Three-Fin Snelson
Robert Galejs
 

What's a snelson anyway? Not a what, but a who....Kenneth Snelson is an artist who first demonstrated the principle of "tensegrity" (a term coined by Buckminster Fuller, but disliked by Snelson, who prefers "floating compression"). Tensegrity involves a set of three or more compression struts connected with a network of tension members that make up a self-supporting semi-rigid structure where none of the struts touch one another. One of his sculptures employing this principle is shown below (right).

For much more detail on the concept of tensegrity and more of Kenneth Snelson's work, have a look at his web site: Kenneth Snelson Web Site

I thought it would be cool to try to incorporate some of Snelson's ideas into a model rocket. Potentially, one could attach fins to a rocket body using only thread only and not have any of the individual fin units touch. As you can see below, it really is possible, although not easy (details in the build section below).

How to build

Parts list

• BT-20
• Balsa
• 3/16" dowels
• button hole twist thread
• clear packing tape
• nose cone
• shock cord
• parachute

Start off with three equal length dowels. Mine were about 8" long. Split the ends with an exacto knife into four quarter round sections. The splits will allow you to easily connect the thread to the dowel ends and adjust the tension as well. Wrap some thread around the dowel, about 1/4" from each end. I used a clove hitch tied off with an overhand knot. This will stop the splits from going too far down the dowel.

Now comes the tricky part (to do as well as explain)...

Connect one end of each of the three dowels with thread so that the distance between each pair of dowels is the same (about 6" or so). Pull the thread taut into an equilateral triangle and tape the middle of each thread onto a smooth surface ( I used a kitchen counter). Bring the dowels together in the middle in a twisting motion so that each dowel rests on top of the dowel next to it and the free dowel ends are rasied into the air, about 6" apart. This could either be clockwise or counter-clockwise. For a single fin unit it doesn't matter which. Tape the three dowels together temporarily. This will make the next step much easier.

Connect the three free dowels ends together in another equilateral triangle of thread. Attach another thread to each of the three upper dowel ends. Connect the other end of the thread to the lower end of the dowel on which the upper dowel was resting. Increase the tension on these connector threads and untape the dowels. Increasing the tension more will cause the dowels to rise to a more vertical position and the dowel centers to move apart.

You've just made the most basic tensegrity unit!

I attached the fin unit (without fins for the moment) to the body tube with three threads from the top of each dowel and three threads from the bottom of each dowel. In order to adjust the tension of these attachment threads, I made two collars around the body tube by first wrapping a sheet of paper around the tube and then wrapping the paper tightly with clear packing tape. This was then cut into two 1" collars and three small holes 120 degrees apart made in the upper collar and six holes (three 120 degrees apart with the other three about 20 degrees away from the first set) made in the lower collar. Put the threads through the holes before sliding the collars onto the body tube. The end of the thread that is pressed against the body tube should come out of the forward end of the upper collar and out of the rear end of the lower collar. Slide the body tube down the middle of the fin unit. Take the threads that extend from the middle of the upper collar and attach them loosely to the dowel ends at the forward end of the fin unit and three of the lower threads to the dowel ends at the rear of the fin unit. Slowly increase the tension of these attachment threads until the fin unit is suspended symmetrically about the body tube. At this point, I slid the forward collar forward and the rear collar rearward to uniformly increase all of the tension at once. Tape the collars and loose thread ends to the body tube.

Cut out three fins of your favorite shape and glue them to each of the three dowels so that the face of the balsa is parallel with the body tube length. At this point you have three very floppy fins that probably woulnd't do much to stabilize a rocket. Rotate the three fins outward until they touch the lower triangle of thread in your fin unit. Mark a point where the thread touches the fin at the end closest to the body tube (see below). Drill a small hole at this mark . Loop the remining thread through this hole and around one edge of the triangle thread and tie it tight. Pull the other end of the thread attached to the fins. This will rotate the fins back to parallel with the body tube. Tape these threads in place.

At this point, you would finish the rocket just as any other model rocket with a motor mount, nose cone, shock cord and parachute.

Where's the CP?

That's the ususal conundrum with my rockets...

VCP Estimate

I tried to be conservative here. I assumed that only that part of the fin that projected beyond the body tube with the fin horizontal was "effective". This resulted in the CP estimate indicated by the bullseye in the pictures. The dowels were ignored, but should help stabilize the rocket as well as impart a spin due to their offset nature.

Swing test results

I needed a quick nose cone, so I stuck in a used E-15 nozzle forward with masking tape as a shoulder and the nozzle ground down into a somehat conical shape. With that in place, the rocket balanced at the forward black dot as seen in the first picture. I then attached some hefty fishing line at that point, took it outside and swung it in a big circle. It appeared very stable with the rocket quickly recovering from deviations induced by upward or downward tugs. This was not unexpected since this point has about 6-7 calibers of stability margin. I then added a C6-3 engine to the rear, using a spent D12 casing as an adapter. this moved the CG to the black dot just behind the white ring. This configuration was not nearly as stable. If I started the swing test with the rocket vertical, it remained vertical during the test. However when the swing was started with the rocket facing forward, it appeared nearly as stable as in the first test. It looks like if I don't launch my rocket at 90 degree  AOA I should be OK. When I finally launch it, I'll try for a CG around the forward tape ring.

Flight Tests

None yet...but I'll keep you posted.

First flight is planned with C6-3. wRASP indicates that a 5 second delay should be better, but with all of those strings and dowels there should add a lot of drag. I'd rather have an early deployment than one after it hits the ground.

Nose cone is spent Aerotech E-15 motor nozzle forward with a masking tape shoulder that was sanded down to semi-pointy shape.  Nose cone shock cord attachment is a loop of wire through center of motor hold in place with small screw in nozzle.  First flight was on an Estes C6-5.  I made a motor mount out of a spent Estes D-12 motor with the clay nozzle drilled out to 1/2".  The motor mount and engine were friction fit as well as taped into place. Te body shock cord attachment is a kevlar thread looped around the outside of the motor mount.  3 feet of sewing elastic was used as a shock cord and an old Estes parachute with the center logo cut out.

The final weight was just under 3 ounces without the motor.  The flight CG ended up just at the rear of the forward white band.
 

Launch Day...

On the launch rod...  Click on the picture above to download a 4 Megabyte Mpeg video of this flight that Doug Gardei filmed.  Six stills from video are shown below.

First launch was at the first CMASS launch of the year in Amesbury, MA.  It was fairly breezy with 5-10 mph winds. The LCO called a heads-up flight for the first flight of this unusual design.

The motor ignited right away and the rocket flew very straight.  There appeared to be plenty of stability  margin.  As expected, the rocket had a significant spin that probably helped the nice straight flight. However, this spin is hard to see in the video.

The fin placement, esentially mounted tangentially to the rocket body, presented little resistance to the spin induced by the canted dowels.  The C6-5 looked like just about the right motor and the parachute ejected close to apogee.  The rocket did not land hard, but it had a significant horizontal velocity.  When I got to the rocket, the fins were flopping around.  The thread did not break, but had pulled out of one or two of the slots in the ends of the dowels.  It would have been easy to fix had I not cut the threads so short.  Next time, I'll make sure the thread ends are more securely attached to the dowels.

I was very happy with the flight and will work on finding a more secure way of attaching the threads.  I will also start work on a high power version to fly at LDRS...  See you there!