# How High Will My Model Rocket Go? Reading Motor Codes

What C6-5 actually tells you, why a C motor is not automatically 10 newton-seconds, and how mass, drag and body tube diameter each move the apogee.

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- **Canonical URL:** https://dothecalculation.com/blog/hobby/model-rocket-altitude-guide
- **Category:** Hobbies & Leisure Utilities
- **Author:** Do The Calculation Team
- **Published:** 2026-09-20
- **Reading time:** 17 min read
- **Publisher:** Do The Calculation (https://dothecalculation.com)
- **Methodology:** https://dothecalculation.com/methodology

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Everything about a model rocket flight is encoded in two places: the three-part code stamped on the motor, and the mass and shape of the thing you glued it into. Read both properly and you can predict the apogee to within a sensible margin before you drive to the field.

## Reading the motor code

The National Association of Rocketry set the three-part code that every motor sold in the United States carries. Using their own example of a C6-3:

- **C** — the letter gives the total impulse class. NAR: "The letter C in our example motor above tells us there is between 5.01 and 10.0 n-sec of total impulse in this motor."
- **6** — the average thrust in newtons. This is how fast the energy comes out, not how much there is.
- **3** — the delay in seconds between burnout and the ejection charge firing, so the rocket can coast to apogee before the parachute comes out.

Each letter doubles the impulse range of the one before it, which means the scale climbs far faster than the alphabet suggests.

**NAR motor impulse classes (impulse limit in newton-seconds)**
| Class | Impulse limit | Category |
| --- | --- | --- |
| 1/8A | 0.3125 | Micro power |
| 1/4A | 0.625 | Low power |
| 1/2A | 1.25 | Low power |
| A | 2.5 | Low power |
| B | 5 | Low power |
| C | 10 | Low power |
| D | 20 | Low power |
| E | 40 | Mid power |
| F | 80 | Mid power |
| G | 160 | Mid power |
| H | 320 | High power, Level 1 |
| O | 40,960 | High power, Level 3 |

> **A C motor is not 10 newton-seconds** — This is the point most altitude estimates get wrong, and NAR states it outright: "you cannot assume that the actual total impulse of a motor lies at the top end of its letter's power range — an engine marked C might be engineered to deliver only 5.5 Newton-seconds, not 10." Two motors both marked C can differ by a factor of two in the energy they deliver. Look up your specific motor on NAR's certified motor list and use its tested figures.

NAR also give the rule of thumb that ties the two numbers together: "the thrust duration of a motor can be approximated by dividing its total impulse by its average thrust." A C motor with 9 N·s of impulse and 6 N average thrust burns for about 1.5 seconds.

## First question: will it leave the pad?

Before altitude, thrust has to beat weight. A 60 g rocket weighs 0.060 × 9.80665 = 0.59 N, so a 6 N motor gives a thrust-to-weight ratio of about 10:1 — plenty. Load that same motor into a 400 g rocket and the ratio is 1.5:1, which technically lifts off but leaves the rocket slow, unstable off the rod, and at the mercy of any crosswind.

> **The 5:1 habit** — Most fliers want a thrust-to-weight ratio of at least 5:1 so the rocket is moving fast enough for its fins to stabilise it by the time it leaves the launch rod. A rocket that crawls off the pad weathercocks into the wind and can arc over dangerously. This is a rule of thumb rather than a published limit, but it is a widely followed one and it exists for a good reason.

## How high: the three things that move it

Apogee comes in two parts — the height gained while the motor burns, and the height gained coasting afterwards. On a light rocket the coast is by far the bigger share.

**A 60 g rocket, 25 mm body tube, Cd 0.75, on three motors**
| Motor | Impulse used | Max speed | Boost height | Coast height | Apogee |
| --- | --- | --- | --- | --- | --- |
| A8 | 2.5 N·s | 38 m/s | 6 m | 59 m | 65 m |
| B6 | 5.0 N·s | 70 m/s | 30 m | 140 m | 170 m |
| C6 | 9.0 N·s | 109 m/s | 91 m | 228 m | 319 m |

Notice the coast column. On the C6 flight the motor only lifts the rocket 91 metres; the remaining 228 come from coasting. That is why the delay number on the motor matters so much — pick a delay that fires the ejection charge at the top of that coast, not halfway up it.

