An Analysis of the New Apogee Micro Motors -- What Makes Them so Much Fun to Fly?

By Tim Van Milligan, President of Apogee Components, Inc.
NAR 35872
URL: www.ApogeeRockets.com
email: tvm@apogeerockets.com

In this article I thought it might be a good idea to review the characteristics of the Apogee 10.5mm diameter motors for people who are not familiar with them. Before I begin, I'll tell you my reasons for doing this article. First, there is that self-serving motivation to sell more rocket motors. I clearly admit this, but there is another reason too. Being a modeler myself, knowing the good and bad points of the motors would allow me to use them to their best advantage; as well as staying away from those situations where the motors are not useful or that could be unsafe. By letting other modelers know these opinions too, I believe that we'll keep the hobby much safer and more fun.

First I'd like to list all the general characteristics of the motors. Then I'll give my opinions and reasons for the advantages and disadvantages that each trait offers, compared to motors made by Estes or Quest. I'll try to keep the orientation of the comparison geared toward "sport" models, because that is where most modelers get their enjoyment.

Here are the characteristics that I'll go into depth about:

  1. Small diameter
  2. Longer lengths
  3. Low thrust level
  4. Longer burn duration
  5. Low case mass
  6. Price
  7. Specific Impulse
  8. Total Impulse

From these characteristics, you can make your own conclusions as to whether these motors would meet your own needs.


1. Small Diameter

The most obvious trait of the new motors is their skinny diameter -- 10.5mm. From a competition modeler's point of view, the smaller the diameter of the model, the higher it can fly because it has lower drag. So this is a very large advantage in that situation.

And smaller models are typically less expensive to build compared to big rockets needing heavy-duty construction. Therefore the advantage of having a smaller motor can mean that the overall model cost can be cheaper too. For example, you don't need any expensive tools, equipment or supplies like epoxies to build small models. Typically all you need is a knife and some wood glue.

But the skinny diameter also allows the development of some unique sport models. A good example is the "peanut-size" models. Already many modelers are building scale rockets in tiny sizes that are cute and friendly. The new Micro V-2 kit from Apogee Components can only be made as small as it is because of the small diameter of the 10.5mm motors.

The aerodynamics of bigger models can be improved by using a smaller motor too. A simple modification that can be made to "non-minimum diameter" models is to put a boattail on it to make a streamlined shape. Then those 1950's style "Buck Roger" rockets can be made super slick to fly a lot higher.

I like to think that one of the greatest advantages the smaller size gives for sport models is the ability to have sliding mechanisms carried internally. An example of this is the "sliding pod" type rocket glider. The motor's small size means that it can slide easily inside a larger tube, so repositioning the mass of the model after burnout is far easier. And it can be done while keeping the overall size of the model small to reduce the construction costs. Another example of a sliding mass is the "rear-ejection" type models. Here the "slider" is the motor itself. And by careful positioning of the centering rings around the motor, you can create a pocket for the recovery device. This compartment will help protect the parachute from the heat of the exhaust gases, so recovery wadding won't be needed.

The disadvantage of the "odd" size is that the motors do not simply slip into your existing rockets. But this drawback is easily overcome with a motor mount adapter to increase the diameter of the motor to make it fit larger models. Transitioning from "micro" to "mini" is easily accomplished with a few wraps of masking tape over the diameter of the motor.


2. Longer lengths

The next most obvious trait of the new motors is they are longer in length than modelers are familiar with. The reason for the longer length is a result of the skinny diameter. The extra length is needed to hold the proper amount of propellant inside the motor. To get a full power "B" motor, you need around 6 grams of black powder, and if the motor is skinnier, it must be made longer.

The advantage from the longer length is that it moves the Center-of-Gravity (CG) of the motor forward; and hence the model's CG forward too. This helps make the model more stable when it is launched, hopefully increasing the safety of the flight. So for a hobby modeler, they might not have to add extra nose weight into a model to get it to fly straight; allowing a wide variety of rocket shapes that can be launched. And as the motor burns propellant, the CG continually moves forward and the stability factor of the rocket increases; hence the overall safety of the model is enhanced.

From the competitor's standpoint, this is absolutely GREAT! With the stability of the model increased, he has the option of decreasing the area of the fins slightly without having the model go unstable. And by reducing the area of the fins, the drag of the model lowers, and it can fly even higher!

