Dec. 21, 1999
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