Model Rocket Engine Sizes and Classifications | The Model Rocket (2024)

When I first entered into the world of flying model rockets,I tried my hardest to research all of the different classifications and motorsavailable. If you are not a complete aeronautics genius, and maybe you justwanted to get into a new hobby, the information available is mind-boggling.Equations and nomenclature that you might not have ever heard of before arethrown at you from all directions.

It’s important to have a deep understanding of how rocket motorsare classified and what the various different types are so that you cansuccessfully launch your rocket, know what to expect, and even compete incompetitions with other rocket aficionados. But first, at a high level, whatare the model rocket engine sizes and classifications?

Model rocket enginesare classified by engine sizes A through O, with the higher letters being morepowerful. The numbers associated with each engine letter describe the burnduration and delay before firing an ejection charge. Certain engine sizesrequire further certifications and licenses in order to use.

Here’s a simplistic chart of the varying model rocket engine classifications:

You might have seen a similar chart like this before when researching model rockets, but what do all these numbers and letters really mean? In this article, I’ll walk you step-by-step on understanding the engine sizes, classifications, and nomenclature used.

Are you still using the standard Estes controllers for your launches? Have you considered building your own custom launch controller? We just built our own beautiful launch controllers that make launches SO much more fun, and we documented EVERY single step and item purchased and put it into a step-by-step course that teaches you how to do the exact same thing.

Click here to learn more about how you can build your own launch controllers!

What is a Total Impulse?

It does no good to review the total impulse of a rocketmotor without first understanding what that number reflects, as well as it’simportance. Total impulse is obtained by taking the average thrust of therocket and multiplying it by the total burn time of the motor. It can bemeasured in either newton seconds or pounds seconds.

Burn time is the total allotted time that the motorproduces thrust altogether. It’s the time measured that shows how long themotor actually propels the rocket.

Average thrust is measured by dividing the completethrust over the complete burn time of the motor. Thrust is the force the motorproduces during its complete burn time. Finding the average thrust will helpyou determine how much weight a specific motor can handle.

Using burn time and average thrust in your calculation fortotal impulse will help you figure out the complete amount of momentum themotor can provide to the model rocket.

Thrustcurve.org offers a great visual chart of these calculations here.

Or, check out NASA’s official graph here for calculating model rocket engine designation.

How to Decipher the Motor Codes, What Do the Numbers and Letters Mean?

While figuring out the specific class of the motor issimple, and we’ve already gone over it’s relation to total impulse, you mayhave noticed that all model rocket engines being sold have designated numbersfollowing the letter. What do these numbers mean?

For example, let’s look at the model rocket engine numberA8-3. This is a commonly used single stage motor. What exactly does the, “8-3”mean?

The complete motor code is built from the followingspecifications:

Total Impulse (Letter) + Average Thrust in Newtons (FirstNumber) + Time Delay in Seconds (Second Number)

An A8-3 motor, has a total impulse of 2.5 Newton Seconds,with an average thrust of 8 Newtons, and a Time Delay of 3 seconds.

Some A motors have a “1/2”, “1/4” or, “1/8” in front of theletter itself. This simply means that motor is exactly one half, one fourth, orone eighth the size of an actual A motor.

We’ve already discussed what total impulse and averagethrust are, along with how they are calculated, but now you might be wondering:

What Is A Time Delay?

Rocket engines are constructed with a delayed chargemechanism that helps prevent the recovery system from deploying too early. Each rocket engine will on average burn upits propellant in a designated time frame. This is usually about 1 second.However, even after the propellant is gone the rocket continues in an upwardpath, and you would not want the recovery system to activate.

Activating the recovery system this early would likelyresult in a failed recovery and the parachute would be rendered useless, thusleaving your model rocket to crash and become damaged. You’ll notice when thetime delay charge has begun that it sends out smoke from within the rocket thatyou can visually track. This also helps you to see where the rocket is. Oncethe smoke is gone, the recovery is activated.

Altimeter

Larger and higher-powered rockets may be equipped with an altimeter. This device determines that altitude the rocket has attained and makes the determination based on this figure of when to activate the recovery system successfully. We put together a whole starter guide for Model Rocket Altimeters and Accelerometers if you’d like to know more about this topic and see some affordable options to adding these to your rocket.

Here’s a chart I’ve put together so you can have a quick visual of the common motors and their codes.

