I haven't flown a single balloon yet, but I plan on making one super-cheap, utilizing second-hand or my own materials. The goal here is to be able to get into the stratosphere, so at least above 10km, and take a few pictures, maybe even a video, all within 100€.
A store nearby sells 90cm diameter natural rubber balloons filled with helium for about 30€, including the helium. Since it's more close to a toy balloon than a proper weather balloon, I'll use a toy balloon as reference point. These balloons can reach about 9-10km on paper; however, if I only inflate it with half the helium gas (ie. 225L instead of 450L), there's a small chance it might fly higher before popping. With a half-inflated balloon, in theory, we should get some leeway into it, so if the maximum a toy balloon can withstand without bursting is about 3x the normal size (which it reaches around 9-10km), if we inflate it only half-full, we should be allowing it to expand 6x. That would put the maximum height of a half-inflated 90cm balloon to roughly 14,5km, considering the high pressure drop after 9km. That's a theoretical maximum, so in reality we would be around 12km, which still puts us into stratosphere! Natural rubber is more durable than toy latex, so it might actually reach this, and with some good luck go even higher. The balloon company stated they also do a special treatment on these balloons so they will last longer.
I also contemplated on getting two balloons filled with one's volume of helium, to maintain the expansion capability but to get double the lift of just one half-filled balloon. A fully inflated balloon takes about 424 liters of helium, which can lift 455g of weight. With the weight of one balloon (45g), that's 410g payload and with two half-filled balloons that's down to 365g. The companies might not even fully inflate the balloons, which can take the payload of a single balloon down to 340g. Are there any risks in flying with a twin-balloon setup instead of one? If the tether is too long, the balls will bounce against each other, cause friction or static, which can easily lead to a premature burst of one of the balloons, leading to a very slow descent and drift. Even if I tie them closely together, they might still rub against each other as they expand. Is there any way to alleviate this risk? Maybe have them set up on the cord in sequence, so one higher than the other? Of course, then the rope would cause friction, but just how much?
For radar reflection, I might use just stiff paper and aluminum foil. Obviously, I would need a permission from the local air traffic organization to release this, but that should be free, right?
I've got some extra XPS insulation foam left over, so I could make a box out of it. Can I use just glue and packaging tape, or will the G-forces from the balloon burst rip it apart? I made a calculation that for the equipment I plan to have on board, I would need about 1000 cubic cm of foam as a shell, with 300-400 cubic cm hole in the middle. That's basically two foam sheets attached together and the space carved out in the middle. The weight of the box would be about 50g. I could potentially make it even lighter, but I'm concerned with the forces breaking it. I also read from your FAQ that using a styrofoam box contains a risk of losing GPS when it lands on its side (unless I get a GPS reading just before it lands). What if I stick 4-8 very light plastic rods or cardboard straws to each side of the box, so there's a better chance it wouldn't land on its side, but more likely semi-upright (or facing down)? Another option is to get a small canvas cooling bag and fill that with bubble wrap.
I have an old Canon Ixus camera which still functions. The camera would be set up to take a photo every 5 seconds or so. That way I should be able to fill about a 4Gb or 8Gb card within the 2-3 hours the balloon is up in the air, as long as the battery lasts (I need to check that it does...). So that's a 0 cost on the camera equipment and I'm happy to toss it in. If needed, I could buy a new pocket camera as second-hand, should probably get one for 10€ or so. All it needs is to be able to take a picture without the back screen on for every 5-10 seconds as long as it can. One question though, should I buy an acrylic sheet as a window or would that just be a bad idea? I suppose it would just get condensed water on it and get the images all fuzzy, right?
