Descon Two Fin Original Design / Scratch Built

Scratch - TwoFin {Scratch}

Contributed by Evan Ross

Manufacturer:	Scratch
Style:	Spin-Fin

TwoFin

A Spin-Stablized Competiton Design

Background

When I first started attending CMASS's meetings, one of the recurring topics of discussion was the possibility of building rockets with only two fins. Many ideas were tossed around, but it was a fairly one sided debate with all parties reaching the conclusion that you did, in fact, need to have fins in at least three planes.

Being the troublemaker that I am, I couldn't leave well enough alone. I decided to take on the challenge. Granted, the arguments all centered on having two standard, axial mounted fins, but I decided to take a slight departure from that design.

After several evenings of rough cutting, shaping, and sanding, I created a spin stabilized rocket with two canted fins. Since I had no way to predict the rocket’s stability, I decided to start with a small design to minimize any damage due to unexpected flight paths.

My first launches with A8-3s and B6-4s turned out to be absolutely wonderful. The rocket flew perfectly straight and you could easily see the spin on both ascent and, to my surprise, on descent.

Competition

Last summer I was fortunate enough to attend NARAM-41 in Pittsburgh. After several days of humiliating defeats on the competition range, I decided to give up and just stick to sport flying. I had just prepped my TwoFin and was proudly displaying it to some other fliers, when I was jokingly asked if I was going to fly it in the A-Streamer Duration event.

As I mentioned before I have a tendency to be a troublemaker, and I knew that the TwoFin flew well on A motors.

I rushed over to one of the vendor's tents and purchased a handful of streamer material.

Needless to say, I did not win the event. In fact, the only reason I didn’t come in last was that I actually completed all three flights.

There is, however, an important lesson to be learned from this experience. If you can't win with skill, you can at least lose with style.

Therefore, the TwoFin is now a competition veteran. Not a good one, but a veteran nonetheless.

Parts List

1 Body Tube, BT-50, 10 inches
1 Nose Cone (50Y or similar)
1 Motor Mount Tube, BT-20, 2 ¾ inches
1 Engine Hook
2 Centering Rings (BT-20 inside of BT-50)
1 Engine Block (18mm)
1 Launch Lug (1/8” x 1”)
1 Fin stock (3/32” balsa)
1 Kevlar shock cord (12”)
1 Elastic shock cord (12”)
1 Swivel (not a snap swivel)
1 12” parachute or 30” streamer
½ Ounce of clay

Construction

The TwoFin is a very simple design, the only tricky part is attaching the fins to the body.

Fin Assembly

Cut the fins according to the drawing.
On the body tube, mark four lines, as if you were going to install four fins. To make these directions easier to understand, number the lines 1 through 4.
Sand the inside edge of the fins to conform to the body tube. This means that they need to be angled one way at the top of the fin and the other way at the bottom of the fin.
Fit one of the fins around the side of the body tube such that its trailing edge is where line #1 meets the end of the tube and the leading edge is just touching line #2. Tack it in place using a tiny drop of CA.
Tack the other fin with its trailing edge at the bottom of line #3 and its leading edge on line #4.
Now, eyeball the fins from both ends. They should be lined up like the diagram. If necessary, undo the temporary tack (by cracking it or using a bit of CA dissolver) and reset the fins so that are evenly placed and angled around the body.
Glue the fins in place. Since the fins will be subject to more strain than normal fins, I chose to use epoxy to bond and fillet them. Medium CA with an accelerator should also work fine.

Motor Mount and Shock Cord

Mark the motor tube at 1", 2 ¼", and 2 ½" from the tail end.
Cut a slit at the 2 ½" mark and insert the engine hook.
Cut a small notch out of one of the centering tie one end of the Kevlar around the ring. Slide the ring over the motor tube and glue it at the 2 ¼" mark.
Glue the other centering ring at the 1" mark.
Glue the engine block into the end of the motor mount tube
Tie the other end of the Kevlar to the swivel
Tie one end of the elastic to the other side of the swivel
When the glue is dry, feed the shock cord through the motor tube
Glue the motor mount into the main body tube so that the end of the motor tube is flush with the body.

Final Assembly

Push the clay into the nosecone and push it into the tip using a pencil or dowel. Drip a little CA or Epoxy in to hold it in place.
Tie the end of the elastic shock cord to the nose cone.
Attach a launch lug to the side of the body about 4" from the aft end. Make sure that it is in a position that is clear of the fins.
Attach the parachute to the nose cone.

I added the swivel to the middle of the shock cord because the body spins during descent and unravels the Kevlar. This greatly shortened the life of my first shock cord.

I have painted my TwoFins with a candy cane stripe around them. It helps to enhance the look of the slanted fins.