X-37B OTV 3 search elements

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Mon Dec 10 2012 - 22:41:49 UTC

  • Next message: Brad Young: "BY O 121112"

    X-37B OTV 3 is scheduled for launch from Cape Canaveral on an Atlas V-501, on 2012 Dec 11, between 18:03 UTC and 21:56
    1. Search Elements and Visibility
    The following search elements assume launch at 18:03 UTC, into the same initial orbit as OTV 2:
    OTV 3                                                    318 X 344 km
    1 79211U          12346.77222222  .00007731  00000-0  38679-4 0    00
    2 79211  42.8650 137.9330 0019599 297.4114 274.0984 15.80392350    04
    The actual orbit may well differ somewhat in inclination and altitude. I chose OTV 2's initial orbit over OTV 1's,
    because it seems to better correlate with the Centaur's de-orbit hazard area, but it is unlikely to exactly match
    I have not made an exhaustive evaluation of visibility windows; however, it appears that launch at window open on Dec
    11, would provide some of our northern hemisphere colleagues with morning visibility, and some in the southern
    hemisphere with evening visibility.
    The OTV's standard visual magnitude is 4.2 (1000 km range, 90 deg phase angle).
    2. Centaur De-Orbit
    As with the first two X-37B launches, the Centaur will be de-orbited soon after it deploys its payload. OTV 1's Centaur
    was sent into solar orbit. OTV 2's Centaur was de-orbited into the Indian Ocean, as will OTV 3's. The hazard zone is
    centred near 29 S, 95 E. It more or less encompasses that of OTV 2, but greatly expands it southward. The orientation of
    both zones seems somewhat askew the probable ground track, but OTV 3's seems more reasonable, assuming I have guessed
    the orbit correctly. 
    OTV 3 Centaur's hazard time periods appear to begin significantly late relative launch window open. I won't go into
    detail on this, but I suspect the information may be erroneous, at least in part. Another possibility is that the
    intended time of launch is later in the window that is believed. Time may tell.
    The time and magnitude of the de-orbit manoeuvre has not been made public. OTV 1's occurred at the southern apex of the
    orbit (T+45:09 for 2m28s). Total delta-V was about 3569 m/s. The delta-V to de-orbit into the Indian Ocean is far less.
    The exact magnitude depends on the location of the burn. At the southern apex, delta-V of about 628 m/s would cause the
    orbit to intersect the Earth near the centre of the hazard zone, about 56 min after launch. Here is the corresponding
    TLE (epoch at time of ignition):
    Centaur de-orbit                                       -1491 X 321 km
    1 79212U          12346.78252315  .00000000  00000-0  00000-0 0    03
    2 79212  42.8650 137.8699 1564000  90.0000 180.0000 19.70000000    02
    3. Confusing Launch Windows
    If the launch slips to Dec 12, then the window will open more than an hour earlier. The reason is not obvious. It
    reminds me of the confusing situation with OTV 2's windows, such that on at least some days the true windows proved to
    be of just 10 min duration, at intervals of about 97 min. Bob Christy and I discussed it a briefly on the list:
    We discussed it quite a bit more off-list, which resulted in my writing the following, which I shared with a few
    correspondents. It may not prove relevant to OTV 3, but this seems like a good time to get it on the record:
    <<<< As for OTV 2-1's weird windows, here is a compilation of the window open/close times (we did not always get both
    close and open, so there are gaps, but all seem to have been of 10 min duration):
            Start    End   Start    End
    Mar 04  20:50 - 21:00  22:27 - 22:37
    Mar 05  21:09 - 21:19  22:46 - 22:56
    Mar 06  21:27                  23:15
    Mar 07  21:47                  23:34 
    Bob Christy pointed out that the data clearly suggests coordination with an existing satellite of period ~97 min. That
    is supported by the interval between the start of the two daily windows, and the progressively later window-open, which
    averages 19 min/d; assuming it completes 15 revs in one day plus 19 min, then period = (1440+19)/15 = 97.3 min.
    I suspect that OTV 1-1 may have been subject to the same constraints, considering that on the date of its launch, 2010
    Apr 22, the launch period was from 22:44 to 01:08 UTC, but the window was 23:52 to 00:01 UTC, just 9 min. If there was a
    97 min earlier window, it would have opened at 22:15 UTC, but that would have fallen outside the launch period. My guess
    is that the launch period is determined by orbit illumination constraints, and the launch windows are determined by the
    close encounter with some observer satellite.
    Bob Christy pointed out that most of the KeyHoles have roughly 97 min periods, and we have been kicking around various
    explanations to explain how the two orbits interact. The ten minute window clearly is too wide to preserve any given
    conjunction, given how far the objects travel in that time. Bob suggests some kind of phasing with the perigee of the
    KH's. >>>>
    4. Possible Co-ordination With ORS-1
    I offer the following with the caution that meaningless orbital coincidences of various kinds are common.
    While preparing this analysis, I noticed that for the last few months that OTV 2 was in orbit, its RAAN precessed at
    almost exactly the same rate as that of ORS-1. As a result, they remained in a nearly fixed RAAN separation of just over
    90 deg, and planar separation about 82.5 deg. During this period, OTV 2 was in an almost exactly 31:2 resonance, which
    means that its ground track repeated every 31 revs, or about every two days. ORS-1's was close to 61:4, but not as exact
    as that of OTV 2. A cursory examination of their ground tracks suggests that one very roughly repeated the other's
    ground track after about 12 h. 
    Here is a recent ORS-1 TLE:
    ORS-1                                                    418 X 421 km
    1 37728U 11029A   12330.71869331  .00000000  00000-0  00000-0 0    00
    2 37728  40.0030 218.0296 0002000 141.9234 218.1718 15.49955659    06
    Launch at OTV 3's window-open on Dec 11 would put its RAAN about 18 deg east of ORS-1, and their planes would be about
    12.3 deg apart. However, launch at window-open on Dec 11 would reduce the separation to about 9 deg in RAAN, and to
    about 6.7 deg in plane. I see no apparent rhyme or reason, but they would be rather close in plane. Once OTV 3 settles
    into its operational orbit, it would be worth looking for signs of coordination with ORS-1. Also, they seem likely to be
    sufficiently close in plane to make a rendezvous feasible at some point. I am not predicting that will happen. Let's see
    what happens.
    Happy hunting!
    Ted Molczan
    Seesat-l mailing list

    This archive was generated by hypermail 2b29 : Mon Dec 10 2012 - 22:43:00 UTC