Australian Rocketry Association INC

(AEROTECH SHORT MOVIES)  can help new people to the hobby understand reloades

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The codes are broken down into 4 sections:

• G - This specifies the TOTAL IMPULSE of the motor and is measured in Newton seconds
• 40 - This number specifies the average thrust of the motor in Newton's
• 7 - This is the ejection charge delay in seconds. Faster rockets want longer delay times.
• T - This is the propellant type

TOTAL IMPULSE

This is an indicator of the total power potential of the motor. Each letter has twice the power potential as the previous letter as you will see in the following table.

AVERAGE THRUST

This, as the name implies, tells you how much thrust the motor generates during it's burn as an average. The G40 generates 40 newtons of thrust on average. It also provides an indication of the rate at which the motor burns. A higher number equates to a faster burn and a lower number to a slower burn. In our example of a G-40 motor, we can see that this is a fairly slow burn motor. You can get an idea of the rate at which a motor will burn, by dividing the total impulse by the average thrust. We could expect our example motor to burn for about 4 seconds *(3 seconds is normal). If it were a G80 we could expect it to burn for about 2 seconds *(1.5 seconds). So we can see that the higher the number, the higher the initial thrust, but a shorter burn time results.

Sometimes you will see a motor that has a higher average thrust than the total impulse for that motor - like an F100 for example. A full F motor has a potential of 80N/sec of thrust. This does NOT mean it can generate no more than 80 newtons of thrust. It means that an F100 will generate 100 newtons of average thrust and burn for less than a second.

* Because most motors fall short of their total power potential, most burns times are actually less than what you work them out to be - as with everything related to rocket motors, there is only a fine line of 'certainty'.

EJECTION CHARGE DELAY

This is the time in seconds between motor burn out and the ejection charge. As a basic rule, if a rocket is fast or flies high, you'll want a longer ejection charge delay. If the rocket is slow and heavy, you'll choose a short delay. If it's a booster for a staged rocket, you don't want any delay at all.

PROPELLANT TYPE

There are three common types of propellant:

1. Black powder
2. Amonium Perchlorate
3. Plastic / N2O

Most motors in the "A" through "C" range are black powder. You can also get "D" and "E" black powder motors. Black powder motors are typically low thrust motors and are relatively inexpensive compared to composite amonium perchlorate motors.

Amonium perchlorate (AP) composite)) propellant is the standard propellant used in mid and high power rocketry. In Australia, you need a license to posses composite motors. Amonium perchlorate motors range from the 18mm D21, right up to 98mm "P" impulse motors (which cost thousands of dollars each)

Our example motor has a T designation, which makes it a Blue Thunder. There is also a  'sparkling' propellant

Plastic / N2O or Hybrid motors as they are commonly referred to, have a small bottle of nitrous oxide connected to a fuel grain of plastic (or just about any other material (rubber and cellulose also being popular)). The plastic is the fuel and the N2O is the oxidizer. All components of a hybrid motor are totaly inert and they are intrinicly safe and can only be deemed 'dangerous' when the right sequence of ignition events take place. Hybrids are usualy low thrust motors and require expensive ground support equipment (GSE) and they take longer to prepare for flight. They must also use electronic deployment (altimeter) because they do not have a motor initiated ejection charge. One of the great things about hybrids is once you have the GSE, the fuel grains are much cheaper than AP motors and you can buy them over the net because they are an inert substance.

COMPOSITE SINGLE USE MOTORS & RELOADS

Composite motors come in three different types - Single use, loadable and reloadable. It should be noted that black powder motors burn from the nozzle up, while all composite motors burn from the top down. With composite motors, you are required to slide the ignitor into a slot - the ignitor rests against the delay element. It should also be noted that composite motors vary greatly in length, unlike black powder motors. This is why most mid and high power rockets don't have any form of motor retention (thrust ring & motor clip)- this gives you greater flexibility in what sort of motor you can use. Retention is usually via friction fit (wrapping tape around the nozzle end of the motor) - as to how you keep the motor from ejecting when the ejection charge fires is up to you and is something you should be able to figure out for yourself when you get to this level of rocketry but there are commecial solutions available.

