Upcoming CCAFS Titan IVB may not be carrying a Lacrosse

From: Ted Molczan (molczan@rogers.com)
Date: Tue Mar 29 2005 - 08:31:48 EST

  • Next message: Leo Barhorst: "ATV Jules Verne"

    Six months ago, based on its 66 ft fairing, and having originally been located
    and intended for launch at VAFB, I speculated that the upcoming CCAFS Titan IVB
    would launch a Lacrosse:
    
    http://satobs.org/seesat/Sep-2004/0234.html
    
    I now have some doubts, arising from the recent announcement of the launch
    period: 2005 Apr 07, 00:00 - 02:30 UTC. That date has since slipped, and a new
    one has yet to be announced.
    
    The announced launch period appears not to result in favourable initial orbital
    plane spacings relative Lacrosse 3 or Lacrosse 4, hence my doubts. I caution
    that it is nearly impossible for onlookers to be certain of favourable plane
    spacings, and my tracking record deducing/guessing them is not good.
    
    The four Lacrosse's launched to-date have employed two standard orbit-types; one
    inclined 57 deg, the other 68 deg:
    
    Spacecraft  Year  Vehicle  Site    Inc    Present status
    ----------  ----  -------  -----  ------  ---------------
    Lacrosse 1  1988  Shuttle  CCAFS  57 deg  de-orbited March 1997
    Lacrosse 2  1991  T-IVA    VAFB   68 deg  in orbit
    Lacrosse 3  1997  T-IVA    VAFB   57 deg  in orbit
    Lacrosse 4  2000  T-IVB    VAFB   68 deg  in orbit
    
    Lacrosse 3, the only remaining Lacrosse in a 57 deg orbit, appears ripe for
    replacement or augmentation with a sister spacecraft. I do not know whether or
    not the combination of a T-IVB and Lacrosse have sufficient performance to reach
    a 68 deg Lacrosse orbit from CCAFS.
    
    The 57 deg and 68 deg Lacrosse orbital planes precess at considerably different
    rates, making it impossible to establish a fixed spacing between them. However,
    in principle, a fixed plane spacing is feasible between two or more 57 deg, or
    two or more 68 deg planes - as long as they orbit at the same altitude.
    
    The first opportunity to do so arose in 2000, with the launch of Lacrosse 4,
    which has the same 68 deg inclination as Lacrosse 2.
    
    The optimal Lacrosse plane spacing is known only to their operators. Prior to
    launch, I had confidently theorized that their planes would be separated in the
    same precise manner as the two primary KeyHole planes, but this proved to be
    totally incorrect. However, now that we know the outcome, it may offer a clue as
    to what to expect from a Lacrosse 5.
    
    Lacrosse 4's initial plane was 69.3 deg east of Lacrosse 2's. Although they had
    the same inclination, Lacrosse 4 entered a higher orbit than Lacrosse 2. As a
    result, Lacrosse 2's plane drifted west relative to Lacrosse 4, at the rate of
    about 0.039 deg/day, causing their planes to separate at the same rate. Within
    about 18 months of launch, they were 90 deg apart; today, they are 140 deg
    apart.
    
    Apparently, the intent was to enable the two spacecraft to operate with a plane
    spacing near 90 deg for the first few years of Lacrosse 4's mission, after which
    time, presumably Lacrosse 2 would be at or near the end of its useful life. 
    
    A rational argument to prefer quasi 90 deg spacing is that it provides more or
    less even coverage during a 24 hour period, with good passes occurring at
    roughly 6 hour intervals.
    
    Why not manoeuvre one or both spacecraft to maintain a fixed plane spacing, as
    do the KeyHoles? It may be that the Lacrosses do not carry sufficient propellant
    to support the amount of manoeuvring that would be required over their lifetime.
    
    Supporting evidence exists in the apparently passive way in which the spacecraft
    maintain ground tracks that nearly repeat every two days, at intervals of 29
    revolutions, i.e. 29:2 resonance. U.S. imaging reconnaissance satellites
    commonly nearly repeat their ground tracks every 2, 3 or 4 days, depending upon
    the type of spacecraft.
    
    Instead of manoeuvring regularly to maintain the altitude required for precise
    29:2 resonance, Lacrosses enter initial orbits roughly 10 km too high, causing
    their ground track to nearly repeat at intervals of about 28.94 revolutions, and
    allow natural decay to drag them through resonance about 4 to 5 years after
    launch, when they briefly achieve exact 29:2 resonance. Thus, over a roughly 8
    to 10 year mission, they passively maintain approximately 29:2 resonance, within
    a certain tolerance.
    
    Lacrosses 3 and 4 followed this exact regime. Lacrosse 1 differed only in that
    its initial altitude was a bit higher, resulting in an initial 28.91:2
    near-resonance. Lacrosse 2's initial orbit was slightly lower than that of exact
    resonance, resulting in 29.01:2 near-resonance. Perhaps it could not reach a
    higher altitude.
    
    If my analysis is correct, then new Lacrosses cannot establish fixed plane
    spacings relative to others of the same inclination, because their initial
    orbits must be higher. In which case, they can achieve a temporary quasi 90 deg
    spacing by launching well to the east of an existing sister spacecraft. The
    April 7 launch period does not support such a plane spacing relative either
    Lacrosse 3 or 4.
    
    On April 7, for a new Lacrosse to establish the same initial plane 69.3 deg
    spacing east of Lacrosse 3, as Lacrosse 4 relative Lacrosse 2, would require
    launch near 09:03 UTC. That is far from the announced launch period of 00:00 -
    02:30 UTC.
    
    Assuming launch roughly 90 min into the launch period, at 01:30 UTC, the plane
    of a new Lacrosse would be roughly 46 deg west of Lacrosse 3. If it were 46 east
    deg east, this would not be a bad spacing, since Lacrosse 3's roughly 0.031
    deg/d greater rate of precession would gradually separate them, reaching 90 deg
    within 4 years. Instead, the 46 deg west spacing would result in their becoming
    co-planar within four years.
    
    The announced launch period of April 7 is even less favourable to establish a
    new plane relative Lacrosse 4. To achieve 69.3 deg east, would require launching
    near 17:15 UTC - far from the announced launch period of 00:00 - 02:30 UTC.
    
    Assuming launch at 01:30 UTC, the plane of a new Lacrosse would be roughly 169
    deg west of Lacrosse 4 - orbiting in opposite directions within nearly the same
    plane. Lacrosse 4 has not decayed much relative the required initial orbit;
    therefore, its rate of westward precession would be only 0.012 deg/d greater
    than that of a new Lacrosse, prolonging this unfavourable plane spacing for many
    years.
    
    So, although the 66 fairing argues strongly for a Lacrosse, the launch period
    appears to argue to the contrary.
    
    Ted Molczan
    
    
    
    -------------------------------------------------------------------------
    Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive:  
    http://www.satobs.org/seesat/seesatindex.html
    



    This archive was generated by hypermail 2b29 : Tue Mar 29 2005 - 09:07:22 EST