Hello Allen.

I've been thinking about how to reduce the cross sectional area of the VBP's lift cell array during the ascent. Certainly there's lots of extra volume under the shroud to play with -- at least half the fully inflated volume being empty below 5km. But a means of allowing the balloons to still lift their loads and of allowing for further expansion is difficult if we want them to begin compressed. For example, if we use conical balloons, the shape of the full balloon is different from their shape at the beginning of the ascent.

Fully Inflated

Partially Inflated

The partially inflated lift cell's shape is mainly formed by a “pool” of hydrogen at the top, and the majority of its length consists of nothing but folded fabric with little or no gas contained within. In fact, this length of folded fabric is difficult to force air through when inflating the balloon. Many modern high altitude balloons are filled through sealed tubing that hangs down from orifices on their tops, rather than being filled from their bottom end.

There is a negligible amount of hydrogen in the uninflated folds of a large balloon. A balloon whose folds are bound together by a collar or sleeve contains just as much gas as one whose folds are allowed to hang free. If the fabric is allowed to make a shallow enough angle at the point where the folds are gathered, this does not dramatically affect the load the balloon can handle.

Partially Inflated with Sleeve

Because little pressure is applied to a collar before the lift gas begins expanding through it, a set of light-weight collars can be made to keep the lift cell fabric gathered during the ascent yet be weak enough to break off and free the balloon fabric as expansion progresses. This configuration dramatically reduces the cross sectional area of the partially inflated balloon while making nearly no change in the loads it can lift.

An external sleeve, placed over a set of collars, need not even be a complete cylinder to keep the folds gathered. A flat sheet wrapped around the uninflated portion will do if the collars are placed outside of it to keep it in place. A sleeve can further reduce drag for exposed balloons. It can also increase abrasion resistance for balloons whose position would otherwise require them to rub against each other or the shroud during their expansion.

Since a partially inflated balloon with a sleeve assumes a linear form with strength equivalent to a thinner rope line, it can also be wound like line as well. It may be possible to deploy the lift cells from reels during the ascent. Reel deployment would take up far less space than the conventional method of stretching large balloons out across the ground to inflate them, and would prove an easy means of raising new lift cells once the platform was in place.

Reeled lift cell

The sleeve would aid with tailing the wound lift cell fabric, but the reel would still have to be fairly large so that the entire wound length of the lift cell could be wrapped in as few layers as possible. Otherwise, binding could be a problem. Also, the reel would have to be able to support the loads involved. That may be a problem for a lightweight reel.

But placing sleeves around the folded lift cells is a useful tactic regardless of whether reel deployment is a viable proposition.

Thanks for your time.

CME