12.11 How do I cluster rocket motors? When igniting a cluster of rocket motors, should the igniters be wired in parallel or in series? Why?
Description
This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
12.11 How do I cluster rocket motors? When igniting a cluster of rocket motors, should the igniters be wired in parallel or in series? Why?
The advent of composite model rocket motors in 'standard' black power
sizes (18 and 24mm) has led to an increase in the use of composite motors
in cluster rockets. Mixed black powder/composite clusters are also
becoming popular. In particular, clusters of 3 or 4 composite
motors, or a composite core motor with outboard black powder motors,
are being seen more. These offer special ignition challenges. The old
black powder techniques don't work when composite motors are
involved. The most common method for clustering Estes type black
powder motors is to use multiple Solar igniters and clip whips. Flash
bulb to sheathed thermalite is the most common composite ignition
method. Although flash bulb ignition has been used for years, there
have been safety concerns over its use. Here are some suggestions from
rmr posters:
From PeteAlway@aol.com (Peter Alway):
I cluster black powder motors with Solar igniters wired
in parallel and a car battery for power. I stuff igniters
with little balls of tissue paper wadding to insure they
stay in place. My general rule is only to cluster with
a technique I use regularly for single-engine models,
as reliability has more to do with experience and my
current state of skill than with the particular technique.
[Editor's note: Estes plastic plugs work well in place of tissue
wads. The igniter plugs can be reused several times, as well.]
From: glenn@lightning.nsc.com (Glenn Newell)
My technique for clustering composite motors is to use equal length
pieces of thermalite with 1/16" heat shrink tubing as a sleeve. I
leave about a 1/2" unsheathed in the motor and about one inch unsheathed
on the other end (I don't shrink the heat shrink, it just happened to be
around and the right size). I tape all the ends together around a single
solar igniter. No flashbulb problems here!
From: billn@hpcvaac.cv.hp.com (Bill Nelson)
I prefer to use a short section of Thermalite, with igniter wires,
inserted into each motor - the wires are taped to the motor for security.
There is no need for an igniter for the Thermalite. Simply remove the
cloth wrap, and all but one of the spiral metal wires. Wrap the end of
one wire to one end of the thermalite and the end of the other wire to
the other end. You can use anything from about 22 gauge wire (if it will
fit in the grain slot) to about 28 gauge. The free ends connect to the
controller ignition wires. When the relay closes, the Thermalite wire
wrap is essentially vaporized instantly. I have never seen the
Thermalite fail to ignite.
From: burkefj@kodiak.ee.washington.edu (Frank J. Burke)
The main reason for using parallel igniters is that as one ignites, the
others are still in the circuit. As one igniter breaks in a series circuit
the circuit is broken and the others will not get any more current. It may
be that with a 12V system, and low impedance wire, that the current
provided is high enough that they flash so fast that it doesn't matter....
I have never had a failure with parallel circuits. I ... prefer using a
parallel system, knowing the limitations, using a meter to verify that the
igniters are "good" before using them, and using good connections when
wiring them up.
From: buzzman@netcom.com (Buzz McDermott)
The biggest concern with wiring cluster igniters in series is that one
igniter might burn through and break the circuit before all of the
igniters have fired. Once the circuit is broken, no more igniters will
fire.
On the other hand, it was mentioned by several posters that series wiring
is extensively used in the explosives and pyrotechnics industries
because of the added reliability you get. With series wiring you can
verify the complete igniter circuit and you will know if *any* igniter
is improperly wired. Also, you would be able to ignite many more (fast
igniting) igniters with series wiring, especially if the resistance in the
igniter is high.
From: kaplow_r@eisner.decus.org (Bob Kaplow)
For a 4 engine cluster I like to wire the ignitors in a "bridge":
X======B
/ \
/ \
/ \
/ \
/ \
/ \
/ \
A ======X X======A'
\ /
\ /
\ /
\ /
\ /
\ /
\ /
X======B'
Clips A and A' come from one clip whip. B and B' are from the other whip.
I use a manual wire wrap tool for twisting the Solar ignitors together
AFTER installing the "earplug" (tm). Be sure your wraps are nice and tight
so they all touch where they are supposed to. Having a clip on each joint
certainly helps. For multiple wire clipping, I've found that the clips
with teeth hold better than the standard micro-clips.
I've used this several times now on 4xD12 in a BT-80 rocket with
100% success.
Editors Note:
The bottom-line-consensus of the 'net' seems to favor parallel wiring for
most clusters of 7 or fewer motors, using a 12V (or more) launch system
capable of dumping plenty of amps to the igniters. This generally means
a relay based system with the primary ignition power source close to the
launch pad.
Readers are also directed to check out the NCR Technical Reports #1 &
#2, on black powder and composite clustering, respectively. Although
they are a few years old, they still contain valuable information.
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