I've said this many times but it is worth repeating to anyone NEW to BP clustering:
RELAY IGNITION SYSTEMS are not A way to ignite clustered BP motor, It is the ONLY reliable way to ignite them.
There are 3 parts to reliable Cluster Motor ignition:
1) Check, Check, Check all igniters (Starters). Before installing in the motors, After installing in the motors, and one last check with the model on the pad.
This can be done easily with a Ohm meter or a pocket continuity checker (see diagram attached).
2) The Battery: Use as large a 12volt high Amp/hr battery as you can afford, for 1-3 motor BP clusters a Hobbico 7amp/hr Gel-Cell will do, For larger clusters I wouldn't use anything smaller than a 26amp/hr Gel-Cell for 4-12 motors. (REMEMBER 2amps per Igniter is the rule of thumb).
3) Most important "THE RELAY": For smaller 1-4 motor clusters a 10amp D.P.D.T. relay works just fine, for larger clusters I use a 40amp/contact power relay.
Why a relay? First the relay moves the power source from the controller side of the circuit to the Launcher side, shortening the amount of cable the power must go through to get to the igniters. Connect the relay to the battery with the heaviest and shortest length, stranded wire you can get (I use #14 to #2ga stranded copper). Use 18-20" micro clip leads mechanically connected to the Load side of the relay. 18" of micro clip lead allows a round of wire under the model to allow first motion of the model to raise some while all motors ignite before the clips are ripped off. Below is a drawing and photos of a "Range box Relay" that I've used successfully to ignite up to 10 motors.
4) Clip whips are NEVER the answer to cluster ignition! No matter how well made, a clip whip is ONLY an extension of the existing Launch Control system, In fact a clip whip actually decreases power transfer efficiency by adding inferior connections and added length to the already to long, small gauge controller wiring.
There is also an in depth Tech Tip-006 "Clustering BP Motors" a 6 page study of what has to happen in the first millisecond to gain successful Cluster ignition. Go to
www.narhams.org look for the library tab on the left hand menu, click on the library tab then look for Tech Tips.
Since switching over to Relay ignition I've launched over 2000 2-10 motor clustered models with a total of 13 unlit motors in all those thousands of motors clustered. I haven't had an unlit motor since 1997. The system works!
View attachment RangeBox Relay-a2c_Rev Drawing & Wiring_01-31-96 Rev.06-23-10.pdf
View attachment Cluster Wiring Diagrams-2c_Dwg(Revised)_09-17-12.pdf