large cluster mounts and stability

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Well-Known Member
Apr 7, 2004
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Hello all! Just curious what you all think about this idea.

So here's the idea. 1x 29mm central motor surrounded by 6x 18mm motors. It'll come with a 29-24mm adapter, and plugs for all the motor tubes when not in use.

Well the question. If we make the rocket stable for 6x C motors and 1x E motor (the heaviest possible combination, if not mixing bp and composite), will it be okay if it's over stable with a 29mm by itself, or even a 24mm (if there's enough thrust, some caluculation will be needed)? It'll be hard to figure the CP because of the very odd shaped fins...

By the way, this won't be a kit or anything ;)

Thanks you all.
You have to set the stabiltiy for the heaviest possible motor combination. I've had no problem flying a 5 motor model on 3 inboard motors only, or 4 of 5 motors leaving a core motor mount blocked off. Truely depends on the thrust to weight ratio for your model and the motors selected.
OBTW: I've heard combining composite and BP motors in a clulster is not a good idea at all. Composite motors always take longer to ignite then BP so your chances of getting everything going before the model leaves the pad isn't very good. Stick to one propellant or the other in your clusters.
Yes, one of the questions that can come up in the UKRA RSO exam covers exactly this point - if you try to fire a combined AP/BP cluster using separate igniters, the BP will go first, the rocket will leave the pad seriously underpowered, and probably drop to the ground just in time for the AP motor to light.

The solution is a sort of fuse called Quickmatch. I don't know if this stuff is available in the US, but it's often used in the UK for clustering. You run pieces of Quickmatch from each outboard BP motor to a common point right under the core motor's exhaust, and put a normal igniter in the core motor. If the core motor fires, it sets off the Quickmatch and thus the BP outboards. If the core motor doesn't light, neither does anything else. I haven't done this myself because I only use BP, but I've seen it done successfully by other people.
Another solution would be to build the rocket as stable (with no nose weight) for less than a full cluster, and then to add modular (removable) nose weight as you add engines. Just be sure you've done enough sims and/or testing of your design to have confidence that the appropriate amount of nose weight that is added or removed for a given cluster configuration is correct to maintain stability.
Very Good point ForeCheck as long as somehow; I use pull plugs in the motor tubes to remind me to add the extra payload section or screw in ballast, as well as having a big note in red on my Flight log sheet to remind me "Don't forget the Ballast" in the heat of on site flight preperation. Can't tell you how many times I've forgotten to put in the counterweight. Not a pretty sight or flight:(