Yesterday, Flying Apple Space Technologies (FAST) launched our 20th mission. The goal of the FAST-20 mission was to put up a neutrally buoyant, “floater” balloon—something that a latex weather balloon is not designed to do. People hack computers, smartphones and software, but here at Flying Apple, we are trying to hack a weather balloon.
The giant latex weather balloons we use are designed to contain a lifting gas, take off the ground an continue to rise and expand until the balloon gets so large that it bursts. (To give you an idea of how large that is, the balloon is about 6 feet in diameter when on the ground, but when it bursts, it can be over 30 feet in diameter–that’s large enough to (uncomfortably) fit a full symphony inside, including the percussionist.)
But we don’t want the balloon to continue to rise and grow. We want it to stop rising and float in the strong winter winds at around 110,000 feet.
Our goal is to add just enough hydrogen for the balloon to get off the ground, but not much more. Using the Alicat Flowmeter of Science, we add hydrogen until the balloon and payloads can be lifted, and then we add just a tiny bit more. The amount of lift that the gas has above and beyond the power to lift the payloads is called “free lift.” We give the balloon just between 50 and 100 grams of free lift, which means that a family mice would be enough to tether it.
In other words, the balloon barely gets off the ground.
Here’s the hack: the latex balloon, which encloses the hydrogen always squeezes down on that gas, but when the balloon reaches 110,000 feet (or so), where the pressure is low, that squeezing force becomes important. That latex squeezing force compresses the hydrogen gas into a slightly smaller space, displacing less of the surrounding air. If the balloon displaces less of the surrounding air, the upwards, or buoyant force, is slightly smaller as well. That’s what Archimedes was shouting about in the bath.
That’s exactly we are going for. We use the balloon itself to “get rid” of the extra 50-150 grams of free lift that we gave it at launch. That way it will stop rising and start floating. Once up there, the balloon will catch the East wind and fly across the country. All the way from Las Vegas to the Atlantic ocean and, we hope, beyond.
We’ll be attempting to hack weather balloons all winter. We invite you to check out our launch schedule or follow along here.