Like the radar reflector, a flight termination unit (FTU) is not required if your balloon launch is “exempt” under FAR Part 101 Subpart A, which is discussed in detail in the FAR Part 101 chapter. Also like the radar reflector, even if an FTU is not required, it may be a worthwhile exercise to make one. Including the FTU can actually add a possible failure point in your mission. So, although it can be a great way to stretch your engineering and electronics skills across many areas of your HAB mission (telemetry, radio, recording, logging, GPS, etc.), but it is not an additional project to take on without some consideration. Some projects have worked on an FTU, only to ultimately not use it because it seemed like a bigger risk to use it than to not use it.
Imagine that we are operating under FAR Part 101 Subpart D, even though this is not required for simple balloons. If you are going to make an FTU, you should make it in accordance with the rules that would actually apply when you launch heavier balloon missions. Under Subpart D, you are required to have two methods for bringing the balloon down. For sounding balloons like ours (in contrast to zero pressure balloons), they quite reliably burst, as long as they are not under filled, so balloon burst can be one of the termination methods.
Other methods for bringing down the balloon with an FTU system can be a thermal knife (heating nichrome wire), actuating a blade, or using a solenoid to release a knot. If you do create an FTU, you need to make sure that it is attached between the balloon and the parachute on your flight train. Since it is hard to run wires through or around a parachute, you can avoid this by attaching the parachute to the side of the flight line. As the balloon ascends, the parachute will technically be able to fill with air. The slow ascent rate will hardly be affected by the parachute, while on the way down the parachute can make much more of a difference.
The most likely reasons for needing an FTU are by becoming neutrally buoyant from under-filling the balloon or veering off course toward a hazard like mountains, lakes, or restricted areas. Watch out for current limiters in batteries and make use of other resources for tips, diagrams, and code.
An interesting way to “deflate” a zero pressure balloon (ZPB) is to put a small weight on top so when the payload is cut down, the balloon will flip orientation and all the gas will vent through the bottom vent that is now on the top.
: The SparkFun tutorial is a good place to start learning about this: https://www.sparkfun.com/tutorials/187.
Copyright 2013–2021 Bryan Costanza