Multi motor Crayon

[CharlaineC]

while browsing the local dollar store I found a plastic yellow crayon bank. Well I nabbed it up allong with 2 sheets of foam core. I'm planing on building her to have 4 different motor mounts that are held in buy duel locking bolts. So my questions are?

1. should I line the plastic body with poster board, a bt that fits it or use a stuffer tube.
2. inorder to handle interchangeable moter mounts should light plywood be used for the centering ring that holds the mount or should I make the the it so the bottem locks it in.
3. should I make the moter mount holds the fins?
4. i'm thinking on 2- 24mm cluster, 3- 24mm cluster, single d/e, and a 29mm mount. is this to much? or should i stick to a single setup.

 

[FROB]

If you want to play with clustering, airstarts etc my favorite config is 6 motor tubes arranged around a slightly larger central tube- in this case 6x24mm around a 29mm, if it fits. You don't have to use 7 motors at once, but its one of the most flexible arrangements, it lets you do the following:
clusters:
2x 24 w/optional 29mm middle (or 24mm middle with adapter)
3x 24 w/optional 29mm middle (or 24mm middle with adapter)
4x 24 w/optional 29mm middle (or 24mm middle with adapter)
6x 24 w/optional 29mm middle (or 24mm middle with adapter)
Airstarts:
Launch on 29 or 24mm central,
fire 3x 24mm at T+x
fire 3x 24mm at T+Y
Or
Launch on 29 or 24mm central,
fire 2x 24mm at T+x
fire 2x 24mm at T+Y
fire 2x 24mm at T+z

lots of others possible too... no need for interchangeable motor mounts, which are a lot more trouble to build.

This arrangement around a larger central motor leaves a small gap between the outer motor tubes, which is perfect for mounting 3 or 6 fins directly to the central and outer motor tubes, so it makes everything extremely strong & rigid, plus you only need 1 centering ring really.
If space allows its a good idea to cant the outer motor tubes towards the expected CG point- that way if an engine doesn't light in a cluster chances are it will still go straight.

 

[AndyC]

My son and I built a couple of crayon rockets. Ours had fairly thick cardboard (~4") tubes. If the plastic on yours is flimsy, I'd stuck a tube through it, and after centering it with a couple of rings, maybe use some urethane foam to hold it in.

We built ours with just 29mm motor mounts. Since it is difficult to get a good bond to the plastic cap on the end of the crayon, we stuck 3 rings in - 2 wedged inside the the ends of the cap, and one epoxied into the cardboard tube. This way the plastic does not carry any load from the motor, but the cap really is just wrapped around the motor mount. We also filled the empty space aournd the motor mount with 2 part foam so the plastic cap stuck on good, and did not get deformed by bumps, etc. Also filled the plastic cone with foam, which also keeps it from getting dinged and helps keep the plywood bulkhead in place. If you don't use an internal tube, you will need to do a real good job of scruffing up all the plastic parts where you want glue to stick.

If we had added cluster mounts to ours, I'd have used fixed motor mounts, like FROB suggested. That way you can glue the fins to the central MMT. I'd probably stick to (3) 24mm mounts around the 29 mm if you plan to use 3 fins, 4 tubes for 4 fins. Unless you plan to fly electronics, you will need to be careful with motor ejection to be sure the charge does not vent through an unused motor tube. Either plug unused tubes, or only have the 29mm vented to the body tube, and always use the central motor.

My 2 cents. Sounds like fun. We really like our crayons, and fly them a bunch, mostly on G and H motors.

 

[Micromeister]

Charlaine:
You shouldn't need anything inside the tube, I've been flying a 2-D12 clustered 2.5" dia crayon and a 5-d12 Clustered 4" dia crayon for years.

At least the rear ring should be Lite ply if your planning on switching motor mounts with T-nuts or threaded inserts. the forward can still be foamcore or heavy cardboard for that matter to save a little excess weight.

These tubes are kinda funky, If you really want to do TTW fins try to support the Inside of the tube while cutting as they tend to seperate while sawing. Personally I didn't preferring to use epoxy rivets instead. If you'll stick with a clipped Delta or trapizoidal fin form located a bit forward of the aft end of the tube it seem to help prevent fin damage on landings.
As mentioned i've been flying these crayons a long time and used to be pretty often without a single fin break or damage. My fins are .063 polycarbonate which I think also helps a bunch

You really need to check the weight of the model deciding on motor selection. if yours is a 2.5" dia crayon You'll be fine, if one of the larger dia crayons 2 D12's may not be enough.