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Thread: Instea of "Where's Wally?" here's Where's Webb

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    LSemmens
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    Default Instea of "Where's Wally?" here's Where's Webb

    was launched on Christmas day heading to an L2 orbit 1.5 million km above earth.
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    Reading on about the orbit management, the lack of redundancy is disconcerting:

    Because the thrusters are located solely on the Sun-facing side of the observatory, all station-keeping operations are designed to slightly undershoot the required amount of thrust in order to avoid pushing the JWST beyond the semi-stable point, a situation which would be unrecoverable.
    Why the heck not have thrusters also on the opposite side incase this 'undershoot' strategy goes awry wrong?
    I would have put all the thrusters on all 4 'outside corners' in pairs pointing both towards and away from the sun.
    Too many times have I seen software/user errors occur but good old hardware comes along to save the day... or in this case an $8 Billion telescope !
    Last edited by Uncle Fester; 03-01-22 at 09:26 PM.
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    I agree, however, I bet their "good" reasons were weigh!
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    According to The Goddard Spaceflight Center, Webb is now fully deployed. It is 3/4 of the way to it's destination.
    The fact that there's a highway to hell and a stairway to heaven says a lot about the anticipated traffic flow.

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    Quote Originally Posted by Uncle Fester View Post
    Why the heck not have thrusters also on the opposite side ....
    yep, first guess is always weight, but there are probably other reasons like contamination. You really do not want any gas in the cooled regions of the telescope else this becomes like frost in your freezer. Imagine having a thin layer of frost on your camera lens that builds up every time you maneuver.

    The explanation for how they deal with it does make sense though. I'm trying to think of a similar way to describe it. Ah, got an analgoue.
    If you're a scuba diver, do you err on the side of caution of positive or negative bouyancy? This is actually a real world problem. Divers are always in a semi-stable bouyancy. If they put no air in their BCD, they might float at the surface, but as soon as they get a bit of pressure, they start to sink faster and faster.
    This normally isn't a problem if you're diving where the bottom will stop you. In open water were the bottom is a long way down you could "theoretically" descend fast enough that you cannot inflate the BCD fast enough. though that is fat chance in reality with >100bar behind a generous regulator.

    The reverse situation is far more common. When at depth, if the diver does not dump gas from their BCD before or as they ascend they can easily get into a runaway scenario. The solution which they don't properly train recreational divers for is to stay negatively buoyant so you sink.
    This prevents the runaway but you have to swim up rather than float up.

    In the case of the JWST, it's the same thing. Gravity is pulling them towards the Earth and sun. So provided they stay on the down side of the L2 they can always use fuel to push up. Using fuel to push down is like the diver trying to swim against his buoyancy. It's much harder and uses more energy.

    The way I dive describes it quite well now I think of it. I dump gas and swim up to just below bouyance and dump more gas and swim up more.
    If I were to overshoot, it becomes a runaway situation and I have to dump gas fast or flip over and swim down fighting the buoyancy as it gets worse the longer I don't fight it. Never getting to buoyancy (the L2) means I have to use fuel, but I'm safe. I'll sink slowly rather than ascend fast.
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    Good analogy Trash.
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