It's burst into flames! It burst into flames, and it's falling, it's crashing!

April 20, 2010 2:44 AM

What is the cheapest non-deadly way to assemble a balloon-based aerial photography rig?

I'd like to use a balloon to lift a fairly large expensive camera 100-200 m into the air. Gear and tether weigh about 4 to 5 kg. I've found a balloon with 2.5 m diameter for sale locally. How much will this lift? How much gas do I need? What is the maximum amount of wind in which I should attempt flight?

Helium costs three times as much hydrogen. How crazy would it be to fill the balloon with hydrogen? I'll be on a third world beach so don't have to worry about laws and rules, but will likely attract curious cigarette-smoking onlookers. What are the real dangers here?

posted by Etaoin Shrdlu to science & nature (7 answers total)

Hydrogen is still pretty common in weather balloons. Provided you read up on and follow safety precautions (nobody smoking anywhere your gas cylinder, basically), you should be fine. You're outdoors after all, so there's zero risk of any buildup of hydrogen.

According to this site, the maximum lifting capacity for an 8-foot balloon is 15.5lb, although they don't say whether that's for hydrogen or helium (I'd guess the latter).
posted by le morte de bea arthur at 3:04 AM on April 20, 2010

Using some weights of gases at atmospheric pressure and room temp that I just found online:

Helium has a density of 0.18kg/m³
Air has a density of 1.2kg/m³

So 1m³ of helium in air will give a net upward force of 1.2-0.18 = 1.12kg

Assuming your 2.5m balloon is sperical, is has a volume of

Volume of sphere = (4/3)*pi*radius³ = (4/3)*3.142*1.25³ = 8.181m³

...and therefore exerts an upward force of 8.181m³ * 1.12kg/m³ = 9.16kg

So if the combined weight of your camera rig and the balloon is less than 9.16kg, the balloon should lift it. Assuming you're planning to tether the balloon (otherwise it'll go far higher than 200m and you'll need GPS to chase it), you also need to include the weight of your tether.

Note that this calculation assumes that your hydrogen is at one atmosphere (=1bar) of pressure. The weight and elasticity of the balloon will mean that, when full, the gas inside will probably be at a higher pressure. Higher pressure = higher densiry = less lift. It probably won't be a big effect, but you might want to look into that.

I don't know enough about pressure changes at altitude to calculate how high your balloon will get. In a perfect system, the maximum height would be the height at which the ambient air is at a density where there's no net force exterted, i.e. lifting force = payload force. It's pretty easy to calculate, but you'll need to find a table (or know how to work out) the density of air at various altitudes. However, I'd be willing to bet that 100-200m will only make a tiny difference, so your balloon will keep pulling upward almost as strongly as it would on the ground.
posted by metaBugs at 4:30 AM on April 20, 2010 [1 favorite]

Your other questions:

For buying gas, 1m³ = 1000 litres. So if I'm right that your balloon holds about 8.18m³, that's 8180 litres.

Gas cylinders normally tell you the size of the cylinder (in litres) and the pressure at which the gas is stored (in bar). You just multiply the numbers together to get the total volume of non-compressed gas. For example, a 15 litre tank at 250bar = 15*250 = 3750 litres of uncompressed gas.

Wind speed, I'd guess you want very little. Experience flying kites tells me that the wind 50-100m up tends to be much stronger than at the surface, even in exposed places like a beach. You'll need to make sure that your balloon is firmly tethered to something. If you really need to know the altitude at which your photos were taken, knowing your thether's length and measuring the angle that the wind blows it to will let you do some simple trig to work out the approximate height of your balloon.
posted by metaBugs at 4:46 AM on April 20, 2010 [1 favorite]

The cheapest way is likely with a kite instead of a balloon - one of my friends has done a ton of work in this area and his site is a very good resource.
posted by tmcw at 5:03 AM on April 20, 2010

Probably not quite what you're looking for, but this guy seems to have a nice rig :)
posted by reptile at 6:08 AM on April 20, 2010

Your balloon is too big and your tether may be too heavy. Are you planning on something heavier than kite line? If so, reconsider. 50 lb test line should offer a large safety factor, and you can limit altitude by tying on washers or some other light weights every few meters as you let line out until the altitude begins to stabilize. This will allow you to let out more line from the reel without the balloon continuing to climb higher and higher. The result will be a larger lateral range without you having to run around, and better altitude control.

Referring to metaBugs' post, with 1 m^3 of Helium, you can lift 1.12 kg. From your link, it looks like your camera has a mass of about 1.2 - 2.0 kg, depending on what you've got attached to it (I'm guessing on the upper range). You want to look for a balloon that will hold around 2 m^3 of gas, not 8. So shoot for a balloon with a diameter of about 1.5 m. Heck, call it 3 m^3 and go for a 1.8 m diameter. Or, go ahead and use your big balloon, but just don't fill it completely. Don't use Hydrogen, it's not worth the risk. We're talking about $10 worth of Helium. If you're that concerned with costs, you shouldn't be putting an expensive camera in a balloon.

Also, keep in mind that you don't want your heavy camera crashing down on somebody's house or head, regardless of local laws. So keep away from populated areas.

And if you want to go into near-space, the $750 rig mentioned above is too expensive. Try $150.
posted by dsword at 9:37 AM on April 20, 2010 [1 favorite]

Can I ask why you want to send a 1 series up? How about a 5D?

Or if you've got some money to spend, why not a much smaller camera? You can fly a remotely controlled micro four thirds camera on a helicopter. You'll have much more control over the photos you're taking.
posted by Brian Puccio at 10:38 AM on April 21, 2010