Black Powder Rocket Motors VS Composite Motors

There are two types of rocket motors that can be used blackpowder rocket motors and composite motors. Black powder rocket motors are themost commonly seen engines.

How Does A Black Powder Rocket Motor Work?

It may seem obvious by the name, but these little motors arestuffed with black powder propellant. They have an iconic brown paper-likeshell in the form of a tube. Along with the propellant is the delay charge withsmoke and the ejection charge. In front of the propellant is a clay-likematerial known as the nozzle.

When it comes time to launch the model rocket, an igniterwill be inserted into the nozzle, and the launch device will use this igniterto effectively explode the propellant, resulting in a launch.

How Does A Composite Motor Work?

A composite motor works nearly the same as a black powderrocket motor except instead of utilizing black powder these engines are builtwith a composite propellant that is nearly three times as powerful as blackpowder motors the same size. This extra power is extremely beneficial ascomposite motors can be made much smaller and lighter but drive the same amountof power.

Composite motors are both sold as single-use and reloadable.The reloadable type features an aluminum casing that can be used multiple timesand cleaned, helping the model rocket user save money and have the ability tobuild their own engine. Reloadable engines are seen more with high-poweredmodel rockets.

What are the Different Sizes and is Bigger Better?

Larger engines can, in fact, produce greater average thrust volumes, and drag force which directly corresponds with how much weight the engine can pull and the maximum thrust that might be achieved. You’ll commonly find that large model rockets use large engines because they can pull more weight. That doesn’t mean they will fly higher than a smaller rocket that is using a smaller engine, though.

All things such as weight, drag force, average thrust, andmaximum thrust should all be considered. A smaller engine could have a verysimilar maximum thrust and achieve the same height, but it may not be able topull as much weight as a larger engine.

Here is a chart of standard sizes based on the motor class.

Note: These sizes are standard only, and many manufacturers may have different diameter and length specifications than what is listed on this chart.

What are Multi-Stage Engines?

Some model rocket designs utilize two or possibly threemotors in stages. There are single-use motors, and then there are multi-stagemotors. Multi-stage motors will have a lower and sometimes intermediate stagemotor that will show a value of 0 instead of a delayed time. This is becausethey do not have a delayed time mechanism; they only contain propellant.

The final stage or, “upper stage” engine will include thedelay and ejection system. The lower and intermediate stages burn through thepropellant that applies heat through the nozzle of the next stage successfullyigniting it in succession.

Only black powder engines can be used in multi-stagerockets. Composite engines are designed with a propellant that requires highpressure to sustain burning. This need makes it difficult for the heat andflame to pass from motor container to the next motor container. Thus, theynormally just burn out.

The main benefit of having a multi-stage motor is that itwill fly much higher. This also means though that it will drift further in thewind, so special consideration has to be taken when choosing the appropriatelysized launch field.

Can You Build Your Own Model Rocket Engine?

You may run into some conflicts building your own modelrocket engine if you have to ask permission for use of land to launch, or youhave to discuss approval with your local fire marshal. If you don’t look like aprofessional who knows what you’re doing, it might rub a few local agencies incontrol of your park grounds the wrong way.

The main reason it can be an issue with obtaining approvalfrom fire marshals and local agencies is that you can’t necessarily tell themhow high or how far the model rocket may ultimately go. This makes it hard tofigure out if you have the appropriately sized launch field to keep you andothers in proximity safe.

In fact, in the state of California, you legally can not usean engine that is not certified by the NAR (National Association of Rocketry)first. California is officially the only state that has such a strict rule onmodel rocket engines and what is allowable. Not only do they have to becertified by the NAR, but they must also be certified by the state itselfbefore use.

You’ll probably run into some conflicts if you plan to usethe motor in a NAR competition or launch area as well, their safety codesrestrict the use of any engine on a specified field that is not certified bytheir association. They also have a designated Range Safety Officer on everysite that makes sure all model rockets being used are first certified and donot conflict with their safety rules.

If you plan to make your own rocket fuel, exercise extreme caution. The materials you are working with can be explosive and extremely dangerous. This website does not advise you to make your own rocket fuel.

If you’re curious, here is a good overview of someone else making rocket fuel. Again, exercise extreme caution if you plan to do this as it can be very dangerous.

