This page describes some equipment I devised for a parachute workshop at Glasgow Science Centre.
The parachutes were simple devices made of the bottom half of a thin polythene bin liner, string, and a reusable payload consisting of a metal rod encased in closed-cell foam, with an attached cord. The launching system had to achieve two aims:
- It had to launch the parachute and payload to a height of about 15 metres (the height of the atrium at GSC).
- It had to allow the parachute to deploy at the maximum altitude or close to it.
The launching system
For safety reasons, I wanted to avoid using stored energy, for example in a stretched bungee cord. The final design was pneumatic. The parachute was fired out of a tube when the user stamped on an airbag attached to the bottom of the tube.
The tube was a piece of polythene domestic waste pipe (36mm ID) about 1.2 m long. Attached to the bottom of the pipe was a push-fit right-angle connector. The air bag was a wedge-shaped bag made of tarpaulin, glued using contact adhesive. A short sleeve on the thick end of the air bag was fixed to the right-angle connector using a jubilee clip. The volume of the bag was calculated to be about twice that of the tube, guaranteeing that the entire volume of the tube would be swept.
The parachute itself couldn't provide the seal inside the tube - if it was tight, it tended to get damaged or unrolled on the way up the tube. Instead, the parachute rested on a cylindrical wad or plug made of closed-cell foam, that was an easy sliding fit in the tube. The wad was retained by a piece of elastic shock cord that ran down inside the tube. The small leakage where the shock cord was passed through the tube at the bottom was unimportant.
To load a parachute, the workshop leader first blew down the tube to inflate the bag (this is easy). The parachute was folded/rolled and placed on the wad in the tube. It was then pushed down the tube using a rod. There is enough leakage in the system for any excess air to escape easily.
To launch the parachute, you stamp on the air bag. Even quite a measured stamp will send the parachute about 15m into the air.
The parachute deployment system
It is important to deploy the parachute at the right moment. If it deploys very early, when the package is travelling at high speed, the parachute can be ripped off its strings. Slightly early deployments stop the parachute reaching maximum height, while slightly late ones make the actual "parachuting" time shorter than it could have been. Finally, if the deployment is very late, the entire flight is ballistic.
Many trials showed that leaving things to chance does not work. In the final arrangement, the parachute package was encircled by a band of velcro. The overlap of the velcro was only a few millimetres, so that a small tug will pull the band off. The loose end of this band was attached to a long, light, nylon cord, the other end of which was held at ground level. The length of the cord was chosen so that, when the cord went tight on the upward flight, the parachute package still had enough speed for the velcro band to be ripped off. The parachute package would continue upwards and the parachute would deploy shortly thereafter. This arrangement worked very well.
Cord management
To store the cord so that it can be got out without tangling, start at one end of the cord and feed the cord into a box. Remember which end of the cord went in last: if you pull on that end the cord will come out of the box without tangling. Similarly, before a launch, feed the cord into a loose pile on the floor, starting at the bottom end of the cord. It will then unpile itself without problems when the parachute is launched. Don't coil the cord.
Alternatively, you can wind the cord onto a drum, by turning the drum. Do not wind the cord onto a stationary drum by hand. This will put a twist in the cord for every turn around the drum, which will cause you no end of tangling trouble later.