Cut down mechanism

[cjkogan111]

I just built a system to cut down a high altitude balloon using 2 HAM radios to communicate from ground to the balloon & a basic stamp to read the info from the HAM radio.

Anyway, now I need to use the stamp to cut through some 100-lb test cord. I was thinking of using nichrome wire to burn through the cord. My boss, however, is a little worried about cutdown system malfunctions (i.e. burning down a wheat field if the thing triggers on the ground.) It seems to me that there are many easy steps that can be taken to prevent this, but I decided that I'd also look into cutting down the balloon with scissors or razor blades etc.
I also thought about using electromagnetics to seperate the balloon from the payload (i.e. solenoids and pieces of metal.)

So anyway, I tried to use some nichrome wire to burn through the string using a power supply, and after 10 minutes I'd only just browned the string. The power supply immediately maxed out on amperage. We will be using 12V batts to do the job, but I'm worried that just putting a the nichrome wire between the leads of the battery will make things explode (these batteries are monsters - one time I actually did put the leads together & it did explode.) In addition, I'm not excited about draining a battery every time that I try to see if the cutdown system will work.
So anyway, I was wondering if anyone could give me any tips on the nichrome wire. I don't know exactly what I was using, but the professor who gave it to me said that it was standard nichrome wire. I also think that the power supply maxes out at around 2-3 amps. Is there a better (thinner?) type of nichrome wire I should get? Should I test it with the battery?
Are there any scissor like things I could use? Any ideas on how to build some scissor like things?
Any simple solenoid ideas? Anything else?

Thanks for your help!
-cjkogan111

[Robert Kubichek]

cjkogan111 said...
I just built a system to cut down a high altitude balloon using 2 HAM radios to communicate from ground to the balloon & a basic stamp to read the info from the HAM radio.


Anyway, now I need to use the stamp to cut through some 100-lb test cord. I was thinking of using nichrome wire to burn through the cord. My boss, however, is a little worried about cutdown system malfunctions (i.e. burning down a wheat field if the thing triggers on the ground.) It seems to me that there are many easy steps that can be taken to prevent this, but I decided that I'd also look into cutting down the balloon with scissors or razor blades etc.

I also thought about using electromagnetics to seperate the balloon from the payload (i.e. solenoids and pieces of metal.)



So anyway, I tried to use some nichrome wire to burn through the string using a power supply, and after 10 minutes I'd only just browned the string. The power supply immediately maxed out on amperage. We will be using 12V batts to do the job, but I'm worried that just putting a the nichrome wire between the leads of the battery will make things explode (these batteries are monsters - one time I actually did put the leads together & it did explode.) In addition, I'm not excited about draining a battery every time that I try to see if the cutdown system will work.

So anyway, I was wondering if anyone could give me any tips on the nichrome wire. I don't know exactly what I was using, but the professor who gave it to me said that it was standard nichrome wire. I also think that the power supply maxes out at around 2-3 amps. Is there a better (thinner?) type of nichrome wire I should get? Should I test it with the battery?

Are there any scissor like things I could use? Any ideas on how to build some scissor like things?

Any simple solenoid ideas? Anything else?



Thanks for your help!

-cjkogan111


Use a solenoid to release the cord.

Or the solenoid to trigger a mechanical bow release that the string is attached to..

It is safe, " No Heat" cheap, parts are easy to procure. Plus very easy to fabricate.

Bob N9LVU http://forums.parallax.com/images/smilies/scool.gif

[steve_b]

Be sure you are "dead man safe" in that if you lose power, you won't go in to an unsafe mode.

You can buy 12Volt release mechanisms that basically go against or around cord/cable and when you apply voltage it detonates and fires blades thru the cord/cable.

A guy from work found one of these things and thought it was a relay. Poor bugger hooked it up unknowingly and it flew thru his hand. No major damage (scarring) but it did make a mess (yes I had to clean it up! haha).

So, just take note if you go this route. no idea on costs....is there an issue if the cutaway fails and the balloon fly's away?

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Steve
http://ca.geocities.com/steve.brady@rogers.com/index.html
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

[bobledoux]

Soak a small area of the cotton cord with a concentrated solution of potassium nitrate. That will allow the nichrome wire to light it as a wick. It will burn and separate. Embed a very small amount of fine black powder in the wick if you want it to catch fire at 500 degress F. With careful design the wick will extinguish after the soaked area is burned.

There are common knife edge cutting techniques that use a spring loaded blade in a covered mechanism. These are sometimes seen in skydiving equipment.

Bob, N7SUR, Advanced

[cjkogan111]

Any links where I could get that relay blade string cutter thing?
Thanks!
-cjkogan111

[Alex Hunter]

if you don't mind spending a few bucks take a look at this:

64.226.12.24/defygravity/tether.html (http://64.226.12.24/defygravity/tether.html)

it is meant for releasing large parachutes on high power rockets. i have never used this device, but it looks like it would be pretty reliable.

[cjkogan111]

Thanks for the link!
-cjkogan111