Bummer on the weather. The payload is looking great though!
I've been watching a few launch/flight vids recently trying to figure out what I want to try to do this summer. A couple of observations and thoughts regarding tethering that may or may not have any basis in reality. You've probably already carefully considered each one of them but they aren't doing much good stuck in my head.
- Payload stability seems to increase with a) more attachment points and b) more distance between attachment point and center of payload gravity. Payloads that are just hung from a single eye hook or whatever in the middle of the lid have very chaotic motion with rapid yaw/pitch/rolling. Videos are nearly unwatchable. Two attachment points (e.g. handle straps) are better, but can still roll on the axis between the points where the straps grab the payload. Four attachment points (one on each corner) tend to be the best, with yaw and pitch of the payload somewhat constrained. (The whole rig still does a lot of swinging though). A couple of teams have created frames/hoops that the payload is tied to and then tether to those. These are probably the most stable.
- The pendulum effect of the entire system tends to slow down and be less jarring with longer distances between balloon and payload. However, the swinging seems to persist longer with fewer moments of complete stability. (This could just be environmental issues of course)
- The balloons sometimes have a very strong snapping effect (like a rubber band) when they pop...i think it depends on where they fail. The parts of the latex that doesn't shred off tends to wrap around downward and, if the chute and/or payload are too close, slam into and entagle with them. This is one of the longest tethers i've seen and it looks like there was minimal influence from the balloon when it burst: https://www.youtube.com/watch?v=5cRgBPqpJmA
(launch 6:08 burst 12:00) Despite using a 'payload frame' this one has a lot of wobble, but it looks like they have very short lines connecting the payload to the chute tether. If they would have run individual lines from each corner to a couple of feet below the chute, i think it would have been more stable. Also keep in mind that whatever balloon is left is going to be hanging from the chute as well, so you'll want to keep that lead shorter to reduce the chance it hangs in front of the camera.
- I've experimented a bit with increasing moment of inertia (carbon rods with small weights at the end extended from the payload). It does seem to help with the herky jerky response but again more mass means more pendulum effect.
- I've also experimented with rigid tethers (fiberglass rods). It eliminates payload wobble almost completely, but again more mass = longer periods of swinging.
- I'm also thinking of adding a swivel to the tether to keep the payload from being twisted in circles by the balloon, haven't tried that yet.
- Question, do you have any idea what purpose those rings serve that you sometimes see along the tether line?
Whatever you do, check your tether before launch - https://www.youtube.com/watch?v=7dxdfh4OLAE