technology maturation phase to evaluate and, as necessary, guide continued development of the selected combustor design. If a commitment is made to conduct full-scale testing, it may be feasible to eliminate the subscale core engine tests planned for Phase II. A suggested schedule is presented in Table 3-3.

Finding 3-3. Significant uncertainties regarding the viability of potential ultralow NO_x combustor designs—and the materials needed to implement those designs—are likely to remain at the conclusion of Phase II, as currently planned.

Recommendation 3-3a. During the recommended technology maturation phase, the HSR Program should test a full-scale demonstrator engine to reduce uncertainties regarding the viability of the selected ultralow NO_x combustor design. Combustor development during Phase II should focus on preparations for full-scale tests.

Recommendation 3-3b. In order to increase the potential market for silicon carbide CMC liners—and thereby ensure their availability for use in HSCTs—the HSR Program should encourage other engine research programs sponsored by NASA, the Department of Defense, and the Department of Energy to include more CMC materials.