The webmaster's Viper III cluster rocket wearing the Aerotech Mustang decal. Click for larger image.

Clustering is the act of using multiple motors in parallel. Clustering is interesting, unique and a sure way to keep you on the poverty line! There are a great many ways to cluster - you're only limited to a few basic laws of nature, your budget and your imagination.

If you've never clustered before, we recommend that you buy a cluster kit rather than scratch build one. The LOC Viper III and Viper IV are good starter kits. The biggest range of cluster kits I've found so far are made by US Rockets.

Ok, the first thing to note about clustering is the potential problem of asymmetrical thrust; that is to say, all lit motors need to balance each other out, with the majority of the thrust being towards the center line of the rocket. In some configurations, if one or more motors fail to light, the rocket can go into a suicidal flight attitude and smash to pieces. This really is just common sense.

Let's have a look at some common cluster configurations:

This is the most basic cluster configuration. 2x in-line. This configuration requires that both motors be rated the same:- i.e. 2 x D12-5	3x triangle. A fairly common config. This config also requires all motors to be of the same rating.	3x In-line. Also very common. You could use say, an E-9 for the central motor, however, both outboard motors need to be of the same value as each other. Maybe 2 x D12's. Or you can use the same motor in all tubes.
The 2 x 2 square. Very popular. All motors can be of the same value, or motors diagonal from each other can be of same value, with the other pair of motors being of different values. You can even fly on 2 motors if you plug the remaining motor tubes.	2 different sized motors can be used if you keep asymmetrical thrust in mind. Here we see a central 38mm motor with 4 x 29mm motors.	There's no limit to 'mix n matching' with motor sizes. How about central 54mm motor, with 4 x 38mm and 4 x 29mm motors!

I might also mention that to ignite clusters all igniters should be joined together in parallel. If you join the igniters in series, then the second igniter is not going to go off because the circuit is broken when the first igniter burns. This means that if you have say, a 3 motor cluster, one lead from each igniter should be connected to one lead on the other two igniters and the same for the other lead, so you end up with six leads (each igniter has 2 leads) with three leads twisted together and another three leads twisted together. I hope that makes sence! What you dont want is one lead connecting to the next in a chain (or row).

Also, you can not cluster Quest tiger tail or Aerotech Copper head igniters - these are the single lead type igniters. You must use twin lead igniters. If you are using black powder motors, you can use 'flash pan' ignition which involves placing a small amount of black powder in a tray directly under the motors and placing an igniter in the powder and firering the igniter as normal. The black powder will ignite in a big flash and (hopefully) set off your cluster of black powder motors. Note that you can not use this technique for composite motors. To ignite AP clusters you need twin lead igniters such as the Aerotech 'First Fire' igniters.

For more information, including how to successfully ignite clusters, download this PDF from www.rocstock.org. It's probably the most informative document about clustering.