Walk up to the rocket engine pegboard with a seven-year-old and you'll hear the question within ten seconds: "Which one goes the highest?" The labels look like a secret code. A8-3. B6-4. C6-5. E12-6. But each one tells you three things about the engine, and once you can read it, you can choose an engine for any rocket on the shelf.
Here's what the letters and numbers mean, why the delay number matters as much as the letter, and how to choose an engine for a first flight that comes back down in the same county it went up from.
The Short Answer
A model rocket engine code like C6-5 has three parts: the letter (C) is the total impulse class, the first number (6) is the average thrust in newtons, and the last number (5) is the delay in seconds between burnout and the ejection charge that deploys the parachute.
- Each letter can be up to twice the total impulse of the one before it. A is 1.26 to 2.50 N·s, B up to 5, C up to 10, D up to 20, E up to 40.
- Higher thrust gets heavier rockets off the pad. Longer delays suit lighter rockets that coast higher.
- A "0" delay means a booster stage only, and "P" means a plugged engine with no ejection charge.
- First flight: use the smallest engine your kit recommends, often an A8-3 or B6-4.
1. Decoding the Code: Letter, Thrust, Delay
The format is the same on every certified model rocket engine. Estes' explanation uses B6-4 as its example. We'll use the three on the label of this article.
The letter: total impulse
Total impulse is the engine's total push: thrust multiplied by how long it lasts, measured in newton-seconds (N·s). Each class has a range, and each letter's upper limit is double the one before it. The NAR Model Rocket Safety Code sets the minimum launch-site size by impulse:
| Class | Total impulse (N·s) | Minimum launch site (NAR) | Typical Estes example |
|---|---|---|---|
| 1/4A | 0.3126–0.625 | 50 ft | Mini engines |
| 1/2A | 0.626–1.25 | 50 ft | Mini engines |
| A | 1.26–2.50 | 100 ft | A8-3 (2.50 N·s) |
| B | 2.51–5.00 | 200 ft | B6-4 |
| C | 5.01–10.00 | 400 ft | C6-5 (10.00 N·s) |
| D | 10.01–20.00 | 500 ft | D12-3 |
| E | 20.01–40.00 | 1,000 ft | E12-6 (29.50 N·s) |
| F | 40.01–80.00 | 1,000 ft | F15 series |
| G | 80.01–160.00 | 1,000 ft | Mid-power G motors |
1/4A and 1/2A ranges come from NAR's model rocket motor standards. A model rocket can't exceed 320 N·s of total impulse or 125 g of propellant, and can't weigh more than 1,500 g at liftoff, under the Safety Code. Above that is high-power rocketry, which has its own code and certification.
The first number: average thrust
The middle number is average thrust in newtons. A C6 averages about 6 N. A C11 delivers the same C-class impulse at nearly twice the thrust, so it burns for a shorter time. Higher thrust gets heavy or draggy rockets moving faster off the rod, which keeps them stable. Note that the code is rounded: the C6-5 delivers 10.00 N·s over 1.60 s, which works out to about 6.25 N average.
The last number: the delay
After the propellant burns out, a slow-burning delay grain lets the rocket coast up without thrust. Then the ejection charge fires and pops the parachute or streamer. The last number is that delay in seconds. Too short and the chute opens while the rocket is still flying fast, which can strip the chute or snap the shock cord. Too long and the rocket arcs over and deploys on the way down, sometimes too low. Estes notes two special cases: -0 means no delay and no ejection charge, for booster stages only (like the B6-0), and P means plugged, with no delay or ejection charge.
2. A8-3 vs C6-5 vs E12-6: Same Code, Very Different Rockets
Here are Estes' published specs for the three engines in the title. Same code format, very different jobs.
The numbers side by side
| Spec (Estes) | A8-3 | C6-5 | E12-6 |
|---|---|---|---|
| Total impulse | 2.50 N·s | 10.00 N·s | 29.50 N·s |
| Thrust duration | 0.80 s | 1.60 s | 2.70 s |
| Delay | 3 s | 5 s | 6 s |
| Diameter × length | 18 mm × 2.8 in | 18 mm × 2.8 in | 24 mm × 3.8 in |
| Propellant | 4.1 g | 12.2 g | 36.9 g |
| Max liftoff weight | 3.0 oz (85 g) | 4.0 oz (113 g) | 14.0 oz (397 g) |
Sources: Estes product pages for the A8-3, C6-5 and E12-6.
