This balloon burst calculator we have made is quite simple to use.
We have been using it here at Near Space Sciences for almost all the twenty years of doing these flights.
Prior to the home computer being able to be used, we had a giant chart that had all this data in print form.
Using the method described below, we have been withinn +/- only 1000 feet for a prediction vs the actual burst altitude.
To use the above calculator some simple math needs to be done first. I eventually plan on being able to just enter the data into a field and have it spit out the needed data for me, I just haven't come up with the oomph to do it because the mathwork just doesn't take much time to do. Or in the spirit of the "Open Source" Amateur Ballooning, any of the other groups feel free to try to do this for us all.
First you need to know what the burst volume of the balloon you are planning to use is. Not the burst Diameter, but the Volume of the balloon at Burst. This will of course work with any balloon, but for this example I'll only talk about Paul and his Kaymont balloons.
Example flight parameters,
1500 Gram Balloon
4 pound payload
6 ounce parachute
1 ounce misc. support lines, tape, etc.
First lets get the burst volume of the balloon.
A Visit to Pauls site at Kaymont on this page. http://www.kaymont.com/pages/sounding-balloons.cfm
shows a table of most of his balloons.
And the data for the 1500 Gram balloon shows that it bursts with a diameter of 944 centimeters.
We here at Near Space Sciences mainly still deal with Inches, and cubic feet. so some conversion is needed here.
A 944 cm diameter equals to a diameter of 30.971128609 feet.
Now lets get the volume of this sphere. We use the formula of the radius cubed times 4.18879.
radius of the 1500 gram at burst is, 15.4855643045 feet. Cube that and you have,, 3713.4801595627928992808206463161
Take that answer and multiply it by the 4.18879, and you have the answer of, 15554.988557575031268578508715083
Or for our purposes 15555 cubic feet is the balloons volume at burst. save this number! you'll need it later.
NEXT STEP Total System Lift.
Add up EVERYTHING that will be flying, if it's going on the flight it MUST be added. And convert them to ounces.
1500 Gramm Balloon = 52.910 942 924 ounces, lets say 53, close enough.
4 pound payload is 64 ounces. then add in the rest and we have a total weight of everythinfg that is flying is 124 ounces.
But of course that is totally neutral lift state. it won't be going anyplace. Here at Near Space Sciences we use far less what we call "Positive Lift" than most of the other groups. On Calm days we use 8 ounces and on windy launch days a full pound.
So in this case last say it's windy and we add the full 16 ounces of extra lift.
So now we have a total system lift of 140 ounces.
Now how much lifting gas does it take to lift this much weight? That all depends on what gas you use.
At the "Standard" Sea Level, HE lifts 1.072 ounces per cubic foot, and H2 can lift 1.216 ounces per cubic foot.
So again some simple math will tell us how many cubic feet will be needed for the flight.
We here at Near Space Sciences have always for twenty years have used only H2 for our lifting gas so, in these calculations we will also continue this,
Need 140 ounces of lift divided by 1.216 ounces per cubic foot equals just barely over 115 cubic feet of gas.
This is where the magic comes in. Take that 115 cubic feet of gas, and divide it by the burst volume of the balloon, 15555. and the answer is,,, 0.0073931211828993892639022822243652 this I call the volume percentage number.
Yeah i stretched it way out, but copy and paste is soo much fun! now that number is what you need to look for,
Go up to the calculator, change the altitude to feet, and enter say 100000 feet,
ignore the temp field,
Look at the pressure field and change the pull down menu to the atmospheres value, and look at the number it is displaying,
In this case it's showing,, 0.010759000000000001
Compare that to the calculated number, of the volume percentage number, which is bigger? In this case the number in the calculator is bigger, which means we need to go higher, try 110000 feet,, and we now have 0.006832, Opps too small but close, try 108000 feet. and we get 0.007474, for me thats close enough! you can whittle it down to an exact number if you wish but I'd stop there.
ahh what the hell huh,, 108250 feet should be the predicted Burst altitude!
So with this set of parameters for this flight I would state that the balloon will burst at 108,000 feet.
It's been for almost every flight spot on less than 1000 feet off ever.
Joe WB9SBD And Near Space Sciences