 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
Scratch
Goblin Upscale
scale: 3x
parts list:
Airframe: LOC
BT-3.90
Motor Mount:
LOC MMT-1.14 (29mm)
Centering Rings:
3 LOC CR-3.90-1.14
Nose: THOY
ACE PNC-3.9
Fins: 3/32"
aircraft plywood
Recovery: 36"
Topflight round parachute
various hardware:
u-bolt, t-nuts, quick-links, eye bolt, 12 feet of 3/8"
elastic for
shock cord
I was looking
for a good candidate for my first scratch-built upscale project, and after
a little bit of looking around on JimZ's rocket plans site, I considered
the Estes Goblin. Is it just me, or does this little rocket just beg to
be upscaled? I know it's not a terribly original idea, but it seemed well-suited
for my first upscale. I got even more inspiration for doing an upscale
of this particular rocket while browsing Yitah
Wu's rocket web page. The scale factor of 3x was a perfect match for
the 4" body tube. After deciding on the scale factor and choosing
an airframe size, it was just a matter of ordering all the parts to build
it.
I measured
the original Goblin and came up with 27 inches for the main airframe length.
I marked the tube and carefully cut it with a sharp X-Acto. Then I cut
the motor mount/stuffer tube to 18 inches. I drilled the aft centering
ring for "Kaplow Klip" motor retention and installed the t-nuts,
then I drilled two holes in the forward centering ring and installed the
U-bolt I'm using for the shock cord anchor. I marked four lines at the
aft end of the airframe tube for four fins, then I marked four more lines
offset 3/32" from the original lines for the fin slots. I slid a
couple of centering rings into the tube to support the inside of the tube
while I cut the fin slots.
I got the fin
pattern for the Goblin from JimZ's
web site, which I also used to make my normal-scale clone.
I loaded the pattern into Paint Shop Pro and enlarged it by a factor of
3. I cropped the fin section of the image and printed it out, then used
the cutout to mark the plywood for the fins. I added a tab to the leading
edges of the fins large enough to reach the 29mm motor mount tube. I cut
out the fins on my dad's radial-arm saw and rounded the leading and trailing
edges with a block sander.
Then it was
time to start epoxying stuff together. With my first batch of epoxy (Bob
Smith's 30 min. cure), I attached the forward centering ring to the motor
mount tube. I carefully measured for placement of the middle centering
ring (I wanted it in just the right place so the top edge of the fin tabs
would rest against the underside of the centering ring) and applied a
generous fillet to the forward joint only. Then I dabbed some epoxy on
the nuts that held the U-bolt, and some on the t-nuts. I did not attach
the aft centering ring at this time.
After letting
the motor mount assembly cure, I placed a large bead of epoxy in the airframe
about where the forward C.R. would be. I slid the assembly part-way into
the airframe and applied another bead of epoxy where the middle C.R. would
be. Then I slid the motor mount assembly the rest of the way into the
airframe. After the epoxy had set a bit, I added a nice fillet to the
front C.R. where it met the airframe. I did not add a fillet to the rear
of the middle C.R. because that would've interfered with the fin tabs.
Next, I installed
the fins one at a time. I used a little CA to hold each fin in place,
then after the CA set, I applied good epoxy fillets on the outside fin/tube
joints, the inside fin/tube joints, and the inside fin/motor mount tube
joints. After all the fins had been attached and the epoxy cured, I epoxied
the aft centering ring into place. When I was done, I had a completed
airframe/motor-mount/fin assembly. The last step of the major construction
was adding two pieces of 1/4" launch lug to the airframe, each 3"
long.
Finishing
was fairly straight-forward. Two coats of Krylon primer, followed by two
coats of Rustoleum yellow on the body and Wal-Mart black on the nose.
The Rustoleum required a full 24-hours to dry before I could mask off
a fin and paint it black, as well as the stripe around the body just above
the fins. After all the paint was dry, I applied the decals I got from
Tango
Papa. The finished rocket is really sharp-looking. The final
step was attaching the parachute and shock cord.
If you make
an upscaled Goblin based on my construction, be sure you use motors with
short delays. On the first flight of my Goblin, I used an F40-7, and it
was almost a disaster. It flew much slower and lower than I'd expected,
and resulted in a low-altitude, high-speed ejection that, fortunately,
resulted in very little damage. I'd recommend flying it on: F40-4, F52-5,
G33-5, or G64-4. These are all Aerotech 29/40-120 reloads. The Fs should
take it to about 800 feet, and the Gs should boost it to about 1,200 feet,
maybe a little more.
To view the
Goblin’s flight reports, follow these links: