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There are two types of staging and one type of clustering.
Parallel staging is very similar to clustering, as you will see in a minute, except for one small difference. More commonly used is series staging which is also similar to clustering but at the same time very, very different.
Let's start off with clustering, since it's easier for most people to understand. Clustering is igniting two or more motors (in a cluster) simultaneously. For example, you could put three C6-7 motors into a rocket and ignite them all at the same time. It's having more than one motor, side by side.
Parallel staging is the same thing except that when these motors burn out, the are jettisoned. This means that they are 'dropped' from the rocket and the simply fall back to the ground. The rest of the rocket coasts upward and usually ejection is accomplished through a means other than by the motor (since there are no longer any motors on board). Parallel staging is actually very rare, and I've personally never even heard of a kit that uses it.
Much more common is series staging. When one motor burns out, the next one kicks in and the first is jettisoned and falls back to the ground. In staging, the first motors to be fired are called the boosters. The airframe that houses them is usually known as the booster section and the main airframe is known as the sustainer.
A common stage configuration is a C6/C6-7. The first motor usually doesn't specify a delay because IT MUST NOT HAVE ONE! To stage, you must have a motor that ends in -0 instead of some other number so that it will have no ejection charge. That's because you don't have anything to eject. Instead, you have to ignite the upper stage motor. This is accomplished as you can see in the diagram at right. The most difficult thing in staging is upper stage ignition. It should be ignited at the point of maximum speed (not altitude) in order to gain maximum altitude. The reason is illustrated in detail in The Handbook of Model Rocketry by G. Harry Stine. In order to do this, you want to vent the booster airframe by making two holes on either side of it. This allows pressure to escape, giving the burning particles from the booster engine time to ignite the upper stage. A lack of venting VERY OFTEN causes pressure to build up and jettison the booster stage without upper stage ignition, causing a lawn dart.
In clustering, the outside engines are called the outboards and the central one (if you have one) is called the core. A common cluster is a D12 core motor with several C6-7s as outboards.
The hardest part of clustering is igniting all the motors at the same time. It is not a good idea to hook up igniter-to-igniter because this means that when one engine is ignited before another one, the second one won't be lit (because the first has broken the circuit). It is better to wire them in parallel, as shown at left. When one burns out, the others will still pass current until they ignite.
The diagram at right shows a typical staging configuration. It shows exactly what to do. The one at left shows what to do if the engines are of different sizes.
All of these diagrams have been taken (with permission) from Peter Clay's web site.
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