Room Temperature Tensile Behavior and Damage Accumulation of Hi-Nicalon Reinforced SiC Matrix Composites

Composites consisting of woven Hi-Nicalon fibers, BN interphases, and different SiC matrices were studied in tension at room temperature. Composites with SiC matrices processed by CVI and melt infiltration were compared. Monotonic and load/unload/reload tensile hysteresis experiments were performed. A modal acoustic emission (AE) analyzer was used to monitor damage accumulation during the tensile test. Post test polishing of the tensile gage sections was performed to determine the extent of cracking. The occurrence and location of cracking could easily be determined using modal AE. The loss of modulus could also effectively be determined from the change in the velocity of sound across the sample. Finally, the stresses where cracks appear to intersect the load-bearing fibers correspond with high temperature low cycle fatigue run out stresses for these materials

Nicalon/siliconoxycarbide ceramic composites

A series of silsesquioxane copolymers was synthesized by acid hydrolysis and condensation of trimethoxysilanes of the form RSi(OCH3)3, where R = methyl or phenyl. By varying pH, water/methoxy and methyl/phenyl ratios, the molecular structure, polymer rheology and ceramic composition can be controlled. The polymers form an amorphous siliconoxycarbide on pyrolysis. Composites of Nicalon/siliconoxycarbide were fractured in four-point flexure and in tension to evaluate the influence of matrix composition, final fabrication temperature and use of filler on composite mode of failure, modulus, strain capability and strength. Incorporation of filler was found to increase matrix compressive strength. Employment of processing temperatures of 1375 to 1400 C enhanced strain to failure and reduced the tendency toward brittle fracture. Mixed mode (compression/shear and tension/shear) failures were observed in flexural samples processed to the higher temperatures, giving rise to nonlinear stress-strain curves. Tensile samples pyrolyzed to 1400 C showed linear-elastic behavior and failed by fracture of fiber bundles. Matrix material was found to be adherent to the fiber surface after failure. These results demonstrate the need for tensile testing to establish composite behavior

Polymeric routes to silicon carbide and silicon oxycarbide CMC

An overview of two approaches to the formation of ceramic composite matrices from polymeric precursors is presented. Copolymerization of alkyl- and alkenylsilanes (RSiH3) represents a new precursor system for the production of Beta-SiC on pyrolysis, with copolymer composition controlling polymer structure, char yield, and ceramic stoichiometry and morphology. Polysilsesquioxanes which are synthesized readily and can be handled in air serve as precursors to Si-C-O ceramics. Copolymers of phenyl and methyl silsesquioxanes display rheological properties favorable for composite fabrication; these can be tailored by control of pH, water/methoxy ratio and copolymer composition. Composites obtained from these utilize a carbon coated, eight harness satin weave Nicalon cloth reinforcement. The material exhibits nonlinear stress-strain behavior in tension

LEO to GEO (and Beyond) Transfers Using High Power Solar Electric Propulsion (HP-SEP)

Rideshare, or Multi-Payload launch configurations, are becoming more and more commonplace but access to space is only one part of the overall mission needs. The ability for payloads to achieve their target orbits or destinations can still be difficult and potentially not feasible with on-board propulsion limitations. The High Power Solar Electric Propulsion (HP-SEP) Orbital Maneuvering Vehicle (OMV) provides transfer capabilities for both large and small payload in excess of what is possible with chemical propulsion. Leveraging existing secondary payload adapter technology like the ESPA provides a platform to support Multi-Payload launch and missions. When coupled with HP-SEP, meaning greater than 30 kW system power, very large delta-V maneuvers can be accomplished. The HP-SEP OMV concept is designed to perform a Low Earth Orbit to Geosynchronous Orbit (LEO-GEO) transfer of up to six payloads each with 300kg mass. The OMV has enough capability to perform this 6 kms maneuver and have residual capacity to extend an additional transfer from GEO to Lunar orbit. This high deltaV capability is achieved using state of the art 12.5kW Hall Effect Thrusters (HET) coupled with high power roll up solar arrays. The HP-SEP OMV also provides a demonstration platform for other SEP technologies such as advanced Power Processing Units (PPU), Xenon Feed Systems (XFS), and other HET technologies. The HP-SEP OMV platform can be leveraged for other missions as well such as interplanetary science missions and applications for resilient space architectures

Solar vs. Fission Surface Power for Mars

A multi-discipline team of experts from the National Aeronautics and Space Administration (NASA) developed Mars surface power system point design solutions for two conceptual missions. The primary goal of this study was to compare the relative merits of solar- versus fission-powered versions of each surface mission. First, the team compared three different solar power options against a fission power system concept for a sub-scale, uncrewed demonstration mission. The 4.5 meter (m) diameter pathfinder lander's primary mission would be to demonstrate Mars entry, descent, and landing techniques. Once on the Martian surface, the lander's In Situ Resource Utilization (ISRU) payload would demonstrate liquid oxygen propellant production using atmospheric resources. For the purpose of this exercise, location was assumed to be at the Martian equator. The three solar concepts considered included a system that only operated during daylight hours (at roughly half the daily propellant production rate of a round-the-clock fission design), a battery-augmented system that operated through the night (matching the fission concept's propellant production rate), and a system that operated only during daylight, but at a higher rate (again, matching the fission concept's propellant production rate). Including 30% mass growth allowance, total payload masses for the three solar concepts ranged from 1,116 to 2,396 kg, versus the 2,686 kg fission power scheme. However, solar power masses are expected to approach or exceed the fission payload mass at landing sites further from the equator, making landing site selection a key driver in the final power system decision. The team also noted that detailed reliability analysis should be performed on daytime-only solar power schemes to assess potential issues with frequent ISRU system on/off cycling. Next, the team developed a solar-powered point design solution for a conceptual four-crew, 500-day surface mission consisting of up to four landers per crewed expedition mission. Unlike the demonstration mission, a lengthy power outage due to the global dust storms that are known to occur on Mars would pose a safety hazard to a crewed mission. A similar fission versus solar power trade study performed by NASA in 2007 concluded that fission power was more reliable-with a much lower mass penalty-than solar power for this application. However, recent advances in solar cell and energy storage technologies and changes in operational assumptions prompted NASA to revisit the analysis. For the purpose of this exercise a particular landing site at Jezero Crater, located at 18o north latitude, was assumed. A fission power system consisting of four each 10 kW Kilopower fission reactors was compared to a distributed network of Orion-derived Ultraflex solar arrays and Lithium ion batteries mounted on every lander. The team found that a solar power system mass of about 9,800 kg would provide the 22 kilowatts (kW) keep-alive power needed to survive a dust storm lasting up to 120-days at average optical depth of 5, and 35 kW peak power for normal operations under clear skies. Although this is less than half the mass estimated during the 2007 work (which assumed latitudes up to 30o) it is still more than the 7,000 kg mass of the fission system which provides full power regardless of dust storm conditions