In the previous post I talked about balloon technology. Here I'll describe how we attach the HAPP to the balloon. To set the stage, recall that we are attaching a 10 kilogram / 22 pound load to a piece of thinly stretched latex rubber and we require it to survive turbulent winds and the like as it ascends to the target altitude of 30 kilometers / 100,000 feet.
My finished solution looks like this, with the balloon neck looped through a steel ring and clamped off with carbon fiber plates and steel screws:
A quick search of other people's hobby balloon projects reveals the "standard" method of tying off using rubber bands and electrical tape. Follow the link and check out the pictures. This solution is not going to be capable of handling our loads; we need something more robust.
My idea was to use the steel ring in lieu of a loop of string formed from the tether itself. This prevents the abrasive Kevlar string from cutting through the latex. And to resist the pull-out force, I clamped the balloon neck back over onto itself using hand-cut carbon fiber (scraps from construction of the HAPP structure!) as pressure plates.
To enhance the grip of the plates onto the rubber while avoiding sharp, lacerating edges, I lightly scored the inner mating surfaces of the plates with a Dremel rotary tool. See frame (3) in the composite image below. After tightening the screws, this baby is going nowhere. I haven't actually done the test, but I'm pretty sure the rubber will shred before the steel ring slips out of this assembly.
One advantage of using the steel ring is that we can accommodate two different setups quite easily. The main setup is, of course, for flying the HAPP, with a single tether line attached to the craft. See the left frame below. You can also see a small generic fishing swivel I spliced in-line to decouple any rotations of the balloon and avoid transmitting them to the HAPP. No sense in making the stabilization jets work any harder than they need to - we do have a limited supply of gas! This swivel has a rating of 600 pounds which is almost 30X the static system weight, providing a comfortable safety factor even under extreme dynamic loading.
The alternative setup is for low-altitude test flights where the system remains tethered to the ground while still allowing the HAPP to hang freely below the balloon. See the right frame. In a tethered test we have three long lines running to tie-downs on the ground to prevent the balloon from moving around in any winds. See the video of the first test flight. I used three of these large kite reels to stow the ground lines.
In both cases we can use small screw-lock caribiners to connect to the steel ring for quick assembly / disassembly during testing. On flight day we can eliminate these carabiners and simply tie the single main tether directly to the steel ring for additional weight savings.
What about all those knots tied in the kevlar lines? They're technically hitch knots. I like the Grapple Hitch for a variety of reasons, including its security under load or not, its adjustability, and the ease of untying.
Finally, let's look at the attachment to the HAPP itself. The main tether is fairly long at 5 meters. Why 5? I chose this length to achieve a nice slow natural oscillation of the HAPP. From the formula for a simple pendulum you can show that the period of back-and-forth swinging will be about 4.5 seconds (the reality is a bit more complex as this system is actually a compound pendulum - you can verify the details :-).
The thether is tied off to the machined aluminum tether ring on the HAPP. In-line with the tether are the doubly-redundant cutdown pyros. I also tied a small opaque cap to the tether. This cap is placed over a small photoresistor that's affixed to the HAPP. During normal flight the photoresistor does not sense any light and we can be sure the HAPP is still attached to the tether. When the pyros fire and the HAPP falls away, we will get a positive signal that separation is complete. More on this in a future post discussing the HAPP's sensor suite.
OK, that's it for the harness and tether. Thanks for hanging around!