Sent 02-Apr-03by Chris Koehler
The Dacron braided string we and EOSS use can be purchased from www.synthetictextilesinc.com and has a breaking strength of is 260 lbs and works great. They have other diameters too. I would recommend using it.
I think Mike uses a bamboo type ring to separate the parachute lines.More by Mike Manes
You are prudent to have perused a copy of FAR 101, but it does take a bit of verbal contortioning to read it. If you reduce the gobbledegook that ends at para 101.1(a)4 to plain English, you'll find that this law does not apply to any unmanned balloon carrying a payload that does not exceed the 4 limits in paragraph (i) - (iv). One of those stipulates a maximum "impact force" of 50 lb to separate the balloon from the payload string. Evidently you've succeeded in navigating that tortuous route.
The operative term here, as EOSS reads it, is "impact", which we infer to mean the rapid application of kinetic energy rather than the gradual application of tension which is conventionally used to measure the "tensile strength" of a support member. In other words, a line rated at 50 lbs. tensile strength is capable of supporting a 50 lb static load, but it most certainly will break if that 50 lb load is suddenly accelerated to a speed typical of an aircraft in flight. And we read this limit as it might be applied in the latter case. Thus if one assumes that a payload string struck by an aircraft traveling at a modest 120 kt true airspeed might experience a peak acceleration of 30 Gs, then this 50 lb "impact force" limit translates to a tensile stress of 1500 lb. So in that sense, the 260 lb line that Chris and EOSS have used is well within the stipulated limit, even for the light payloads that we fly.
A search of the FAA web site did not reveal any definition of "impact strength" or its ilk, nor did a similar search of rope and cord manufacturers' sites. So in that sense, this interpretation is not contradicted.
Empirical experience goes further in its support. Novice balloonists who have read "impact" as "tensile" have one several occasions have woefully observed their costly bubber and gas soar skyward immediately after release, unburdened by the payload string now at their feet.
EOSS has experienced failure of its customary 250 lb tensile nylon line at a knot during the dreaded "post-burst chaos" event. This was diagnosed as embrittlement of the CA glue used to secure the knots, and since then, we don't use glue and have experienced no further line failures.
Abiding advise from Norm Kjome, perhaps the world's most experienced research balloonist, we have adopted the double bowline as our standard knot; it minimizes stress concentration and is self-securing.
Chris is correct in pointing out that we use a spreader ring on the parachute shroud lines, about halfway between the 'chute skirt and the load line tie-off point. The ring ensures rapid inflation of the parachute on burst or cutdown. It's made of wicker rather than bamboo, however, but that's a minor distinction. Fiberglas might make a lighter and equally effective ring.
EOSS has done some cold-soak testing of electronic payloads, but our mechanical cold testing has fallen more in the category of "the proof is in the pudding", i.e., if it doesn't break in flight, then it must be OK. And we've seen a lot of things break, but mainly from post-landing stresses. However, we have learned that silicone adhesives (RTV) and hot-melt glues do not embrittle when cold to the extent that epoxy does.
The "tangle" that Chris referred to was one of the several consequences of "post-burst chaos", a term which I use to describe the effects of objects placed lower on the string soaring above those above during the high speed initial descent from altitude. In most cases, this is associated with parachute oscillation, where the 'chute dumps its bolus of air out the skirt and turns sideways, thus losing drag area. This may be exacerbated by the weight of burst balloon shards tugging on the 'chute apex as well. One effective countermeasure is to place the fastest-free-falling payloads on the bottom of the string and the slowest at the top. And cutting the shards away also helps tremendously, but this must be done shortly after burst to be effective.