551 temperatures to voltage supply . During testing, the team n oticed that the motors actually ran hotter at a  lower voltage. This was because the motor driver would send a higher current at the lower voltage, which  was counter to intial assumptions.  

The assumed most favorable conditions were selected for the initial  test, being lower voltage and coolest  temperature. A duty cycle was determined, and then a total of 15 minutes of run time was performed at that  duty cycle. The voltage was increased and the test repeated. Then chamber temperature was increased,  and the t est repeated for both voltages, and the entire series run again at the third chamber temperature.   A duty cycle was recorded for each specific motor at the given voltage and chamber temperature in which  it was determined, so that each motor had six duty cyc les established. At the end of this series, the motor  was considered “conditioned”.  

Duty Cycle D iscovery Method    • Run motor for a few seconds   • Measure and record a ll coil  resistances  immediately after operation ends . Recommend to have a  multimeter in the loop of the driver circuit.   • Continue to monitor resistance until reading is below the cool -down value.   o Note that cooling in vacuum is asymptotic in nature.   o Cooling to within 5- 10°C of ambient gives a good restart point for the following cycles   • Run motor a few seconds longer than the previous run , depending on motor performance trends.   o If the motor max coil temp is below maximum allowable, the following cycle can be longer   o If the motor max temp is met or exceeded, either reduce the run t ime or extend the cool  down time. Again, note that extending the cool down time is more time expensive than  reducing the on time.   • Continue until three runs are made with the same runtime, cool -down, and resistance values. This  indicates a steady state duty  cycle.  

A sample of the Duty cycle discovery data is shown in Figure 10.  

The data shown in Figure 10 is extensive, but still reduced from the total data set (a sample of which is  shown in Figure 11) the team used to finalize a duty cycle for flight operations. The most pertinent data is  the red and green boxes, which show the increasing run and cool down times as the approach a final value.  For the left sample, this time was about 20 second run time, followed by a 55 second cool down. The right  sample had  a 35 second run time and a 70 second cool down. The teal line shows the duty cycle as a  percentage of run vs. cool down time. Note the differences between the two identical motors, further  indicating some thermal differences resulting from manufacturing v ariance. A final data point of note is the  light green line, showing the current draw during a duty cycle. The upward sloping nature during each on  cycle is indicative of the heating coil. As the coil temp rises, the resistance does as well. The motor driv er  hardware sends more current as a response. The peaks of the green line during a cycle could be charted  to give a maximum coil temp trend. The peaks remained horizontal for this data set, hinting that the coil  temperature maximums were holding constant across the cycles. If the peaks were rising gradually, the coil  temp could be assumed to be rising as well and the duty cycle could be adjusted accordingly. The same is  true if the peaks were falling, except the coil temps would be decreasing and the duty c ycle could be  adjusted in kind.