Balloon cutdown device

[MikeK]

I'm looking into making a cutdown device for a high-altitude balloon, which will sever the line to the balloon after it bursts to prevent it getting tangled in the parachute, or in case the balloon goes buoyant instead of bursting.

Anyway, it'll consist of pushing at least 1A of current through a coil of nichrome wire for 5-10 seconds.  I picked up a disposable camera thinking I could use the flash circuit, but my calculations show that it only provides 6 or 7 joules and I need closer to 100 joules.

Camera: 0.5*C*V² = 0.5*100u*350² = 6.125 joules

Cutdown estimate: V*I*t = 9*1*10 = 90 joules

Am I going to be able to do this?  Do I need a supercap?  The reason for the separate self-contained circuit is that I don't want to run long power wires from the payload battery to the cutdown.

EDIT: Renamed the thread.

[IanB]

I think you are looking at this problem all kinds of backwards.

Start at the beginning: what is a cut-down device and how does it work? What control do you have over its construction and power requirements?

I have the impression there is a coil of wire that when heated by an electric current melts (or burns?) through a cord and releases the balloon.

Are you making this coil yourself or does it come ready made? What voltage and current requirements does it have? If you make it yourself, do you have control over this?

The simplest way would be to use a small battery like an AA size NiMH or NiCd rechargeable cell. These cells can supply at least 5-10 amps and with a suitable thin resistance wire can make it glow red hot without much trouble.

Why do you have a working voltage of 9 V in your formula? Is this what ready-made units require, or is it just an assumption?

A camera flash unit is entirely  the wrong approach, however. You have a total source-load mismatch there. On the one hand a camera flash is designed to produce a high current at a high working voltage for a tiny fraction of a second. On the other hand your resistance coil is designed to consume a high current at a low working voltage for a long time. No way can the camera flash do what you want.

[amspire]

Definitely the flash circuit will not work, and IanB is right - a small battery will probably be the best answer.

What is the resistance of the nichrome wire?  - that is the important question. Without knowing that, we cannot really help.

Richard.

[mariush]

What about the temperatures at high altitudes? Would the nicrome wire heat up enough at the cold temperatures at the top?

What I'm thinking right now on the spot is that use a nut and bolt and a small DC motor



Solder/weld bolt on the top of the dc motor, solder/weld a piece of wire on the bottom of the nut and get the nut on the thread with just a few turns... put the nut between two pieces of plastic that form an U shape so that when you start the motor anti-clockwise, the nut won't spin with the bold (but there should be some leeway for the nut to move sideways.
When the thread spins counter clockwise, the nut is locked in place and can't rotate, but as the thread spins, the nut will go out, will be pushed out from the thread and then just flow up with the balloon.

Not sure if there's going to be problems with the metal contracting with the colder air temperature... imho both the nut and the bolt should contract a bit but  I don't think

[Time]

The nichrome might be initially cold but the thermal losses at lower pressure are poor so unless the air is flowing than it should technically heat up faster.

[Jeff1946]

Why not use very small nichrome wire wrapped around a match head.  Ignite the match head and several others next to it.  Should be able to burn thru the cord.

[BravoV]

Similar to Jeff's suggestion, but instead of nichrome wire, get those cheap 2 cells torch light incandescent light bulbs, crack or make a hole at the glass bulb, then fill with crushed match head or gun powder and tie up to the cord, with just 3 volt you will have a nice low voltage and no fuss igniter.

Try it 1st to see how much powder is enough, cause if too much or with the "right" powder, it will turned into a poorman diy/improvised detonator. 

[mariush]

Do you guys really think there's enough air at high altitude to ignite matches? Really...

[BravoV]

Do you guys really think there's enough air at high altitude to ignite matches? Really...

Yep, cause "certain" powder can be ignited even there is no oxygen at all, cause the needed oxidizer is in the powder it self to fuel the combustion.

[Psi]

Yeah, i'm not sure if match heads contain that though.

A hot wire and some plastic based cable is what i'd try first.
Obviously the hot wire should be insulated from the cooling effect of the wind

A small model rocket engine with the jet aimed at the cable might also be something worth looking at.

[HLA-27b]

How much weight can the element inside 1A fuse pull?
You can see the lines I am thinking along of course...

[IanB]

Yeah, i'm not sure if match heats contain that though.

They certainly do. Match heads ignite perfectly well when wrapped in foil where no air can get to them.

[G7PSK]

Do you guys really think there's enough air at high altitude to ignite matches? Really...

Black powder or double base gun nitro contains its own oxygen otherwise it would not work. As for the hot wire insulate it but to make sure of the break why not put the wire in as part of the suspension from the balloon take a pice of fuse wire make an inverted u shape tie the balloon to the loop end and solder the power wires to the other end when the battery is switched on the fuse blows and the balloon separates,Job done  don't over complicate.

[Codemonkey]

Why not just make a release device with a small solenoid. Solenoid energises, pulls its pin back which the cord to the baloon was secured around, baloon releases, job done.

[MikeK]

Thanks for all the good suggestions.

