Limited testing of precracked ceramic rocket nozzles.

When the deterioration of ceramic rocket nozzle throat inserts is governed by the thermal stress conditions, the performance may be markedly enhanced by precracking the ceramic insert. Limited testing showed no instance where precracking degraded performance of an initially monolithic insert. In the presence of extreme melting or corrosion (as eutectic formation), precracked inserts behave the same as the initially monolithic inserts. The factors that are considered to account for the improved performance accompanying pre- cracking are the improved articulation capability and the consequent accommodation of thermal displacement, the increased redundancy for load transmission and resulting fracture tolerance, the reduction of maximum compressive stress, and the improved fracture surface geometry obtained in precracking. UDGE4 describes nozzle failures attributable to cracking; however, the nozzle support structure was very nearly statically determinate, so there was little inherent fracture tolerance. Lynch, Quick, and Duckworth5 found cracking to be one of the causes of failure in ceramic nozzle inserts. Others have shown that cracking in a rocket nozzle is not necessarily detrimental to single-cycle performance. Baskin and Greening1 found that fracture induced in operation was sometimes fatal, whereas in other cases cracking did not dominate single-cycle performance. They report no multiple firings of such inserts and are pessimistic about the prospects of survival. Olcott and Batchelor2 exercised some control on the cracking of tungsten inserts by prenotching. In firing cycles lasting several seconds, this cracking did not control performance. Duckworth and Krochmal3 allude to the ad- vantages of segmented construction of combustion chambers and nozzle inserts, but give no data. There are many instances where nozzle fracture during a firing has resulted in severe fragmentation, whereas in other cases fractured nozzles have performed satisfactorily. Usually, however, the presence of such fracture was quite commonly regarded as inimical to successful multiple firings. Fracture in rocket nozzle inserts must be evaluated with some caution since, not infrequently, the fracture had occurred not during the firing, but as a consequence of residual stress appearing during cool-down. Preliminary dynamic tests of high-density alumina nozzle inserts at this Laboratory demonstrated that the spalling and gross deterioration that occurred with solid (monolithic) ceramic inserts could be avoided by precracking the nozzles before the test. This paper reports the results of those tests and of tests of ceramic nozzles under much more severe rocket motor conditions. Table 1 shows the candidate materials that were considered for these tests. The materials selected for the rocket motor tests, alumina (A1203), magnesia (MgO), and zirconium carbide (ZrC), were all sensitive to thermal shock, and were chosen to combine the effect of thermal shock with melting (A1203), corrosion (MgO), and neither Received May 3, 1968; revision received September 16, 1968. This work was performed under the auspices of the U.S. Atomic Energy Commission. The author wishes to express appreciation for the able assistance rendered by staff members of the U.S. Air Force Academy Aeronautical Test Laboratories, R. D. Marker, I. V. Spann, T. D. Fultz, C. F. Hollenbaugh, and F. E. Jayne. This experiment was made possible through a coopera- tive agreement between the AEG and the U.S. Air Force Acad-