Combustion Stability Characteristics of the Project Morpheus Liquid Oxygen/Liquid Methane Main Engine

The Project Morpheus liquid oxygen (LOX) / liquid methane rocket engines demonstrated acousticcoupled combustion instabilities during sealevel groundbased testing at the NASA Johnson Space Center (JSC) and Stennis Space Center (SSC). Highamplitude, 1T, 1R, 1T1R (and higher order) modes appear to be triggered by injector conditions. The instability occurred during the Morpheusspecific engine ignition/start sequence, and did demonstrate the capability to propagate into mainstage. However, the instability was never observed to initiate during mainstage, even at low power levels. The Morpheus main engine is a JSCdesigned ~5,000 lbfthrust, 4:1 throttling, pressurefed cryogenic engine using an impinging element injector design. Two different engine designs, named HD4 and HD5, and two different builds of the HD4 engine all demonstrated similar instability characteristics. Through the analysis of more than 200 hot fire tests on the Morpheus vehicle and SSC test stand, a relationship between ignition stability and injector/chamber pressure was developed. The instability has the distinct characteristic of initiating at high relative injection pressure drop (dP) at low chamber pressure (Pc); i.e., instabilities initiated at high dP/Pc at low Pc during the start sequence. The high dP/Pc during start results during the injector /chamber chillin, and is enhanced by hydraulic flip in the injector orifice elements. Because of the fixed mixture ratio of the existing engine design (the main valves share a common actuator), it is not currently possible to determine if LOX or methane injector dP/Pc were individual contributors (i.e., LOX and methane dP/Pc typically trend in the same direction within a given test). The instability demonstrated initiation characteristic of starting at or shortly after methane injector chillin. Colder methane (e.g., subcooled) at the injector inlet prior to engine start was much more likely to result in an instability. A secondary effect of LOX subcooling was also possibly observed; greater LOX sub cooling improved stability. Some tests demonstrated a lowamplitude 1L1T instability prior to LOX injector chillin. The Morpheus main engine also demonstrated chug instabilities during some engine shutdown sequences on the flight vehicle and SSC test stand. The chug instability was also infrequently observed during the startup sequence. The chug instabilities predictably initiated at low dP/Pc at low Pc. The chug instabilities were always selflimiting; startup chug instabilities terminated during throttleup and shutdown chug instabilities decayed by shutdown termination

Combustion Stability Characteristics of the Project Morpheus Liquid Oxygen / Liquid Methane Main Engine

The project Morpheus liquid oxygen (LOX) / liquid methane (LCH4) main engine is a Johnson Space Center (JSC) designed ~5,000 lbf-thrust, 4:1 throttling, pressure-fed cryogenic engine using an impinging element injector design. The engine met or exceeded all performance requirements without experiencing any in- ight failures, but the engine exhibited acoustic-coupled combustion instabilities during sea-level ground-based testing. First tangential (1T), rst radial (1R), 1T1R, and higher order modes were triggered by conditions during the Morpheus vehicle derived low chamber pressure startup sequence. The instability was never observed to initiate during mainstage, even at low power levels. Ground-interaction acoustics aggravated the instability in vehicle tests. Analysis of more than 200 hot re tests on the Morpheus vehicle and Stennis Space Center (SSC) test stand showed a relationship between ignition stability and injector/chamber pressure. The instability had the distinct characteristic of initiating at high relative injection pressure drop at low chamber pressure during the start sequence. Data analysis suggests that the two-phase density during engine start results in a high injection velocity, possibly triggering the instabilities predicted by the Hewitt stability curves. Engine ignition instability was successfully mitigated via a higher-chamber pressure start sequence (e.g., ~50% power level vs ~30%) and operational propellant start temperature limits that maintained \cold LOX" and \warm methane" at the engine inlet. The main engine successfully demonstrated 4:1 throttling without chugging during mainstage, but chug instabilities were observed during some engine shutdown sequences at low injector pressure drop, especially during vehicle landing