The big disadvantage of the longer length is similar to having an odd size diameter; the motors don't readily fit in existing models. But again, this is a fairly easy modification to "un-built kits" -- just move the position of the motor block forward to accommodate the longer motors. Another option is to let a portion of the motor hang out the rear of the model, and then compensate for it with extra nose mass (if necessary).


3. Low thrust level

The 2-Newton average thrust level of the motors is a big advantage for models that are fragile and can't withstand high liftoff stresses. Models can be built using lighter-weight materials -- which again improves the overall performance of the model by allowing them to fly higher. And again, light-weight materials are typically less expensive. You can use paper (heavy cardstock) fins instead of balsa, and balsa instead of basswood or plywood.

One example of a typically fragile model is the boost glider. I think that we have all seen models shred at liftoff, and the problem is that the model was over-powered. A lower thrust motor like the micro motors increases the success of the launch by lowering the forces trying to tear the model apart.

The other characteristic that low thrust motors offer is the model will lift off from pad slower. This could be either good or bad depending on the situation. If the model is "neutrally stable," taking off slower would probably mean that it would go unstable after clearing the rod. But for models that are stable, the slower liftoffs are nice; it gives the model a "realistic" appearance, like real NASA style rockets that also lift off slow. And when they lift off slow, the roar of the motor seems to be around longer and sounds louder; enhancing the pleasure of the lift-off.

And slow lift-offs are great for spectators who might have a hard time following the flight of a very fast model. You've probably stood next to a person at a launch who asked: "Where did it go?" And since slow flights are easier to track, this lowers the chances that they might be lost in the haze of the sky. And it is always nice to return home without losing your favorite model.

The low flight speeds are fantastic for competition modelers. Because the drag on the model stays lower, the model can fly higher. You can read more about this in Apogee's Technical Publication #1. Even for sport models, there will be a slight but noticeable altitude increase because the drag on the model is lower.

The main drawback of the low thrust from the motors is that it limits the size model that can be safely lifted. So typically, models using the new motors are limited to BT-55 size and smaller. But there is still more "grunt" force from the motors than you would expect, and some heavy or draggy models can still be flown. It is best to check the maximum liftoff masses listed on the instructions that come with the motors.


4. Longer burn duration

The increased length of the motors allow them to burn for a significantly longer period of time. This is really noticeable for most models, as the smoke column is really a nice crowd pleaser. I think a lot of people love long burn motors because they remind them of high powered rockets which also produce sky-reaching smoke columns when they lift off. The longer 'motor roar' duration is an added plus too (more rumble for the ruble - or dollar).

The longer burn could be a disadvantage if the model goes into an erratic flight, as it could land in some dry grass while still under power. But hopefully modelers will take precautions to minimize this situation in any models they might fly by performing proper stability checks.

The other disadvantage is that the case is hotter after the propellant is burned. Most times this isn't a problem; by the time the model descends to the ground, it has cooled to where it isn't noticeable. But this heat can cause some types of glue to soften or to shrink. This is one reason Apogee doesn't recommend friction fitting the motors into engine mount tubes. The adhesive on the tape can turn to goo making it difficult to remove the motor after flight.


5. Low weight case

After the motor burns out, the case mass becomes dead weight and reduces the performance (altitude, speed, and duration aloft) of the model. So having a lightweight case like the micro motors offer will increase the performance of the model.

The low mass also allows many different science fair experiments to be performed. One such example would be "optimum weight" experiments. With the low weight case, now you have to add mass to the model to get it to an optimum mass for best altitude. So comparing different motors to each other is simple and fun to perform.

And when the motor weighs less, the overall model mass will be lighter too. Therefore it doesn't need to be built as strong to survive hard landings, since it will descend slower. For example, you don't too often see "G" powered helicopters, because the helicopter blades would be easily damaged when the model lands hard on the ground. But if lighter motors could be used, the blades are more likely to survive to fly again.

As a comparison, the typical Estes 18mm motor has a case that weighs about 10 grams, compared to less than 5 for the Apogee "B" motor. The Estes mini motor (13mm dia) has a case mass of 3 grams, which is still over the 2.5 gram mass of the Apogee "A" motor.

It is hard to think of a disadvantage to having a lightweight motor and model; so I can't list anything here. If you need more mass to make the model more stable, or to get up to optimum weight, it is better to add mass to the forward part of the rocket by using tracking powder.