Addition to the camera, I would put in a single hand warmer, which weights about 43g. Then, for tracking, monitoring and communications, I would put in a Raspberry Pi Zero in the box. I've got an old low-quality camera for the zero so I could toss that in as an extra image/video feed, saving the contents on the SD card. For GPS, I've got a second-hand Adafruit v3 GPS module, but for an even cheaper fit, I could get a NEO-6M module for a few euros from China. I would also attach a GSM module (SIM800C GSM GPRS Module Quad-band Development Board) to the PI and set it up to transmit the location data via SMS. This obviously will only work when the balloon is in range of a GSM ground station, so I wouldn't get any data until the balloon bursts and the thing floats back to Earth. To power the Pi, I have an old 2000mA battery bank, which should be enough to get the job done, as long as I disable the HDMI and leds on the Pi. Battery banks cost about 10€ as new anyways. There's no backup tracking, so I'll just put in my contact details to the box as backup. A final module to the Pi is a BMP280, just to get some barometric/altitude data stored. If the GSM module has a 3G connection, I could probably even set it up to send some of the data directly to my own ftp server while it's ascending or descending, but that depends on battery consumption. GSM itself can transfer 9,6Kbps, so at least I could get it to transfer some data. Just need to make a trigger for it, maybe time-based, so once the flight is over, it would start sending the data using this slow method for as long as there's battery juice left.
As a parachute, I plan on using a plain old plastic bag! However, I'm a little concerned with the setup. I was initially thinking of dangling it behind the whole thing during ascend, but I'm concerned that it would affect the camera position, or will it? We're super-light here, so anything can affect the stability of it. I'm even thinking if I dangle the bag behind, I should probably attach it to the bottom and let the box flip over when the balloon bursts, so that the cord doesn't pull the box in an odd position during ascend. Another setup would be to have the balloon at the end of a 5m rope and about 2m behind that attach two plastic bags to the same rope. Would that work? Would a plastic packaging tape be a good idea to enforce the bags?
I even contemplated on making holes at the bottom of the bags and running the cord through them, which ensures the bags open fully. But would the cord just rip the bags when the balloon bursts, considering the G-forces?
I've seen some cheap paracords, but to minimize the weight, I'm also pondering whether using Spectra would work. That's basically fishing cord. It's a 4 strand braided cord which can withstand about 10-15kg pull for the thinnest line. With 8G forces working on a 300g payload, the maximum pull would be about 2,5kg, so the fishing cord should hold. A 5 meter paracord weights about 42g, but a fishing cord the same length weights maybe just 1g. They have poorer knot-holding ability, so I need to ensure the knots are triple-secure. But would a fishing cord work or does it just translate into a razor whip when the balloon bursts?
For weight calculation, the Canon Ixus weights about 128g with battery and card. The battery bank is about 75g and I could actually strip the shell off for reducing the weight to around 50g. GPS, BMP280 and GSM modules weight about 10g and the Pi Zero is about 9g. A hand warmer is 43g, the box 50g. That's about a 300g payload, so we are just within the lifting ability of the 90cm balloon or two of them. I read somewhere that a good margin for a payload vs. lift is 15%, which means that I'm well above that with two half-filled balloons. If I use only one balloon, I'm still above it, but I would have to fill the balloon to at least 87% of the standard 424 liters, which means it can't expand as much. By my calculations, that's a difference of about 1,5 km in burst altitude between a dual-balloon setup vs. single. If I reduce the margin to just 5%, ie. fill the single balloon to 80% / 340 liters, it would only add about 500m to its max altitude but cause a lot of unnecessary horizontal movement. Considering the high risk of mission failure related to dual-balloon setup, I think the best would be to accept a lower theoretical max altitude and just see where the one balloon goes. It should still reach the lower edge of the stratosphere! With some good luck in the mix, it might actually get to the 12km, maybe even further!
The total cost of the project: Balloon(s)+gas 30€. Camera 0-10€. Box 0€. Cord 1€. Raspberry Pi Zero 10€. NEO-6M GPS 4€. GSM module 5€. Prepaid GSM contract 5€. BMP280 1€. SD-cards 0-10€. Battery bank 0-10€. Hand warmer 2€. Plastic bags and tape 2€. TOTAL: 90€.
For 90€, I think it's totally worth it. I can easily just lose it all, have it float to sea and never be found. Or I could get lucky and it lands safely on a field and I can retrieve it. For this cost, I'm willing to take the chance. What do you think?