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ROCKET MOTOR CODES
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Rocketeers have their own language, and you often hear them talking about motors: "This little guy flies great on a B6-6" or "This rocket screams on a G-40-7T". So what does it all mean? The codes are broken down into 4 sections:
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.
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:
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). Aerotech are the big fish in the composite motor game. Their motors have an additional code, which designates the propellant characteristics. Here's a table listing the various Aerotech propellants and their characteristics.
Our example motor has a T designation, which makes it a Blue Thunder. There is also a new 'sparkling' propellant (which I doubt we'll ever see in Australia). 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 You MUST be a member of an approved rocketry club if you wish to purchase composite rocket motors in Western Australia. Approved rocketry clubs include:
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. 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: RMS 29/60
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).
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 D12 has greater average thrust at 12 N/sec compared to the 9 N/sec of the E motor but the E motor has more initial thrust. What this means to you is that the E9 can launch a heavier rocket than the D12 because the higher initial thrust will help get the rocket stable before it leaves the launch rail, but the D12 will average more thrust. You can think of it as being the difference between a push and a flick. Sometimes one is more desirable than the other. 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.
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