Reliability as an Independent Variable Applied to Liquid Rocket Engine Hot Fire Test Plans

Abstract

Manufacturers lack an adequate method to balance performance, reliability, and affordability. The reliability-asan-independent-variable methodology is the solution proposed by expressing quantitatively the reliability trade space as ranges of a number of hardware sets and a number of hot-fire tests necessary to develop and qualify/certify a liquid rocket engine against a stated reliability requirement. Therefore, reliability-as-an-independent-variable becomes one of the key decision parameters in early tradeoff studies for liquid rocket engines because the reliability trade space directly influences the performance requirements and, as a result, the affordability. The overall solution strategy of reliability-as-an-independent-variable is based on the Bayesian statistical framework using either the planned or actual number of hot-fire tests. The planned hot-fire test results may include test failures to simulate the typical design-fail-fix-test cycles present in liquid rocket engine development programs in order to provide the schedule and cost risk impacts for early tradeoff studies. The reliability-as-an-independent-variable methodology is exemplarily applied to the actual hot-fire test history of the F-1, the space shuttle main engine, and the RS-68 liquid rocket engine, showing adequate agreement between computed results and actual flight engine reliability.

A reliability as an independent variable (RAIV) methodology for optimizing test planning for liquid rocket engines

“…Based on the fundamental hot fire test strategy definition, through the functional node representation, the mission equivalents are needed to relate the planned hot fire tests at the various system integration levels to the mission requirement as well as to capture the two fundamental strength-reduced and stress-increased failure mechanisms into a single metric, the equivalent mission [1]. The mission requirement not only includes the actual flight but also any other hot fire tests aggregated throughout the product life cycle.…”

“…The functional node representation of a physical architecture of a liquid rocket engine not only describes the structural relation of components known from the fault tree (FT) or reliability block diagram (RBD) techniques but also defines the fundamental hot fire test strategy [1].…”

“…The total number of STH sets needed to complete the overall hot fire test plan is given in Eq. (7) and is based on elements defined by the RAIV methodology [1].…”

“…(4). It is based on the Bayesian formulation to estimate parameters (probability of success p i ) given a set of data (the number of hot fire tests) [1].…”

“…1 Hot fire test plans are found for two scenarios: (1) a reduced thrust scale engine demonstrator precedes the flight engine development and (2) a flight engine development is executed from scratch (without a demonstrator). The results of these two scenarios are used to support the claim that a reduced thrust scale demonstrator is cost beneficial for the subsequent flight engine development.…”