What goes up must be made to come down.
On mission day the HAPP will be carried to an altitude of 30 Km / 100,000 ft by a weather balloon. Once it reaches apogee, we need to cut the umbilical cord between the HAPP and the balloon. There are two possible scenarios.
First, the balloon may burst as it expands in the near-vacuum conditions, in which case we need to cut the cord and release the floppy balloon to ensure a stable descent and a clean deployment of the parachutes. This is the most likely scenario, and in fact is our Plan A for returning the HAPP to Earth.
If the balloon does not burst, we need to cut the cord to ensure the HAPP comes down and doesn't turn into a perpetual "floater" that proceeds to circumnavigate the globe until ultraviolet radiation finally degrades the latex skin of the balloon. In fact, FAA regulations require a redundant Plan B for unmanned balloons for exactly this reason (Federal Code of Regulations, Title 14, Part 101, Subpart D, 101.33).
Most folks I've seen on the internet use a NiChrome cord cutter. This type of system takes a small piece of NiChrome wire, spirals it around the cord (usually made of Nylon), and at the moment of truth sends a relatively high current through the wire, thereby heating it up and melting the cord. Here is one commercially available system, and here is a video of someone's hobby version in action.
As the 9 kg HAPP is heavier than most hobby balloon payloads, we need to use a fairly robust umbilical cord: Kevlar instead of Nylon. However, Kevlar has a melting temperature of about 500C/930F, whereas Nylon 6 melts at 220C/428F. Thus, we'd need to get the NiChrome almost twice as hot to melt the Kevlar, which means more current, bigger batteries, more weight, and lower reliability.
Here is where we have another opportunity to use pyros and blow stuff up (and those are always welcome opportunities :-)
After exploring a few alternatives for home-made, explosively actuated guillotines and the like, I decided to keep it simple and use the lovely little Cable Cutter from Prairie Twister Rocketry. This device is designed to cut plastic zip ties, but I figured it might work for Kevlar string as well.
You can glean the operating principle from the picture below. Moving left to right, we have 500-lb yellow Kevlar braided string threaded through two small radial holes in a blue anodized aluminum cylinder. A small steel fire pin goes into the cylinder. Next comes about 0.1 cc of black powder (not shown, but it goes where my finger is located in the picture). After that we insert a J-Tek electronic igniter which has been threaded through a black steel end cap. To complete the assembly we simply tighten the endcap onto the cylinder. The entire assembly weighs only 10 grams. The igniter is triggered simply by hitting it with 5V from the flight control system.
You might wonder whether the powder charge will burn effectively in a nearly zero-oxygen environment at flight apogee... it's a reasonable question. The answer is that the potassium nitrate (KNO3) in the black powder provides the oxygen needed for combustion, so the powder is essentially self-oxygenating. Guns can be fired underwater for the same reason.
So does it successfully cut the Kevlar cord? Check out the video below.
Looks like a solid solution. We'll use two of them on flight day for redundancy.
FYI, a rough analysis of the video shows the cable cutter jetting away at about 90 kph / 55 mph as the hot combustion gas shoots out of the endcap...