest motor's bars, why each motor fails in plain words, and the weight that would pass; the full numbers are under Grown-up details. The Guide button opens a quick start, which links to this full guide.
Advanced
Every setting: body length instead of overall height, wall thickness, a body OD offset for a fatter rocket (the actual OD is shown under it), fin count, thickness, fillets and custom fin size, nose fineness, launch lugs, filament, and the flight check (target altitude, which motor to check, recovery mass, drag, launch rod). Switching modes carries the size across.
The report
Stability margin (calibers)
How far the centre of pressure (CP, red marker) sits behind the centre of gravity (CG, blue marker), measured in body diameters ("calibers"). The CG must be ahead of the CP for the rocket to fly straight. 1–2 cal is ideal. Under 1.0 is unstable and downloads are blocked; over 3 is over-stable (it flies, but turns into the wind). It's checked with the heaviest motor of the size loaded and a parachute allowance, and it also lists the length and liftoff mass.
Motor picker
A quick flight simulation for each motor of your size, using thrust curves from ThrustCurve.org. Peak: highest altitude (m and ft). Off rod: speed leaving the launch rod. Thrust ×wt: average thrust ÷ weight. Stability: margin with that motor. Liftoff: total mass. A motor passes (✓ Good) with thrust at least 5× the weight, at least 15 m/s off the rod, at least 30 m of altitude for the parachute to open, and a stable margin. The highlighted chip is the recommendation for your target altitude; click a row to check that motor instead.
Parts
Each printed part with its colour in the view, its mass (solid, in the chosen filament) and its print height, which must fit your printer. Long bodies are split into sections automatically.
Build notes
How the parts go together, the recovery and anchor details, and anything the builder changed to make the design work (e.g. fins enlarged for stability).
Key dimensions
The main measurements in mm: body OD and ID, motor bay, shoulders, the twist-lock thread, body length and number of sections, and how much the fins were scaled. Useful for fitting the motor and parachute, and for OpenRocket.
Key terms: model rocketry basics
The ideas behind the numbers in the report, from the ground up.
Stability: will it fly straight?
Stability margin = (CP distance − CG distance) ÷ body diameter D. Here the gap is about 2 diameters: 2 calibers.
CG: centre of gravity
The balance point: where the rocket would balance on your finger. Weight in the nose moves it forward; the motor and fins pull it back. Always measured with the motor loaded, because the motor is heavy.
CP: centre of pressure
The point where the air's push on the rocket effectively acts. Fins are big flat areas at the back, so they pull the CP rearward. Bigger fins move it further back.
Caliber (cal)
One body diameter, used as a unit of length. A 25 mm-wide rocket has 1 cal = 25 mm. Using calibers lets the same rule work for small and large rockets.
Stability margin
How far the CP is behind the CG, in calibers. If the rocket tilts, the air pushes harder on the tail (behind the CG) and swings the nose back into line, like a weathervane or a dart's flights. Positive and 1–2 cal is the target. Below 1 the correction is too weak (or, if the CP is ahead of the CG, it tumbles or loops). Above about 3 it over-corrects into the wind.
Unstable / over-stable
Unstable: too little margin; fix with larger fins, a longer body or a lighter motor (or nose weight). Over-stable: so much margin it turns into any crosswind and flies off at an angle (weathercocking); smaller fins help.
Barrowman method
The standard hand-calculation (developed by James Barrowman in the 1960s) for finding the CP from the nose and fin shapes. It's what this app and most hobby tools use. It assumes a slender rocket at low speed and small angles, which suits model rockets well.
Motors
Motor size (M13, M18, M24, M29)
The motor's diameter in mm. The rocket's motor tube is built to fit; you can then fly any motor of that diameter (and length).
Motor code, e.g. C6-5
C: the impulse class, the total push. Each letter is up to twice the one before (A up to 2.5 N·s, B up to 5, C up to 10, D up to 20…), so a C is roughly twice a B. 6: the average thrust in newtons. 5: the ejection delay in seconds (below). A -0 motor has no delay (for a booster stage); P means plugged, no ejection charge.
Thrust and impulse
Thrust is the push, in newtons (1 N ≈ the weight of 100 g). Total impulse (N·s) is thrust × burn time: how much the motor can speed the rocket up overall. A thrust curve is thrust plotted against time; the app uses real measured curves from ThrustCurve.org.
Thrust-to-weight ratio
Average thrust ÷ the rocket's weight at liftoff. At least 5:1 is the usual safe minimum, so the rocket leaves the pad briskly instead of wobbling off.
