Repairing a Poor Thermal Design  After the engineering development unit (EDU) hardware was manufactured, accepted, and assembled on  center, the bench top and environmental testing began. The team planned full functional tests, small lab  vibration tests, thermal bake out, and a thermal vacuum test. The thermal vacuum test followed the same  temperature profile as the solar sail subsystem and utilized NASA Marshall Space Flight Center  environment test chambers. The on-site location was beneficial for off-nominal hour support.   The purpose of the thermal vacuum testing was to demonstrate the full functionality of the system in  estimated limits of the cold and hot environments to be experienced on orbit. The engineering unit went  through a thermal bake out for contamination control reasons at 80°C. The engineering unit was expected  to see cold temperatures at -70°C and hot temperatures at 70°C. The unit would cycle through the hot and  cold temperatures four times in vacuum with functional tests completed before test, during each cold and  hot cycle, and after the test was completed.   The first thermal vacuum test had a successful functional test completed before the chamber door was  closed. The ambient temperature was 23°C. Once the door was closed and before vacuum pump down,  the AMT performed a limited functional test to ensure connection to the hardware was maintained. The test  was completed successfully. After an 8-hour vacuum pump down, the AMT performed a limited functional  test at ambient temperature with a pressure of 1.0E-5 Torr. After initial movement, the motors displayed  abnormal behavior, then stalled. The test was halted.   After the TVAC failure, the NEA Scout design team met and constructed a detailed fault tree to determine  as many failure causes as possible. This was crucial to ensure that no stone was left unturned, nor that the  personal strengths and biases led to unconscious oversight of less obvious sources. The team produced  95% answer almost immediately: The stepper motors’ internal coils had overheated and shorted. This was  confirmed with impedance data and a day-of conversation with the vendor.   For the next few weeks, the team unearthed 3 root causes that led to this failure. First, the control board  had provided 2.25x the motors’ rated current. Second, the thermal design did not provide enough contact  area to allow heat to escape the motors via conduction. Third, there was a clear misunderstanding of the  motor’s internal heat path and how residual heat could be dissipated.  Control Board Input Error  The first issue, the motor overcurrent, was remedied rather quickly. The board was reprogramed to  gradually accelerate the motors speed using a simple ramp. The overcurrent error had been hidden for two  reasons. First, human error had the control board programmed to 2.25x nominal current. At low speeds,  the ramping function is not critical, and a nominal current supply can accommodate for an instantaneous  change from rest to operational speed, known as a step input. The vendor noted that these motors could  handle a step input up to 1000 RPMs, but our motor was set to run at about 8000 RPMs.   At lower currents, the motor was stalled because there was not enough magnetic force to accelerate the  motor from rest to the desired speed. The control board current output was increased until the motor  operated, and the current output was not checked against the data sheet. This was the primary error.   Insufficient Thermal Conduction  Second, the small stepper motors predominant thermodynamic process is convection. Even at 2.25x power,  the motors surface temperature barely fluctuates in atmosphere giving no tactile evidence that the motors  were under thermal or mechanical stress. Once the motors entered vacuum, the overcurrent compounded  by a poor thermal path led to motor failure in just seconds. Figure 2 shows the actual size of the motors  and Figure 3 shows results from bench top tests comparing surface temperature rise versus time for multiple  current inputs.   NASA/CP—2018-219887 285