Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Basic rule for Atmospheric Reentry of Glider Class vehicles

  1. Apr 7, 2018 #1
    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
     
  2. jcsd
  3. Apr 11, 2018 #2

    DaveC426913

    User Avatar
    Gold Member

    This was posted verbatim on another forum, and my answer here is the same as there.

    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.
     
  4. Apr 11, 2018 #3
    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?
     
  5. Apr 12, 2018 #4

    DaveC426913

    User Avatar
    Gold Member

    Relevance of initial three paragraphs is ambiguous. I have not challenged any of those allegations. Your thesis is that solar panels can cause the craft to behave akin to a glider. Enough that it's worth factoring in. I think you need to show that they are up to the task.

    Solar panels on a satellite are not a load-bearing structure. This is a fact.

    Their strength to bear a load against the tumultuous assault of re-entry is inversely proportional to how wasteful the Chinese would have been in adding strength (and thus weight) to a part that had no reason to bear a load.

    Furthermore, solar panels rotate. To do so, they can only be secured at a single point.

    I present this diagram not with any expectation of technical accuracy, simply for reference.

    satellite.png
    I am not asserting that there cannot be any ephemeral effect on orientation, at least while the panels remain attached, but I think it is quite safe to assume that these panels will not turn the craft into anything resembling a glider dynamically. Certainly not enough to affect calculations of its landing

    Safe enough, I think, so as to place the onus on you to show how or why they should be strong enough.
     
  6. Apr 13, 2018 #5
    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

    metals.jpg

    http://ceramicrotaryengines.com/
     
  7. Apr 13, 2018 #6

    DaveC426913

    User Avatar
    Gold Member

    Regardless of what material is used - if the Chinese are doing their engineering correctly, they won't waste extra mass on over-engineering parts to do more than they need to do.

    Mass is the most precious thing in a space launch. Any extra mass is mass not devoted to payload.Those joints won't be any stronger than they need to be to tilt the panels, slowly, in microgravity.

    Can you demonstrate why they would be any stronger than that?

    What strength would they need to be (thus how over-engineered would they need to be) to inadvertently act as the wings of an 8-tonne glider hitting the atmo at 17,000mph?



    Or, look at it the other way. You compare it to the space shuttle:
    Let's grant for a moment that a solar-powered sat might superficially have a vaguely passing resemblance to a space shuttle.

    If the sat's razor-thin, single-point of attachment panel configuration could act like gilder wings, then why would the space shuttle need its wings to be 7 feet thick and attached to the fuselage along its entire 56 foot length when something about a foot or two in diameter would (apparently) suffice?

    shuttle.png


    You may have a case, but I think you've got your work cut out first demonstrating that an 8-tonne sat with minimal attachments points comes even close to being modelable with a Glider Class configuration.

    And remember:
    Your Glider Class is spec'ed for a re-entry vehicle. So your analysis requires the panels to actually stay attached for a significant duration the descent. Which makes it even less plausible.
     
    Last edited: Apr 13, 2018
  8. Apr 14, 2018 #7

    Filip Larsen

    User Avatar
    Gold Member

    If the idea is to analyze the thesis that an uncontrolled satellite like Tiangong-1 potentially could have an re-entry trajectory which, due to lift (and not just drag) generated on the solar panels, would be significantly different than without those panels present, then one would also need to consider the attitude stability as a lifting body during re-entry. A tumbling body tend to have a ballistic trajectory, i.e. a trajectory dominated by drag where any net lift on the body averages out over time as it tumbles or rotates in the air stream.

    From the position of the solar panels show in the diagram of Tiangong-1 it seems unlikely that there exist an aerodynamically stable re-entry attitude (i.e.. center of mass aft downstream of center of pressure) where any lift (i.e. a force transverse to velocity vector) generate by the solar panels would act near the center of mass. If the net lift on a body in an air stream does not pass through center of mass there will be a destabilizing "pitching" torque that works against any stabilizing torque there might be from drag. And, as mentioned above, without an attitude stable body it is difficult to imagine a non-ballistic trajectory.
     
  9. Apr 14, 2018 #8
    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

    imagetiangon.jpg
    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.
     
  10. Apr 14, 2018 #9
    Please refer to my post above.
     
  11. Apr 14, 2018 #10

    Filip Larsen

    User Avatar
    Gold Member

    I do not see anything in your post regarding aerodynamic attitude stability when in high-drag low orbit or re-entry. Can you specify more precise (perhaps in mechanical terms) what you mean by "Glider Class vehicle"? Are you perhaps thinking of Sänger type skip-glide vehicles (see [1])?

    [1] https://en.wikipedia.org/wiki/Boost-glide
     
  12. Apr 15, 2018 #11
    If you look at Ngoc-Thuy Dang Nguyen's Masters thesis "Performance Analysis of Skip-Glide Trajectories for Hypersonic Waveriders in Planetary Exploration" from the University of Tennessee - Knoxville in 2008 below, you will see many interesting diagrams and comparisons of both types, some listed below. I don't want to re-invent the wheel or plagiarise Ngoc-Thuy Dang Nguyen's impressive and comprehensive work so you are probably better off looking there.

    (1) Figure A-8: Sänger-Bredt Silbervogel Aircraft
    (2) Figure A-9: Parameters of a Periodic Hypersonic Cruise Trajectory
    (3) Figure A-16: Velocity Ratio vs. Range Parameter for a Skip Trajectory
    (4) Figure A-17: Velocity Ratio vs. Range Parameter for a Glide Trajectory
    (5) Figure A-39: Sketch of a Vehicle Executing a ‘Skip’ Trajectory
    (6) Figure A-40: Sketch of a Vehicle Executing a ‘Glide’ Trajectory

    http://trace.tennessee.edu/cgi/viewcontent.cgi?article=1449&context=utk_gradthes

    Also, the type of Glider Class methodology I have described is probably a more suitable way to retrofit heat resistant aerodynamic capabilities to various large objects to allow them to glide in safely from any planetary orbit as opposed to going between planets, although there is not that much real difference between the two, and Tiangong 1 didn't really bounce around that much during reentry either.

    https://en.wikipedia.org/wiki/Gliding_flight
     
  13. Apr 15, 2018 #12

    Filip Larsen

    User Avatar
    Gold Member

    This is all fine, but I fail to see how this relates to Tiangong-1 being a lifting body? As mentioned, non-ballistic re-entry trajectories do in general require lifting bodies, that is, a body that has a stable (and preferably also controllable) attitude with significant lift (significant L/D ratio) during re-entry. Just because a re-entry body has solar panels attached it does not follow that it must be a lifting body.

    Again, it is not clear to me what purpose this thread has. Can you state more clearly what it is you want to discuss? Reading your original post there is no questions, but it seems you still want input on something. If so, perhaps you can state this as one or more specific questions?
     
  14. Apr 15, 2018 #13

    berkeman

    User Avatar

    Staff: Mentor

    The OP has left the building, so this thread is closed. Thank you to everybody who tried to help the OP.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook