[aprssig] Transatlantic amateur radio balloon in flight

Scott Miller scott at opentrac.org
Wed Mar 12 16:43:31 EDT 2008

I thought some sort of anti-freeze was the usual ballast for this sort 
of thing.

I'm going to have to come up with a better demo of my cut-down system - 
I wanted a super-cheap and reliable system to release a load from a 
balloon, and I wound up using a 1-watt metal film resistor run at about 
3 watts for several seconds.  It'll cut through 200-lb test Spectra 
cord, and you've got plenty of margin before it starts melting its own 
solder.  I had the load pull a jumper loose for feedback.

Carbon film resistors will work (if you want to do it for 2 cents 
instead of 40) but they'll blacken and maybe smoke a bit.

For a ballast-dropping system, you could use a whole line of them with 
something less durable than UHMWPE, like maybe Nylon fishing line, to 
get faster burn-through for a lighter load.  And since you can choose 
from many resistor values, you can get it to work with just about any 
available supply voltage.

You can get cheap driver ICs that'll handle 8 cut-down resistors each, 
so interfacing is easy.  And mechanically, there's very little to go 
wrong.  For something like this, I'd probably use two PCBs (or 
perfboards) - one with the resistors, and another spaced an inch or so 
from it with holes for the lines to pass through to keep them from 
getting tangled or slipping off the resistors.  You can also pass a loop 
of cord through two holes to help reduce the mechanical stress on the 
resistors - they'll take a 20-lb load directly with a clove hitch tied 
around them, but I had no trouble holding a 200-lb load with a couple of 
loops of cord.


Keith VE7GDH wrote:
> Scott N1VG wrote...
>> If it's averaged across the whole flight, and it took off
>> at about sea level, wouldn't it be zero?
> You would think so, but...
>> Things don't look very good right now... must have splashed down 
>> already.  What's with the ballast drop count?  I'm guessing that once
>> it ran out of ballast it just left the valve open.
> Yeah, no update since 1851 UTC. That's still a pretty good effort 
> though. If at first you don't succeed... ! I liked the blog where they 
> discussed what to use for ballast, discarding frozen bananas and ice 
> cubes. Paper seemed like a reasonable choice.
> 73 es cul - Keith VE7GDH
> -- 
> "I may be lost, but I know exactly where I am!"
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