Nuts and Volts - November 2013

to tear a couple of Dacron straps off the jacket. After the second loop was torn, I stopped attaching the modules to the bottom split rings and began attaching both the parachute and lower modules to the split rings at the top of the airframe. Four lift lines — or short lengths of woven Dacron kite line — span the distance between the lower module and the split rings.

After switching to using only the top split ring to tether parachutes and lower modules, I didn’t do away with the bottom split rings. I repurposed them. I run the link lines from the lower module through the bottom split rings without actually attaching the link lines to the bottom split ring.

The bottom ring’s small interior diameter seems to prevent the airframes from swinging wildly like a pendulum. I like this so much that in the new design, I created restraining loops at the bottom of the harness.

In this case, however, I added four loops of Dacron kite line to the harness. On the harness at a point one inch above the bottom of the airframe, I melted a hole through all four arms of the harness. To strengthen the hole for the Dacron loop, I inserted an opened rivet through the hole. Then, I cut a short length of woven Dacron kite line and melted the cut ends with a lighter to prevent the kite line from unraveling.

I tied a loop with the kite line by folding it in half and tying a simple overhand knot in the end.

Next, I pulled the folded end of the Dacron kite line loop through the center of the rivet using a pair of tweezers. The knot at the other end of the loop is so large that it can’t be pulled through the rivet. The loop of Dacron left sticking out of the rivet replaces the split ring I used to restrain the swinging of modules.

Before attaching the harness to the airframe, I added a bumper to the bottom of the airframe. My older modules had a landing bag at the bottom of the airframe. The landing bag protected it in case it landed on a hard surface like a road. The landing bag consisted of a one inch thick sheet of foam rubber placed between the Styrofoam bottom of the airframe and the inside of the bottom of the abrasion jacket.

I took a hint from hard hats when I replaced the landing bag in the new design. Hard hats protect heads two different ways. First, the elastic band inside of them spreads out an impact’s length of time, thereby reducing the force of the peak impact. That’s how the landing bag functioned in my previous airframe design.

Second, hard hats have a hard shell that spreads out the area of the force of an impact, thereby reducing the peak pressure of the impact. I chose the second method to protect the airframe in the new design.

I spread out the area of the landing impact by adding a lightweight plastic plate to the bottom of the airframe. The plastic was a 1/4 inch thick sheet of Correplast, or corrugated polypropylene plastic. This impact bumper attaches to the bottom of the airframe with bolts that also attach the harness to the bottom of the airframe.

The largest and oldest conference for amateur near space explorers is the Great Plains Super Launch (GPSL). It takes place every summer in a location in the Midwest. The 2014 Great Plains Super Launch will take place in Hutchinson, KS — the home of the second largest air and space museum in the United States: the Cosmosphere. The dates for the conference are June 12 and 13, with the 12th set aside for presentations and the 13th reserved for near space launches. If you would like to begin your own program or investigate how and what near space explorers do, then plan to attend the next GPSL. While you're there, be sure to look for me. By the way, I want to give Zack a big hand for hosting GPSL again.

A six inch loop of Dacron was folded in half and tied at the end with a simple overhand knot. Note that the cut ends of the kite line are melted to prevent them from unraveling over time.

In this bottom view of the airframe, you can see the impact bumper attaches to the bottom of the airframe using the same bolts that attaches the four arms of the harness to the bottom of the airframe.