A cutdown in high-altitude balloon (HAB) parlance refers to detaching a payload, and can refer to the act of severing a line or to the mechanism itself. How is this done? The most common way is the “hot wire” method: a segment of wire is heated rapidly with a high current, causing it to melt through something like a nylon line.
But there’s more than one way to solve a problem, and while documenting different cutdown methods, [KI4MCW] found that a caliper-style archery release plus hobby servo could be used as a high strength cutdown mechanism. An archery release (or bow release) is a tool to assist in holding the string of a bow in the drawn position, and cleanly release it at the touch of a lever or button. It occurred to [KI4MCW] that these features might be made to serve as a payload release as well, and you can see here the crude but successful prototype for a reusable cutdown.
The archery release [KI4MCW] obtained opens its jaws when a trigger-style lever on the side is pulled. The force required to trigger this is remarkably low, and a low-torque economical hobby servo easily does the job. In fact, the force needed to trip the release is so low that [KI4MCW] added a short rubber band to provide some opposing tension on the lever, just to be sure no spontaneous triggers occurred. The device hasn’t flown yet, but the prototype looks promising. Maybe a mechanism like this would be appropriate for a payload like dropping a high-altitude RC glider from a balloon.
Another advantage of a bow string release is that they are made to handle the “weight” of a bow. Or in other words a tension of around 30-80+ pounds of force, or 150-400+ Newton.
A fairly heavy payload could reliable be held in place with this typ of mechanism.
That should be fine for typical hobby balloons, but don’t underestimate the forces involved with turbulent air. We had a ~6 pound balloon pull a medium sized spilt ring (key ring) apart due to turbulence.
3 ring cutaway is a long time proved system, very easy to sew, with no load limit, just need a super light cable puller or a nichrome very thin thread cutter.
This could cut one year’s growth of tree twigs and possibly cut out film-plastic bags that get caught in such growth beyond reach of a pole hook? Tethered balloon or drone either way a close growing tree could every year be kept away form the house etc.
it’s pretty reliable tbh; my team and i had tested it out a few years ago (https://www.youtube.com/watch?v=WD9MhWVV_KU) but dropped this mechanism in favor of nichrome wire cutdown mechanisms due to weight constraints; unfortunately you need to be careful with nichrome as you can set your payload on fire. don’t ask how i know T_T
Works well, one of our student teams came up with it years ago. We flew them for over a decade.
I did a PCB design to feed the receive audio to a DTMF circuit and TX audio connection to an Argent Data open tracker. A CNC machined piece of expanded PVC provided structure for the whole system (including the nigh indestructible tape measure antenna). I’ve even got the CAD/STL for the printed “trigger finger” servo horn around here somewhere.
what about a system to deflate the balloon instead, and get it back with the payload ?? (like not letting it float and land “somewhere” as hazardous pollution)
Mylar balloons can be very troublesome as jetsam. Latex balloons decompose readily, if I’m not mistaken. Of course, a six-foot balloon is a fair amount of latex.
Large latex envelopes can be mighty fragile, too, so inserting a mechanism inside or on the surface of the balloon can be problematic. I have not read of any latex balloons with self-deflation mechanisms. Has anyone here ever flown one? Maybe a valve in the neck, or some kind of contraption to cut or burn the envelope?
Latex balloons are traditionally used both as altimiter and release mechanism — they’re specced to pop at a certain volume, and you calculate the initial fill to determine at what altitude they hit that volume. It’s not hyper accurate, but we ran five or six balloons without any particular surprises.
But re: hazardous pollution. The weather bureaus launch up a balloon a day, for who-knows-how-many-years, with a sonde package that they don’t even retrieve most times. Compared to what we ran, they also use undersized parachutes, so the sonde package is coming in hot. I’m always surprised that they haven’t dented up someone’s car roof.
Latex balloons are not reusable or recoverable. They generally rise until they burst. It is possible to get one neutrally buoyant but it’s unpredictable and varies greatly with temperature (altitude, time of day, and cloud cover would cause large changes in buoyancy).
They are also very fragile. Ground crews generally wear gloves and go to a great deal of trouble to ensure that the balloon never touches anything other than a clean tarp, and that only until it gets enough gas to float. If it stays up for long, UV (which is far worse at altitude) will kill it.
They aren’t reusable; anything that comes back is garbage, even if by some near-miraculous combination of engineering and luck you manage to recover one with the envelope intact.
Latex is a natural biodegradable material.
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