Tuesday, 21 December 1999, 7:30 PM
Today's attempt to unlatch the instrument cover did not occur successfully.
Although we received an initial indication that the cover had transitioned
to the "unlatched" condition, a short time later telemetry indicated that
it returned to the "latched" state. Based on the available data, we believe
the actuator that performs the unlatch is operating properly, but that the
cover returned to the latched state as a result of a procedural error in
the pre-canned command sequence. A corrective action has been identified
and we intend to unlatch tomorrow.
Unlatching occurs through the use of "wax thermal" actuators. These
actuators operate by heating up a wax pellet which then expands and pushes
against a spring loaded piston. When the piston reaches a certain point, a
pawl falls into a detente and holds the piston in the extended (unlatched)
position. Indication that the piston has extended is provided through a
microswitch. However, we know from preflight testing that there is not
exact mechanical synchronization between the "unlatch" condition as
indicated by the microswitch and actual capture of the piston by the pawl;
therefore, an additional 45 seconds of power is supposed to be applied to
the actuator after the switch indicates "unlatch" in order to insure that
the piston has extended far enough so that the pawl can fall into place.
This has been tested successfully many times before launch. The pre-canned
command procedure that was issued today unfortunately caused power to be
removed from the actuator immediately upon receipt of the "unlatch"
indication, rather than 45 seconds later, with the suspected result that
the piston had not extended far enough to be caught by the pawl, the wax
cooled down, and the spring pushed the piston back to the latched
condition. This scenario is consisent with the observed telemetry. The fact
that the time between application of power to the actuator and receipt of
the initial "unlatch" indication is consistent with preflight test results
lends further credence to our belief that the problem was with the
procedure as executed and not with the hardware.
This afternoon lengthy consultations and meetings between project and
program management, mission reliability, mechanical and system engineering,
instrument operations, and myself took place to assess this situation. We
are being careful to follow measured steps to rectify the problem. Based
upon our conclusions, we plan to try the cover unlatch again tomorrow with
operator intervention to override the command that caused power to the
actuator to be removed too quickly. Because we did not get the cover
unlatched today, the unlatch of the calibration panels was delayed in order
to give us time to evaluate the situation. Following a successful cover
unlatch, we plan to go ahead with unlatch of the calibration panels and
also to initiate the instrument outgas period.
As mentioned in my earlier status report, the MISR instrument is on and
telemetry looks normal. Additionally, normal operations of the Terra High
Gain Antenna have resumed. A definitive diagnosis of the earlier problem
has not been announced, but the HGA is once again operating properly.
For earlier status reports please check the "News" link of
the MISR web site at http://www-misr.jpl.nasa.gov.
David Diner
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