6. Price

Even though the micro motors are much cheaper than composite motors, the obvious disadvantage to the new micro motors is their higher price compared to other black powder motors made by Estes and Quest. It would be great if you could have all the advantages already listed and a lower price too, but there are always tradeoffs.

But if you compare performance versus price, the new Apogee motors are reasonable.


7. Specific Impulse

The measure of how much thrust is produced per pound of fuel consumed is called "Specific Impulse." This number is used to compare motors to each other to see which is more efficient. The higher a motor's Specific Impulse number, the more efficient it uses the propellant. So at this time we'll compare different motors.

The composite propellant motors have all black powder motors beat "hands-down." They are almost twice as efficient as the black powder motors. Here is a quick comparison of the Apogee B7, the new B2, and the Estes B6.

Motor		Specific Impulse
Apogee B7	1760.56 m/s
Apogee B2	833.3 m/s
Estes B6	725.0 m/s

As you can see, even though the B2 and B6 use the same type of propellant, the B2 is slightly more efficient. How does the Apogee B2 get more efficiency out of the same type of propellant? The answer lies in the chamber pressure of the motor. The Apogee black powder motor uses a stronger paper case, so it can hold more internal pressure -- which forces the burning gases out of the motor faster, which makes the motor efficient. You can learn more about this by reading Apogee's book called "Model Rocket Propulsion."

But operating at a higher chamber pressure does has its disadvantages. If the motor should fail, you'll hear a louder pop. But so far, the micro motors are operating as designed, so we'll all have to wait and see if the paper is capable of keeping the failure rate low in spite of the high pressures trying to blow it apart.


8. Total Impulse

The Total Impulse of the motor is found by multiplying thrust created by the motor (in Newtons) by its burn duration. So a motor that produces an average of 2.5 Newtons of thrust for two seconds has a Total Impulse of 5 Newton-seconds. This is equivalent to the total power level of the motor. If you remember how motors are classified, a "B" motor can have up to 5 Newton-seconds of Total Impulse. Since a "C" motor is twice as powerful as a "B" motor, its Total Impulse would extend up to 10 Newton-seconds.

So how does this number fit into our comparison? Isn't a "B" motor from one manufacturer the same power as a "B" motor from another? The answer is "not always." The Total Impulse can be adjusted by the manufacturer of the motor and it is affected by a variety of conditions. But mostly it is a quality control aspect.

The Apogee micro motors were designed for high performance, so the Total Impulse of each has been set to the maximum allowable for the motor. So a "B" motor from Apogee is very close to 5 Newton-seconds. To prove this for yourself, you can get a copy of the NAR's motor certification tests. In theses tests, you can see that the Apogee motors have higher total impulses than motors from other manufacturers within the same motor classification. Therefore, they are slightly more powerful.

How does this benefit you? For one thing, it will make your rocket fly higher! It also tells you that you are getting the most value for the money you paid for your motors. And since the micro motors cost slightly more, you can feel confident that you are getting everything you expected from the purchase, and not being ripped off.


Other Qualities of the Micro Motors

Good Tracking Smoke

Many people have commented that the little micro motors have better tracking smoke than motors from other manufacturers. This is great for making it easier to follow your rockets into the air -- meaning you'll probably have a better chance of a successful recovery for another flight.

Healthy Ejection Charge

Another comment that has been made about the micro motors is that they seem to have a good ejection charge. This is characterized by a loud pop when the parachute is ejected from the rocket. While the ejection charge sounds "strong," it has been designed to pressurize a larger tube that has more internal volume than a skinny rocket. So if you use the motors in a larger diameter rocket, you can be assured that the motor will most likely kick the parachute out to ensure a successful and safe descent.


Conclusions

As you have read, the micro motors from Apogee have a lot of qualities that make them a lot of fun to use -- even for "sport" rockets. And these same qualities make them the best for competitions flights too. If the opportunity presents itself; why not give them a try!

You can get more information about the Micro Motors from the Apogee Components web site. The address is: www.ApogeeRockets.com. You can also get a printed version of the Apogee catalog by sending $1 to:

Apogee Components, Inc
1431 Territory Trail
Colorado Springs, CO 80919-3323

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Last modified January 27, 2006

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