I'm not really an expert, but I think rocket fuel would require slightly more than a pair of AA batteries, and ReNNo has a point: the batteries need to be heat-insulated at the very least, lest they freeze. Although I don't know how ling the balloon will be spending at that altitude, but I'm thinking the batteries will be frozen by the time it gets there...

Uhm, i had some meteo radio probe around (i gifted them away, had no balloons, just the electronic units) ..... they was using common 9V batteries, for work, and they go high enough for freeze too (also if not high as a good rocket )

But, as docs02d said, it's probably an one-time use, so, just as suggestion, insulating the battery inside a block of polystirene, may be enough for the use he want, and polystirene also weight almost nothing, and in these applications, also weight is important

I can't tape anything to the balloon. The latex is so thin and fragile it would burst. The balloon will expand from maybe 5 feet across at release to about 50 feet across at peak altitude.

I have toyed with using a rocket igniter in line on the string from the balloon to the payload and it seems that may be the best option. I have to carry 7.2 volts in lithium batteries for the camera, flight computer and radio anyway. 7.2 volts is plenty. I believe rocket igniters have an oxidizer built in but I'm not sure. The laser is attractive because it can reside on the payload and just point generally up at the huge balloon to burn a hole. In addition, I don't have to worry about a lack of oxygen.

From the last flight I know that extreme cold will drop the voltage on a battery (retards the chemical reaction) and of course I have plans to try and keep it warm.

Just so you know... in a near vacuum there is not really much air to carry away the heat so its less a problem than you might think.

Thanks for the input guys!

Putting the burster on the lines is a good idea too, I was going to say estimate the size of the balloon, and place it accordingly.

Good luck with the launch then!

Well, for the cold, the problem can be in the initial ascension part, when you're still in atmosphere, and the temp go under zero, but as i said, a good polistirene insulation can take care about that part (and 7,2V Li-Po batteries now are also cheap enough, and more light than steel cells)

Yes, the igniters, the commercial ones, have a part of oxidizer in the mix, but you can easily build yourself something that burn also in lack of oxygen, if needed ..... lots of mixtures contains oxidizers

Also the idea to cut the string is ok, if you don't care about the balloon (but i suppose that, without load, it can ascend enough to self-pop for expansion) ..... only, maybe use 2 of them for safety (redundance, redundance ), and add some burning mixture, for be sure that the flame is enough to cut the string without uncertain results (make some tests)

Interestingly enough.... On the last flight things were fine on the ascent which is about 800 Feet Per Minute. I expect the next flight to take about 2.5 hours to reach peak altitude.

On the descent however, the initial descent rates were close to 10,000FPM (a parachute doesn't do much above 80,000Feet) and as a result the temperature plummeted inside. I believe the increased airflow due to the high speed was wicking away the heat. As a result the voltage dropped on the battery and my camera shut down.

I'm thinking of buying a pool noodle to build payload bays. Eventually all this has to be shoved into a rocket.

Oh yes... I will have my very own DIY satellite. It's just a matter of time.

I was going to say, Estes model rocket igniters are basiclaly 4 AA-sized batteries running current through a small wire. That's less than the 7.2 volts you've got to run for your camera, anyway.(My dad was big on model rocketry until we remodeled the basement and the rocket room became my bedroom.)

The engines themselves are gunpowder-based, and gunpowder can ignite in a vacuum. It includes its own oxidizer.

Good luck.

Hmm. Ya don't use a laser for this. I doubt the laser will work at -70 degrees.

Here is a simple idea. Why not attach a really long needle to a servo.
I assume you have the ability to send a signal up to the balloon and trigger a servo or something. It can then push the really long needle up into the balloon lol.

EDIT: Double post. My first post took a while to appear, weird...

wow! thats damn impressive!

I would as well use a smashed lightbulb with blackpowder to burn the string. could be as small as a 10mm long thin drinking straw. if its all blackpowder and plastic, you could connect two strings to it, the "cap" in the middle. when it burns away, the strings definitely will be disconnected. with a 1.5v bulb, it will definitely ignite, how low the voltage might ever sag. 0.5v should be enough to ignite it.

do you want to use a rocket as "stage 2" to further lift the cam/payload?
i guess its all timer-based, and not directly (radio) controlled?

how far does the payload/cam drift while falling down? do you use gps or a sender or something to recover it?

really great project! wouldnt have expected some private hobbyist could pull that off! :-)

manuel

Interesting project

Will it be a day launch? Perhaps you could use a solar cell as a power source? I'm not sure if a small one would generate enough current, but it might do, and the sun is more intense the higher you climb.

Another possibility might be a small charged capacitor. Not sure, but I have a feeling they might work at much lower temps than batteries.

Or you could just use a slow burning fuse cut to the correct length and attached to a small charge... or even fuse on it's own if you used it in place of rope/string, depending on what you have in mind.

If you work it out well, I think you might be able to perform both the tasks you want using fuse alone, providing you can find a suitable type of fuse that burns at the correct rate for what you want to do.

You can make a simple ignitor by grabbing a filament of steel wool and shorting a 6+ volt battery across it.. The steel will glow red and melt. If you pack the area with gunpowder it'll easily burst into flames. Estes model rocket ignitors are pretty much just this, only I think they use nichrome wire instead of steel.

I actually think you could just wrap a very thin wire around the base of the balloon, pass a heavy current through it and it should heat up enough to melt the latex and just sever the balloon from the payload. I think something like this: 100ft Quality Nichrome Model Rocket Igniter Wire 36AWG - eBay (item 300343386985 end time Sep-06-09 17:52:50 PDT) should serve you quite well.

^ but the latex expands, when the balloon rise ..... lowering the external pressure, the inside gas expand, and this may also be 8 or 10 times than at ground level, so probably stick something on latex can risk to break it just when it expands .....

Probably the better thing is use a cotton rope or string for connect the apparate to the balloon, like the ones that are used for DIY candles (not waxed !) ..... just made a mix of potassium chlorate and sugar, dip the part of the rope that you want to burn, and left it dry ..... stick one or two igniters (estes are good, if you don't want to made your own ones, too) in the point of the rope that was dipped in the mix, and wrap with a bit of tape, or some domopak plastic, just for protection against humidity, and all is done