2 ©2015
MadCow
Rocketry™
mini
DX3™
Assembly
mini DX3™ Assembly
©2015 MadCow Rocketry™
3
Please make sure you read all directions and understand how to assemble your model
before you start construction. It is also a good idea to test fit each part before assembly
– some manufacturing tolerances may require light sanding before final assembly.
Laser cut parts will exhibit varying amounts of charring on the edges depending on the
density of the plywood. The charred edges do not interfere with bonding and do not
need to be cleaned before assembly. In most cases the charring will be cleaned up
during sanding for finishing and painting.
Step 1 – Motor Mount Assembly
Step 2 – Insert Motor Tube Assembly into Body Tube
Test fit each of the fins into the pre cut fin slots. Because
plywood thickness varies, you may need to sand the slot
to the correct width. Wrap the shock chord into a small
bundle and stuff it inside the motor tube for this next step.
Test fit the motor tube assembly into the body tube to
ensure a snug fit. Sand the centering rings if necessary.
When you are satisfied with the fit, spread some epoxy on
the inside of the body tube and slide the forward centering
ring of the motor assembly into the body tube.
Make
sure you have the motor assembly facing the right
way!
Spread some more epoxy on the inside edge of the
body tube before sliding the rear centering ring into the body tube. Continue sliding the
assembly inside the body tube until the aft centering rings are just clear of the fin slots.
It’s a good idea to test fit a fin in each slot here before the epoxy sets. Hold the body
tube with the motor tube assembly down until the epoxy sets. Make sure the weight of
the motor tube doesn’t cause it to slide out of alignment.
Step 3 – Fin Assembly
Using a door jam or small section of angle stock, pencil a line halfway between two of
the fins that extends from the front to the back of the body tube. This line will be used
later to align the launch lugs. Test fit each of the fins into the pre cut fin slots. The fin
should seat firmly against the motor tube - sand each fin or slot if necessary. When you
are satisfied with the fit, apply some epoxy to the end of the fin tang that will contact the
motor tube. Also, spread a thin layer of epoxy on each side of the fin tang. Slide the fin
into place and check the alignment. Continue rechecking the fin alignment until you are
sure the epoxy has set. Clean any excess epoxy from around the fin joint. Repeat for
the remaining fins. Next, apply epoxy fillets to both sides of each fin by applying a thin
bead of epoxy at the fin-body tube joing. Carefully smooth the epoxy fillets with your
finger before the epoxy sets. Allow each fillet to set before rotating the airframe for the
next fillet.
Test fit centering rings over the motor mount tube and sand motor tube if necessary. Also
test fit the centering rings in the body tube and sand if necessary. Spread some epoxy on
the outside of one end of the motor tube and slide the ring (without the notch) until there
is approximately 1/2” of motor tube exposed. Make sure you clean the motor tube of any
epoxy so as not to interfere with the fin tangs later. After the aft ring is dry, make a mark
1/2” from the other end of the motor tube. Spread some epoxy on the motor tube and
slide the forward ring until it aligns with the mark.
VERY IMPORTANT: make sure there
is not any epoxy on the motor tube that would interfere with the fin tangs later on.
Fin
Fin Tang
1/2” Exposed
1/2” Exposed
Forward Ring Notch
Push cord in corner and
tack with epoxy
The shock cord in this kit consists
of a shorter section of Kevlar and a
longer section of nylon cording. The
two sections should be tied together
using a single overhand, ring bend
or double fisherman’s knot. The
Kevlar section will be attached to
the motor mount and the nylon
section will be attached to the nose
cone.
Wrap the end of the Kevlar shock
cord around the forward end of the
motor tube and tack in place with
CA. Make sure the cord lays flat
enough so it will not interfere with
the body tube when you slide the
motor tube inside. Apply some
epoxy to the cord to hold it in place.
Make sure the cord is secure
and will not come loose later with
ejection forces that will pull on the
shock cord.
Step 5 – Launch Lug
Mark the CP point along the launch lug line you made in the previous step. Make sure
you measure the CP point from the tip of the nose cone and NOT the end of the body
tube. Apply a small amount of epoxy on the launch lug line about ¾” long on the CP
mark. Press one of the launch lugs into the epoxy and ensure that it is aligned with the
launch lug line previously drawn on the body tube. You can site down the tube and look
through the launch lug to make sure it is straight. Similarly epoxy the second launch
lug about 2” from the aft end of the body tube (aligned with the aft end of the fins). Site
down both launch lugs and make sure they are both aligned. If you have a ¼” launch
rod, you can use this to ensure that both lugs are aligned properly.
Ensure rings are clear of the fin slots
Step 4 – Forward Section Assembly
Insert the eyebolt through the hole in the center of the bulkhead and secure using the
washer and nut.
IMPORTANT: Apply some epoxy to the nut and eyebolt threads
to ensure the nut doesn’t come loose later.
Apply some epoxy to the inside of the
coupler and push the bulkhead in so there is about a 1/8” to ¼” of coupler exposed.
After the epoxy has set, apply a fillet of epoxy around the inside edge of the coupler
bulkhead joint.
Next mark the coupler 2” from the forward edge. Apply some epoxy to the inside of the
forward body tube section and slide the coupler up to the mark. There should be 2”
of coupler exposed. Make sure the coupler is straight and the body tubes are aligned
properly when they are assembled later.
Step 6 – Balancing Your Model
Assemble your model and insert the largest motor you intend to fly (or simulate the
weight with a substitute) and ensure that the CG is at least 1 body diameter in front of the
estimated CP point specified on the first page. The CP point is measured from the tip of
