Tiangong 1 Reenters Over South Pacific

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Yazata, Apr 2, 2018.

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  1. Yazata Valued Senior Member

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    About two hours ago. Verified by the US Space Command. Some reports have it impacting somewhere southeast of Tahiti in the vicinity of Easter Island (very appropriate for Easter day).

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  3. Yazata Valued Senior Member

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    Jonathan McDowell seems to think that it came down northwest of Tahiti in Polynesia somewhere. If so, there are more islands around and somebody might have captured some cellphone video of it burning up in a fireball like a bolide.

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  5. DaveC426913 Valued Senior Member

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    Paving the way for a Gilligan's Island Reboot Pilot episode.
     
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  7. LaurieAG Registered Senior Member

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    But did any of them project an accurate atmospheric reentry date on March 1?

    Basic rule for the Atmospheric Reentry Time of non powered Glider Class vehicles

    The Chinese Space Station, Tiangong 1, looks very much like a glider i.e. a bit like the US Space Shuttle (solar panels are made of similar materials to Space Shuttle tiles with much less friction), as the German radar images show in the link below, so it most likely wasn't tumbling but yawing from left to right and pitching and rolling just like a glider would, as it comes down to land.

    https://www.space.com/40089-china-space-station-tiangong-1-radar-images.html

    Here's a basic summary of how to project the reentry time of non powered Glider Class reentry vehicles, similar to Tiangong 1, as it might prove useful in the future.

    It's probably a little bit easier to look at rough approximates, as I never managed to find out how the US Strategic Command, or anybody else for that matter, calculated their Altitude of Nominal Burst (ANB). The ANB's used below are those provided by Satview.org, so here's the basic methodology.

    The basic rule of thumb is that when the natural descent becomes a Pythagorean right triangle with a ratio of 3:4:5 over 8 days. i.e. it drops 6km over 8 days, the time to atmospheric reentry is approximately 1 kilometre per day until the average ANB (over those 8 days) is reached plus an adjustment factor.

    I made rough plots and a projection on the link below on March 1, and wasn't sure how recent the data was as the Satview.org UTC timers (and the ANB) were all over the place for most of the reentry, so the end figure I used was 252.5 - 220.25 = 32.25 days - an adjustment from my last plot (i.e. 09:00 UTC March 1), because I didn't know how accurate the timing/altitude was.

    http://www.scienceforums.com/topic/24218-near-earth-objects/page-11#entry354689

    Note that my figures don't actually reflect the eventual altitude of atmospheric reentry (ANB), they just indicate the time of atmospheric reentry based on the last data point, that satisfies the criteria in bold, and the average ANB projections over the previous 8 days. Even without the timing adjustment this projection was much more accurate than the forecasts by the US Strategic Command and the European Space Agency (ESA) for most of March.

    The eventual atmospheric reentry time given by the US Strategic Command via Satview.org was April 2, 2018 at 00:16 +/- 1 minute UTC (below) and China Manned Space give the reentry altitude as approximately 132.75 km (below).

    http://www.satview.org/?sat_id=37820U
    http://en.cmse.gov.cn/col/col1763/index.html
     
  8. DaveC426913 Valued Senior Member

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    I think this assessment is grossly flawed right from the get-go.

    The comparison to a glider is entirely visual and superficial. The solar panels are not load-bearing in the slightest - kind of exactly opposite to a glider.

    The moment they met any appreciable resistance from the atmo, they just ripped right off. They simply do not have the strength to aerodynamically alter the orientation of the 8 tonne satellite.

    All the rest follows from that flawed premise.
     
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  9. Gawdzilla Sama Valued Senior Member

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    Basic rule for the Atmospheric Reentry Time of non-powered Flying Brick Class vehicles
     
  10. LaurieAG Registered Senior Member

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    Are you aware of the advantages of a V tail glider, i.e. reduced drag for a start?

    Are you aware that the solar flares that occurred during its decline had no effect on Tiangong 1 despite everybody modelling extra friction and reducing their atmospheric reentry forecasts incorrectly?

    Are you aware that the variation of Tiangong 1's altitude during orbit prior to and including March 24, 2018 was less than 1 kilometre?

    The Chinese have not released any technical documentation with regards to the design, components or the composition of the components of Tiangong 1 so how can you make that judgement?
     
  11. DaveC426913 Valued Senior Member

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    Demonstrate that these attachment points are sufficiently load-bearing to turn an 8 tonne satellite into an aerodynamic gilder during re-entry.

