When can steel nozzles be used in research motors?

Steel nozzles can be used in research motors specifically when the nozzle throat is recessed within the case of a sugar motor. This configuration is crucial because the recess of the nozzle throat helps in managing the thermal and structural loads that occur during operation. In sugar motors, which can produce significant combustion temperatures, the recession provides a protective barrier that can prevent the nozzle from directly absorbing excessive heat, thereby reducing the risk of structural failure.

While high-performance nozzles and protective coatings can enhance the performance and safety of steel nozzles, they do not specifically address the unique requirements and implications of using steel in sugar motors. Moreover, while safety is always a priority in rocket motor design, it does not inherently validate the use of steel nozzles in all situations; therefore, it does not apply as a definitive criterion. The correct usage scenario is focused on the specific design and operational characteristics of the nozzle in relation to the motor type.