After mass properties, we decided that we needed to take the ACE box off of the spacecraft to troubleshoot a problem we had discovered during thermal vacuum and could not reproduce on the engineering test unit (ETU). Over a narrow temperature range around 50 degrees Celcius, the y reaction wheel would not start up properly when the box was first turned on. This wheel must spin up for the spacecraft to properly acquire the sun. We quickly eliminated some of the most serious possible causes. After getting a better look at the problem at the box level, we were able to reproduce the same problem on the ETU. The anomaly was caused by a timing problem. We demonstrated sufficient margin at 35 degrees C, which is more than ten degrees above the temperature the box will be at startup. Once the wheel is spinning, this condition does not occur. We decided not to change anything in the box, but we performed a thermal vacuum cycle and a complete functional on the box since we had done a fair amount of disassembly.
After returning the box to the spacecraft and performing a spacecraft
functional, we moved the spacecraft to the Magnetic Test Facility (MTF). The
MTF is located at a remote part of the Goddard Space Flight
Center property to minimize the magnetic field disturbances caused by cars and
other human activity. These three pictures show various views of the
spacecraft within the 40 foot coils of wire. The coils cancel out the ambient
magnetic field from the earth and produce a known field. The building is
constructed with non-magnetic materials such as cinder-block walls, a wooden
roof and floor, and copper pipes for the fire sprinklers. The magnetic fields
are accurate to less than one ten-thousandth of the earth's magnetic field.
We use this facility to test the spacecraft magnetometer, compensate the
magnetometer readings for the fields produced by the magnetic torquer bars on
the spacecraft, and verify the proper functioning of the rate-damping portion
of the safehold system.
In the early part of June, some of the team traveled to Vandenberg to meet
with range safety and inspect the Astrotech facilities where WIRE will be
integrated to the Pegasus and loaded with hydrogen in preparation for launch.
The first photo shows the clamshell structure outside the building. This
temporary structure will shelter the Pegasus when it arrives. This clamshell
will also be used by other missions as an airlock, preventing contamination of
the Astrotech highbay. The second photo shows the inside of the clamshell
and the door to the highbay. The third stage of the Pegasus will protrude
through this doorway while WIRE is integrated and loaded with hydrogen. The
last photo shows the hydrogen safety vent on the side of the building.
Previous updates:
August 18, 1997
October 20, 1997
October 31, 1997
November 7, 1997
November 14, 1997
November 21, 1997
December 5, 1997
December 23, 1997
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January 16, 1998
January 23, 1998
January 30, 1998
February 6, 1998
February 20, 1998
March 12, 1998
March 23, 1998
April 3, 1998
April 20, 1998
April 25, 1998
May 29, 1998
June 3, 1998