Some commonly expressed tips were:

• If using a pre-manufactured model rocket, you’llhave to build your engine to fit the stock engine mount your rocket comes with,or you’ll need to modify the engine mount to make it work.
• Be prepared for failure, and maybe opt to use aninexpensive rocket or two for testing.

Model Rocket Engines That RequireCertification

There are some model rocket engines that requirecertification, licensing, or permits in order to operate/use because they areconsidered high-powered. Here is a list of these certification levels and thecorresponding rocket engine classifications they are assigned to.

Level 1 Certification

Level 1 certifications allow the purchase of motors that arein the H, and I total impulse category. These classifications have motors thatboast total impulses of 320-640 Newton Seconds. On average the cost of thesemotors might run you anywhere from $40-$65.

Level 2 Certification

Level 2 certifications allow the purchase of motors that arein the J, K, and L total impulse category. These classifications have motorsthat have total impulses of 1,280-2,560 Newton Seconds. You cannot apply for alevel 2 certification without at least obtaining a level 1 first. On averagethe cost of these motors might run you anywhere from $70-$230.

Level 3 Certification

Level 3 certifications allow the purchase of motors that arein the M, N, and O total impulse category. These classifications have motorsthat show total impulses of 10,240-40,960 Newton Seconds. This certificationalso requires that the model’s design and construction be thoroughly reviewedprior to flight. These motors will run anywhere between $300 to $1,000+.

FAA/AST Permit/License Required

P motors that have a total impulse of up to 81,920-newton seconds require an FAA/AST permit or license to purchase and operate. These rockets are considered “suborbital rockets.” Find out more information on the experimental permits required here.

Largest Rocket Motor Used by Amateurs

Classified as the S motor with a total impulse of 655,360 Newton Seconds, this motor is the largest kind to be used by amateur rocket builders. The team behind the rocket that used the motor was known as CSXT they were the first entity to actually launch an amateur rocket in space. They did this on May 17^th of 2004. We wrote an article covering the CSXT and if model rockets can reach space. Check it out!

How High Can My Model Rocket Go Based on The Motor?

So, we’ve explained the different model rocket motorclassifications, sizes, types and what total impulse means, but in simpleterms, maybe you just want to know how high the rocket can really go based onengine size.

How high the model rocket goes will depend on a few things:

• The maximum thrust of the engine
• The maximum lift weight of the engine
• The weather conditions during the launch
• The design of the model rocket body
• Is the rocket using a multi-stage motor?

Model rocket engine manufacturers like Estes list out their maximum thrusts and lift weights on a helpful chart here. If you’re buying a complete assembly of a model rocket, it might give you a specific idea of how high the model rocket will travel. If you’re building your own, and simply purchasing a certified engine, you’re going to have a hard time determining beforehand how high it will really travel.

The most common model rocket engines can fly from 100 feetup to several thousand feet. I’ve put together a chart on the most common modelrocket engine classifications and their average expected heights based on a fewof the top manufacturers’ listed specifications. Again, be advised these arejust average ranges and ultimately the model rocket body, weather, and moremight change these figures.

How to Select the Right Motor

Selecting the right motor is not as simple as determininghow high you want your model rocket to go. If you’ve already purchased a modelrocket kit, most manufacturers will list the recommended engine for this rocketto have optimal performance and altitude.

If you’re building your own model rocket, it may take sometrial and error in figuring out the right motor for use. These things have tobe considered:

1. The weight of your model rocket total and themaximum weight pull of the engine.
2. The size of the engine compartment.
3. The delay time your rocket will need for a successfulrecovery.

Apogee Rockets offers a great walkthrough guide on determining the best motor to use with your model rocket, read it here.

You might also explore available rocket software likeRockSim, which Apogee recommends in their walkthrough guide. This softwareallows you to design your rocket with different motors installed and thesoftware will let you know your expected statistics and suitability based onyour choices and design. This software does cost about $125, but they offer afree 30-day trial or your money back.

Learn more about RockSim, or purchase it here.

Exploring Model Rocket Kits for Purchase

The easiest way as a beginner to set out into the world oflaunching and flying model rockets is to buy one already designed and ready forassembly and launch. Usually, the manufacturer will even give you a prettyaccurate estimate of how high the model rocket can go based on the recommendedengines.

