69,462 research outputs found
Design, cost, and advanced technology applications for a military trainer aircraft
The potential impact is examined of advanced aerodynamic and propulsive technologies in terms of operating and acquisition costs on conceptual mission and performance requirements for a future undergraduate jet pilot trainer aircraft
Resonant Tunneling in Truly Axial Symmetry Mn12 Single-Molecule Magnets: Sharp Crossover between Thermally Assisted and Pure Quantum Tunneling
Magnetization measurements of a truly axial symmetry Mn12-tBuAc molecular
nanomagnet with a spin ground state of S = 10 show resonance tunneling. This
compound has the same magnetic anisotropy as Mn12-Ac but the molecules are
better isolated and the crystals have less disorder and a higher symmetry.
Hysteresis loop measurements at several temperatures reveal a well-resolved
step fine-structure which is due to level crossings of excited states. All step
positions can be modeled by a simple spin Hamiltonian. The crossover between
thermally assisted and pure quantum tunneling can be investigated with
unprecedented detail.Comment: 5 pages, 6 figure
Countryside Survey 2000 Module 8: Airborne Scanner Applications. Classification of airborne CASI and LIDAR data of selected CS2000 sample squares. Final Report
The optical counterpart of SAX J1808.4-3658, the transient bursting millisecond X-ray pulsar
A set of CCD images have been obtained during the decline of the X-ray
transient SAX J1808.4-3658 during April-June 1998. The optical counterpart has
been confirmed by several pieces of evidence. The optical flux shows a
modulation on several nights which is consistent with the established X-ray
binary orbit period of 2 hours. This optical variability is roughly in
antiphase with the weak X-ray modulation. The source mean magnitude of V=16.7
on April 18 declined rapidly after April 22. From May 2 onwards the magnitude
was more constant at around V=18.45 but by June 27 was below our sensitivity
limit. The optical decline precedes the rapid second phase of the X-ray
decrease by 3 +/- 1 days. The source has been identified on a 1974 UK Schmidt
plate at an estimated magnitude of ~20. The nature of the optical companion is
discussed.Comment: 5 pages, 3 figures; published in MNRAS, March 15th 199
Integration of a mean-torque diesel engine model into a hardware-in-the-loop shipboard network simulation using lambda tuning
This study describes the creation of a hardware-in-the-loop (HIL) environment for use in evaluating network architecture, control concepts and equipment for use within marine electrical systems. The environment allows a scaled hardware network to be connected to a simulation of a multi-megawatt marine diesel prime mover, coupled via a synchronous generator. This allows All-Electric marine scenarios to be investigated without large-scale hardware trials. The method of closing the loop between simulation and hardware is described, with particular reference to the control of the laboratory synchronous machine, which represents the simulated generator(s). The fidelity of the HIL simulation is progressively improved in this study. First, a faster and more powerful field drive is implemented to improve voltage tracking. Second, the phase tracking is improved by using two nested proportional–integral–derivative–acceleration controllers for torque control, tuned using lambda tuning. The HIL environment is tested using a scenario involving a large constant-power load step. This provides a very severe test of the HIL environment, and also reveals the potentially adverse effects of constant-power loads within marine power systems
Apsidal advance in SS 433?
Context. The Galactic microquasar SS 433 launches oppositely directed jets at
speeds approximately a quarter of the speed of light. Both the speed and
direction of the jets exhibit small fluctuations. A component of the speed
variation has 13 day periodicity and the orbital phase at which its maximum
speed occurs has advanced approximately 90 degrees in 25 years. Aims. To
examine the possibility that these variations are associated with a mildly
eccentric orbit and conditions necessary to achieve this apsidal advance.
Methods. The advance of the orbital phase for maximum speed is taken to be
advance of the apses of the putative elliptical orbit. It is compared with
calculations of the effects of tides induced in the companion and also with
gravitational perturbations from the circumbinary disc. These calculations are
made in the light of recent results on the SS 433 system. Results. The 13 day
periodicity in the speed of the jets of SS 433 might be attributed to a mildly
elliptical orbit, through periodic approaches of the donor and the compact
object. Advance of the apses of such an elliptical orbit due to tidal effects
induced in a normal companion looks to be to small; if caused by the
circumbinary disc the mass of the inner regions of that disc is ~ 0.15 solar
masses.Comment: 2 pages, no figures To appear in A & A. No significant changes from
original version poste
Photovoltaic array: Power conditioner interface characteristics
The electrical output (power, current, and voltage) of flat plate solar arrays changes constantly, due primarily to changes in cell temperature and irradiance level. As a result, array loads such as dc-to-ac power conditioners must be capable of accommodating widely varying input levels while maintaining operation at or near the maximum power point of the array. The array operating characteristics and extreme output limits necessary for the systematic design of array load interfaces under a wide variety of climatic conditions are studied. A number of interface parameters are examined, including optimum operating voltage, voltage energy, maximum power and current limits, and maximum open circuit voltage. The effect of array degradation and I-V curve fill factor or the array power conditioner interface is also discussed. Results are presented as normalized ratios of power conditioner parameters to array parameters, making the results universally applicable to a wide variety of system sizes, sites, and operating modes
Advanced thermoplastic resins, phase 2
High temperature structural resins are required for use on advanced aerospace vehicles as adhesives and composite matrices. NASA-Langley developed polyimide resins were evaluated as high temperature structural adhesives for metal to metal bonding and as composite matrices. Adhesive tapes were prepared on glass scrim fabric from solutions of polyamide acids of the semicrystalline polyimide LARC-CPI, developed at the NASA-Langley Research Center. Using 6Al-4V titanium adherends, high lap shear bond strengths were obtained at ambient temperature (45.2 MPa, 6550 psi) and acceptable strengths were obtained at elevated temperature (14.0 MPa, 2030 psi) using the Pasa-Jell 107 conversion coating on the titanium and a bonding pressure of 1.38 MPa (200 psi). Average zero degree composite tensile and compressive strengths of 1290 MPa (187 ksi) and 883 MPa (128 ksi) respectively were obtained at ambient temperature with unsized AS-4 carbon fiber reinforcement
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