## Mass, drag and diameter

**Same C6 at 9 N·s — changing one thing at a time**
| Change | Max speed | Apogee |
| --- | --- | --- |
| 40 g | 137 m/s | 345 m |
| 60 g (baseline) | 109 m/s | 319 m |
| 80 g | 86 m/s | 272 m |
| 120 g | 57 m/s | 173 m |
| 18 mm body tube | 120 m/s | 442 m |
| 34 mm body tube | 95 m/s | 227 m |
| 41 mm body tube | 84 m/s | 183 m |
| Cd 0.30 (very clean) | 123 m/s | 496 m |
| Cd 1.00 (draggy) | 103 m/s | 273 m |

Body tube diameter is the surprise. Going from a 25 mm tube to a 41 mm one at the same mass and motor costs 136 metres of apogee — more than doubling the mass does. That is because drag scales with frontal area, which scales with the square of the diameter. A 41 mm tube has 2.7 times the frontal area of a 25 mm one.

Drag coefficient matters nearly as much, and it is the one you can improve for free. Sanded fins with a rounded leading edge, a smooth filled body tube and a properly fitted nose cone are worth more altitude than a lighter motor is.

Tool: [Estimate apogee for your rocket](https://dothecalculation.com/calculators/model-rocket-altitude-calculator) — Apogee, maximum velocity and the boost and coast heights from mass, body diameter, drag coefficient and your motor's average thrust and burn time.

## What the model does not know

The calculation above is a closed-form solution for a rocket flying straight up with constant mass and a constant drag coefficient in still air at sea level. Every one of those is an idealisation, so treat the output as a planning figure rather than a prediction.

- Mass is not constant. The motor loses propellant as it burns, so the real rocket is lighter at burnout than at liftoff, and the model uses one figure throughout.
- Drag coefficient is not constant either, and it is not something most fliers measure. Values around 0.45 to 0.75 are typical for a conventional model rocket, but that is a range you pick from rather than a number you know.
- Air density falls with altitude and changes with temperature. The model uses 1.225 kg/m³, the sea-level standard.
- Rockets do not fly straight up. Any wind makes the rocket weathercock, and a tilted flight path converts some of that altitude into downrange distance.
- The launch rod matters. A rocket still accelerating as it leaves a short rod has less stability margin than the model assumes it does.

The honest way to close the gap is an altimeter. They are cheap, they fit in the payload bay of most kits, and one flight with one tells you more about your rocket's real drag than any amount of arithmetic. Fly the same rocket on the same motor, compare the altimeter reading with the prediction, and you have calibrated your drag coefficient for every future flight of that airframe.

## Choosing the delay

The last number on the motor is the one people get wrong most often, and NAR give the principle: "Longer delays are best for lighter rockets, which will coast upwards for a long time. Heavier rockets usually do better with shorter delays — otherwise the rocket might fall back down to the ground during the delay time."

An ejection that fires too early pops the parachute while the rocket is still moving fast, which shreds it. One that fires too late deploys on the way down, sometimes very near the ground. The right delay puts the charge at apogee, where the rocket is momentarily near stationary.

> **And a motor with a 0 delay is not a delay at all** — NAR: motors marked with a time delay of 0, such as a C6-0, "are booster engines. They are not designed to activate recovery systems." They are for the lower stage of a multi-stage rocket and will ignite the next stage rather than deploy a parachute. Flying one in a single-stage rocket means no recovery deployment at all.

## Choosing the delay from the coast time

The delay number on the motor should roughly match the time the rocket spends coasting from burnout to apogee. That coast time comes from the same drag model as the altitude, and for a rocket coasting straight up it has a compact closed form.

**Coast time from burnout to apogee**

```
k = ½ × air density × frontal area × drag coefficient
Coast time = √(m ÷ (g × k)) × arctan(v × √(k ÷ (m × g)))
```
- m is the rocket's mass in kilograms, g is 9.80665 m/s², and v is the speed at burnout, which the calculator reports as maximum velocity.
- With no drag the arctangent term reduces to the familiar v ÷ g. Drag makes the coast shorter, which is why real delays are shorter than a vacuum calculation suggests.

