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History Making First Recovery for SpaceX - Orbcomm-2 Launch

Discussion in 'SpaceX' started by hockeythug, Dec 3, 2015.

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  1. pmadflyer

    pmadflyer Member

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    It's because the site was originally supposed to mainly feature weird stuff that he said. The home page has a humorous picture of Elon and top quotes.
    Examples:
    Some very interesting things to consider:

     
  2. doug

    doug Administrator / Head Moderator

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    Are we sure it's not mostly from the initial launch?
     
  3. evp

    evp Nerd

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    Reviewing the video and checking the math, the scenario here plays out as follows:

    First stage cutoff was at about 5988km/h or 1663m/s at T+2:26 (146 seconds, somewhat less than the website suggests probably due to low payload weight). That gives us (180-146)/180 seconds of fuel left or 18.8% (somewhat more than my previous estimate).

    Boostback burn: 30 seconds to kill forward velocity and end up at 1300m/s towards launch site: delta vee 1300+1663 = 1963m/s
    Entry burn: 20 seconds to slow from 1300m/s to 250m/s between 70 and 40km altitude: 1300-250 = 1050m/s
    Fall from 40km adds 885m/s in vacuum minus an unknown amount for drag. But in the video, the boostback burn starts at about 8:10 (ends 8:30) and landing is at 9:46 so it covers 40km in 76 seconds for 526m/s
    Landing burn: 526m/s
    Total delta vee: 1963+1050+526=3539m/s
    The first stage weighs 25600kg empty
    Rocket equation says m0 = m1*exp(dv/ve), where m0 is starting mass, m1 is ending mass, dv is delta vee, and ve is exhaust velocity.
    ve can be calculated by the engine Isp*g, 311s*9.8m/s = 3050m/s.
    so, to touch down with zero fuel remaining, m0 = 25600*exp(3539/3050) = 81689
    subtract the mass of the stage: 81689-25600 = 56089kg of fuel burned
    Fuel load is 395700kg, so the required fuel at MECO is 56089/395700 or 14.9%.

    18.8% fuel remaining (from video timing), 14.9% required (from rocket equation); therefore fuel margin 3.9%
     
  4. lolachampcar

    lolachampcar Well-Known Member

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    Ben,

    Fantastic job.
    Would you or someone with access to Elon's twitter feed kindly post the pic? I think his followers would enjoy the added historical reference to the existing logo.

     
  5. AudubonB

    AudubonB One can NOT induce accuracy with precision!

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    evp: Initial Delta vee is 2963 m/s; you used 1963, for a total Delta vee of 4539 m/s. Best re-do your numbers.
     
  6. Johan

    Johan Ex got M3 in the divorce, waiting for EU Model Y!

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    #226 Johan, Dec 26, 2015
    Last edited: Dec 26, 2015
    Good catch. Correcting for this gives m0=113385 kg

    Subtract mass of the stage 113385-25600 = 87785 kg burnt

    Assuming fuel load is correct at 395700 kg then the percentage of fuel required at Main Engine Cut Off would be 87785/395700 = 22,2%

    So something doesn't add up with this calculation...

    Perhaps the drag when reentering the denser part of the atmosphere is actually quite substantial and helps brake the rocket a lot, perhaps even more so coming down bottom first (the aerodynamics must be quite different to when going up, pointy nose first).

    Another thing is these back-of-the envelope calculations may fail to take in to consideration that all these velocity changes are vectors and that they are not completely perpendicular to each other, even though mostly you're going one way then turning back the opposite direction. Another thing, which I'm unsure if has any bearing is the earth's rotation.
     
  7. SW2Fiddler

    SW2Fiddler We Are Cognitive Dissidents

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    #227 SW2Fiddler, Dec 26, 2015
    Last edited: Dec 26, 2015
    I'm not sure about anything, but would reason that the biggest opportunity to get carbon "stuck" there would be during the (Entry) Deceleration Burn - air flow has to leave the sooty exhaust far behind in the first and second big burns.
    So it could indicate how much LOX was touching tank sides during the "longest a$$-backwards burn." Or on second thought, merely what part was STILL COLD during that and the last landing burn. Hmmmm.

    I am still giddy about the whole thing having succeeded so well! Love having people to still discuss it with. Even here in Space City, I'm in a minority with my enthusiasm :)
     
  8. Johan

    Johan Ex got M3 in the divorce, waiting for EU Model Y!

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    Have to agree with this common sense wise. Going away from the exhaust should give way less burn marks than going backwards braking in to the rocket exhaust, right?

    Problem is there very little frame of reference at least for us as spectators. They're covering new ground.

    One thought I have is if there's any particular reason why space rockets are often white? The soot (however I do prefer carbon scoring) would show a lot less on a black rocket. Elon's Model S and X are black...
     
  9. Cosmacelf

    Cosmacelf Well-Known Member

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    I believe the landing legs (or their coating) catch fire a bit after they retract, so that would create some soot.
     
  10. HVM

    HVM Savolainen

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    #230 HVM, Dec 26, 2015
    Last edited by a moderator: Mar 13, 2016
    o.jpg
    No soot visible, yet.

    Reentry burn

    Ice and soot.
     
  11. Ben W

    Ben W P85 #61, Roadster #108

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    Thanks! I sent it to him on Twitter: @elonmusk New logo? Congrats on an epic launch/landing!!

    Let's hope he notices!
     
  12. Patrick W

    Patrick W Active Member

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  13. Grendal

    Grendal SpaceX Moderator

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    Commenting on the soot patterns, the pattern at the base is a perfect line. So perfect that there must be something artificial causing it. It's possible that there is a different coating of paint that resists the soot. Some of the pattern at the top of the booster could have occurred as the second stage ignited. If you watch the video, I believe you can actually see the first stage get hit by the flame wash of the second stage ignition.

    Thanks to everyone for the various contributions. All of it is exciting to me.
     
  14. physicsfita

    physicsfita Member

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    When you're trying to keep the oxidizer super-cold it's kind of counter-productive to paint it black and end up fighting the extra heating from the Florida sun because the black paint absorbed the sunlight.
     
  15. Johan

    Johan Ex got M3 in the divorce, waiting for EU Model Y!

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    Thanks. Makes a lot of sense.
     
  16. HVM

    HVM Savolainen

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    #236 HVM, Dec 27, 2015
    Last edited: Dec 27, 2015
    That's the common bulkhead between RP-1 and LOX tanks, and unlike Falcon 1, there isn't any visible insulation on LOX tank wall. And why only paint half of the LOX tank height?
     
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  17. ggr

    ggr Expert in Dunning-Kruger Effect!

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    My guess is that the part around the LOX tank was still covered with ice during the reentry burns, and it subsequently melted or fell off.
     
  18. HVM

    HVM Savolainen

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    Maybe bottom half of LOX tank have more equipment in it, (slosh baffles and helium bottles e.g. COPVs and their struts with the problematic bolts) and those keep the tank walls cold, even if LOX surface is lower.
     
  19. StephenM

    StephenM Active Member

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  20. HVM

    HVM Savolainen

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    #240 HVM, Dec 31, 2015
    Last edited: Jan 1, 2016

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