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  12. LaurieAG Registered Senior Member

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    Are you aware of the properties and advancements in ceramic bonded metals and why they are used in hot environments?

    The links below are from publicly available documents that point to their potential uses. I cannot supply documents about composite material specifications that would answer your question just like you cannot provide the Tiangong 1 design specifications.

    http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/strength-temp/basic.html

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    http://ceramicrotaryengines.com/
     
  13. Yazata Valued Senior Member

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    Predictions as to time were pretty good. By the end the time predictions were down to a window of hours, not days. But predictions as to the location where it would fall weren't really possible because the thing was moving at orbital velocity (~17,000 mph) so that a time uncertainty of only one hour would mean a position uncertainty of 17,000 miles.

    https://twitter.com/planet4589/status/980620760753606656
     
    Last edited: Apr 13, 2018
  14. DaveC426913 Valued Senior Member

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    You're missing the point. Given any material you choose - the joint for each panel is no stronger than it needs to be. If you posit a stronger, lighter material for the joint well, that's great; then they can use less of it, and use that gain for a bigger payload.

    The most precious commodity on a rocket is mass. If the joints could support any extra weight, then they're over-engineered. Which means they're wasting precious mass. This is unlikely.


    So, I'm asking you: before you spend too much time calculating what might happen to a glider-like object on re-entry, can you show that there's reason to expect it to act like a glider at all? That a satellite's zero-g panels should plausibly be strong enough to support an aerodynamic force?
     
  15. LaurieAG Registered Senior Member

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    I forecast 'just after midnight on April 2' on March 1 using the method shown in my first post and did not change my forecast later.
     
  16. LaurieAG Registered Senior Member

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    Tiangong 1 weighed approximately 8,500kg where the Space Shuttle weighed approximately 75,000kg empty so comparing their respective 'wing' loads etc as you do is not practical. Also all the final mass does not have to be limited to just 1 launch.

    Note that the ceramics do not appear on the German radar images of Tiangong 1 from Fraunhofer FHR below. Ceramics, as shown in the image in my previous post, have high strength as well as high operating temperature. How do you know how the ceramics may be formed around the metallic parts of Tiangong 1 without knowing the full Chinese specifications? Not only do ceramics have their own properties they also protect the metallic frame from heat allowing different alloys than may be possible with just metal alone. Any 'extra mass' not required on the original launch can be delivered later and Tiangong 1 itself is shielded during the actual launch.

    https://phys.org/news/2018-03-image-tiangong-space-radar.html

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    Credit Fraunhofer FHR.

    The Phys.org article above states that the Fraunhofer FHR images were taken in the week before and, as the article was dated March 29, it refers to sometime within the period 16-24 March where the glider wings and V tail are obviously intact and still attached to the body of Tiangong 1.

    Also, the Shenzhou Tiangong docking(s), for all we know, could have delivered appropriately modelled ceramic components including wing strengthening/forming panels covering other metallic strengthening bracers that could have been attached to Tiangong by the Chinese Astronauts prior to their departure. It seems that Shenzou has much more space than just required for the test docking and no extra space station equipment was necessary for what was essentially a 'proof of concept' mission.
     
  17. Gawdzilla Sama Valued Senior Member

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    I just keep picturing that early 20th C. airplane with a whole stack of wings. The video shows it collapsing before any serious ground speed is achieved.
     
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  18. DaveC426913 Valued Senior Member

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    You need to understand how a rotating joint must be constructed. You can't "layer" it or "brace" for strength" It comes down to the diameter of a single axle, made of a single material.
     
  19. LaurieAG Registered Senior Member

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    Like a 4WD Locking Hub or a Revolute Joint?

    Once you have the joint locked the ceramic materials and reinforcements made to Tiangong 1 would stop the wings from rotating, should the locked joint get hot, and provide further bracing for the purposes of a glider landing.
     
  20. DaveC426913 Valued Senior Member

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    The OP has been banned at the other forum where this was posted, and the thread has been locked.

    Ideally s/he has decided to reconsider his/her thesis.

    Time will tell if the OP returns here.
     
  21. Write4U Valued Senior Member

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    This may be of interest.

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  22. DaveC426913 Valued Senior Member

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    This is totally off-topic. Why not start a new thread?
     
  23. LaurieAG Registered Senior Member

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    If you bothered to ask the moderators at the other forum you would find that my old account could not be restored (after the recent upgrade) and I was given permission to use the new account name.
     
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