Balloon cutdown device - Page 1
Forum section: electronics / projects

author: MikeK
author_rank: Super Contributor
timestamp: May 11, 2012, 02:09:24 am
content: 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 joulesCutdown estimate: V*I*t = 9*1*10 = 90 joulesAm 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.

author: IanB
author_rank: Super Contributor
timestamp: May 11, 2012, 03:11:59 am
content: 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.

author: amspire
author_rank: Super Contributor
timestamp: May 11, 2012, 03:30:57 am
content: 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.

author: mariush
author_rank: Super Contributor
timestamp: May 11, 2012, 03:48:45 am
content: 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 motorSolder/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

author: Time
author_rank: Frequent Contributor
timestamp: May 11, 2012, 04:18:11 am
content: 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.

author: Jeff1946
author_rank: Regular Contributor
timestamp: May 11, 2012, 05:05:16 am
content: 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.

author: BravoV
author_rank: Super Contributor
timestamp: May 11, 2012, 05:18:23 am
content: 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.

author: mariush
author_rank: Super Contributor
timestamp: May 11, 2012, 05:21:34 am
content: Do you guys really think there's enough air at high altitude to ignite matches? Really...

author: BravoV
author_rank: Super Contributor
timestamp: May 11, 2012, 05:25:39 am
content: Quote from: mariush on May 11, 2012, 05:21:34 amYep, 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.

author: Psi
author_rank: Super Contributor
timestamp: May 11, 2012, 05:35:23 am
content: 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 windA small model rocket engine with the jet aimed at the cable might also be something worth looking at.

author: Unknown
author_rank: Unknown
timestamp: May 11, 2012, 06:01:50 am
content: How much weight can the element inside 1A fuse pull?You can see the lines I am thinking along of course...

author: IanB
author_rank: Super Contributor
timestamp: May 11, 2012, 06:55:18 am
content: Quote from: Psi on May 11, 2012, 05:35:23 amThey certainly do. Match heads ignite perfectly well when wrapped in foil where no air can get to them.

author: G7PSK
author_rank: Super Contributor
timestamp: May 11, 2012, 07:43:55 am
content: Quote from: mariush on May 11, 2012, 05:21:34 amBlack 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.

author: Codemonkey
author_rank: Regular Contributor
timestamp: May 11, 2012, 08:51:07 am
content: 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.

author: MikeK
author_rank: Super Contributor
timestamp: May 11, 2012, 01:28:47 pm
content: 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.

author: T4P
author_rank: Super Contributor
timestamp: May 11, 2012, 02:10:33 pm
content: Umm... A massive cap?

author: Codemonkey
author_rank: Regular Contributor
timestamp: May 11, 2012, 02:12:30 pm
content: 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.

author: MikeK
author_rank: Super Contributor
timestamp: May 11, 2012, 02:33:03 pm
content: Quote from: Codemonkey on May 11, 2012, 02:12:30 pmThat'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.

author: Codemonkey
author_rank: Regular Contributor
timestamp: May 11, 2012, 03:04:45 pm
content: 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!

author: MikeK
author_rank: Super Contributor
timestamp: May 11, 2012, 03:19:05 pm
content: 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.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 11, 2012, 04:21:50 pm
content: 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 Alloys8 oz 160F (70C) Alloy for $14.95

author: SeanB
author_rank: Super Contributor
timestamp: May 11, 2012, 04:32:19 pm
content: 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.

author: jerry507
author_rank: Regular Contributor
timestamp: May 11, 2012, 08:36:23 pm
content: 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.

author: MikeK
author_rank: Super Contributor
timestamp: May 11, 2012, 09:30:09 pm
content: 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.

