Cutdown Mechanisms

Hot wire

A hot-wire cutdown mechanism is the most common system used to remotely or automatically detach a payload string from a high-altitude balloon. In this design, a large current is passed through a piece of nickel-chromium ("nichrome") wire. This wire, which is wrapped around a section of synthetic rope, heats up quickly to an orange glow and melts through the rope, literally cutting the payload down from the balloon.

Hot wire systems typically use a lithium battery of some description to provide the high current, since lithium batteries are less affected by the extreme cold of high altitudes. The battery used for cutdown is typically isolated from the other flight systems, both to ensure that there is enough power available to heat the wire, and also to avoid negative effects of voltage sag or current fluctuations on sensitive computers and radios. The triggering circuit is then a simple relay or transistor switch that is used to make the high-current connection from battery to wire.

The effectiveness of a hot-wire system depends on getting the rope nice and hot. This means that the heating area should be protected from wind and ambient air. This is often done by insulating the area with a re-engineered styrofoam coffee cup (eg, cut off the bottom, and wrap the side of the cup closely around the rope-and-cutting-wire assembly). Nichrome wires are prone to shorting out if not wrapped carefully around the cord. Some experimentation with different wrapping techniques and spacings may be useful. Hot wire systems work best with at least a modest amount of static tension on the cord to be severed. Even so, it is common for flight systems to trigger their hot wire cutters after bursting, so that the buffeting of the initial free-fall will pull away the shredded remains of the balloon and avoid fouling the parachute system. Hot wires are usually the option with the lowest mass - the wire itself (typically 30 gauge) weighs almost nothing, but the high-current battery and related connections are non-trivial.

Though not common, it is certainly conceivable to use a hot wire system to cut the balloon envelope directly, in order to terminate a flight. This may work best for fixed-volume floaters, since the expansion of a latex envelope would pose some mounting challenges.

Pyrotechnic

Please note that in the US, home-made pyrotechnic devices are considered explosives, and are regulated by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (BATFE). If you do not already have a license to manufacture explosives from the BATFE, you should not consider building any kind of pyrotechnic devices.

Even toy fireworks can be very dangerous, and flying them in from one jurisdiction to another by balloon might cause some terrible legal headaches. If you choose to use pyro devices, make absolutely sure that you comply with all of the laws for such items for all of the areas your balloon could conceivably fly over.

That said, it may be feasible to use firecrackers or other pre-made pyrotechnic device to perform the cutdown function. One such design might use a short pipe to hold the flight string up to the end of a rocket engine, such that the engine is triggered electronically to begin the cutting operation (I have seen this demonstrated in a video on teh Interwebs). It may also be possible to tie the envelope and flight string to opposite ends of an acrylic tube, and use a small firecracker within the tube, electrically triggered, to shatter the tube at cutdown time. Besides the obvious safety concerns (for example, the risk of a false triggering on the ground during launch or after landing), such systems may also risk damage to the systems they are trying to protect. A rocket engine-based system, for example, might cut the rope, then go on to swing around and burn a hole in the parachute. Or worse, set the flight computer or radios on fire, thus dropping flaming electronics about the countryside, complete with lithium battery-bombs.

Pyro cutdowns can be very light, and depending on the details of the implementation, might not require any static tension on the flight string. Safety and legal issues abound, though, so this is not the best choice for a hobbyist flight.

A similar system could be used for a mechanical cutdown. The image below shows such a system, using sections of rope and a series of brass rings of various sizes. As shown in the middle diagram, the release pin is holding up one end of the green section of rope. By pulling the release pin free, now only one side of the green section is being supported (the left side). The weight pulling down on the green string (with Ring 3 applying that pressure) now pulls down until Ring 3 slips off the end of the green section. Now only one side of the orange section is being supported. The tension on the red section pulls Ring 2 downward until Ring 3 passes through Ring 2. Now the red section is only supported on one side. Ring 1, bearing the entire weight of the payload, pulls down on the red section until Ring 2 passes through Ring 1. At this point, the payload is free, with only Ring 1 and a short length of rope attached to the top of the parachute.

