Nuts and Volts - November 2008

Connecting bearing swivels is fast and easy with split rings like these.

The parachute apex with its bearing swivel.

ing the balloon or you’ll get a very fast descent after the balloon bursts.

AN ELECTRONIC RECOVERY AID

Now let’s look at an electronic device that you might add to your parachute as an aid to recovery. Normally, a burst balloon remains attached to the apex of the parachute during the descent to the ground. This doesn’t harm the descent, but does make it more chaotic due in part to the weight and drag of the burst balloon trailing behind the parachute at the end of a 20 foot load line. As the burst balloon swings around, it tugs on the parachute causing it to swing and tip. If we cut away the balloon, there’ll be no off-balanced forces pulling on the parachute and the descent is more like an elevator ride.

Many near spacecraft carry an audio beacon to help locate it when

it lands in tall grass or a corn field. The downside of the audio beacon is that it starts wailing away while you’re launching the near spacecraft. It’s not pleasant to be the one nearest the audio beacon during a near spacecraft launch. (Man, it sure would be nice if the audio beacon would only start up on descent or later.)

radio signal (or lack thereof) from the near spacecraft. This device cuts away the balloon by melting the load line running through its nichrome coil. An I/O pin of the PICAXE operates the coil by controlling a simple SPST reed relay. The coil only needs to get hot for a few seconds to melt the load line. After melting, the balloon is free to continue ascending if it hasn’t burst yet. This gives the Smart Cutdown the ability to terminate the ascent of a near space mission at any point. If the balloon has already burst, then the balloon and load line drop away from the parachute, making the descent easier on the near spacecraft.

To enable the nichrome coil to melt the load line in just a few seconds, the coil needs insulation from the cold near space air. Not

THE SMART CUTDOWN

The recovery aid I designed to counter the two problems mentioned above is the Smart Cutdown. It’s PICAXE controlled and can cut away the balloon and start the audio beacon based on either mission elapsed time (MET) or a

Each hole is centered vertically in the hoop’s side and 90 degrees apart. Don’t sweat it if your holes are mildly misaligned.

The outer hoop’s clamp was removed and the gap was filled with a block of basswood. The letter visible on the hoop’s side was part of a test. I was experiencing too many knots in the shroud line and thought labeling every connection would help reduce their numbers.