author: pickle9000
author_rank: Super Contributor
timestamp: May 11, 2012, 09:59:43 pm
content: 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

author: IanB
author_rank: Super Contributor
timestamp: May 11, 2012, 10:17:41 pm
content: I feel the length and gauge of the wire can be controlled to match the desired power source. An NiMH AA cell can deliver 5-10 W easily, and I'm sure a suitable piece of iron wire can be made to glow red hot with that much power. Then you have the mechanical arrangement to get right, with springs, tension, weight, whatever. For example I am sure a short, thin hot wire will cut through a polypropylene cord if the tension is set up the right way.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 11, 2012, 10:19:15 pm
content: In terms of weight, pyro of some sort is probably going to give you the most work for the weight approach, and can be ignited by a small current. Match heads probably as good as anything, and can be quite explosive if confined.The way I'd try is sometning like a thin-wall plastic tube containing the pyro material, used as a link in the cord, so once ignited, it gets burnt all round from the inside.Using a spring to store energy for a mechanical cut may also be worth exploring, but you need to test at low temps.

author: free_electron
author_rank: Super Contributor
timestamp: May 11, 2012, 10:29:33 pm
content: magnesium wire then ? that burns VERY well ...

author: nctnico
author_rank: Super Contributor
timestamp: May 11, 2012, 11:19:56 pm
content: Quote from: MikeK on May 11, 2012, 02:09:24 amIn satellites they use so called thermal knifes to deploy solar panels and antennas. Those are extremely reliable (especially compared to explosive bolts).Here is some info:http://www.dutchspace.nl/pages/products/content.asp?LangType=1033&id=258In your case I think using a li-poly or li-ion battery is a good idea if you can keep its temperature around 20 deg. C. A small battery would have more than enough energy to power the knife and keep itself heated.

author: Unknown
author_rank: Unknown
timestamp: May 11, 2012, 11:26:52 pm
content: How about a Tantalum cap as an ignition device?With lowest available rating of 2V these should be ignitable by 2 coin cells.

author: Psi
author_rank: Super Contributor
timestamp: May 11, 2012, 11:39:37 pm
content: A CR2032 coin cell (the most typical) has such a bad internal resistance that all you can really draw is ~5mA.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 11, 2012, 11:52:46 pm
content: So far I think Jerry507's idea is the best since it has been proven.Having said that, here's another half-baked, unproven idea...You could use fusible alloy metal (Wood's metal, for example) as a stop for a spring-loaded pin. The cord goes around the pin and the pin supports the payload. Once the payload is to be separated, a heating element (such as your Nichrome wire) is activated. This melts the stop and the compressed spring, no longer held back by the stop, forces the pin into an open position, thus separating the payload from the balloon.Like nctnico pointed out, the circuit will likely need to heat itself to prevent the battery's chemistry from becoming ineffective.

author: jerry507
author_rank: Regular Contributor
timestamp: May 12, 2012, 01:47:00 am
content: In our experience, coiled nichrome has a tendency to fatigue easily. First, you've already bent it way past it's min bend radius with any reasonable dacron line, and now you're subjecting it to a lot of additional bending unless you secure the line above and below it. Frankly, if you mount it in a similar way (inside a T or similar so the line stays straight over the section you want to cut) then either way is probably just as good. The advantage of the single tensioned wire is that it's waaaaaaaaaay easier to rig than threading the line through the coil. It's faster to replace the line as well, though if you're using a constant current method and operating below the max temp of the wire you will get a lot of burns out of a single wire.Knife or latch systems have been tried a lot, it's actually the only way we've actually found documentation of. Variations of them usually use a hobby servo that moves the latch or the hot knife. I think they're significantly more complicated, and generally unreliable. However I'm sure that's likely 90% because it's actually pretty hard to maintain good engineering processes on projects like this. When our group originally started it was funded by our state NASA Spacegrant. The engineering was secondary and all the people involved were obsessed with process. It's incredibly difficult to reliably follow even a good process.FYI, pyro is a big no no. Obviously it's hard for the FAA to actually catch you but it's illegal under the part 101 rules or whatever the number is. You're technically supposed to be under 6 pounds as well unless you have a waiver

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2012, 02:46:18 am
content: This really seems like an area that would be amenable to engineering input. There must be many problems that don't have a documented solution out there that have to be engineered from first principles. I'm not sure who usually joins or gets involved with these ballooning groups, but a professional engineer (aeronautical? mechanical?) as an adviser or consultant or willing volunteer could probably help enormously with solving what is by all appearances a fairly ordinary design problem.It seems like jerry507 has a working solution nailed down rather well.