A8-3: the first-flight classic
The Estes A8-3 Engines (3-pack) is a full A at 2.50 N·s, with a 3-second delay for small, light rockets up to 3 oz. Estes calls it one of its most popular engines. It keeps the flight low enough to watch the whole way and land close. It needs a 100-foot field under the NAR table.
C6-5: four times the impulse, same size
The Estes C6-5 Engines (3-pack) is the same 18 mm diameter and length as the A8-3 but carries 10.00 N·s, the top of class C. In the same light rocket, that's a much higher flight, so the 5-second delay lets it coast to the top before deploying. If your rocket is heavy or draggy and arcs over early, try the C6-3. If it's light and slim, consider the C6-7. Plan on at least a 400-foot field.
E12-6: a bigger engine for bigger rockets
The E12-6 is a 24 mm engine with 29.50 N·s and a 14 oz maximum liftoff weight, so it's for larger rockets with a 24 mm engine mount. Estes notes E engines can't ship to post office boxes and need a physical address. They also need a 1,000-foot field. Availability rotates: if the E12-6 isn't on our shelf, ask about the E12-8 (longer delay, for lighter rockets) or the D12-3.
3. Choosing Your First Engine (and Moving Up)
The kit decides, not the pegboard. Every Estes kit lists the engines it was designed and tested with.
Start with the smallest recommended engine
- Read the kit's engine chart and pick the lowest one listed, often an A8-3 or B6-4.
- Fly it on a calm day (the NAR code limits launches to winds of 20 mph or less) and watch where the chute opens.
- If the rocket was still going up when the chute opened, try a longer delay next time. If it was going down, try a shorter one.
- Step up one letter at a time, and only to engines the kit lists.
Match the field to the letter
The NAR table above is the minimum field size. A school ballfield may be fine for A and B engines and too small for a C6-5. If your rocket keeps drifting into trees, drop a letter or use a streamer instead of a parachute.
Safe handling
Store engines cool and dry, away from igniters. Use the correct igniter (Estes Startech starters) and a pad with a blast deflector, like the Estes PortaPad II Launch Pad. The NAR code calls for spectators at least 15 feet back for D engines and smaller (30 feet for larger), a launch rod within 30 degrees of vertical, and a 60-second wait after a misfire before anyone approaches. Our beginner launch guide walks through a full launch day.
Conclusion
Read the code left to right: letter for total impulse, first number for average thrust, last number for the delay before ejection. Pick the smallest engine your kit lists for the first flight, tune the delay after you watch it fly, and size your field to the letter. That's all a C6-5 is: a C-class engine pushing about 6 newtons, with a 5-second coast before the chute pops.
Browse rockets and engines online, or bring your kit to the store and we'll help you match engines to it. Remember that engines often have shipping restrictions, so plan your purchase before launch day.
Gear Mentioned in This Guide
Frequently Asked Questions
What does C6-5 mean on a rocket engine?
C is the total impulse class (5.01 to 10.00 N·s), 6 is the average thrust in newtons, and 5 is the delay in seconds between burnout and the ejection charge that deploys the recovery system.
Which engine should I use first?
The smallest engine your rocket kit recommends, usually an A8-3 or B6-4. It keeps the first flight low and easy to watch and recover.
Is a C engine twice as powerful as a B?
In total impulse, a C can have up to twice the impulse of a B. Class B tops out at 5.00 N·s and C at 10.00 N·s. Thrust, how fast that impulse is delivered, is the separate middle number.
What does a 0 at the end of an engine code mean?
No delay and no ejection charge. Engines like the B6-0 are boosters for the lower stage of multi-stage rockets. Never use one in a single-stage rocket. A "P" suffix means a plugged engine with no ejection charge.
How do I choose the right delay?
Start with the kit's recommendation. If the chute opens while the rocket is still climbing, use a longer delay. If it opens after the rocket has started falling, use a shorter one. Heavier or draggier rockets usually need shorter delays.
Do I need a bigger field for bigger engines?
Yes. The NAR Safety Code minimum is 100 feet for A, 200 for B, 400 for C, 500 for D and 1,000 feet for E through G engines.
