Re: Question about STS-116 launch times

From: Christian Kjrnet (ckjarnet@broadpark.no)
Date: Fri Dec 08 2006 - 19:01:34 EST

  • Next message: Ralph McConahy: "Re: Question about STS-116 launch times"

    Hello all,
    
    This is my first post to this list in many years, but I have spotted
    satellites for many years. I live in the town of Kongsberg, Norway.
    
    > Could someone explain why the delta-time jumps like
    > this instead of a constant change from day to day?
    
    The short answer to this question is the elliptic nature of the orbit of
    ISS. The ISS's current orbit has a perigee of 327 km and an apogee of 370 km
    above the Earth ellipsoid. Therefore, it moves with varying speed with
    respect to the Earth's surface.
    
    NASA must launch the shuttle at the time when the orbital plane of the ISS
    passes through KSC in Florida. This choice of time is done to save fuel,
    because the shuttle would have to use a lot of energy to "turn" from one
    orbital plane to another in vacuum. It is not like turning an airplane by
    banking the wings inside the atmosphere.
    
    The orbital plane is the tangent of the ISS's orbit at any time, and because
    the Earth moves at a constant speed with respect to the orbital plane, but
    the ISS moves at varying speed in the orbit, then, when the ISS is at
    perigee closest to the Earth, the orbital plane "moves faster", and when the
    ISS is at apogee, the orbital plane "moves slower" with respect to the
    Earth's surface.
    
    The orbital plane of the ISS passes through KSC at 12 Dec 7:36 pm EST when
    the ISS is in the south Pacific East of New Zealand at an altitude of 330
    km. The next day, the orbital plane goes through KSC on 13 December at 7:11
    EST  pm, when the ISS is over England at an altitude of 354 km. And so on...
    
    This situation/explanation requires some "3D goggles", but I hope it helps.
    
    Best regards,
    
    Christian
    
    -- 
    Christian Kjaernet
    Kongsberg, Norway
    59.6408 N, 9.6402 E, 176 m
    
    
    
    
    On 08-12-06 18:43, "Ralph McConahy" <rwmcconahy@earthlink.net> wrote:
    
    > 
    > A question for the orbital mechanics gurus:
    > 
    > I plugged the STS-116 nominal (ISS In Plane) launch times from
    > Spaceflightnow.com into a spreadsheet, then created a column that displayed
    > how much time earlier each day the launch will occur compared to the previous
    > day (see below). Comparing the day by day "Earlier by" times, it is
    > interesting how it is equal every other day, with a +/- ~3m10s alternating
    > from one day to the next. Could someone explain why the delta-time jumps like
    > this instead of a constant change from day to day?
    > 
    >   Ralph McConahy
    >   38.3306N,  75.6970W, -25m (WGS84)
    >  
    >  
    > STS-116 Nominal Launch Times 
    > 
    > PM (EST)  Earlier 
    >  9-Dec  8:47:34   by:  
    > 10-Dec  8:25:02  22:32 
    > 11-Dec  7:59:20  25:42 
    > 12-Dec  7:36:48  22:32 
    > 13-Dec  7:11:06  25:42 
    > 14-Dec  6:48:34  22:32 
    > 15-Dec  6:22:52  25:42 
    > 16-Dec  6:00:20  22:32 
    > 17-Dec  5:34:37  25:43 
    > 18-Dec  5:12:06  22:31 
    > 19-Dec  4:46:23  25:43 
    > 20-Dec  4:23:52  22:31 
    > 21-Dec  3:58:09  25:43 
    > 22-Dec  3:35:38  22:31 
    > 23-Dec  3:09:55  25:43 
    > 24-Dec  2:47:24  22:31 
    > 25-Dec  2:21:41  25:43 
    > 26-Dec  1:59:10  22:31 
    > 
    > -------------------------------------------------------------------------
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    > 
    
    
    -- 
    Christian Kjśrnet
    
    
    
    
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