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143 In order to bring the depth-varying concept evaluation to same reason, loading oscillations could potentially be more the maturity point of the other candidates, metamodel data severe in the linear case since it might not prove as robust for would need to be created for the full suite of aircraft tires. preventing over-penetration. The loading oscillations with Subsequent arrestment simulations could then be conducted the aggregate foam system required control through the bed with the APC to quantify the overall improvements to the geometry design. Similar control would probably be required arrestor bed lengths and aircraft exit speeds. for a stratified glass foam arrestor bed as well. 12.5.2. Transition to Glass Foam System 12.5.4. Estimated System Cost and Maintenance The depth-varying evaluation was carried out using a gen- eralized ideal crushable foam material. Before conducting the If the depth-varying crushable foam concept were to be metamodel generation recommended in the preceding sec- implemented using a glass foam material, the system cost tion, it would be beneficial to link the concept with actual and maintenance issues would be the same as discussed in materials and calibrate to match them. Section 9.6.2. The nature of the glass foam concept's layup The glass foam material appears to be a strong candidate and construction would readily permit the use of several for implementing this concept (Chapter 9). It can be pro- strengths of foam material in stratified layers without altering duced in a variety of strengths, is manufactured in relatively the construction methodology. thin blocks, and can be adhered to itself in stratified layers matching the concept layup. It also has fairly ideal crushable 12.6. Summary foam characteristics that would lead to a good performance match with the idealized material predictions. The depth-varying foam material concept was assessed as part of a parallel study to the overall research effort. The focal point of the investigation differed from the other arrestor 12.5.3. Comparison with Aggregate concepts investigated, and the final concept evaluation is less Foam System mature than for the three primary arrestor candidate systems. The aggregate foam material manifested a depth-varying The comparative optimization study involved a protracted material property (Section 11.3.2), which should be compared set of tire and arrestor simulations, which investigated linear, with the concept investigations of this section. quadratic, and exponential material hardening profiles. A The aggregate foam material exhibited the best overall fleet- down-selection of these alternatives found that the linear wide performance of the three candidate systems evaluated strength increase profile was the most promising, and it was (Section 11.5). The fleet design arrestor bed produced the clos- compared with the incumbent homogeneous material. est overall exit speed rating for the three test aircraft, showing The results for this investigation proved positive: a linear strong improvements in performance for the B747, which depth-varying material offers several advantages over the typically lagged in the one-size-fits-all beds. This improve- current homogeneous material. The depth-varying approach ment was possible because the foam aggregate essentially had helped bridge the geometry and loading gaps between large and an exponential depth-varying strength profile. small tires, reducing the performance disparities between During the depth-varying profile investigation, it was deter- them. A bed constructed using the linear depth-varying mate- mined that the linear strength profile offered more advanta- rial could produce shorter arrestment distances and/or higher geous behavior than the quadratic or exponential profiles. If design exit speeds for the aircraft fleet. this observation holds true during a broader assessment involv- In practice, this methodology could be readily imple- ing the three test aircraft, it is possible that a stratified glass mented using a glass foam block material, with minimal foam bed could perform even better than the foam aggregate. impact on anticipated cost, construction methods, or main- However, there are several design coefficients for either depth- tenance needs. varying profile, and a more extensive examination would be Finally, Chapter 11 demonstrated that the exponentially required to firmly establish the best fleet-wide approach. depth-varying aggregate foam material could achieve perfor- The rut depths previously shown for the aggregate foam mance leveling between aircraft. Although the exponential bed tended to be fairly shallow. The penetration depth into profile was not ideal per the findings of this study, the aggregate the linear gradient arrestor bed would likely be deeper, which foam concept illustrates the potential results when extrapolated could reduce the amount of unused material. Yet, for the through full aircraft arrestment simulations.