84 K. J. Pederson et al. Fig. 9.3 Exploded force-limited test setup Fig. 9.4 “Free end” of load cell Table 9.1 NEXT-C test part masses Part Mass per part (lbs) Quantity Total mass Ion thruster 36.2 1 36.2 Bolt 0.14 3 0.42 Washers 0.01 9 0.09 Load cell 0.6 3 1.8 reacted at the corners with linear forces. Similarly, in the NEXT-C test setup, moments could be reacted by linear forces at the three interface locations. In the exploded figure, the hardware referred to as being on the “free end” of the load cell (as defined in Fig. 9.4), can be seen as the purple “Test Article”, the furthest right red washer, and the part of the yellow bolt that extends beyond the load cell. The NEXT-C thruster and some auxiliary mounting hardware were weighed on the scale. The weights are tabulated in Table 9.1. The weight of the test article alone was 36.2 lbs. The total mass on the “free end” of the load cell includes the test article, three washers, and a small portion of the three preload bolts, totaling ~36.3 lbs. The “free end” is depicted in Fig. 9.4. Unfortunately, it is not immediately evident whether and what other mounting hardware should be included in the measured mass. The total mass of the test article and auxiliary hardware includes nine washers, the bolt, and three load cells, totaling 38.61 lbs. Depending on whether the measured mass includes free-end hardware only, all mounting hardware, or somewhere in between, the mass varies from 36.3 to 38.61 lbs, a 6% difference. Ultimately, any deviation from the correct measured mass will directly contribute to a shift in the acceleration spectrum achieved by test article due to the force limits that engage in certain frequency bands. It is hard to say with certainty, but a 6% difference in force measurements could have caused the NEXT-C test to not meet various test requirements such as input acceleration levels staying within tolerance bands. This crux of the problem is identifying which auxiliary components should be included in the measured mass. At first glance, the measured mass would equal that of the test article; however, it is unclear if the load cell, washers, and preload bolt’s mass should be considered in their entirety or at least in some portion. It is still unclear which components and proportions should be included in the measured mass. This chapter does not provide an answer to this question; rather it brings the issue to light and suggests how important it is to properly document and account for the uncertainty. Perhaps, the best way to approach this situation is to perform the calculations with only the test article’s mass and again with all of the
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