**Coast time for the example rocket (25 mm body, Cd 0.75)**
| Rocket and motor | Speed at burnout | Coast to apogee | Nearest common delay |
| --- | --- | --- | --- |
| 60 g on an A8 (2.5 N·s) | 38 m/s | 3.3 s | 3 |
| 60 g on a B6 (5.0 N·s) | 70 m/s | 4.9 s | 4 or 6 |
| 60 g on a C6 (9.0 N·s) | 109 m/s | 5.9 s | 5 or 7 |
| 40 g on the same C6 | 137 m/s | 5.4 s | 5 |
| 120 g on the same C6 | 57 m/s | 4.9 s | 3 or 5 |

When the coast time falls between two available delays, it helps to know which way to err. An ejection slightly before apogee happens while the rocket is still rising slowly; one slightly after happens as it begins to fall. The worst outcomes are well away from apogee in either direction. Motor manufacturers publish recommended delays for their kits, and those recommendations, plus the first flight of a new design with an altimeter, are better guides than any single calculation.

## How fast the rocket leaves the launch rod

The thrust-to-weight ratio earlier in this guide matters because of what it does at the end of the launch rod. Fins only stabilise a rocket once air is flowing over them fast enough, and until then the rod is doing the steering. Ignoring drag, which is small at these speeds, the speed at the top of the rod is easy to estimate.

**Speed at the end of the launch rod**

```
Acceleration = (thrust − weight) ÷ mass
Speed at rod exit = √(2 × acceleration × rod length)
```
- Use the motor's initial thrust if you know it; average thrust gives a rough figure.
- Rod length here means the distance the rocket travels while still guided, not the rod's total length.

**Rod exit speed on a 6 N average-thrust motor, ignoring drag**
| Rocket mass | Guided length | Acceleration | Speed at rod exit |
| --- | --- | --- | --- |
| 60 g | 0.9 m | 90 m/s² | 12.7 m/s |
| 60 g | 1.0 m | 90 m/s² | 13.4 m/s |
| 120 g | 0.9 m | 40 m/s² | 8.5 m/s |
| 400 g | 0.9 m | 5 m/s² | 3.1 m/s |

The 400 g rocket leaves the rod at about 3 m/s, which is slower than a moderate breeze. At that speed the fins can do very little, and the wind turns the rocket into itself as soon as the rod stops guiding it. This is the physical reason behind the 5:1 thrust-to-weight habit, and why heavier rockets need either more initial thrust or a longer rod.

## Coming down: parachute size and descent rate

What goes up has to come down at a speed that does not damage it. A parachute's descent rate follows from the balance between weight and drag, using the same drag equation as the ascent.

**Descent rate under a parachute**

```
Descent speed = √(2 × m × g ÷ (air density × Cd × canopy area))
```
- Canopy area is the area of the parachute's flat circle: π × (diameter ÷ 2)².
- Parachute drag coefficients vary with design. The table below assumes 0.75 for illustration; your parachute's maker may publish its own figure.

**Illustrative descent speed, assuming a parachute Cd of 0.75 (m/s)**
| Rocket mass | 12 in chute | 18 in chute | 24 in chute | 30 in chute |
| --- | --- | --- | --- | --- |
| 60 g | 4.2 | 2.8 | 2.1 | 1.7 |
| 120 g | 5.9 | 4.0 | 3.0 | 2.4 |
| 250 g | 8.6 | 5.7 | 4.3 | 3.4 |

Doubling the parachute's diameter quadruples its area and halves the descent speed. That looks like an obvious win, but slow descent has its own cost, which the next section shows. Choosing a parachute is a trade between landing gently and landing somewhere you can reach.

## Wind drift: why rockets land so far away

**Drift under a parachute**

```
Drift ≈ (apogee ÷ descent speed) × wind speed
```
- A simple estimate that treats the wind as the same at every height, which it rarely is. Wind usually increases with height, so real drift is often larger.

**How far a rocket drifts from a 319 m apogee**
| Descent speed | Time to land | Wind 3 m/s | Wind 5 m/s | Wind 8 m/s |
| --- | --- | --- | --- | --- |
| 4 m/s | 80 s | about 240 m | about 400 m | about 640 m |
| 8 m/s | 40 s | about 120 m | about 200 m | about 320 m |

A C6 flight on a gentle parachute in a moderate breeze can land 400 metres away. Halving the time in the air halves the drift, which is why fliers use smaller parachutes, streamers, or a reefed parachute on windy days, and why a large parachute is not automatically the safe choice. The rocket still needs to land slowly enough not to break; the art is finding the smallest recovery device that does that.

