The engine exhaust nozzle envisioned for an HSCT propulsion system is quite large, about 18 feet in length. In order to meet aircraft and propulsion system weight goals, the HSR Program has established performance and weight goals for the nozzle that cannot be achieved using materials, designs, or manufacturing processes typically used for engine exhaust nozzles.

The main components of the engine exhaust nozzle are the primary structure, convergent flaps, divergent flaps, noise absorption system, and thermal blanket. The current HSR nozzle concept features a large nickel-base superalloy primary structure with a thin-walled casting to meet weight goals. Although complex, this design appears to be manufacturable, and the mechanical and thermochemical properties of the superalloy seem to be acceptable.

Candidate materials for the convergent and divergent flaps are thin-walled castings of a nickel-base superalloy and a titanium aluminide intermetallic, respectively. Secondary processes to remove material from the initial castings will be required to achieve weight goals. Casting demonstrations indicate such structures are feasible. Areas of ongoing concern include joining, which will be important during manufacture and repair; the impact of thermal fatigue, oxidation, and creep on the durability of the superalloy convergent flaps; and the impact of acoustically driven high cycle fatigue, oxidation, and creep on the durability of the titanium aluminide divergent flaps.

The exhaust nozzle design also includes an internal noise absorption system constructed from CMC acoustic tiles and an insulating thermal blanket. The durability of the CMC in the harsh environment within the exhaust nozzle is a major concern. Failure can result from interfacial oxidation, acoustic fatigue, thermal fatigue, or erosion. Moisture can also degrade the CMC acoustic tiles.

The life goal for the primary exhaust nozzle structure is equal to engine life, about 36,000 hours. The life goal for the acoustic liner and thermal blanket is one-half engine life or about 18,000 hours.

Finding 3-4. Development efforts for the exhaust nozzle may achieve the specified level of technology readiness (TRL 6) by the end of Phase II. Nonetheless, uncertainties about nozzle materials and manufacturing processes will require additional work during the recommended technology maturation phase.

Recommendation 3-4. The HSR Program should fabricate and test full-scale nozzles during the recommended technology maturation phase to validate nozzle manufacturing technology, noise levels, and material performance.

The fuel efficiency of the HSCT propulsion system will depend largely on the efficiency of the air intake, engine turbomachinery components, and exhaust

FRONT MATTER (R1-R10)

EXECUTIVE SUMMARY (1-10)

1 INTRODUCTION (11-25)

2 REQUIREMENTS ANALYSIS (26-45)

3 PROPULSION (46-60)

4 AIRFRAME (61-90)

5 INTEGRATED AIRCRAFT (91-109)

6 SUMMARY OF PROGRAM PLANNING ISSUES (110-126)

A LIST OF FINDINGS AND RECOMMENDATIONS (127-141)

B BIOGRAPHICAL SKETCHES OF COMMITTEE MEMBERS (142-146)

C STATEMENT OF TASK (147-147)

D PARTICIPANTS IN COMMITTEE MEETINGS (148-149)

ACRONYMS (150-151)