The cut-down mechanism is an extremely important component of the high altitude balloon. Getting a reliable cutdown mechanism is something that was considered difficult. Initial thoughts were to have some external ‘sleeve’ that would wrap around the balloon throat, using a rubber band, or string, velcro or some other material to keep it mated to the balloon. It was however decided that a piece of PCB tube inserted into the balloon to provide a means of attaching the payload could be modified to provide a way to bring a Nichrome wire in touch with the balloon.
The only issue I can see is tying of the bolloon end to stop the Helium gas from escaping. The wires _may_ lead to gaps being present. However if we have sufficient cable ties, each tied up sufficiently, and twist the balloon end around the wires, we should be able to stop any significant amount of gas from leaking.
Take a look at some of the pictures below.
Initial tests show that 6 volts is more then adequate to get the nichrome wire red-hot in a few milliseconds. Enough to heat and burst the balloon.
Current flowing through the wires was measured at about 3.5amps.