# Actarus Project

## Open Source Arduino & Android-based High Altitude Balloon with Gps Assisted auto "Return To Launch" ## How it works – a little theory

How does the balloons fly in near space work?
The idea is quite simple.
The helium gas is lighter than air. The air density is in fact 1,225 kg/m3 while the helium is 0,1785 kg/m3. This roughly means that 1 liter of helium, weighs “-1 gram”,  namely that a liter of helium is able to lift approximately 1 gram.
In this way, if we fill a balloon with about 2,000 liters of helium, we have an upward thrust of about 2,000 grams. Without the weight of the balloon that is 1,200 grams, there are another 800 grams of weight to be lifted. If the payload will be 800g, the situazione would be stable, so in order to have a little push we need to leave at least 550 grams free. There are so 250g of weight to be lifted. In these 250 grams we have to put strings, the camera, the GPS tracker and the onboard computer.
That said, at what speed will lift our balloon?
The calculation to be done is quite complex, because we have two conflicting forces. One is graviity force upwards caused by helium, and the other is the friction of the balloon against the air that increases as we go up due to the increase of volume of the balloon (which will be discussed shortly).
Using an online calculator, we can say that the average speed with the parameters above, is about 5.3 m/s.
How do we know at what altitude will go the balloon?
Here too it is to do two calculations. Because the higher you climb, the more the atmospheric pressure decreases, our balloon will get bigger according to the physical law on the gas, so halving the pressure doubles the volume. In this way the balloon we inflated with 2,000 liters (and therefore with 2m3 of volume), arrived at 5000 meters above sea level will double the volume (4m3), according to an exponential progression…  Arrived at about 33,000 meters, the volume will be so high (approximately 130 times the initial volume), the balloon will burst, and at this point our payload will begin to fall. At what speed?
Also here we have to calculate the two forces, the gravity that “pulls” downward and that of friction of our parachute … The calculations are quite complex, but even here there is a usefull online calculator. For a payload of 250g and with a parachute of 40cm in diameter, the speed of fall will be about 6.5 m/s.
At this point we just have to calculate the trajectory of our ball, and here will help us a site that using the predictions of the winds, gives us a fairly accurate path with landing point, once specified the starting point, the ascent and descent speed and altitude of bursts.
Et voila!!! 🙂