Using Two Party Balloons for the payload
So, you can get a bit more free lift out of the system, and perhaps a bit more altitude, by using two party balloons in tandem to fly the payload. In my most recent flight, my payload was 18g, and I wanted 5g of free lift. Using the float calculator (assuming helium), I came up with these numbers:
|(1) 36" mylar, 18g payload, 5g free lift||Projected: 7630|
|(1) 36" mylar, 9g payload, 2.5g free lift (each doing 1/2 the work)||Projected: 8860|
|(1) 36" mylar, 9g payload, 2.5g free lift (each doing 1/2 the work)||Flight 1 Actual: 9156|
The third row is an edit following the first flight, confirming the expectation that we would have a much nicer altitude with two balloons.
How to attach them
Consulting with Dave/VE3KCL, who is also flying dual balloons, he said he was using a Tyvek tape. I don't think I found exactly what he's using. I found this:
I sealed the upper balloon with the Tyvek tape. Note the attempt to keep the stem and valve section very flat. I then found the center line of the lower balloon, and put a strip of Tyvek tape along the seam edge facing me. I tried to keep the tape on the seam, and not extending down onto the balloon proper. I then pressed the upper balloon into the tape. The picture below shows the strip of tape attached to the lower balloon, and the upper balloon attached to the sticky side, facing me. There is a little bit of the tape from the lower balloon extending past either side of the upper balloon, providing a sticky surface on each end.
I then placed a second strip of tape on the lower balloon closest to me, again trying not to hit the inflatable section of the lower balloon, and capturing the upper balloon. The front and back strips are stuck to each other on the end, and via the upper balloon in the middle.
Dave also makes a secondary connection between the two balloons. The runs a loose cord down from the upper to the attachment point at the bottom of the lower balloon. I didn't do that for fear that the string rubbing on the lower balloon might cause problems.