This valve is an experiment designed to replace the tail ducts of traditional constant-altitude ZP balloons. A tail duct is a large tube extending from an opening on the bottom of the plastic balloon. For example, Spirit of Knoxville II's tail duct measures 91cm (3ft) long by 26cm (0.85ft) in diameter. It was made of 0.75 mil plastic, the same as the rest of the balloon.
How it works: The duct's purpose is to allow rapid overflowing of the helium lift gas as the balloon climbs past it's neutral buoyancy pressure altitude. This also occurs as the sun heats the balloon in the mornings. At sunset the helium contracts, and creates a negative pressure in the balloon. This collapses the lower portion of the balloon, as well as the tail duct. This collapsing of the tail duct is supposed to keep air from entering the balloon envelope at night.
The problem with the existing duct design is that it prevents only high rate air entry, it does not prevent air entering at a slow rate.
Goal: replace with a vent similar to japanese paper balloon vents, with a spring holding a single plate with silicone seal. Video of japanese paper balloons Pics of paper balloon vents, 17inches (43cm) diameter, click for full size:
Needs to have as much of an unrestricted flow as possible. The danger with this is that if the flow rate is even slightly less than the expansion rate of helium, the rate of helium expansion will continue to outpace the flow rate as it climbs, and the balloon will burst instead of floating.
The valve seat should be smooth. Seal material should be continuous silicone rubber sheet cut into ring. ~2mm thick Remove center section to save weight.
Weight must be as low as possible. All components should be reviewed to find lighter-weight alternatives.
The valve should actuate physically powered by the differential internal envelope gas pressure of less than 5mb (2 inches of water, 0.0725psi)1). Japanese paper balloons were 3.878mb (0.05625psi)
For now we will try to make a valve that is less than the Japanese balloon release pressure, so LESS THAN 3.878mb ( 0.05625psi).
Icing in the form of frost may form on the valve area. A system that would be unaffected by this is preferrable.
Operating temperature range should be -50C to +40C. Cold temperatures will be tested in a cryo chamber. Pay special attention to avoid use of lubricants which may solidify, contraction of hard materials in linkages, and the stiffening of flexible materials such as gaskets/elastics.
The valve will experience the highest flow rate during the initial climb to float transition. Thereafter it will be used primarily at sunrise, to vent excess gas, and periodically if too much ballast is dropped, to arrest the climb. Expect at least 10 actuations over the course of the 72 hours flight.
Spring force estimate needed: 3 lbs McMaster spring, use two: 1692K31