The oxidizer-rich staged combustion (ORSC) and full-flow staged combustion (FFSC) rocket engines currently under development offer dramatic improvements in fuel efficiency and thrust over the traditional gas generator engine cycle. ORSC and FFSC power cycles use a preburner to produce an oxidizer-rich stream of combustion gases that drives the turbine, then burns with the remaining fuel in the main combustion chamber. However, operating the preburner of a rocket engine under oxidizer-rich conditions presents significant materials engineering challenges; most materials are susceptible to ignition and combustion under a high-pressure oxygen-rich environment. One of the ignition mechanisms of greatest concern to rocket engines is frictional ignition due to rubbing of high-speed rotating components in high-pressure oxygen-rich environment. Two notable recent launch failures, Sea Launch?s NSS-8 and Orbital?s Orb-3, are believed to have arisen from frictional ignition of metals. This work focuses on determining the physical mechanisms that drive frictional ignition of engineering alloys in high-pressure oxygen. Experiments on high-speed sliding wear suggest that frictional ignition results from the onset of severe oxidational wear, corresponding to a breakdown of a lubricating oxide tribolayer at the rubbing interface. The underlying causes of this wear transition are poorly understood and likely vary according to the specific material system. This proposal will address this gap in understanding, revealing, for the first time, the effects of test conditions and alloy chemistry on the properties, structure, and stability of oxide tribolayers that form on several important aerospace engineering alloys. These insights will connect micro-scale structural evolution processes with macro-scale materials phenomena, such as frictional ignition, with important scientific implications for the fields of tribology, mechanochemistry, and physical metallurgy. This understanding could be used by materials scientists to design new alloys resistant to catastrophic frictional ignition, and by rocket engine designers to implement design and manufacturing modifications that reduce the risk of engine failure due to unintended ignition of metal engine components. These developments will enable robust reusable launch vehicles that will serve as the foundation for key emerging space technologies, such as satellite mega-constellations for high-speed space-based internet, reliable interplanetary crew and commerce transport, and large in-space RF telescopes for low-frequency radio astronomy, all of which promise to transform commerce and our understanding of the universe.
Predicting the conditions under which oxide tribolayers break down during high-speed sliding wear remains a significant challenge because the underlying mechanisms that control the growth, properties, and thermomechanical stability of tribolayers are poorly understood. This is partly because oxide tribolayers appear to grow via an iterative transient oxidation process, which complicates direct comparison with oxide scales formed during conventional static oxidation. Using a combination of wear experiments, contact mechanics theory, and metallurgical thermochemistry, this proposed research will test the hypothesized relationship between the onset of severe oxidational wear and frictional ignition, as well as reveal the mechanisms of oxide breakdown during high-speed sliding. The project will begin with high-speed wear testing experiments to characterize the friction, wear, and ignition behaviors of several important model materials under high-pressure oxygen environments. Next, the wear surface and mechanical properties of recovered samples will be characterized ex situ to gain insight into the micro-scale mechanisms that drive oxide breakdown. Finally, the experimental results will be combined with physics-based models of frictional heating, oxide growth, and contact mechanics in order to determine the relationship between oxide breakdown and frictional ignition. The results of this work will reveal an important relationship between tribolayer breakdown and frictional ignition in mission-critical aerospace alloys, as well as the physical mechanisms that drive oxide tribolayer growth and degradation at the onset of severe oxidational wear.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.