author: Rerouter
author_rank: Super Contributor
timestamp: May 12, 2012, 03:18:46 am
content: prsonally i would still think a solenoid pulling a pin would be the better method, you can make very very small solenoids, the only primary issue would face is icing up, however i suspect if the exposed shaft and pin is plastic and the catch is plastic, the the current through the solenoid should be enough, to overcome the issue(when air pressure falls temperature rapidly decreases, not to mention the insane wind speeds)

author: EEVblog
author_rank: Unknown
timestamp: May 12, 2012, 03:30:12 am
content: Dave.

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2012, 04:13:28 am
content: Quote from: Rerouter on May 12, 2012, 03:18:46 amThis is why engineering input is important.The temperature drops at altitude, but this is not caused by the drop in air pressure. They are coincidental but unconnected.Balloons experience more or less no local wind speed at all. From the balloon's perspective the air is perfectly still.There is no special reason to believe that plastic is less prone to icing than metal. The question of whether a mechanism might get jammed by ice depends heavily on unknown details and speculation is pointless.

author: kaptain_zero
author_rank: Regular Contributor
timestamp: May 12, 2012, 04:32:18 am
content: Sometimes one needs to step out of one field and into the next. Such devices have been used for decades in model aviation, notably the free flight models where the aircraft is released and allowed to fly to either a specific height, time or what have you. The device I'm linking to is more complex than what is needed for this project, but it might be of interest anyway.http://tli.tl/P239PKOf course when I was a lad, we just used 3 eyelet screws, with the middle one being the release, and ran a piece of cotton string through it. Lite the end of the string and it would smolder away until it released the center eyelet screw (attached to whatever you wanted to release which in my case was the rudder so the plane would spiral back down to the ground).RegardsChristian

author: Rerouter
author_rank: Super Contributor
timestamp: May 12, 2012, 04:47:20 am
content: ok i worded that rather horribly :/ and you are correctso lets approach it from an engineering standpointhttp://www.lsbu.ac.uk/water/phase.htmlthis is the phase relation vs temperature and pressure, at the altitudes the baloons can reach lets say 30KM, which is about 1KPa, the boiling point starts getting very close triple point about 2-10 degrees, meaning barely any heat is required to melt ice, from 0 degrees,then lets look at how much heat, atmospheric moisture is pretty clean, so lets base it as plain water, which is 4.186 joule/gram °C say surface moisure of about 0.1 grams then depending on the expected temperature one can calculate the amount of heat to melt ice,simialr charts can ba applied to types of plastic to better characterise a controlled melting of a length of balloon cord,just covering a point, hope your happier with this approach

author: jerry507
author_rank: Regular Contributor
timestamp: May 12, 2012, 05:38:53 am
content: We used to run temperature sensors on every run, and it would regularly fall below the freezing point rather quickly. On our videos I don't believe we ever recorded any icing, but we didn't run cameras on every flight nor did we point them at the balloon every time either. It took 30? years for them to realize ice buildup on the Shuttles external tank was a problem though.Mechanical pin systems just require through engineering to make sure they move easily under all conditions, which is certainly possible. The downside is that you're adding a whole mechanism which weighs quite a bit. Again remember you're supposed to be under 6lbs. We dropped them not just for reliability concerns, but because we started wanting that weight for payload.

author: MikeK
author_rank: Super Contributor
timestamp: May 12, 2012, 05:39:34 am
content: Quote from: IanB on May 12, 2012, 04:13:28 amI'm not sure local wind speed means, but I do know that a balloon going to 90kft will pass through two jet streams, both of which have high wind speeds causing it to travel quite a bit. The payload chain will almost lay horizontal at some points, due to what I presume is the wind. Granted is weak wind, like on Mars.But I plan on enclosing the cutdown in a block of styrofoam, both to help keep the battery warm and protect against wind. Also aliens.Quote from: jerry507 on May 12, 2012, 05:38:53 amI'm shooting for under 4 pounds, because of the size of the parachute I made. I know the cutdown isn't necessary, I just wanted to try it out.

