Abstract: The mechanical properties and environmental durability for a gelcast in-situ reinforced silicon nitride in a simulated rocket engine environment was investigated. The results summarized in this dissertation include tensile, flexural strength, compressive strength, interrupted stress rupture, thermal cycle, fracture toughness, low cycle fatigue, elastic modulus, and Poisson's ratio determinations. In addition, the results for a series of investigations determining the effect of simulated engine environment on AS800 are included. Specimens were exposed to hydrogen, water, and oxygen at approximate temperature and pressures of engine operation. After exposure the retained flexural strength of the specimen was measured and compared to the strength of as-received material. Environmental effects for as-sintered specimens and machined specimen surfaces were measured. The investigation temperatures included cryogenic (−320°F), ambient and elevated (2000°F). The environmental exposure data showed that there are minimal effects on the retained properties of AS800 from exposure to oxygen and water combinations.