There are a lot of balloons to consider; balloon sizes, materials, thicknesses, costs, and even age. Generally, the older the balloon, the less elastic it is and the less reliable. A balloon that’s been sitting in a harsh environment for years might not be able to inflate as much and will pop at a lower altitude than expected. Similarly, the cold air of the upper atmosphere can have an effect on the balloon.
The balloon will expand as it rises until it pops.
The size of the balloon determines how much gas the balloon can hold and thus how much mass the balloon can lift. Too small and the balloon might not be able to pick the payload off the ground, too big and you’re wasting money and gas that you don’t need to. You can intentionally use a bigger balloon than necessary if you want to go up very fast.
The balloon needs to be filled with a gas that has a density that’s lighter than air so that it will rise. One way to do this is with hot air. The hot air excites the particles, making them spread out and less dense inside the balloon. Because it’s less dense, it will rise. This is why oil floats on water (oil is less dense than water). Hot air balloons need to stay hot in order to keep rising, though, and in the upper atmosphere temperatures get down to -60, so hot air is not a good option. We need to fill it with a gas that is lighter than air.
The atmostphere is made up of 78% Nitrogen and 21% Oxygen and some other elements, so we need something that’s lighter than Nitrogen and Oxygen. Looking at the periodic table, these are our options:
- Carbon – Solid at room temperature, so this won’t work.
- Boron – Also solid at room temperature.
- Beryllium – Also solid, and highly dangerous.
- Lithium – Also solid, and very unstable.
- Helium – Gas!
- Hydrogen – Gas!
- Other (not on the periodic table but lighter than air):
- There are some other gases that are lighter than air but highly reactive and dangerous.
So we have four options. Each has different properties and prices.
Helium is most often used because it doesn’t react and is very safe to handle, but there’s a very limited supply on Earth, and it’s running out. Within the next couple decades it will run out, and its price will continue to rise.
Because helium is running out and will only get more expensive, and hydrogen is very abundant and has more lifting power anyway and can be handled safely, we chose to use hydrogen. The cost of hydrogen won’t change, we won’t dwindle the world’s supply, and when done properly, isn’t dangerous (see the hydrogen explosion test).
Helium will work just fine if Hydrogen is not an option for you.
To acquire hydrogen, it’s easiest to contact a local gas supplier. Airgas is a common supplier of gases. A single tank of HY 200 is sufficient to fill one 8′ balloon or two 6′ balloons. Hydrogen tanks will be red on the outside. It is not uncommon to be charged a small daily fee to ‘rent’ the tank on the order of a few cents per day. You will also need to get a regulator, which is the piece that connects to the tank and has a gauge, handle, and nozzle. Some gas suppliers rent them out.
You will need to connect the regulator to the tank. Connect it tightly so that no gas can escape. Turn the knob slightly to verify that it works and gas comes out the nozzle. Attach a hose to the nozzle. As gas goes through the nozzle, the nozzle will get extremely cold. The purpose of the hose is to keep the balloon away from that extreme cold. During our run, the regulator was malfunctioning and we had to regulate the flow using the knob on the tank instead. It’s entirely possible to do, but the slightest mistake could send very high pressure hydrogen through the hose and burst it or the balloon.
Roll up the balloon to get all the air out of it, or better yet, leave the balloon sealed in the bag until you are ready to fill it. Turn on the hydrogen for a while to flush the air out of the hose, then turn it off and pinch the end to prevent the gas from leaving. Insert the hose into the balloon and turn the knob to allow gas to inflate the balloon.
The balloon is inflated when it is no longer slack, but before it is stretched. Remember that it will expand as it rises, so the more the balloon is stretched in the beginning, the earlier it will pop. If you underfill the balloon, it may not lift the payload or if it does it may ascend very slowly, which means more drift and a longer trial. If you overfill it then it will rise very quickly but pop at a lower altitude than intended. You can use the Ballon Calculator to determine how full the balloon should be and when it is expected to pop. For our payload weight, balloon size, and gas choice, we had to inflate ours to roughly 1.5m diameter, which we measured using a tape measure, making sure not to touch the balloon.
Once the balloon is filled, tie the end closed and attach one end of the tether. Make sure this is a good and tight knot. The balloon and payload will bounce during the ascent and can put strong forces on the tether.
Filling the balloon is a step that needs to be performed by an adult. Any spark near the hydrogen may ignite it, including static. Since we are using pure hydrogen and not mixed hydrogen, an explosion would be fairly un-dramatic. The ball of gas would ignite from the outside in, burning fairly quickly as the oxygen from outside the balloon rushed in to mix with the burning hydrogen. It’s a fast fire, not an explosion. If a mixed gas was used, something that contained hydrogen and some air, then a spark would be dangerous, as all of the gas inside would be able to ignite at once. For this reason, it’s safer to use pure hydrogen than mixed gas. If the balloon pops without a spark, the gas will rise and disperse, and nothing bad will happen. Still, it’s best to fill the balloon outside so that spilled hydrogen doesn’t collect at the top of a building.
Do not inflate or release the balloon near any power lines or other tall structures that could catch on anything.
Hydrogen balloons will only explode if there is oxygen and a flame or spark. Without both, nothing will happen.
Here’s a video of someone else blowing up a pure hydrogen weather balloon using a flame. Note that while you probably don’t want to be near it, it’s not something that would knock you down, and probably would only singe some hairs.
We also ran our own tests exploding a hydrogen filled balloon and filming it in high speed. See the high-speed results on the blog here.