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2012, 06:17:51 am
content: Quote from: MikeK on May 12, 2012, 05:39:34 amLocal wind speed would be the wind speed or air speed as experienced by the balloon and payload. Perhaps it is not the best term, but I was trying to convey the wind speed measured against a local point of reference on the balloon. Since the balloon is free floating it will move at the same speed as the air around it, thereby experiencing no difference in speed between itself and the air. The "air speed" of a balloon as measured by a pitot tube or anemometer is going to be very close to zero.Where some air movement can be experienced is during gusts or within patches of turbulent air, or when moving between stratified layers of different wind speed causing the balloon to accelerate. During these periods of acceleration it will experience wind effects on its surfaces.But apart from those particular cases, the general experience of the payload will be of still air.

author: Rerouter
author_rank: Super Contributor
timestamp: May 12, 2012, 06:39:54 am
content: 2.2kg to a pound, you can get solenoids capable of pulling the pin that are down in the tens to 1-2 hundred grams, as the module is capped at 6 pounds maximum then each baloon would only be holding x amount of pounds so the mechanisms only need to be smallthen we fall back to the issue for a reliable release perhaps a slight taper on the pin it would mean that once its tugged on the forces at work would assist it, though equally i am left wondering if a fusable linkage could be employed instead, calaculating the forces on a bloon in a jet stream is out of my area of expertise,retiring that idea, i think i may have worked out how to improve your rope melter, use an acrylic chain link, and wrap your coil tightly around either side of where you secure the rope to the link for the ballon, acrylic has a low melting point, and is sufficnetly rigid for the expected forces (lets say 12m/s/s acceleration in the jetstream) and also has a rather small specific heat of 1.47J per gram per degree C, so being how its nice thin atmosphere so most of the heat will go into the acrylic, suppose we keep the coil running as a heater coil on the way up keeping it a nice and toasty 60 degrees off say a AAA battery, to get to its melting point of ~120C, that would require only 88.2 J of energy per gram to melt it, the warmer the coil can maintain it the less energy required to in a pulse to melt it, and let the force of the ballon pull it into a fine enough strand that it can no longer support it and break off,just thinking out ideas, measuring up the right diameter of acrylic would be the key to it i would think

author: sonicj
author_rank: Frequent Contributor
timestamp: May 12, 2012, 07:49:59 am
content: rc tow release $2.99weight 4.5gdebur and maybe polish for smoove, reliable release action.the other thing that comes to mind is asnap shackle.(think mythbusters) consistent & reliable release, can freely rotate, designed for use around wind & ropes (sailboats). i have one here and its pretty sweet. no sharp edges, all of the rivets are flush or recessed. the release spring is probably too tight for most hobby servos, but if a servo is holding the release pin in place, the tensioner spring could be completely removed.after looking at this snap shackle a bit more, its looks like the tensioning spring could be installed in reverse. a small solenoid could push against the spring until the battery is discharged enough that the solenoid no longer can resist the spring.hmm... after looking at it again, i'd bet the spring could be easily replaced with a coil; integrating the solenoid into the release mechanism. hmm... interested to see what your final solution is!-sj

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 12, 2012, 08:19:27 am
content: Quote from: Psi on May 11, 2012, 11:39:37 pmAlkaline button cells (e.g. LR44) are much more powerful - you can get a few hundred mA out of them

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 12, 2012, 08:24:32 am
content: Quote from: Codemonkey on May 11, 2012, 03:04:45 pmYou can get some really small & light supercaps, e.g.These from Cellergy<2 grams with ESR down to 300 milliohms, so capable of a few amps - ideal for a wire igniter or solenoid/motor pulse

author: Unknown
author_rank: Unknown
timestamp: May 12, 2012, 08:32:48 am
content: Here they use slow burning fuses. It works like a charm.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 12, 2012, 08:34:52 am
content: Here's an out-there idea - how about an electrostatic system.I'm envisaging a set of interleaved plastic-insulated foil layers, each alternate layer physically connected to either side of the string. You apply HVDC to the layers to make them cling to each other. The grip force is the sum of the attractive force over the whole surface area.Turn off the HV and short the two sides and they no longer cling, and can slide apart. If it's well enough insulated and corona proof you may not even need a constant supply - just charge it up on the ground, and discharge using a HV reed switch, IGBt or triggered sparkgap to release.No idea how practical this would be though...

