Molecular dynamics simulations and experimental measurements were used to investigate the thermal and mechanical properties of cross-linked phenolic resins as a function of the degree of ...cross-linking, the chain motif (ortho–ortho versus ortho–para), and the chain length. The chain motif influenced the type (interchain or intrachain) as well as the amount of hydrogen bonding. Ortho–ortho chains favored internal hydrogen bonding whereas ortho–para favored hydrogen bonding between chains. Un-cross-linked ortho–para systems formed percolating 3D networks of hydrogen bonds, behaving effectively as “hydrogen gels”. This resulted in differing thermal and mechanical properties for these systems. As cross-linking increased, the chain motif, chain length, and hydrogen bonding networks became less important. Elastic moduli, thermal conductivity, and glass transition temperatures were characterized as a function of cross-linking and temperature. Both our own experimental data and literature values were used to validate our simulation results.
Chemical functionalization of carbon nanotubes (CNTs) is critical because it gives them desirable properties such as good solubility in various solvents and compatibility with polymers. We examined ...two approaches to attach hydrophilic groups on the surface of both single-wall carbon nanotubes (SWNTs) and multi-wall carbon nanotubes (MWNTs), thus making them soluble in water and different organic solvents. Our first approach was to covalently attach maleic anhydride groups on the surfaces of high-pressure CO conversion (HiPCO) SWNTs and MWNTs and to study the reaction mechanism. The success of the reaction was demonstrated by solubility tests, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The number of maleic anhydride groups on the CNTs can be easily controlled by reaction time and reagent stoichiometry. Our second approach was a two-step phenylation-sulfonation tandem reaction and was done on non-HiPCO SWNTs with lengths in the range of 5–3 μm and diameters between 1–2 nm. While this process has been shown to be facile with high-pressure CO (HiPCO) SWNTs due to their small diameters, we have demonstrated herein that this process also works on non-HiPCO SWNTs with a diameter significantly above 1 nm. We also conducted a systematic study to examine the influence of different reaction parameters such as concentration, solvent, temperature, and reaction time on the reactivity of SWNTs.
Phenolic resin has extensive heritage as a TPS (Thermal Protection Systems) material, however, alternative resin systems such as Cyanate Ester and Phthalonitrile may offer improved performance ...compared to state-of-the-art phenolic resin. These alternative resin systems may have higher char yield, higher char strength, lower thermal conductivity and improved mechanical properties. In current work at NASA Ames alternative resin systems were uniformly infused into fibrous substrates and preliminary properties characterized. The density of the cyanate ester infused in fibrous substrate ranged from 0.25-0.3 grams per cubic centimeter compared to PICA (Phenolic resin impregnated carbon ablative) having a density of approximately 0.25 grams per cubic centimeter. The density of Phthalonitrile varies from 0.22-0.25 grams per cubic centimeter. Initial formulations of these new resin systems were recently tested at the LARC HyMETs (Hypersonic Materials Environmental Test System) facility to evaluate their performance and data such as back face temperature, char yield, and recession are compared to PICA. Cyanate Ester and Phthalonitrile impregnated carbon ablative samples showed comparable performance to phenolic resin impregnated carbon ablative samples.
The conformal ablative TPS first developed under NASA's Hypersonics Project in the early 2000's demonstrated very low through the thickness conductivity compared to state-ofthe- art PICA. However, in ...initial arcjet testing of Conformal-1, surface recession rates were 2x higher than PICA. Because commercial carbon felts are currently available as very thin substrates, this was a concern if conformal TPS were to be considered for a mission that required thicker material. Discussed in this paper are the results of the development of an Advanced Conformal TPS derived from thicker, higher density carbon felt. Two substrate systems were evaluated, the first material was a needled rayon-based carbon felt and the other a needled PAN-based carbon felt. Both substrates were impregnated with phenolic resin following the PICA/CPICA process to add a low density phenolic matrix to the system prior to aerothermal screening at the LaRC HyMETS facility and larger scale testing in the NASA ARC Interaction Heating Facility (IHF) at heating fluxes ranging from 250-1700 W/cm2.
Ablative thermal protection systems are commonly used as protection from the intense heat during re-entry of a space vehicle and have been used successfully on many missions including Stardust and ...Mars Science Laboratory both of which used PICA - a phenolic based ablator. Historically, phenolic resin has served as the ablative polymer for many TPS systems. However, it has limitations in both processing and properties such as char yield, glass transition temperature and char stability. Therefore alternative high performance polymers are being considered including cyanate ester resin, polyimide, and polybenzoxazine. Thermal and mechanical properties of these resin systems were characterized and compared with phenolic resin.
NASA's future robotic missions to Venus and other planets, namely, Saturn, Uranus, Neptune, result in extremely high entry conditions that exceed the capabilities of current mid density ablators ...(PICA or Avcoat). Therefore mission planners assume the use of a fully dense carbon phenolic heatshield similar to what was flown on Pioneer Venus and Galileo. Carbon phenolic is a robust TPS, however, its high density and thermal conductivity constrain mission planners to steep entries, high fluxes, pressures and short entry durations, in order for CP to be feasible from a mass perspective. The high entry conditions pose certification challenges in existing ground based test facilities. In 2012 the Game Changing Development Program in NASA's Space Technology Mission Directorate funded NASA ARC to investigate the feasibility of a Woven Thermal Protection System to meet the needs of NASA's most challenging entry missions. This presentation will summarize the maturation of the WTPS project.
The Adaptive Deployable Entry and Placement Technology (ADEPT), a mechanically deployable entry vehicle technology, has been under development at NASA since 2011. As part of the technical maturation ...of ADEPT, designs capable of delivering small payloads (10 kg) are being considered to rapidly mature sub 1 m deployed diameter designs. The unique capability of ADEPT for small payloads comes from its ability to stow within a slender volume and deploy to achieve a mass efficient drag surface with a high heat rate capability. The low ballistic coefficient results in entry heating and mechanical loads that can be met by a revolutionary three-dimensionally woven carbon fabric supported by a deployable skeleton structure. This carbon fabric has test proven capability as both primary structure and payload thermal protection system. In order to rapidly advance ADEPTs technical maturation, the project is developing test methods that enable thermostructural design requirement verification of ADEPT designs at the system level using ground test facilities. Results from these tests are also relevant to larger class missions and help us define areas of focused component level testing in order to mature material and thermal response design codes. The ability to ground test sub 1 m diameter ADEPT configurations at or near full-scale provides significant value to the rapid maturation of this class of deployable entry vehicles. This paper will summarize arc jet test results, highlight design challenges, provide a summary of lessons learned and discuss future test approaches based upon this methodology.
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