I constructed a prototype of this system, a (terrible) diagram of which is shown on the right. I chose to use a zip-tie in order to reduce friction on the pin. In practice, the zip-tie pulled the pin downward against the surface of the aluminum bracket, causing much more friction than the zip-tie ever could. Even so, the prototype worked very reliably with the tiny servo.

This system depends entirely on there being significant tension on the line, in order for the rings to pull themselves free at each stage. A design like this would probably work well under stable flight conditions (a controlled cutdown), but is unlikely to work at all after the balloon has burst, at least until / unless tension is restored. It may be best, therefore, to reserve such a design for large flight systems that require multiple independent cutdown options.

The rings obviously must be able to pass freely through each other, even with rope attached, and should be smooth on all surfaces. The welded brass rings that I used came from a craft store. They don't weigh much individually, but weigh several ounces collectively. Again, this design is best suited to large flight systems.

Barrel Release

Still to be written. But briefly, one could use something like a "strap lock" for a guitar, or a quick-disconnect key ring. A small force releases the bearings, freeing the attachment (with the release button motorized somehow). I have not built one of these.

Pinched Rope

Also still to be written. Imagine a hinge, pin side down, with the two sides folded together, with a captive rope/cable held between the plates. There would be minimal side-pressure on the plates, such that they could be held together by a lightweight retaining pin. A small motor would pull the pin, allowing the hinge to swing open, releasing the rope.

Archery release

Some archers, especially those who hunt with a bow, use mechanical devices to hold, and then release, their bowstrings. This device is known as a "release", and is available widely in sporting goods stores. An archery release is made to easily handle a draw force of 200 pounds or more on a narrow cord, so can be pressed into service as a mechanical cutdown device. It has the advantage of requiring only a very small amount of force to open (such as with a "micro"-sized servo), with no concerns about lateral or shear forces jamming the works. Such a system requires at least a small amount of tension in order to pull the flight string from the open jaws. It may also benefit from a positive closure force to ensure that the jaws remain closed during flight, prior to release.

In the name of science, I experimented with constructing a prototype.

Below is the completed prototype assembly, in its closed and open states. Note that prior to assembly, the wrist brace and bolt cover were removed from the release mechanism. The #10 bolt extending upward from the release is an original part of the mechanism (a happy coincidence), though I have removed an angled (bent) section of the bolt where the wrist brace was originally mounted. The bracket attached to the bolt is a piece of scrap aluminum.

The servo motor for actuating the release (which I extracted years ago from a broken toy) is mounted to the bracket with black electrical tape... cheesy, but it worked fine. As you can see, the motor is not yet wired. The holes on the servo arm and the trigger arm did not align in any useful way, so instead of a hard linkage, I used a piece of 18 gauge stranded insulated wire. It flexes enough to make the linkage trivially easy, and is stiff enough that the motor can operate the trigger in either direction.

Clearly visible are the rubber band and leverage bracket mentioned above. When in the closed position, the rubber band exerts only a tiny forward pull on the trigger. This seems to be enough to hold the calipers shut. Some tugging on a nylon rope held within the jaws confirmed that the calipers remained closed under moderate pressure.

Finally, here's the mechanism in action - click on the image below to see the animation (ugh, Google Sites). The simulated payload (barely visible) is a roll of gaffer's tape, roughly 2 lbs worth. It is hanging by a piece of aluminum wire which has been placed in the calipers of the archery release. I did not have a heavier load handy with which to drop-test.

To activate the release, I have connected the servo briefly (via clip leads) to a stale 9V battery. The low current available from the 9V is still plenty for the servo to briskly yank back on the trigger, releasing the "payload". I have no idea what voltage the servo is intended to operate from (it is not marked), but the brief burst of power is not enough to generate any heat that I have noticed.

I was able to perform this same test maybe a dozen times, without any surprises. The same battery worked fine every time. With the rubber band assisting, the jaws always stayed closed, with no spontaneous releases.

Last updated: 20210309