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6.12 Rocketry: I've built several BT-80 based models. How can I strengthen future models to take larger motors, such as Aerotech 24mm or 29mm reloads?




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This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

6.12 Rocketry: I've built several BT-80 based models. How can I strengthen future models to take larger motors, such as Aerotech 24mm or 29mm reloads?

Estes recommends using just D motors in their BT-80 based kits, such as
the Broadsword (Super Big Bertha) and Shadow (Optima). There are a number
of steps that posters to r.m.r have done to strengthen these rockets to
fly with larger motors. Remember in doing this that you are modifying and
using the rocket kit in a manner NOT recommended by the manufacturer.

From: Buzz McDermott (bmcdermo@ix.netcom.com)
Here's a summary of what I've done to strengthen my Shadow and Broadsword
kits to fly on 24mm reloads and up to 24mm G42 motors. You will need
a LOC-style 2.56 inch tube coupler, 3/4 oz fiberglass, 15-30 minute
epoxy, and 20 minute 'coating' or 'finishing' epoxy:
1. If the kit comes with the 'plain' BT-50 type motor tube, replace
that with LOC-type, thick-walled 24mm motor tube. You will only
need about 6".
2. Reinforce each body tube section with 3/4 oz fiberglass. Apply it using
one of the methods discussed in the section on High Power Construction
Techniques. Be sure to completely sand off the glassine coating of the
body tubes prior to applying the fiberglass.
3. Also fiberglass-reinforce the thick paper centering rings supplied
with the kit. Glass both sides of the rings. If you want to build a
payload section in the Shadow, then leave the center cut-out in one
of the rings and glass over it.
4. If you plan to use only 24x70 disposable motors (including Aerotech
E's) and/or 24mm reloads, then yellow glue or epoxy a motor block
2.5" into one end of the 6" motor tube. If you install the motor hook,
file down the part that sticks into the tube. This will let you
fit in 24mm reloads. Be sure and lightly sand the motor tube prior
to installing motor hook. [NOTE: If you plan to use non-standard 24mm
Aerotech motors than skip this step.]
5. Epoxy one fiberglass-and-epoxy-reinforced centering ring 1/2" from the
rear end of the motor tube. Be sure there is a notch in the ring to
allow some movement of the motor hook. Epoxy a second centering ring
in the middle of the motor tube. Epoxy a third ring 1/8" from the
front of the motor tube. Install the motor tube into the main
body tube with the motor tube flush with the bottom of the main body
tube.
6. Fiberglass-reinforce all of the fins. Apply the glass to both sides
of the fins. Be sure that the fins are completely sanded (and any
airfoiling/rounding completed) before applying the cloth. An optional
step is to apply some 1/2" wide strips of glass along all of the
fin edges EXCEPT the root edge.
7. Rough up the epoxy on the main body tube along the lines where the
fins will attach. Use 220 or coarser sandpaper. You really want the
smooth epoxy coating roughed up. You can also drill a few 'rivet'
holes along the fin attachment lines.
8. Apply epoxy where the fins will attach and attach the fins. Do not
fillet at this time.
9. When the fins are dry, apply 1/2" strips of fiberglass cloth along
each fin root edge, with 1/4" on the body tube and the other half of
the width along the fin side. Coat this will coating epoxy. When
dry, YOUR FINS WILL NOT COME OFF.
10. Couple the bottom two body tube sections together. An option step
is to make a payload section out of the third body tube section that
comes with the Shadow. Use the LOC coupler to make a payload section.
You can sand down the solid centering ring to slide inside the LOC
coupler to form a bulk-head. Epoxy a 2"x 2" piece of scrap 1/16"
plywood or 1/8" balsa to the inside surface of the bulkhead to add
strength. Install either a large screw-eye or small eye-bolt to the
center of the bulkhead, to be used to shock cord and parachute
attachment.
The end result of the above is a model which is really too heavy to
fly on a D12. My modified Shadow came out to about 14 oz (I built it
VERY heavy and added the payload section). My modified Broadsword came
out to about 12 oz. Both have been flown on motors as small as composite
D's (D21-4, D13-4R). The Shadow has flown on E15-4 (perfect), E30-4
(a -5 is really needed), F24-7R, F39-7R, and G42-8. The Broadsword
has just been test-flown on the D21-4 and E15-4. It WILL be flown on
G42's, though.

If you want to use 29mm motors in BT-80 based models, I would recommend
either switching to plywood centering rings or sticking to low-thrust
motors, such as the F14. In the end, if you want to fly 29mm, you would
really be better off getting a kit designed as a Large Model Rocket from
the start.

 

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