> **What the safety code says about wind and rod angle** — The NAR Model Rocket Safety Code requires launching "in safe weather conditions with wind speeds no greater than 20 miles per hour", and from a launch rod, tower or rail "pointed to within 30 degrees of the vertical to ensure that the rocket flies nearly straight up". The drift table above shows why the wind limit matters even well below 20 mph: every extra metre per second of wind adds tens of metres of drift for every minute the rocket spends under its parachute.

## How big a field you need

The NAR Model Rocket Safety Code sets minimum launch site dimensions by the total impulse installed in the rocket. They are worth knowing before you choose a motor, because a larger motor quickly needs a larger field than a local park provides.

**Minimum launch site dimensions, NAR Model Rocket Safety Code**
| Installed total impulse | Equivalent motor | Minimum site dimension |
| --- | --- | --- |
| 0.00 to 1.25 N·s | 1/4A, 1/2A | 50 ft |
| 1.26 to 2.50 N·s | A | 100 ft |
| 2.51 to 5.00 N·s | B | 200 ft |
| 5.01 to 10.00 N·s | C | 400 ft |
| 10.01 to 20.00 N·s | D | 500 ft |
| 20.01 to 40.00 N·s | E | 1,000 ft |

Compare those with the drift figures above and the reason for them is plain. A C motor needs a site at least 400 feet across, about 122 metres; the drift table shows a rocket landing hundreds of metres from the pad on a breezy day. The code's other limits are worth knowing too: a model rocket must weigh no more than 1,500 grams at liftoff and contain no more than 125 grams of propellant or 320 N·s of total impulse, and spectators must be at least 15 feet away for D motors or smaller, 30 feet for larger.

## A first-flight checklist built from the numbers

- **Weigh the rocket with the motor installed.** Every figure in this guide depends on mass, and kits often come out heavier than their box figure once painted.
- **Check thrust-to-weight and rod exit speed.** If the rocket leaves the rod slowly, choose a motor with more initial thrust or use a longer rod.
- **Estimate apogee and coast time.** Choose a delay close to the coast time, guided by the manufacturer's recommendation.
- **Match the field to the motor.** Check the minimum site dimension in the safety code for the impulse you are installing.
- **Size the recovery for the day's wind.** On a breezy day, a smaller parachute or a streamer keeps the rocket within the field.
- **Fly an altimeter if you can.** One measured flight calibrates your drag coefficient for every future flight of that airframe.

## Stability: the check that comes before any altitude

None of the altitude arithmetic matters if the rocket is unstable, because an unstable rocket does not fly straight up; it loops, cartwheels or heads off sideways. The NAR Model Rocket Safety Code says that if you are uncertain about the stability of an untested rocket, you will check it before flight and fly it only after warning spectators and clearing them away to a safe distance.

- **Centre of gravity (CG)** is the balance point of the rocket, found with the motor installed by balancing it across a finger or a ruler edge.
- **Centre of pressure (CP)** is the point where the aerodynamic forces act, set by the shape of the nose, body and fins. Rocket design software and kit instructions give it.
- **For stable flight the CG has to be ahead of the CP**, towards the nose. A widely used rule of thumb is a margin of about one body diameter, called one calibre, between them.

If the CG is too far back, the usual fix is to add weight to the nose, which moves the CG forward. It costs altitude, and the drag model shows how much. On the example 25 mm rocket with a 9 N·s C6, adding 3 g of nose weight to a 60 g rocket lowers the apogee from about 319 m to 312 m, 5 g takes it to about 308 m, and 10 g to about 297 m. A few metres of altitude is a small price for a rocket that goes where it is pointed.

## Sources

- National Association of Rocketry, Model Rocket Safety Code — the minimum launch site dimensions, the 20 mph wind limit, the 30 degree launch angle, the 1,500 g, 125 g propellant and 320 N-s limits, spectator distances, and the stability check for untested rockets: https://www.nar.org/ModelRocketSafetyCode
- National Association of Rocketry, Rocket Motor Resources — the three-part motor code, the impulse class table, the impulse-divided-by-thrust rule of thumb, the warning not to assume a motor sits at the top of its class, and the delay guidance including booster motors: https://nar.org/content.aspx?page_id=22&club_id=114127&module_id=669253
- NAR's certified motor list gives the actual as-tested total impulse and thrust for each approved motor, which is what the calculation should use rather than the class limit.
- The altitude model is a closed-form constant-mass drag solution using standard sea-level air density of 1.225 kg/m³ and g = 9.80665 m/s². The drag coefficient is an input you choose, not a measured property of your rocket.

