Possible Improvements for the Insulation Jacket
We hasten to admit that this first insulation jacket is susceptible of considerable improvement, and we would hope that the insulating performance can be considerably enhanced. This is obviously the next target of our testing program.
First, it would be much better to use the thickest grade Primaloft PL1, which is of nominal thickness 1" and nominal weight 6 oz/yd2 (imperial, ugh....) - i.e. 2.5 cm and 200 gm/m2.
Next, this first jacket had no covering over the raw Primaloft, either inside or outside. Now in practice, for an insulation jacket for a Steam Balloon, it will be necessary to have some kind of fabric layer on both sides of the Primaloft, in order to restrain its fine fibers from fluffing out and coming apart - it has a more yielding consistency than cotton-wool. The outer fabric layer also serves for tugging upon the jacket as a whole during ground handling. It will be necessary for this outer layer to have some water-shedding quality, since otherwise the entire water burden of any slight rain shower falling upon the balloon during flight will be absorbed and will quickly drag it down to earth.
Such inner and outer fabric layers can both be expected to afford substantial additional insulating effect. The inner layer will, over many parts of the envelope where it does not contact directly against the envelope material, set up an additional air layer between the jacket and the envelope itself, which will function as an extra heat barrier. And the outer fabric layer will greatly impede the convection of heated air out of the Primaloft to the atmosphere, thus helping to retain heat.
However, this outer fabric layer should not be absolutely impermeable to air, because it is necessary for the Primaloft to be able to breathe; in clearer terms, any small quantity of steam which may escape from the envelope for whatever reason - such as porosity or minor damage - and which will condense into water among the Primaloft fibers, must be allowed to dry out over a time frame of five minutes or so, thus ensuring that its weight does not drag down the balloon over an indefinite period.
It is not clear how lightweight we can get away with making these two fabric layers, but obviously 50 gm/m2 should be more than sufficient for each of them, since neither of them (particularly the inner) needs to be as strong as balloon fabric.
Yet another feature of this first insulation jacket that we are not satisfied is optimal relates to the pre-quilting of the Primaloft PL1. Here is a sectional view:
As can be seen, with these quilting lines about 15 cm apart, the thickness of the material is in fact restricted over nearly a quarter of its width, on average. This must result in a substantial loss of overall insulating performance.
It may be argued that any method of quilting or indeed of controlling the raw Primaloft would be bound to restrict its thickness somehow. We are not convinced, and intend to experiment further with the non-pre-quilted product. For example, perhaps a type of loose quilting could be devised, where the thread is not pulled very tight. Actually the loosest possible method of containing the Primaloft will be the best, always provided that it is not allowed to migrate around and to clump up into lumps.
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