author: jerry507
author_rank: Regular Contributor
timestamp: May 12, 2012, 05:00:40 pm
content: Since I am NOT a mechanical engineer by any stretch, I won't comment on what it would take to make such a system reliable. I'll just comment on balloon dynamics since I am more familiar with them.Wind speed tends to be a very secondary issue, because despite whatever force it has on the balloon, it will be dominated by the dynamics of the balloon itself. The typical amateur ballooning setup is a double pendulum, with the payload connected to the parachute and then a line from parachute to balloon. Pretty much every component of the system has a LOT of spring to it and if you watch a lot of video you will see the payload is typically spinning around it's vertical axis as well as swinging a lot in space. We've never tested to see if the line is under tension the entire time but that was always our assumption.

author: MikeK
author_rank: Super Contributor
timestamp: May 12, 2012, 05:02:53 pm
content: I found that this battery should be capable of doing the job. It's small and light. It's only 3V, but it has a rated continuous current of 1A and a pulsed current of 2.5A. I happened to have one. Although nearly dead and it didn't get the nichrome red hot, it was hot enough to burn me.I'll also experiment with Dave's suggestion, since it doesn't need as much current.CORRECTION: I was too sleepy last night. What I tested was the 10 ohm resistor, which gets very hot. That battery seems to not have a high enough voltage to heat the nichrome.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 12, 2012, 05:36:45 pm
content: In terms of battery power to weight. a small lipo, with no protection circuit, can do an amp or two, which should be enough, but bear in mind battery performance can degrade very significantly at low temperatures - different chemistries will doubtless behave differently. Sticking the battery in a lump of polystyrene would probably be a good start.

author: nctnico
author_rank: Super Contributor
timestamp: May 12, 2012, 07:51:15 pm
content: The best thing to use is a wirewound resistor. This way you can get the most power from the battery by matching the voltage to the load (getting the best power transfer).

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2012, 09:50:13 pm
content: Quote from: jerry507 on May 11, 2012, 08:36:23 pmI just did an experiment with resistance heating of different thicknesses of steel wire. My results suggest a constant current source is a bad idea. A regulated voltage source pre-set to the calculated voltage would work better.The reason is the way resistance increases with temperature in a metal wire. Consider any small portion of your wire. The heat dissipated in the wire for any given small section under consideration is given by W = I²R, where R is the resistance of that local section of wire. Since we have a constant current source I² is constant and the power dissipation is therefore proportional to R.Now suppose that one part of the wire gets hotter than another part (maybe due to a kink or bend, or due to a local narrowing). The resistance of this part will go up, causing the local power dissipation go to up also. This increased power will make that part of the wire get hotter, and so the resistance will go up even more. Very soon (if the constant current source has enough oomph behind it) the local portion of the wire will glow yellow and break.With a regulated voltage source this won't happen so much, since the increased resistance of the hot wire will reduce the current and counteract the local heating effect.The effect is worse with thin wires, and much less of a problem with thicker wires. Thicker wires require much more power to melt and they are likely to saturate the constant current source before the voltage can get that high. Also the thicker wires will conduct heat along the length of the wire much better, evening out local hot spots.Unfortunately thin wires are more suitable for limited power supplies. A thin steel wire (say 32 gauge, I didn't have the exact measurement) glowed red hot and melted at 0.75 A. A thick steel wire (24 gauge) was barely hot to the touch at 5 A.

author: jimmc
author_rank: Frequent Contributor
timestamp: May 12, 2012, 09:56:00 pm
content: Would Nitinol wire (Shape memory wire) be of any use to you?Lots of hits on Google eghttp://www.imagesco.com/articles/nitinol/01.htmlIt can make an actuator much lighter than a solenoidhttp://www.imagesco.com/nitinol/files/NM70Super.pdfJim

author: MikeK
author_rank: Super Contributor
timestamp: May 12, 2012, 11:35:09 pm
content: This 40 AWG nichrome needs at least 8V @1A to get red hot. My 5V power supply @3A will get it hot, but only just. My variable 1A supply gets it red hot at 8V. I need to put this in a fixture with different lengths for testing.Ian's tests suggests that steel wire gets hotter at lower amps. So nichrome's main benefit is that is withstands repeated abuse.

