Assurance of Service Life of the MIRA 150A Variable Thrust Rocket Engine

Reproducibility of service life of an ablatively cooled thrust-chamber design is approached from the viewpoint that the problem is essentially one of assuring consistent injector operating characteristics. For the MIRA 150A, which is a storable bipropellant engine employing a coaxial tube injector, a very meaningful technique was developed for determining the effect of the injector combustion pattern on thrust-chambe r durability. Called the streak test, this technique consists of employing a substitute erodable nozxle throat in a test chamber to register the injector signature during a prolonged static firing. It is shown that the erosion of the streak-test nozzle throat is quite sensitive to the combustion pattern, which, itself, is affected critically by injector controlled variables; the MIRA 150A injector combustion pattern is readily adjustable for optimization of streak-test nozzle throat durability; and the streaktest results correlate reproducibly with flight-chamber service life. Quantitative accept/ reject criteria employed in the MIRA 150A acceptance streak test are defined, and correlation of the acceptance lest results with thrust-chamber service life (as demonstrated in formal prequalificaliori tests) is shown.