Single Use: These are similar to standard black powder motors but are typically three times more powerful. Single use motors are convenient and quick to prep but they are a bit expensive. Most people stepping up from black powder to composites will start off flying on single use motors, but it won't be long before you start looking at reloads.

Loadable Motor System (LMS): These are a cross between single use and reloadable motors. They are a single use motor and all parts are disposed of after use, however you are required to perform some assembly of the motor before you can use it.

Reloadable Motor Cases and Systems (RMS): Reloads consist of two parts:
1) The reload casing. This is the 'shell' for the motor and they are available to suit different levels of total impulse. You only need one reload case for any given impulse range and a reload motor system allows you to use more than one range of impulse. The difference is in the length of the case. Reloads have a slightly different numbering system as follows:

RMS 29/40 - 120
The '29' stands for a 29mm motor. The 40 - 120 stands for the maximum system impulse range in Newton seconds.

RMS 29/60
Again, the 29 stands for a 29mm motor. The 60 stands for the maximum total impulse of the case in Newton seconds.

As you can see from the above table, if you purchase a 29/60 as opposed to a 29/40 - 120 you will be putting severe limits on the type of reloadable motor you can use as it is only suitable for motors in the 60 N/sec range.

2) The reload motor. This is the actual motor that goes into the reload case and is the part that you replace after each launch. It actually consists of a number of components such as O rings, the propellant grain, the delay element plus some other parts that are essential to the motor. All the parts of the motor are assembled into the reload case and it is important that you follow the instructions carefully and assemble the motor in the correct order. Both the reload case and reload motor will have instructions on how to do this. It should also be noted that different motors have different assembly requirements, so do not presume that because you've reloaded a 29mm F motor that the assembly for a G motor is going to be exactly the same, because it probably won't be.

Reloadable Systems are pretty expensive: A 29/40 - 120 will set you back about AU$120.00 Individual cases (such as the 29/60) are a little cheaper but the initial expense is worth it because the motors are cheaper and in the long run you will save money. Provided you look after your reload case and take care so that it doesn't get ejected from your rocket and lost, it should last you many years. The downside of reloads is that it can take 15mins to 45mins to reload the motor after each flight but this is good in it's own way because you get better value out of your day out. You could burn $100 worth or single use motors in an hour or burn the same in reloads over the whole day. You also need to clean the reload case after every launch. Failure to do so can result in a CATO (catastrophic take off).

Single Use Motors
Estes and Aerotech motors above
Left to right: 18mm black powder (B4-4) - Single use. 24mm black powder (D12-5) - Single use. 24mm composite (E30-7T) - Single use. 29mm composite (F50-6T) - Single use.

Loadable Motor System
Typical contents of an LMS	Completed LMS Motor

Reloadable Motors
Left: 24/40 RMS Case Right: 29-40/120 RMS Case	Typical components of a 29mm reload motor.

SELECTING A MOTOR

When selecting the right motor to use, many people who are new to rocketry only tend to look at the total impulse of the motor - after all, a G has to be better than an F right? Not always. As you can see from the chart above, a G impulse motor has a power range of 80 - 160 n/sec. Most motors don't come anywhere near the top range of the motors potential power - 80 - 160 n/sec is quite a range and it gets far worse, to the point of being totally meaningless, as you get into high power motors such as L and M impulse.

The average thrust is more important. Let's say you have a rocket and you have the choice of using a D12 or an E9. The E9 motor will have a much greater total impulse, almost double that of the D12. The D12 however, has greater average thrust at 12 N/sec compared to the 9 N/sec of the E motor. What this means to you is that in some situations the D12 will launch a heavier rocket more safely than the E9. This is because the higher initial thrust the D12 supplies will help get the rocket stable before it leaves the launch rail. Once in the air, though, you can guarantee that the E9 motor would lift your rocket higher. "

 The reality of selecting a motor is to consider the the entire motor code and match it to your rocket's weight and/or diameter. Most rockets come with a recommended motor chart and it is advisable that you stick to these recommendations. If it's a scratch built rocket, well, we hope you've done your homework and remember to advise the range safety officer that you intend to launch an unproved scratch built rocket.

MOTOR CASES AND RELOADS

Following is a table of Aerotech RMS cases (excluding R/C cases) and the reloads that can be used in them. If you can not find the motor you are looking for check the Aerotech products list click * HERE *