Let’s walk through a few of the popular model rockets beingsold on Amazon right now (Note these kits do not come with the rocket modelengine, but all of the rockets featured use one or multiple of the enginesdiscussed above in this article, you’ll find these listed in the specifications):

Beginner Kits

Estes Alpha III Rocket Launch Set (link to read reviews on Amazon)

This beginner’s rocket kit is recommended for anyone 10years and older. It’s very simple to put together and doesn’t require anypainting.

Included in the kit is:

• Launch Controller
• Launch Pad
• Rocket Assembly Kit
• Orange Plastic Nose Cone
• Body Tube
• One-piece plastic tail fin unit
• Decals for decoration

What’s not included:

• Model rocket engine
• Starters
• Recovery wadding
• Glue

Specifications:

• Skill level: E2X, simple to glue, no painting required
• Age Requirement: 10 years and up, adult supervision under 12
• Engines Recommended: 1/2A6-2, A8-3 (first flight), A8-5, B4-4, B6-4, B6-6, C6-5, C6-7
• Projected Max Altitude: 1,100 feet
• Weight of Rocket: 1.2oz

Purchase the Estes Alpha III directly on Amazon here.

Estes 2452 Athena Flying Model Rocket Kit (link to read reviews on Amazon)

As one of the most popular beginner’s rockets from Estes,the Athena comes completely fully assembled, so you don’t even have to worryabout the effort in putting it together, you can get ready for immediatelaunch!

Included in the kit is:

• Fully Assembled Model Rocket
• Parachute Recovery System

Not included in the kit:

• LaunchPad
• Controller
• Model Rocket Engines
• Wadding
• Igniters
• Igniter Plugs

Specifications:

• Skill Level: Beginner, Already Assembled
• Age Requirement: 8 years and up
• Engines Recommended: A8-3 (first flight), B6-4, C6-5
• Projected Max Altitude: 1,125 feet
• Weight of Rocket: 1.4oz

Purchase the Estes Athena directly on Amazon here.

Advanced Kits

Estes 2440 Magician Flying Model Rocket Kit (link to read reviews on Amazon)

This incredibly tall model rocket (33.5 inches tall to beexact) is manufactured for use with D and E rated engine, and can soar nearly1600 feet! The rocket is designed to have a larger than normal payload. Thefins of the rocket are produced with laser cut wood fins, and the rocket comeswith an 18-inch parachute that is already assembled.

Included in the kit is:

• Body tubes
• Laser-cut wood fins
• Waterslide decals
• Payload section
• 18-inch parachute

Not included in the kit:

• Building supplies
• Finishing supplies
• Model rocket engine
• Launchpad equipment

Specifications:

• Skill Level: Advanced, Assembly Required
• Age Requirement: 18 years or older, adult supervision under 18
• Engines Recommended: D12-5, E9-6
• Projected Max Altitude: 1,600 feet
• Weight of Rocket: 3.5oz

Purchase the Estes Magician directly on Amazon here.

Expert Kits

Estes Comanche-3 Shuttle Flying Model Rocket Kit (link to read reviews on Amazon)

For the expert model rocket enthusiast. The Comanche-3features three stages, which mean it uses three engines to achieve itsoutstanding altitude! Assembly may take up to a day, but the wait is well worththe pleasing abilities of this model rocket!

Included in the kit is:

• High-quality body tubes
• Laser-cut balsa
• Cardstock parts
• Plastic nose cones
• Waterslide decals
• 24” preassembled parachutes

Not included in the kit:

• Launch controller and pad
• Tools, construction and finishes supplies
• Recovery wadding
• Model rocket engines

Specifications:

• Skill Level: Expert, assembly required
• Age Requirement: 10 years or older, adult supervision under 12
• Engines Recommended:
  • 3 Stage Launches:
    • 1st Stage: C11-0, D12-0;
    • 2nd Stage: B6-0, C6-0;
    • Rocket: B6-6, C6-7
  • 2 Stage Launches (using 2nd and 3rd Stages)
    • 2nd Stage: C6-0
    • Rocket: B4-4, B6-4, C6-5
  • Rocket Only: B4-4, B6-4, C6-5
• Projected Max Altitude: 2,250 feet
• Weight of Rocket: 2.5oz

Purchase the Estes Comanche directly on Amazon here.

Build Your Own Launch Controller

Don’t forget! We recently built our own beautiful launch controllers that made the launch process SO much more fun! And we documented everything about it and put it into a super easy to follow step-by-step course showing you how to do the exact same thing! It’s called License to Launch and available here! Here’s an overview: