RE: Question about STS-116 launch times

From: Bob Christy (rdc@zarya.info)
Date: Fri Dec 08 2006 - 20:19:07 EST

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

    Sorry to spoil the party but the delta-time for a launch to meet on object
    already in orbit is essentially constant from day to day.
    
    Ellipticity, or any other attribute of the orbit is irrelevant. The
    objective is to get the new satellite (Shuttle or whatever) into the orbital
    plane of the target. Differences in orbital period are then used to achieve
    a rendezvous.
    
    Minor effects from differing rendezvous profiles (two days, three days, slow
    or fast approach) do affect the launch time, but only by a matter of
    seconds.
    
    If software is indicating several minutes variation in the delta-time
    between successive days then there is something wrong with the model being
    used.
    
    Bob Christy
    
    
    > -----Original Message-----
    > From: Christian Kj ś rnet [mailto:ckjarnet@broadpark.no] 
    > Sent: 09 December 2006 00:02
    > To: SeeSat-L@satobs.org
    > Subject: 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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