Archery Release Becomes Reusable Balloon Cutdown Mechanism

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.

11 thoughts on “Archery Release Becomes Reusable Balloon Cutdown Mechanism

  1. 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.

    1. 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.

  2. 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.

  3. 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.

    1. 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?

      1. 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.

    2. 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.

  4. Latex balloons are not NORMALLY reused, but they can be in certain situations. I’ve successfully reused latex balloons on 3 different low-altitude flights where it was possible to grab the balloon after enough lift gas was vented to ensure a slow descent.

    The main benefit of recovering and reusing the latex balloon in these instances was that all of the lift gas is not lost between flights, only the small amount vented for altitude control. A small additional amount of lift gas had to be added before relaunching the balloon to provide the free-lift necessary to get back up to the desired altitude.

    Being able to reuse a latex balloon in this manor will require calm conditions, controlled venting capability (ex., and the need for quick-succession flights. I would also suggest only trying this with light payloads (<10 lbs) as ground personal safety is an issue.

    But for certain flight cases catching and relaunching a free-floating latex balloon can be a useful technique. For instance, I successfully used it for R&D, acoustic bird surveys, and low-altitude icing studies (ex.

    Lastly I would like to add that getting a latex balloon to neutral buoyancy is NOT unpredictable. It is dependent on a variety of factors which can make it seem unpredictable until they are understood. The most often overlooked factor in my experience is the internal pressure from the latex membrane compressing the lift gas. While this is usually only 50-300 Pa, depending on the balloon's size, it is critical.

    With careful inflation it is possible to get a latex balloon to behave like a super pressure balloon (within a small pressure range) which can provide flights that can hold a very stable pressure altitude. When using this, I normally suggest a second tow balloon, which provides extra free-lift, to get the float balloon and payload to altitude quickly before being released. There are scientific papers from groups in Sweden and India that have also frequently used this technique. See or "A New Paradigm in Space Based Experiments Using Rubber Balloons" by Chakrabarti.

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