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2012, 11:52:38 pm
content: Quote from: MikeK on May 12, 2012, 11:35:09 pmThe temperature a given wire reaches depends only on the current, not the voltage.The situation is best analyzed by considering a unit length of wire.For our unit length, the power dissipated is given by the formula:W = I²RWhere I is the current, R is the resistance per unit length (at the operating temperature), and W is the power dissipated per unit length.We can see that for any unit length of wire, the power is proportional to the square of the current and directly proportional to the resistance.Now the resistance per unit length is proportional to the cross-sectional area of the wire. (And temperature: resistance goes up with temperature, but if we are designing for a particular temperature we can assume the resistance to be the value that applies at our working design temperature.)The cross-sectional area of the wire in the AWG system doubles with three steps in wire gauge. So for example, going from 40 AWG to 43 AWG would double the resistance per unit length.So where does voltage come in? This depends on the length of the wire. If you halve the wire length you halve the total resistance and therefore halve the voltage required to obtain the same current.Suppose you need 8 V to make 1 ft of wire get red hot. Then you need 4 V to make 6 in of wire get red hot, 2 V for 3 in, and so on.Combining all these rules together allows you to determine the wire length, the wire diameter, the required current, and the required voltage to make your wire perform as desired. (You can assume that wires of different but similar thickness, within reason, will reach approximately the same temperature when subjected to the same amount of heat dissipation per unit length.)

author: IanB
author_rank: Super Contributor
timestamp: May 13, 2012, 02:21:50 am
content: Just as a quick point of reference, I did an experiment with some 28 gauge steel picture wire, some polypropylene cord, and a "hot wire cuts through cord" arrangement.I found that a 2-3" length of wire was able to cut through the cord when powered by a 1.5 V AA alkaline battery. It required about 3 A to make the wire hot enough and the alkaline battery could supply this, although an NiMH cell like an eneloop would be more reliable. Note that the wire doesn't need to get red hot, not even smoking hot. Polypropylene has quite a low melting temperature.

author: IanB
author_rank: Super Contributor
timestamp: May 13, 2012, 03:22:58 am
content: Based on previous discussions in the thread, here is a picture of a "T" shaped cutter concept. It seems logical to me and it comes rapidly to mind if I think about how to engineer such a system.The T shaped rigid tubing provides stability and protection, and prevents the cord from pulling on the wire and snapping it.The spring loaded cutter wire ensures the hot wire will pull through the cord and separate it when activated.The hot wire can just be cheap ordinary steel wire of an appropriate thinness. It is fixed to the electrical terminals using small bolts and compression washers. To install a new wire you just hook the cord out, feed the wire through the loop in the cord, and reassemble.

author: MikeK
author_rank: Super Contributor
timestamp: May 13, 2012, 03:46:41 am
content: Excellent! I had trouble visualizing the arrangement, but now I understand it.

author: T4P
author_rank: Super Contributor
timestamp: May 13, 2012, 06:48:08 am
content: You can use a very thin nichrome wire or a steel wire ( of course, 32awg or similar, wirewrap wire or the sort ) with a big arse D cell.Those $3 wire foam cutters you buy off the shelf usually use this arrangement but always remember to throw it in a block of polyprop

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 13, 2012, 09:03:57 am
content: QuoteAnd temperature. This can be a significant issue in heating applications.For a true constant current drive, power will increase with increasing temperature, and chances are a hotspot will form somewhere.For constant voltage, power decreases with temperature, so it is self-regulating.

