[Cutdown] Hot wire High altitude Cutdown Mechanism

Or: How to automatically cut a wire at 36km altitude. In this post i will share some of my experiences with cutdown mechanisms. I have used two, a mechanical one, and one using heat to burn a cord.

Mechanical Cutdown

In the “HoHoHo 2” Flight i used a mechanical cutdown by using a servo. This did not work as i had a power failure. Also, using servo’s can be problematic if you want to use them with “arduino’s”. If you use those, be sure to use the old Servo library. The new servo library works with interrupts. Interrupts can be dangerous if you use multiple protocols that use interrupts, like the new “softwareserial” protocol of other accurately timed operations. Anyway, it looked like this, and probably would have worked:

When the servo turned 90 degrees, a nylon cord looped around one of its arms would slip off

If servo’s get cold, the grease in them could freeze easily. Though, in a payload, generally it would not get cold, and if it does, the change is small that there is enough mass to get the grease to a solid state (freezing point). I can’t advise this method though, hot wire cutdowns are easier!

Hot Wire Cutdown

First of all, you are probably wanting to cut through something simple. Simple is foam, or nylon. The smaller the better. The following method was succesfully used in the “Space Camera Live 1” flight.

I had set a Latitude limit, a Longitude limit, and a “fall-sensing” limit – so after a 1000m fall it would cutdown the balloon if there was still any on the wire. This means that it would cut off the rests of the balloon, which can be substantial (~1.5kg!), so the payload will fall more slowly.
Although you can see from the images below my cutdown worked, but it did not let the wire “go” entirely. This is because there was so little of the balloon left, (<100g) that there was not enough force to pull out the broken wire!

A payload with hotwire cutdown (this was the backup payload of Space Camera Live 1)


Before launch (hotwire wrapped around nylon cord)

After use (a 10 second burn)

Hot Wire Cutdown Electrical Schematic

I had a 7ohm/meter wire, and worked will if connected direcly to a battery of around 3~7 volts. That would result in a ~2A current. I did not have proper transistors to channel that, but i can advise relays for this anyway. I only had a 5V relay, while my system was 3V3. So what i did was: A low power 3v3 arduino pin drove a transistor, that would drive full 3v3 power into a 5V booster. The 5V booster would then switch the relay. To the other end of the relay, there was a battery, in series with the hot wire. If you have a 5V system, just use a 5V relais that drives the hotwire+battery instantly. Simple! Though, small relays cant take large currents for too long, of they could “keep itself shut” which can be dangerous and lead to flying fireballs in ours case.

Test Video

Tim Zaman

MSc Biorobotics. Specialization in computer vision and deep learning. Works at NVIDIA.

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7 Responses

  1. ayellet says:

    Hello, what is the diameter of the filament? Thank you

  2. Srikanth says:

    Hi,

    Your systems looks great. Do you have a schematic or a document detailing your cut down mechanism. I am planning to build one as a project, it would be gr8 if you could give some schematics 🙂

    Regards,
    Srikanth

    • Tim says:

      Actually it is a very standard relay (http://www.sparkfun.com/products/100). The arduino just drives it by putting 5v in. then the relay opens the loop in which there is only a lipo battery and the nichrome wire.
      The whole circuit you are seeing in the pictures, is because the arduino (controller) was 3v3, and the relay was 5v (there are very very few 3v3 relays). So i needed to up the power to 5v with a little booster circuit. But that’s nothing fancy (http://www.sparkfun.com/products/8745).

  3. Hi! I am working on my first high altitude balloon. I have been investigating cutdown systems. The only thing that i am still not clear of is where i would place the cutdown. Assuming that the balloon train is:
    balloonchutepayload
    If I place it above the payload then i loose the chute. If i place it above the chute then i need to run wiring through the chute (doesn’t sound like a good idea). The only way i can think of doing it is having a second small cutdown payload between the balloon and the chute, so the train would be:
    ballooncutdown_payloadchutemain_payload

    So, my question is, where did you place the cutdown?

    • Tim says:

      Actually that was fairly easy. do some smart wireing. How about a “n” shaped loop, in which the balloon is attached to the loop of the “n” shape. then cut one of the sides of the ‘n’ shape and make sure you make a nice knot in the bend of the ‘n’ shape. if you then cut one side, then the balloon will slide off.