Magnetic hopfions in solids

Hopfions are an intriguing class of string-like solitons, named according to a classical topological concept classifying three-dimensional direction fields. The search of hopfions in real physical systems is going on for nearly half a century, starting with the seminal work of Faddeev. But so far realizations in solids are missing. Here, we present a theory that identifies magnetic materials featuring hopfions as stable states without the assistance of confinement or external fields. Our results are based on an advanced micromagnetic energy functional derived from a spin-lattice Hamiltonian. Hopfions appear as emergent particles of the classical Heisenberg model. Magnetic hopfions represent three-dimensional particle-like objects of nanometre-size dimensions opening the gate to a new generation of spintronic devices in the framework of a truly three-dimensional architecture. Our approach goes beyond the conventional phenomenological models. We derive material-realistic parameters that serve as concrete guidance in the search of magnetic hopfions bridging computational physics with materials science

Nonlinear Stability of Static N\'eel Walls in Ferromagnetic Thin Films

In this paper, the nonlinear (orbital) stability of static 180^\circ N\'eel walls in ferromagnetic films, under the reduced wave-type dynamics for the in-plane magnetization proposed by Capella, Melcher and Otto [CMO07], is established. It is proved that the spectrum of the linearized operator around the static N\'eel wall lies in the stable complex half plane with non-positive real part. This information is used to show that small perturbations of the static N\'eel wall converge to a translated orbit belonging to the manifold generated by the static wall.Comment: 45 page

A Sensor Failure Simulator for Control System Reliability Studies

A real-time Sensor Failure Simulator (SFS) was designed and assembled for the Advanced Detection, Isolation, and Accommodation (ADIA) program. Various designs were considered. The design chosen features an IBM-PC/XT. The PC is used to drive analog circuitry for simulating sensor failures in real-time. A user defined scenario describes the failure simulation for each of the five incoming sensor signals. Capabilities exist for editing, saving, and retrieving the failure scenarios. The SFS has been tested closed-loop with the Controls Interface and Monitoring (CIM) unit, the ADIA control, and a real-time F100 hybrid simulation. From a productivity viewpoint, the menu driven user interface has proven to be efficient and easy to use. From a real-time viewpoint, the software controlling the simulation loop executes at greater than 100 cycles/sec

Energy transfer mechanism in Gd_2(SiO_4)O:Ce scintillators

The scintillation decay of cerium-doped gadolinium oxyorthosilicate Gd_2(SiO_4)O:Ce is lengthened by the energy transfer from Gd to Ce. To investigate the role of the Gd in the scintillation processes, the Gd was partially replaced by optically inactive rare earth elements, Y and Lu, and the effective transfer rates from Gd to Ce were measured as a function of Gd and Ce concentrations using UV- and gamma-ray excitations. The data clearly indicate the dilution of the Gd by the Y and the Lu further lengthens the migration time through the Gd in the energy transfer process from Gd to Ce

Energy Transfer Mechanism In Gd_2(SiO_4)O:Ce Scintillators

The scintillation decay of cerium-doped gadolinium oxyorthosilicate Gd_2(SiO_4)O:Ce is lengthened by the energy transfer from Gd to Ce. To investigate the role of the Gd in the scintillation processes, the Gd was partially replaced by optically inactive rare earth elements, Y and Lu, and the effective transfer rates from Gd to Ce were measured as a function of Gd and Ce concentrations using UV- and gamma-ray excitations. The data clearly indicate the dilution of the Gd by the Y and the Lu further lengthens the migration time through the Gd in the energy transfer process from Gd to Ce

Light emission mechanism of Lu_2(SiO_4)O:Ce

Lutetium oxyorthosilicate Lu_2(SiO_4)O has two crystallographically independent lutetium sites. When it is doped with Ce, the dopant Ce is assumed to occupy two different sites, and thus the existence of two activation centers is proposed. UV and gamma-ray excited spectra clearly indicate that two activation centers exist in cerium-doped lutetium oxyorthosilicate Lu_2(SiO_4)O:Ce (LSO)