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Jul 16, 2010
    Space Sphere - EOSS Tidbits

    Post handy items found on the EOSS site here. There's a lot of information on there (a lot of it outdated, but probably still quite relevant) and having the best of it in one location will make things easier.
Jul 16, 2010
Me:
    Regarding the Tethering Agent (String)


    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.
Jul 16, 2010
Me:
    Parachute and Knots

    They recommend a large model rocket parachute. Should be fairly simple to source. They also recommend the bowline knot. I have reservations about this as, although it is true that it is a self tightening knot (that is, it won't come undone under tension), bowlines have a tendency to come undone under lack of tension. I'll have a look through my knot book at some point and see if I can find something better.

    They do recommend using a device of some sort to separate the shroud lines of the parachute to provide a speedier chute opening and help minimize entanglement of the shroud lines.
Jul 16, 2010
Me:
    Fabricating a Payload Container

    I won't duplicate what they say about fabrication here as there's a lot of relevant text. Here's a link to their fabrication faq page, though.

Jul 16, 2010
Me:
    Regarding Payload Oscillations


    Package spin during flight is probably unavoidable. Its adverse effects are windup of cables running between strung-together parts of the flight system and motion sickness by those who watch the ATV image of the ground for too long. Spin has a slow, basal rate intrinsic to balloon dynamics, so I’m told, plus a faster oscillatory component. Wind shear causes suspended parts of the system to sway horizontally, creating a horizontal component of airflow around the package. Asymmetrical projections will weathervane downwind, resulting in an oscillatory yaw rates which may be much faster than the basal spin rate. Oscillatory spin might be reduced by making the package nearly cylindrical to reduce the horizontal drag coefficient, and large asymmetrical projections should be avoided.
Jul 16, 2010
John Brown:
    Safety Regulations

    There are quite a few restrictions placed on unmanned balloon flights in 101.35.

    Two devices to cut the payload, flashing lights (for night flight), radar reflective devices, etc. Have these been discussed? It seems like it will take more than just strapping the balloon on a payload.

    None of the news articles I have read regarding others who are taking these sorts of photos have mentioned anything about these, and I haven't seen anything in the photos of the setups.
Jul 16, 2010
Me:
    Good point. It looks like we should strive to be exempt from those requirements by coming under the constraints set forth in 101.1 (a) (4).
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Space Sphere - EOSS Tidbits