## Where to go next

If the thing you are flying has a battery in it rather than a motor, [LiPo batteries: C rating, flight time and the 80% rule](/blog/hobby/lipo-battery-guide) covers what the pack label actually promises and how long it will really fly for.

## Common questions

**What does C6-5 mean?**

C is the total impulse class, between 5.01 and 10.0 newton-seconds. 6 is the average thrust in newtons. 5 is the delay in seconds between the motor burning out and the ejection charge firing. The letter tells you how much energy there is; the number before the dash tells you how fast it comes out.

**How high will a C6 motor take my rocket?**

For a 60 g rocket with a 25 mm body tube and a drag coefficient of 0.75, around 319 metres on a 9 N·s C6 — 91 metres under power and 228 coasting. Double the mass and it falls to about 173 metres. Use a 41 mm tube instead of 25 mm and it falls to 183 metres.

**Is every C motor the same power?**

No, and NAR say so explicitly: a motor marked C might deliver only 5.5 newton-seconds rather than the 10 at the top of the class. Two C motors can differ by a factor of two. Look up the as-tested figures for your specific motor on NAR's certified motor list.

**What thrust-to-weight ratio does a model rocket need?**

Enough that it is moving fast enough for the fins to work by the time it leaves the launch rod. Most fliers aim for at least 5:1. A 60 g rocket on a 6 N motor is about 10:1; a 400 g rocket on the same motor is 1.5:1, which will leave the pad slowly and weathercock badly.

**What drag coefficient should I use?**

Something in the range of about 0.45 to 0.75 for a conventional model rocket, and it makes a real difference: on the example above, 0.30 gives 496 metres and 1.00 gives 273. Because you are choosing rather than measuring, fly once with an altimeter and work backwards to the value that matches your airframe.

**Why does body tube diameter matter so much?**

Because drag depends on frontal area, which goes with the square of the diameter. A 41 mm tube has 2.7 times the frontal area of a 25 mm one, and on the example flight that costs 136 metres — more than doubling the rocket's mass does.

**How do I choose the right motor delay?**

Match it to the coast time from burnout to apogee. For a 60 g rocket with a 25 mm body on a 9 N·s C6, the coast is about 5.9 seconds, between the 5 and 7 delays. Use the kit manufacturer's recommended delays as your guide, and an altimeter flight to refine the choice.

**How big a parachute does my model rocket need?**

It depends on mass and how fast you want it to land. With an illustrative parachute drag coefficient of 0.75, a 60 g rocket falls at about 4.2 m/s under a 12-inch chute and 2.8 m/s under an 18-inch one. Bigger parachutes land more gently but drift much further in wind.

**How far will my rocket drift?**

Roughly the time under the parachute multiplied by the wind speed. From a 319 m apogee at 4 m/s descent, the rocket is in the air for about 80 seconds, so a 5 m/s wind carries it about 400 m. Wind usually increases with height, so real drift is often larger.

**How big a field do I need to launch a C motor?**

The NAR Model Rocket Safety Code gives a minimum site dimension of 400 feet for 5.01 to 10.00 N·s of installed impulse, which is the C class. A motors need 100 feet, B motors 200 feet and D motors 500 feet.

**How fast does a model rocket leave the launch rod?**

Ignoring drag, the square root of twice the acceleration times the guided length. A 60 g rocket on a 6 N motor accelerates at about 90 m/s² and leaves a 0.9 m rod at about 12.7 m/s. A 400 g rocket on the same motor leaves at only about 3 m/s, too slow for its fins to stabilise it.

**How do I know if my model rocket is stable?**

Its centre of gravity, found by balancing it with the motor installed, must be ahead of its centre of pressure. A common rule of thumb is a margin of about one body diameter. If you are unsure about an untested design, the NAR Model Rocket Safety Code says to check stability before flight and to clear spectators to a safe distance.

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_Source: [Do The Calculation](https://dothecalculation.com/blog/hobby/model-rocket-altitude-guide). Quote freely with attribution and a link to this page._