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
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