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From: "Rodger Doxsey"
To:
Subject: FW: Zero gyro background material
Date: Sun, 14 Nov 1999 12:07:40 -0500


Here is some background material on plans for operations and SM3A in zero
gyro mode. I had mis-remembered the plan with regard to capture for SM3A.
The recommendation last spring was to go into hardware sunpoint for that
activity, as it should result in a more controlled HST attitude during
grapple. Grapple during zero gyro mode is possible, as a backup or
contingency if the RMGAs do not perform as expected(RMGAs are the retrieval
mode gyros. They are distinct from the gyros used in normal operations, and
expected to be used only to stablize HST during retrieval operations. They
do not have adequate performance to support observing. They have been
routinely turned on and calibrated, but I believe they have never actually
been used to control vehicle pointing).

John B., you need to review what the use of the PSEA safing will do to the
SI timeline.

Rodger


>From: Art Whipple
>Subject: Zero gyro background material
>
>In view of today's entry into zero gryo safemode a review of some
background
>material is in order. On May 3rd, 1999, a meeting was held with Dr.
>Campbell and other HST Project personnel to review the MOSES
recommendations
>on "Life with Gyro Failures". Attached is the presentation (PPT) given at
>the meeting and a memo (MSWORD) from STScI on their related recommendations
>that was also presented.
>
>Appended to this is an email that Ed Moy sent out on 10/28/99 that outlines
>the SM3A philosophy for gyro anomaly.
>
>Art
>
>X-Sender: moy.ed@lmmail.hst.nasa.gov
>X-Mailer: QUALCOMM Windows Eudora Pro Version 4.1
>Date: Thu, 28 Oct 1999 17:11:54 -0400
>From: "Moy.Ed"
>Subject: SM3A Philosophy for Gyro Anomaly
>
>Based on the current status of the RGAs as we approach SM3A, I would like
to
>make sure that everyone understands the expected plan if another gyro fails
>between now and SM3A. From all contingency procedures, documentations, and
>meetings held up until today, if another gyro was to fail prior to grapple,
>HST is expected to be commanded to PSEA Hardware Sun Point (HWSP) under
>RMGAs for grapple. This mode is preferred because of the following:
>
>1) HWSP maintains stable pointing HST with low vehicle rates
>(current estimate of ~0.002 deg/sec)
>
>2) Crew safety and Orbiter grapple is less impacted under HWSP
>
>3) Flight Rule and fly-around approach have been established and
>verified for HWSP (see Flight Rule X2.1.6-1, Paragraph I).
>The following three cases define the current actions to be taken for SM3A.
>These cases are dependent at the point where gyro anomaly occurs.
>
>1) Gyro Anomaly Prior to Sequence 8
>Currently, the DF-224 safemode response will be to activate Zero Gyro
Sunpoint
>(ZGSP) if another gyro fails. HST should remain in ZGSP until about an
orbit
>before TI-Burn as defined in ACP 8. This time allows a period to assess
HWSP
>performance and take some contingent action if necessary. RMGAs and Gyro
Hold
>will be set prior to commanding HWSP. The setting of Gyro Hold assumes the
>gyro drift rate to be less than 6.2 deg/hr to maintain a low attitude
offset
>from +V3 sunpoint to be within 30 degrees. This offset assumes a successful
>HWSP entry/sun capture for about 4.4 hr followed by a Fly-around grapple at
>the nominal time (~ 2.2 hours after TI-Burn). An RMGA calibration will be
>performed close to SM3A to verify the drift rate.
>
>2) Gyro Anomaly Between Sequence 8 to TI-Burn
>If by chance a gyro anomaly occurs while executing Sequence 8, it is
possible
>that ZGSP entry will result depending on the number of steps executed. The
>purpose of Sequence 8 for gyros is to modify the PSEA configuration memory
to
>use RMGAs, close aperture door, and set Timer B to 40 minutes if HWSP was
>entered. Other steps taken are to modify the two DF-224 Safing test, Two
Gyro
>Configuration and System Momentum, to have a safemode response resulting in
>HWSP. The action described in Case 1 would be taken if a gyro anomaly
occurs
>before modification of the two DF-224 Safing tests. ZGSP is the expected
>safemode response from either Safing test.
>
>A gyro anomaly after Sequence 8 will result in HWSP entry. Note that 'Gyro
>Hold' will not have been set at this point in the Command Plan. Since PSEA
>only acknowledges the Gyro Hold setting at entry into HWSP and it is
>constrained to being set close to TI-Burn to reduce vehicle drift, PSEA
will
>have to be recommanded into HWSP. By setting Gyro Hold on the active PSEA
>configuration memory, and recommanding HWSP, the proper configuration will
be
>defined for grapple.
>
>3) Gyro Anomaly After TI-Burn
>At the time after TI-Burn, a gyro anomaly can impact the ability to capture
>HST at the SMIT defined grapple time. A delay for a fly-around grapple
under
>HWSP will have to be considered until sunpoint capture is completed.
>Depending upon where a gyro anomaly occurs after the TI-Burn, the PSEA may
>have to be
>reconfigured to command HWSP. The PSEA computer is disabled as part of
>Sequence 33 (KA Monitoring Disabled). Therefore, PSEA computer would have
to
>be first re-enabled before commanding HWSP.
>
>After HWSP is initiated, the vehicle will slew to the +V3 sunpoint
attitude in
>daylight. The vehicle will be though at an attitude hold if in orbit night
and
>then slew in orbit day. The solar arrays will also be slewed 17 degrees
away
>and back to the expected 90 degree SA position (SA normal to +V3 axis).
The SA
>slews take about 15 minutes starting from HWSP entry. Once the SA slews
>complete, HST grapple can be considered at the Orbiter commander's
discretion.
>
>As a side note, there has been an assessment of grappling HST under ZGSP.
>Worst case rates in ZGSP are estimated to reach up about 0.15 deg/sec in
the
>V1/V3 axes and 0.103 in V2 axis at orbit dawn. Rates will only reduce in
the
>V1 and V2 axes in day. Rendezvous with HST in ZGSP by the Orbiter has been
>shown to be feasible at these higher rates. But HST grapple in ZGSP (see
>Flight Rule X2.1.6-1, Paragraph J) is only to be considered as a
contingency
>if HWSP performance is not stable with RMGAs.
>
>If there are any questions or comments please forward them to me.
>
>======================================
>Edward W. Moy
>Hubble Space Telescope
>LMTO, MOSES Safing Systems Engineering
>email: Moy.Ed@lmmail.hst.nasa.gov
>phone: (301)901-6060
>pager: (301)303-3048
>fax: (301)901-6161
>======================================