What Is It and How Does It Work?
The passive anti-rotation device helps mitigate rotation of
balloon payloads in the stratosphere by expanding the
tie-point to the balloon neck horizontally, providing
restoring torque when the payload stack attempts to rotate
with respect to the balloon. Balloons often stop rotating once
they reach the stratosphere – this device couples the payloads
to the balloon so the payloads stop rotating too.
The key to the passive anti-rotation device is the “neck insert” shown above. It consists of a section of thin-wall 2-inch diameter PVC pipe (available at vacuum cleaner stores, rather than at hardware stores). A cap is glued on to the top of the pipe to seal the tube and also to give the balloon a ridge over which to stretch. The balloon is secured by zip ties (below the ridge) and also taped to the pipe below (at the balloon neck opening).
The standard “hard tie point” we put inside the neck of the balloon remains in place – see zip ties and “main line” (pink string) connection located above the passive anti-rotation neck insert. The main line runs centrally (i.e. directly below the neck) to the payload below. Close to the bottom of the open PVC tube (which is reinforced with strapping tape running in two directions) are two 8 inch long carbon fiber tubes through which “side lines” (not in place in these photos) also run to the corners of the payload below. We recommend setting up the rigging so the weight of the payload stack is held almost exclusively by the main line and that the side lines are nearly loose and serve just to prevent rotation, not to hold up the payload weight. That said, the side lines don’t have to be particularly strong (but do need to be protected from abrasion by the ends of the carbon fiber tubes – we use more strapping tape for that). Hanging a lot of weight on the carbon fiber tubes might snap them.
For best effect, place the first payload quite close to the neck of the balloon. The whole point is to avoid any place in the stack, right up to the balloon, to be a single line which cannot provide much anti-rotation torque. However if the stack does manage to spin with respect to the balloon then the main line and the side lines will cross, making this device to spread out the lines ineffective. Hence keep the connection lines between payloads as short as is practical.
In the case of video-telemetry eclipse balloons, the first payload down from the neck of the balloon is likely to be the OCCAMS razor cutter. That needs to be mounted so that the razor cutter can sever connections to the main line and all the side lines simultaneously. One way to accomplish this (not shown in photos here) is to put a “belt” around the OCCAMS box and have all the lines from above attach to that belt so that when the belt is cut, all the lines are released.
To preserve the anti-rotation effectiveness, the entire payload stack should be connected below the first payload box with multiple, parallel, rigging lines, as widely separated as practical. We use four lines and run them down the corners of the boxes (if they are rectangular in profile when viewed from above).
This passive anti-rotation device does complicate the placement of the parachute, which typically would be attached to the balloon neck by a single main line – and we are avoiding ever having any single main line anywhere in the stack. Our recommendation is to attach the parachute top to an open-top hook of wire (no barbs!) on the side of the first payload box and attach the parachute shroud lines to the second payload box. Thus the parachute will hang down as normal during the ascent but when the balloon bursts, or is cut away, the parachute will inflate and slip upwards off the hook. Then the OCCAMS box will dangle below the parachute for the descent, beside the rest of the payload stack.
Relevance to Our Eclipse Goals
Payloads often rotate throughout balloon flights (see first video below), making footage hard to watch. Mitigating rotation using this device (see second video below, taken from another balloon but flown at the same time), will improve video quality and help any pointing mechanism used on eclipse flights to not have to work nearly so hard to keep the shadow (or the sun) in view.
Watch the videos below!