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156 15.2. Experimentation Phase will require additional characterization in order to make high- confidence performance estimations. Overall, the combination The experimentation phase of the effort involved an exten- of unknown factors and anticipated cost and performance sive evaluation of four arrestor candidates: improvements make it a moderate-risk/low-payoff concept. The selection of one or more of these alternatives for field- 1. Glass foam arrestor (passive) ing and approval is a task that requires consideration by the 2. Aggregate foam arrestor (passive) relevant stakeholders, which includes the government, airport 3. Engineered aggregate arrestor (passive) community and manufacturers. Each candidate offers differ- 4. Main-gear engagement active arrestor (active) ent advantages, risk levels, and payoff potential. Depending on the development track that is pursued, manufacturer invest- A combined modeling and simulation effort successfully ment may be required. The extensiveness of subsequent devel- replicated each candidate in order to evaluate its merit and opment plans and the associated costs may determine the compare it with the existing EMAS performance. feasibility of such participation. As a precursor to any such development, it is recommended that the concepts for pur- 15.2.1. Passive System Evaluation suit be determined and that the relevant manufacturers be contacted for preliminary discussions of development scope Chapter 9 through Chapter 11 gave specific recommenda- and participation. tions and guidance for transitioning the three passive candi- New arrestor and cover-layer materials will be used for any dates into fielded systems. The findings of this research indicate candidate that is pursued. It would be advisable to pursue that a fieldable system is feasible for all three candidates. full-scale tests for these alternatives, which would allow: The aggregate foam concept offers superior multi-aircraft performance due to its depth-varying material properties. This Assurance of performance and safety, multi-aircraft performance is arguably the most important fac- Characterization of the full arrestor layups, tor for keeping arrestor beds short, reducing land requirements, Down-selection of final cover layer alternatives, and and increasing the rated aircraft exit speeds. Additionally, it Model calibration to match final configuration performance. provides a substantially lower estimated cost per square foot. However, because the aggregate foam concept uses a novel As part of future research, it would be advisable to test cellu- crushable material and cover layer, the number of unknowns lar cement specimens and incorporate a cellular cement arrestor is greater. The arrestor materials require additional evaluation into the APC. Inclusion of cellular cement was omitted during in order to produce high-confidence performance estimations. the current research. However, such a model could be devel- Overall, the combination of unknown factors and anticipated oped in approximate form by testing a generic cellular cement cost and performance improvements make it a moderate-risk/ material of nominally the same density. The incorporation of high-payoff concept. a cellular cement option in the APC would help to establish Conversely, the glass foam concept is the most conserva- equivalency of various systems and provide additional com- tive of the three alternatives in terms of development risk and parison of performance between the arrestor options. payoff. Glass foam provides an alternative to the current EMAS technology with promising improvements regarding service 15.2.2. Active System Evaluation life and maintenance, but at equivalent cost and performance. The mechanics of the material are predictable with relatively The main-gear engagement active system candidate is not few unknown factors. Fielding such a system would be more recommended for additional pursuit. While stopping the air- straightforward than with aggregate foam, and the approval craft proved mechanically feasible, engaging the main-gear process could arguably follow the shorter "equivalent" path struts involved multiple complexities, including damage to (Section 6.3). Overall, the combination of unknown factors landing gear features, damage to landing gear bay doors, and and anticipated cost and performance improvements make it timing and deployment complexity. Additionally, some air- a low-risk/moderate-payoff concept. craft geometries made capture infeasible altogether. Finally, the engineered aggregate concept provides a fea- Nevertheless, active systems remain feasible if barrier nets are sible alternative, but one without a particular distinguish- used for engagement. Using the suggested sensor and activation ing advantage. It provides cost savings and increased material methods, an automated system could be developed that would durability over the current EMAS technology. However, the eliminate the need for manual triggering, but permit pilot over- speed-dependent arrestor performance generally requires rides. The active system approach offers the highest theoretical longer arrestor beds and diminishes the multi-aircraft per- deceleration limits, which could produce substantially shorter formance. As with the aggregate foam, the materials to be used arrestment distances than any passive system alternative.