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. 


Multi-ring

Skydivers are familiar with the three-ring release system used in parachutes, as shown in the animation below. This is a series of inter-connected rings and fabric loops that allow a tightly packed parachute to be opened with just a light tug. 

Animation of three-ring parachute release system















  
Image by ☭Acodered (Own work) 
  [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) 
  or GFDL (http://www.gnu.org/copyleft/fdl.html)], 
  via Wikimedia Commons



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


Hinged Pin


Archery release

Some archers, especially those who bow-hunt, 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 and some big box retailers. 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 does require at least a very 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.

In the name of science, I experimented with constructing a prototype cutdown using an archery release.





I knew I would be drilling into the mechanism in order to connect it
to an actuating servo, so I purchased the most inexpensive release 
that my local Dick's Sporting Goods had on hand. The "Patriot Jr." 
here was $20 on sale. The mechanism is (somewhat) adjustable, 
and is rated for 200 lbs of draw. 

It turned out to be perfect for this project.

















Here is the mechanism out of the box, in it's closed and open 
positions. The nylon wrist strap and plastic bolt cover tube are 
visible.




Note the length of the trigger pull. I had not considered pull-
length while shopping, but the long throw of this unit is easy 
to work with.





I drilled a 1/16" hole in the trigger arm (lower red arrow) to provide 
a connection point for a link from the servo. Once connected, the 
servo was able to easily operate the trigger in both directions - 
pushing forward ("upward" in this photo) to positively close the 
calipers, and pulling backward ("downward") to open them.

The trigger has an astonishingly light pull - the calipers open with
just the slightest touch. Accordingly, the mechanism is =not= 
stable in the "closed" state. To ensure that the jaws do not open 
accidentally (or through jostling during flight), I decided it would be 
best to apply a light forward tension on the trigger. To do this, I
drilled a second 1/16" hole through the trigger-side caliper (the 
upper red arrow in the photo) and mounted a bracket there to
holds one end of a rubber band. I attached the other end of the 
rubber band to the servo linkage on the trigger arm. The bracket 
itself (not shown here) is a carefully bent paperclip. The rubber 
band applies only a slight amount of tension. 






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 standard, 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: July 16, 2017