CGRO Re-entry efforts ...

From: Steve Walter (
Date: Fri Jan 14 2000 - 11:21:20 PST

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    Hi Folks ...
    I'm one of the more quiet "lurkers" out here on SeeSat (and others).
    But, since I work on the CGRO satellite, I can offer some quick info for
    everyone on up-coming events ...
    1). Gyro #3 had a hard failure back on December 6th ... interestingly, it
    was just a few hours into a formal Risk Management Class several of us
    started to attend ...
    2). CGRO can handle 3-axis control with the 2 remaining gyros; there is, in
    fact, hardly any discernable difference in day-to-day operations.
    3). CGRO must be de-orbited in a controllable state, at some point, because
    it is very large and massive.   It is a virtual certainty that more than
    one part will make it to a ground/ocean contact.  Statistics say CGRO has a
    1/1000 chance of human casualty if it comes back UNcontrolled ... and THAT
    is a very real risk.
    4). The threat in keeping CGRO up much longer is IF one of the 2 remaining
    gyros fail.  In this case, there is no current capability at maintaining
    3-axis control.   Without 3-axis control, there is no way to continue
    science operations -- NOR do a controlled re-entry.
    5). In addition to our gearing up for re-entry (doing sims, studying
    manuals, etc), there are parallel efforts going on to see if derived rates
    from the Sun Sensors or Magnetometers can be used in place of another gyro
    failure.   This has been done on other satellites, but has not seriously
    been funded for CGRO.   [NOTE:  The IUE satellite successfully ran on 1-
    and 0-gyro control laws -- but IT was in geosync -- not LEO.]
    6). It would certainly solve a lot of problems if a Shuttle were to either
    repair CGRO or retrieve it.  However, there are also propulsion line
    problems with CGRO that probably eliminate STS involvement (safety waivers
    are also being examined in parallel efforts).
    7). I know several of the CGRO scientists, and I also know they would love
    an extended mission.   Despite having outlived its original mission
    objectives (and an internal clock that just rolled over the first time on
    December 22!) -- there are many more science observations they would like
    to make.
    8). Official chores are for us to examine whatever is feasible, but between
    now and February 16 -- we are to prepare for re-entry.  This involves a lot
    of work and testing, since none of us have experience with it.   If there
    are no magical answers for successfully using derived rates for attitude
    control -- or some other options appear for keeping CGRO on-orbit and doing
    science operations -- it is likely NASA management will then make an
    official decision to de-orbit.
    9). The goal is to have CGRO splash down east (and south) of Hawaii,
    whenever that day comes [currently scheduled for mid-March].   This will
    require many orbit-reduction burns over at least a 2 week period.
    10). There are at least 2 critical propulsion-related problems with CGRO
    ... the A-side propulsion lines are probably unusable, and one of the
    attitude control thrusters has questionable integrity.   Both of these
    problems were managed for the 1997 orbit re-boost, and we expect to be able
    to manage them for the de-orbit activities.  But, what if we stay on-orbit
    a few more years, and we're wrong about how well things can be controlled?
    It's an extra risk that could have major consequences.
    All in all, we're gearing up for re-entry in mid-March -- but NO firm
    decision has been made yet.
    Steve Walter
    PS: If any of you send me an Email, please do NOT use the 'Erols' address,
    but: ... my Erols account is disappearing
    VERY soon.
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