L
K
Each station has a pair of mini alligator clips to
clip onto the igniters, 1 for voltage and 1 for ground.
To wire the voltage clips to the 10-wire cable in the
middle of the bar, I ran a 22-gauge red wire from
each station to a pin on the 10-pin connector. I used
lots of heat-shrink wrap and zip ties, but after 210+
rocket launches, still wound up with 2 broken wires.
Wiring the igniters’ ground sides was a little more
complicated. At the ground cable’s 4-pin connector,
I soldered 4 strands of 14-gauge speaker wire, and
then ran a short and a long strand to the left, and a
short and a long strand to the right. I attached the
alligator clips by soldering to the ends of the speaker
wires where possible, or else soldering mid-wire
after scraping away some insulation, then covering
with heat-shrink (Figure I, previous page).
traces connected to a wire bundle that tapped into
the blocking diodes for each igniter. Turn the knob,
and you can check each one.
This all took as much time to get right as the rest
of the project, and after 180+ launches I realized
that it represented almost no value added. I thought
that seeing the actual voltage at each pad would
help troubleshoot, but the continuity LEDs worked
perfectly on their own, and saved many launches
where there was a short circuit at the pad. You can
also just use a multimeter to check the voltage
across the battery before each launch.
Lessons Learned
The mega-launcher made our Cub Scout Rocket
Derby a blast. More than 120 rockets were fired in
less than 90 minutes — a huge improvement over
single-launch systems! And the klaxon sound was
a big hit.
One lesson learned from the test launches is that
with the close spacing of the rockets, about 9", an
adjacent liftoff could knock an igniter loose or cause
the clips to short out. A few pieces of masking tape
solved this problem.
Another thing we found: with so many rockets in
flight at once (Figures K and L), it was difficult to
track all of them through touchdown!
Design Overkill
Eagle-eyed readers will note in the photographs
an unexplained feature that falls into the overkill
category. I hacked a multimeter to make a knob for
the control console that you can turn to check that
12V DC is running across each of the 10 blocking
diodes.
Long story short, I soldered jumpers to the
multimeter board to keep it permanently at its 20V
DC setting. Then I carefully cut and arranged copper
tape traces on a thin plastic sheet to match the contact points of the multimeter’s knob (Figure J). The
Doug “Beads” Desrochers ( beads27@cox.net) has been
voiding warranties ever since middle school. He graduated
from Georgia Tech with a B.S. in aerospace engineering,
and served in the U.S. Navy as a test pilot and instructor
test pilot as well as on several operational tours. He’s currently a civilian systems engineer and test pilot supporting
the Department of Defense in Washington, D.C.
Photography by Jason Hornick
For the circuit schematic, plus photos and a
video of the 10-rocket mega-launcher in action, visit
makezine.com/20/megalauncher.
64 Make: Volume 20
