22 replies to this topic

[Robert]

Hi All,

I'm in the process of launching a balloon into the stratosphere (CAA approval is in place) to initially take pictures of the UK. However due to wind there is a good chance that my camera may head towards the sea. This is bad news for the camera and my project. What I would like to be able to do is cut down the payload from the balloon using a pyrotechnic device and this is where you all come in.

The balloon is connected to the payload (box) by one piece of kite string about 2 meters above the flight box. I would like to burn though this string using a pyrotechnic ignited by a processor controlled switch. The problems are:

1) There is very low oxygen content at ~35000m
2) I have very limited power to supply to the ignition system 4xAAA or the
equivalent in weight perhaps a lipo from the local model shop
3) The external temperature is ~ -60 deg C (The payload box internal temp is ~0 deg C)
4) The burning has to be contained so as to not affect the parachute


My initial idea is a cardboard tube with a string running through the middle of it. Using a davysfire (electric match) to ignite some slow burning fuse wrapped around the string in the middle. I'm assuming that a davysfire (if that's what they're called) can be ignited with such low power supply and that it will be enough to ignite the fuse at such low temperatures and oxygen content.

Does any one out there have any useful suggestions or comments.


Many thanks

Robert

Edited by Robert, 04 November 2008 - 10:00 PM.

[Arthur Brown]

A daveyfire igniter will fire off a fresh AA cell so 4 AAA cells will be OK. However NO-One will estimate the performance of a cell at -60C --can you wrap them in foam to reduce heat loss?

Usually an igniter will get fired from a switched supply so you will need to check that the circuit will fire 1 amp into the igniter -check the switching device!

Pyros will work in low oxygen

[Robert]

Ah I should have mentioned that the payload box should keep a temperature of ~0 deg C. It's well insulated and there is some heat loss in the electronics which for a change is good.

[Arthur Brown]

Due to the fact that the UK is an Island there is a 100% certainty that the balloon will be heading towards water! There was a guy doing a project like this but in Canada where he had more miles of land to recover the thing from. -But still he had to recover the payload over periods of weeks from wild lands.

Can you get the payload to send the data back to you? then you could use semi disposable cameras like cheap security cameras. or even I/P cameras!

[wjames]

you dont need a pyrotechnic device mate......you just need a peice of wire, like nichrome, or heating element.

firstly......you need lithium ion AAA batterys...they work at -40 deg C. they are also lighter than alkaline ones. you'll pull more than enough current from one or two of these to make the wire melt through the string !!!!! im 100% on that one. big plus is -40 degree working temp

I would suggest testing in your fridge, then freezer.


0.008" (0.20 mm), Kanthal Nikrothal 80 Plus, Resistance 10.66 Ohm/ft (35.0 Ohm/m), Temp. max 2190 F (1200 C), Surface clean shining silver........this'll work........use 2 inches of it.....should give you less than 2 ohms.

if you can afford the extra weight, you want a Li-Ion batterypack(still lightweight), a 6v solenoid.....and rather than cutting through the string....you want to make an off centre latch.

here's an example of the solenoid you'll need

http://cgi.ebay.co.u...O...A1|240:1318

[marble]

I wouldn't use nichrome, the temperature out side will be cold enough to negate any effects the wire may have. An e-match on some visco fuse may work well.

[pjalchemist]

Hi,

I'll try too help out here as it seams i am the only one that does HPR. After 50K' you need to use C02 for your deployment as there is a massive lack of 02 for BP charges.

What would be best would be a shunt, split pin with a solanoid, it could be all built into a box and kept insulated. Simple 555 timer for the duration till it drops away.

I have no idea how you plan to recover the camera but you could do dual deployment recovery. So you could do the above and have a streamer come out with it then work out some math and then have a BP charge or C02 too let off a main after it's reach such a height which will stop it drifting so far!

114 829 feet there is sweet FA in terms of 02 up there! So nothing will burn unless you have it built in a sealed unit! PVC pipe and epoxy to seal i think!!

[Arthur Brown]

There is no useful oxygen at that height but pyro compounds have their own oxidiser compounded in! If you use an ordinary igniter then insert this into a tubular cord the heat from the igniter will melt the cord and the load will separate the parts.

[Arthur Brown]

Just be certain that you have permission to fly at that height!

[wjames]

However NO-One will estimate the performance of a cell at -60C ?

not stictly correct......without causing offence or starting an argument....li-ion's will perform abouve 80% at -40

Li-ions have a great characteristic - they self heat when used in very low tempratures.....

Thats why nasa use them in the space shuttle....the antartic survey team use them....the proper ones are rated to a whopping -80 degrees c !!!!

[paul]

There is no useful oxygen at that height but pyro compounds have their own oxidiser compounded in! If you use an ordinary igniter then insert this into a tubular cord the heat from the igniter will melt the cord and the load will separate the parts.

Once read an scientific article on pyrotechnics in space. The tried it and guess what. Nearly no reaction took place, using BP and other compounds.
Even if all those mixes got their own chemically bound O2, most of them won´t work in a vacuum/very low oxygen environment.

Always tried to dif that article up again, never found it ):

[wjames]

i had thought about that...i remembered something, but could not back it up.......

a simple comp of a oxidiser and fuel.......Surely, for the reaction to take place between them, there has to be enough oxygen for the fuel to combust in the first instance, thus allowing the oxidiser to react.....


im no expert though....im sure there is someone here that has much more knowledge

[rr22]

Have a look here,

http://www.fit.edu/p...rd_chamber.html

[crystal palace fireworks]

Robert, im no expert, but here is what I would try!

Is it not possible to fit a can of compressed air (like those used for blowing dust away from computers etc) in the payload to use as a oxidiser, which is then activated by a relay switch just before another battery deploys a charge to the nichrome or visco match ?

Disposable cheap underwater camera`s can be bought off of ebay.

As Arthur said,.....retrieval is going to be an issue unless you have a big budget for tracking devices etc, I would be tempted to use photoluminescent paint and a few small flashing LED lights for visuals, plus you need to consider buoyancy/water proofing and perhaps fireproofing (depending on your design), maybe a parachute, and name/address tag for anyone who finds it (reward).

I maybe wrong, but I would have thought you would need to get permission from the CAA to launch a weather balloon to such heights?

[wjames]

i just made a phone call to a friend who is "in the buisness" of aviation radar.

He's 100% that you need permission for a balloon of that scale.