So, the reason I chose the nichrome wire solution is that it looked to be the smallest and lightest.  Since this would be additional, not-really-needed, payload I wanted it as small and light as possible.  (And I'm close to my projected payload weight for the parachute I'm using already.)  Also, the bigger and heaver it is the more dangerous it is if it should come loose and hit something other than the ground.

I already have nichrome wire and its weight is practically zero.  And a small circuit wouldn't weigh that much either.  Testing the nichrome with my power supply showed that I need at least 5V at 1A.  I went with 9V for wiggle room.  I haven't tried a 1/2 AA camera battery (pre-digital era); they're more than 1.5V and should be light enough.  A regular 1.5V AA cell isn't enough voltage.  I figured that a coin cell has enough energy to do what I need.  So I thought: charge pump!  But I don't know enough about getting all of that energy out.

The way it works is that the nichrome wire is wrapped around the cord attaching the balloon to the parachute.  When enough current passes through it, it goes bright red quickly and melts the cord in just a few seconds.  Again, I'm going with 10 seconds for wiggle room.

With mechanical devices I'd be concerned about reliability (at around -50F), and I'm hoping for less weight than they would be.

[T4P]

Umm... A massive cap? 

[Codemonkey]

I'll be amazed if you can get a bit of nichrome wire to glow with the energy from a coin cell. They have a high internal resistance so are pretty much useless for supplying high currents. You'd have to charge a massive capacitor instead which would sort of defeat the purposes.

If you don't want to use a solenoid etc, how about instead of using a solenoid pin retracting to free a loop of cord, use the nichrome wire passing through it. That way instead of having to keep the wire heated for a few seconds to cut the cord, all you have to do is blow it open circuit like a fuse with a short jolt to release it.

[MikeK]

I'll be amazed if you can get a bit of nichrome wire to glow with the energy from a coin cell. They have a high internal resistance so are pretty much useless for supplying high currents.

That's why I titled this "charge pump".  I was looking for help on how to get enough energy of out a small cell to do the job.

[Codemonkey]

My point is that you're going to need to charge a capacitor so big (physically, as well as in capacitance) that it will likely be bigger and heavier than a small solenoid! With reasonable size components, you could probably get the thing to glow for a second or so at the most.

*Just realised, G7PSK already sugested the blowing the nichrome wire thing!

[MikeK]

I'll have to experiment with the pyro suggestion.  The solenoid is tempting - or using a pager motor - but mechanical devices tend not to work at cold temps....A little bit of trapped moisture can freeze and lock it all up.

[TerminalJack505]

You might look into low melting point alloys.  There are metal alloys that will melt at temperatures as low as 48C.  Perhaps you could use this along with your Nichrome wire and a battery powered circuit.  It might be quicker (and at a lower temperature) to melt through the alloy than it is to burn through the cord. 

This would require the Nichrome wire to mechanically support the payload.  I'm not sure if that's feasible or not.

A couple of links...

Fusible Alloys
8 oz 160F (70C) Alloy for $14.95

[SeanB]

Use a small piece of tungsten filament as an ignitor for an incendiary charge, either match head material ( cheap) or a proper electric rocket starter which generates the hot gas and flame in a self contained package. the rocket starter will work if you use 2 lithium Cr2032 coin cells as a power supply , which is both light mass and small.

[jerry507]

I was part of a high altitude ballooning group from 2003 to 2010 that had been doing it since the early 90's. We tried a lot of different solutions over the years from knife systems to nichrome. They were all pretty terrible until we finally had a very brilliant guy sit down and actually do some engineering on it, not just think about it and try a random solution.

We settled on nichrome, but instead of looping it around the line (which is a pita and actually pretty unreliable), we attached the wire to a spring and ran current through the combination. This allows the nichrome to be spring tensioned to your line and improves both reliability and ease of rigging a lot. We used a PVC T to house the wire, with the line running along the top of the T and the nicrome running into the bottom and bolted to the top side of the T.

Second you need to look at the datasheet for your nichrome wire and they should have a current vs temperature graph inside, or the numbers that you can use to calculate it. It's very similar to the video Dave put out about resistor self heating. From there you just use a constant current source to regulate the current through the wire to your target temperature and away you go. We used a mosfet to do it, but I don't see why a common 3 terminal linear reg of appropriate current rating wouldn't work equally as well.

The temperature you need to cut a material will depend on the material. Something like Dacron will cut at pretty low temperatures.

Sadly this field is very poorly documented. We looked for YEARS and never found much out there, so hopefully this works for you.

[MikeK]

Hmm...In what way was the nichrome coil unreliable?  I've done a number of tests and it melts the Dacron kite line quite easily.  Wouldn't the coil be good for concentrating the heat?  I've tested it a bunch of times with about a dozen turns of wire, very closely spaced, at about 0.125" in diameter.

I have no data on the nichrome wire, I got it a few years ago from someone on eBay.  I'll check the Googler.

[pickle9000]

I bet a NiCad can deliver the current and they are light. Just thinking out loud now, consider using heavy gauge fishing line as the shear point, insulate the wire against the elements to keep the heat in, limit the current and heat for a longer period of time to soften the plastic.

...mike