Two engineering tests were conducted recently to test the door mechanisms of the HRI (High Resolution Imager). The first test successfully demonstrated that the primary mechanism still functioned after 8 years in space, as documented by positive telemetry indicators. A second test, this time of the secondary door mechanism, was also apparently successful, as deduced from analysis of science telemetry, although a positive indication was not provided by housekeeping telemetry. ROSAT HRC Door Test - February 10, 1999
The HRI door on ROSAT was opened for the first time on-orbit in June of 1990. It remained open for the entire ROSAT mission - a period of 8 1/2 years. With the imminent shutdown of ROSAT in February 1999 due to problems with the spacecraft pointing and instruments, a final engineering test of the HRI door mechanism was planned for early February 1999.
Two tests were performed on February 10, 1999 during two consecutive real-time passes over Weilheim, Germany. During pass 1 the door was closed and then reopened using the primary door mechanism. The primary door mechanism consists of a motorized carriage with the O-ring seal gate attached to it with a four-bar linkage. This allows the door seal to be made with the gate approaching the sealing surface in a normal direction rather than sliding across it. When the motor power is removed, the carriage/gate mechanism is left in an over-center toggled position to prevent any unintended opening of the door. This test was successful as verified by telemetry.
During pass 2, the door was again closed (verified by telemetry) and the redundant door opening command was sent to the spacecraft. This command fires two pyrotechnic devices in series. Coincident with the commanding of the pyro firings a sharp pulse appeared in the detector count rate (see figure), the steady- state count rate increased indicating a higher detector background, and the telemetry verifier indicated that the door was neither shut nor open (a condition indicating that the door is either in transit or stationary at some intermediate position but definitely not closed). We conclude from these observations that the pyros both fired and the door opened. Most likely the door hit the stops at the open position and rebounded slightly and the springs were sufficiently damaged to prevent their functioning in holding the door at the open position (a displacement of only one millimeter from the open position is sufficient to cause the lack of the open indication because of the limited range of switch operation) but the door was functionally open. A description of the opening sequence is given below.
The first pyro is a pin puller which allows the motor/gearhead to be disconnected from the gear train so that the reflected inertia of the motor/gearhead does not act as a brake. Once this action occurs, there are two Negator (constant force) springs which will pull the carriage/gate to the open position in the event that it was at an intermediate position in its travel. In the instance above, however, the door was in the over-center toggled position and the force of the two springs is not enough to pull the door overcenter and to the open position. For this case, the second pyro is required. It is a pin pusher which fires after the motion of the first pyro is completed (controlled by microswitches) and it acts on the carriage to push it overcenter. In ground test of the mechanism, it was noted that the energy released by the firing and imparted to the carriage was so great that the carriage was forced toward the open position without the need for the springs. In fact, the springs could not coil up fast enough to get out of the way and they were permanently crinkled as a result.
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