author: IanB
author_rank: Super Contributor
timestamp: May 13, 2012, 09:23:04 am
content: Quote from: mikeselectricstuff on May 13, 2012, 09:03:57 amIndeed, it is surprising how much the resistance goes up even with seemingly small temperature increases. When I turned up the voltage the temperature increase dropped the current almost back to the initial value. Hot metal wires (or bulb filaments) make somewhat effective current regulators.QuoteVery true. In my experiments constant current drive was a disaster. As soon as a hot spot formed the wire melted and broke at that point. With constant voltage I could get the whole wire to glow evenly red hot.

author: MikeK
author_rank: Super Contributor
timestamp: May 13, 2012, 03:04:10 pm
content: Quote from: Dave.S on May 13, 2012, 06:48:08 amWell, a D cell is bigger and heavier than I'd like. I might wind up with the 3V photo battery (1/2 AA size?). It's small and light, but I need to try a smaller piece of nichrome as I'm not able to get a 4 inch section to heat up.

author: SeanB
author_rank: Super Contributor
timestamp: May 13, 2012, 03:08:11 pm
content: You must have a dead toaster around, an ideal source of nichrome wire. That or an old kettle element ( a little messy but a nice finely coiled wire inside, even has an insulating powder).

author: jerry507
author_rank: Regular Contributor
timestamp: May 13, 2012, 04:05:20 pm
content: You're absolutely correct Ian about the local hotspot issue. The reason it's not a problem in the application is that it's completely UNNECESSARY to reach the red hot temperatures. In fact this is quite bad. With nichrome wire, as opposed to steel wire, you can get the wire quite hot in absolute temperature and the metal won't glow. Nichrome withstands higher temperatures without corrosion or fatigue hence a longer lifespan and why it's commonly called resistance wire. If you've heated the wire up to red hot then it will become more brittle and it will fail faster.As I mentioned, we use a CC source to control the temperature more easily because at normal operating temperatures the wire won't be red and won't form excessive hotspots. Since you're regulating the largest term of the power dissipation, you're more readily controlling the power dissipated and therefore the temperature rise in the wire.If you're only planning on using the wire once, it's still not quite as reliable, but not terrible. We use the same device many times because we found that it it was more reliable to rig it up and do a really good job inspecting it, and then not mess with it much.

author: MikeK
author_rank: Super Contributor
timestamp: May 13, 2012, 05:08:01 pm
content: Wikipedia is wonderful!The attached image is fromhttp://en.wikipedia.org/wiki/Nichrome

author: ejeffrey
author_rank: Super Contributor
timestamp: May 13, 2012, 05:28:12 pm
content: Quote from: jerry507 on May 13, 2012, 04:05:20 pmRed hot is essentially the same temperature for any material. It is quite acceptable (but unnecessary in this application) to heat nichrome to red hot, as you can see from any toaster.

author: kaptain_zero
author_rank: Regular Contributor
timestamp: May 13, 2012, 05:32:33 pm
content: Here is a typical small Lipo pack, 3 cell so 11.1V nominal and rated for 4.5A draw, holds 180mAH and weighs 19g which could be reduced a little bit by removing the connectors if making it single use.http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=23339RegardsChristian

author: jerry507
author_rank: Regular Contributor
timestamp: May 13, 2012, 06:18:46 pm
content: Quote from: ejeffrey on May 13, 2012, 05:28:12 pmYes, you're correct. What I was trying to say is if you get nichrome red hot the chromium in the wire prevents the wire from oxidizing. In addition, the melting point is much higher which isn't exactly important in this application. If you're looping the wire though, you're going to create significantly more hot spots as you're working the metal to that shape. In any case, you don't need to be anywhere near red hot to melt the materials we're typically looking at. Dacron will melt at between 200 and 250C IIRC, and red hot is usually around 700C IIRC. Obviously there is a huge gap there. The lower temperatures cause significantly less fatigue to the wire and it will last longer as well as be more reliable. So from that table from Wikipedia you're looking at around 1A for a typical size wire to get to the necessary temperatures. It's very reasonable.

author: chrome
author_rank: Regular Contributor
timestamp: May 13, 2012, 09:12:28 pm
content: Why not use something like this:http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=13365It's made for that and can be reused super easy, just add a small servo and done, you don't need all this temperature stuff just give it a position and bam done.