Evaluation and Test Requirements for Liquid Rocket Engines

This Standard establishes test and evaluation requirements related to the development, qualification (or certification), and acceptance (flight production unit) of liquid propellant rocket engines and associated propulsion systems.

Qualification strategies for engine elements, excepting qualification by analysis (no test option), and by similarity. (ECF - environmental correction factor; ELCF - external load correction factor; UF - Ultimate Factor; PF – Proof Factor)

The primary objective of any test program is to maximize the probability, within programmatic constraints, that the flight design will function properly and successfully when used in actual service for the intended application. Flight risks are mitigated via prudent and effective analysis and testing. While analysis can sometimes be used in place of test, proper analytical techniques utilize test data as the basis for model correlations. The combination of analysis and test verification is used for both qualification of the LRE design as well as workmanship verification of each LRE flight unit.

Certain key tenets of testing have served the liquid propulsion test community well as it has tried to accomplish that objective. One particular “tried and true” philosophy is the “Test-Like-You-Fly” (TLYF) approach. TLYF means that testing should demonstrate engine operation with flight-representative hardware and under flight-representative conditions, including expected worst-case conditions. The overall test program should encompass and explore as much of the operational flight envelope as possible on the flight design to accomplish the ideal objective to avoid operating any particular flight hardware configuration under any particular set of conditions for the first time in flight.

Some flight environments (e.g., acceleration) cannot be replicated during ground test. Margin testing with respect to the expected flight conditions should be included to protect against known and unknown uncertainties in the flight conditions (e.g., ground-to-flight dispersions), as well as known, anticipated, and unknown hardware variations (e.g., manufacturing tolerances, non-conformances, and undetected deficiencies).

If it is impractical to test or simulate a particular flight condition on the ground, then additional margin may be appropriate. Furthermore, testing should consider and account for engine hardware experience and exposure throughout all phases of the required life cycle, including manufacturing, acceptance testing, transportation, handling, storage, vehicle integration, checkout testing, launch preparations, aborts, liftoff, and flight. Exceptions to this approach should be carefully evaluated and include a risk determination. It is prudent that LREs be deliberately designed to provide margins equal to or greater than the specified margin test requirements so as to provide high probability for test success and high reliability for flight operations.

A corollary to TLYF is the “fly-as-you-test” approach. “Fly-as-you-test” means that flight operation should remain within demonstrated ground-tested and qualified regimes, and that design and process differences between qualified test hardware and flight hardware should be minimized and ideally avoided. A successful development and qualification program will anticipate all potential flight conditions and ensure those conditions are validated by a robust test program. If an engine has multiple applications, ideally the engine should be developed and qualified to the most demanding requirements, however programmatic considerations may dictate a phased approach.

Hot-fire testing to verify that an LRE design is ready for flight typically consists of four phases of major program activity: prototype testing, development testing, qualification testing, and integrated system testing. The first three test phases typically occur at the component level as well as the engine level. The integrated system testing phase is performed at the propulsion system and/or vehicle level.

After an LRE design has completed the qualification program (i.e., production phase), each individual flight engine is acceptance tested by hot-fire to verify that specific engine’s suitability for flight. Prelaunch operational testing is performed prior to engine start and liftoff to verify readiness for launch. Finally, additional testing can be performed or additional data obtained for the system during the operational phase of a test flight or actual space launch mission.

Specific requirements are placed upon qualification, production unit acceptance testing, and integrated system testing. There is an allowance, and in fact an expectation, that engines not meeting the full requirements of qualification will be used as a part of the overall design verification effort to increase the sample size of suitable engines. “Verification engines” include qualification engines and may also include development engines that are structurally and functionally equivalent to the qualification and flight design. Hereafter, references to the qualification requirements are meant to include testing on formal qualification engines as well as development engines that are suitable as verification engines.

This work was approved by Nick Awwad, David Davis, and Thomas Fitzgerald for the Air Force Space Command. For more information, download the Technical Support Package (free white paper) here under the Test & Measurement category. AFSC-0001


This Brief includes a Technical Support Package (TSP).
Evaluation and Test Requirements for Liquid Rocket Engines

(reference AFSC-0001) is currently available for download from the TSP library.

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