The last few posts have covered electronics. In this post I'll go back to the mechanical side and describe exactly how we use a weather balloon to fly a 10 kilogram / 22 pound “spacecraft” off into the great blue yonder.
First let's discuss the balloon. Weather balloons are typically made from natural latex rubber. Literally, this is the material they extract from trees by pounding spigots into the trunks with hammers. While you might think there are suitable synthetic alternatives, the reality is that natural rubber has some unique performance properties, in particular its hyperelasticity - good for a stretchy balloon.
Balloons come in a variety of standard sizes which are categorized by the mass of rubber used to form the balloon, such as 100 grams, 600 grams, 1000 grams, and so on. Which size to use? The engineering trade-offs basically entail balancing four things: How high we want to go, how fast we want to ascend, ensuring the balloon eventually bursts from expansion in thin air, and the weight (and cost) of lifting a heavier balloon.
For the HAPP, we want to lift 10 kilograms to 30,000 meters / 100,000 feet, so we need 10 kilograms of lift, right? Nope. The balloon itself has mass, as does the bridle and tether connecting the HAPP to the balloon. Furthermore, we don't want to merely balance all the gross weight with lift; we need some net positive lift to ensure we ascend at a reasonable rate. Too fast and the balloon will generate turbulence behind it as it rams through the air, leading to a very bouncy flight - the opposite of what we're trying to achieve with a jet stabilized photography platform! Too slow and the HAPP will drift far downrange in the wind before it reaches the target altitude (and the onboard batteries may expire as well). Based on experience from other balloonists, it appears that the sweet spot is about 3 to 4 meters per second of vertical velocity.
Of course I verified the math by hand, but you can use a variety of online calculators to show that we need about 13,000 liters / 460 cubic feet of helium at standard temperature and pressure to provide the requisite lift. Using the 139 cubic foot aluminum pressure tanks that my local welding supply store uses for helium, this equates to about three and a half tanks. It also equates to a sphere of helium about 3 meters / 9 feet in diameter at STP, just like this:
A hint if you wish to try the calculations yourself: The ideal gas law and Archimedes' Principle are your friend, as is knowledge of the relative molecular weights of helium and air (mostly nitrogen). And don't forget about the coefficient of drag on the balloon. Bonus points if you show your work :-)
If we use a tiny balloon, say 200 grams, it will burst before it stretches to 3 meters in diameter as we inflate it on the ground. If we use any size other than the smallest balloons, say 600 grams, we can inject sufficient helium to achieve liftoff. However, there won't be much room left for the balloon to stretch as it expands in the thin air at high altitude. It will burst before reaching our target altitude.
So we should just use a ginormous balloon and be done with it, right? Nope again. An excessively large balloon may not burst before it stops rising, resulting in what is affectionately known as a floater. Floaters are bad because they can do things like circumnavigate the globe before the latex finally degrades in the ultraviolet rays of sunlight. Note that the HAPP guards against this possibility by using doubly redundant pyrotechnic devices to sever the balloon tether after time and / or distance limits have been exceeded. At that point the payload is gone and the balloon will resume ascending until it eventually bursts (and even if it didn't, the North Koreans would likely be less upset with a simple balloon floating overhead as opposed to a high-tech capsule laden with cameras and satellite communication devices!)
Fortunately, the largest balloon size easily available for general purpose, 3000 grams, appears to be about right. That's what we'll use for the main mission. They're also quite expensive, almost $400 apiece. Here are the two I've ordered:
To conserve funds, we'll use smaller ones such as this 1200 gram balloon for initial flight testing (still $115 each!).
While we're discussing money, I should also note that a 139 cubic foot cylinder of helium costs $144 to fill where I live, so four cylinders ring up at $576. That means a full mission flight will cost almost $1000 for the balloon and helium alone, with any test flights, hopefully only one or two, costing somewhat less. Crikey! Well, in for a penny, in for a pound I suppose...
OK, with the balloon selection and helium calculations out of the way, in the next post I'll talk about attaching the HAPP to the balloon.
Onward and upward!