Burnout and coast
Burnout: the motor stops pushing. The rocket then coasts upward, slowing under gravity and drag, until it stops climbing.
Flight
Launch rod (or rail) and rod speed
The rocket slides up a guide rod (about 0.9 m for small rockets) through its launch lugs. Until it's moving fast enough, the fins can't steer it, so the speed leaving the rod matters: at least 15 m/s is the usual rule. Faster is better on windy days.
Apogee
The highest point of the flight, where the rocket stops climbing and starts to fall. "Peak" in the motor picker.
Ejection delay
The seconds between burnout and the motor's ejection charge, a small charge that fires forward to pop the nose off and push out the parachute. The best delay makes this happen at or just after apogee, while the rocket is slowest. Too short: the parachute opens while still going up fast and can rip. Too long: the rocket is already falling fast. The motor picker chooses the delay closest to the coast time.
Drag and drag coefficient (Cd)
Drag is air resistance. Cd is a number describing how slippery the shape is (lower is better); 0.5–0.75 is typical for model rockets, and the app assumes 0.65 unless you change it in Advanced.
Liftoff mass
Everything that leaves the pad: printed parts, the loaded motor, and recovery gear.
Field size
The safety code's minimum launch-site size for a motor class (e.g. 60 m across for B, 120 m for C). Bigger motors need bigger fields.
Recovery and parts
Recovery system
What brings it down gently: usually a parachute or streamer, tied to a shock cord (elastic or Kevlar) that joins the nose to the body via the anchor. Wadding (or a reusable baffle) protects the parachute from the hot ejection gas. The app allows a few grams for these unless you enter a recovery mass.
Descent rate
How fast it falls under the parachute. About 3–5 m/s lands without damage; a bigger parachute falls slower but drifts further.
Nose cone, shoulder and fineness
The nose's shape (ogive, conical, Von Kármán…) and its fineness, length ÷ diameter. The shoulder is the short sleeve that slips into the body tube and holds the nose on until ejection.
Fins: root, tip, span, sweep
Root chord: the fin's length where it meets the body. Tip chord: its outer edge. Span: how far it sticks out. Sweep: how far back the tip is set. A fillet is the rounded joint between fin and body; it makes the joint stronger.
Motor mount and minimum diameter
When the body is wider than the motor, an inner tube (motor mount) holds the motor. A minimum diameter rocket has the motor fitting the body itself, the lightest and fastest layout.
Printing it
Print the parts as they download (each one already rests on its print face), in PLA with a 0.4 mm nozzle, 0.42 mm line width, 0.2 mm layers, 2 walls and no supports. Set the seam to Random, not Aligned: an aligned seam leaves a ridge down every joint that makes it bind. The full list, with where each setting is in Bambu Studio, PrusaSlicer and Cura: Print settings. Not sure of your printer's fit? Print the test-fit pieces under the Download button first.
Your files: personal use only
The STL files, build notes and design file you download are licensed to you to print and fly your own rockets, and to change for your own use. They aren't free to share: please don't upload them, or remixes of them, to MakerWorld, Printables, Thingiverse, Thangs, Cults3D or any other site, and don't sell the files or prints made from them. Share a photo of your rocket and a link to Rocket Builder instead. Every ZIP has the full terms in LICENSE.txt; see also the file license page.
What is OpenRocket, and what should I check?
OpenRocket is a free, open-source model rocket simulator (Windows, Mac, Linux) and the usual hobby tool for checking a design before it flies. This app's numbers are quick estimates (Barrowman stability, a 1-D flight sim), so check your finished rocket there before its first flight, especially a new or unusual design:
Recreate the rocket from Key dimensions and the overall length: nose shape and length, body tube OD, ID and length, fin count and shape (measure the fin STL in your slicer).
Weigh the printed parts and enter the real masses as overrides (infill and walls make prints lighter or heavier than the solid estimate here), plus the parachute, shock cord and wadding.
With the motor loaded, check the stability margin is still 1–2 cal. Also balance the real rocket on a finger to find the CG and compare.
Run a simulation for your chosen motor and check: the speed off the launch rod (at least 15 m/s), the apogee fits your field, and the ejection delay, so the parachute opens near apogee rather than going up or coming down fast. Pick the motor delay to match.
Check the descent speed under your parachute (about 5 m/s or less for a printed rocket).
If OpenRocket and this app disagree, trust the one with your measured masses, and adjust fins or nose weight before flying.