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NASA NTRS1992

Implementation of the TOMS contamination control requirements in the former USSR

The American Total Ozone Mapping Spectrometer (TOMS) was integrated with the Russian Meteor-3 spacecraft and launched on August 15, 1991. Although the TOMS instrument was sensitive to both particulate and molecular contamination, the program for Meteor-3 had not formerly addressed contamination control in ground operations. In order to accommodate the TOMS cleanliness requirements, a contamination control program was successfully established from inception at both the Meteor-3 spacecraft plant near Moscow and at the launch site in Plesetsk.

Abrams, Eve M.↗

NASA NTRS1992

Near-resonant excitation and propagation of eccentric density waves by external forcing

An overview is presented of the astronomical evidence that relatively massive, distended, gaseous disks form as a natural by-product of the process of star formation, and also the numerical evidence that SLING-amplified eccentric modes in the outer parts of such disks can drive one-armed spiral density waves in the inner parts by near-resonant excitation and propagation. An ordinary differential equation (ODE) of the second order that approximately governs the nonlocalized forcing of waves in a disk satisfying Lindblad resonance almost everywhere is derived. When transformed and appended with an extra model term, this ODE implies, for free waves, the usual asymptotic results of the WKBJ dispersion relationship and the propagation Goldreich-Tremaine (1978) formula for the resonant torque exerted on a localized Lindblad resonance. An analytical solution is given for the rate of energy and angular momentum transfer by nonlocalized near-resonant forcing in the case when the disk has power-law dependences on the radius of the surface density and temperature.

Ostriker, Eve C.↗

NASA NTRS1993

The collapse of clouds and the formation and evolution of stars and disks

We consider the interrelationships among the structure of molecular clouds; the collapse of rotating cloud cores; the formation of stars and disks; the origin of molecular outflows, protostellar winds, and highly collimated jets; the birth of planetary and binary systems; and the dynamics of star/disk/satellite interactions. Our discussion interweaves theory with the results of observations that span from millimeter wavelengths to X-rays.

Shu, Frank↗

NASA NTRS1994

Miniature, Cooled Pressure-Measuring Probe

Probe designed to reduce settling time dramatically. Pressure-sensing transducer mounted in probe and connected to tip by short tube having cross-sectional area substantially smaller than conventional connecting tubes. Probe includes stainless-steel cylindrical exterior housing holding closed pressure chamber in which piezoelectric pressure transducer mounted. Open connecting tube passes portion of high-velocity, high-temperature fluid stream into closed pressure chamber. Any change of pressure in sampled stream propagates into closed pressure chamber with settling time inversely proportional to cross-sectional area of connecting tube. Cooling chamber formed around pressure chamber connected to source of water or other cooling fluid via inlet and outlet tubes.

Ashby, George C., Jr.↗

NASA NTRS1994

Fixture For Calibrating Pressure Probe

Fixture in form of specially designed clamshell housing enables in situ calibration of pressure transducer mounted in body of pressure probe in wind tunnel. Includes two metal half shells machined with necks and matching cavities, when put together, define larger neck and cavity accommodating probe. Probe secured to bottom half shell by use of clamp before installing top half shell: necessary to follow sequence to protect probe during assembly. Clamshell calibration fixture attached to pressure probe in few minutes, making it possible to calibrate pressure transducer at convenient times. Calibrations performed before and after wind-tunnel runs each day, between runs in event of delays or suspected malfunctions, and essentially any other time, without having to remove probe from wind tunnel.

Ashby, George C., Jr.↗

NASA NTRS1992

A knowledge based system for scientific data visualization

A knowledge-based system, called visualization tool assistant (VISTA), which was developed to assist scientists in the design of scientific data visualization techniques, is described. The system derives its knowledge from several sources which provide information about data characteristics, visualization primitives, and effective visual perception. The design methodology employed by the system is based on a sequence of transformations which decomposes a data set into a set of data partitions, maps this set of partitions to visualization primitives, and combines these primitives into a composite visualization technique design. Although the primary function of the system is to generate an effective visualization technique design for a given data set by using principles of visual perception the system also allows users to interactively modify the design, and renders the resulting image using a variety of rendering algorithms. The current version of the system primarily supports visualization techniques having applicability in earth and space sciences, although it may easily be extended to include other techniques useful in other disciplines such as computational fluid dynamics, finite-element analysis and medical imaging.

Senay, Hikmet↗

NASA NTRS1994

Magnetocentrifugally driven flows from young stars and disks. 1: A generalized model

We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = R(sub x), where the Keplerian angular speed of rotation Omega(sub x) equals the angular speed of the star Omega(sub *). For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, R(sub x) exceeds the radius of the star R(sub *) by a factor of a few, and the inner disk is effectively truncated at a radius R(sub t) somewhat smaller than R(sub x). Where the closed field lines between R(sub t) and R(sub x) bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as R(sub x) remains significantly greater than R(sub *). Exterior to R(sub x) field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate M(sub w) equal to a definite fraction f of the disk accretion rate M(sub D). For high disk accretion rates, R(sub x) is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.

Shu, Frank↗

NASA NTRS1997

Initial contamination control considerations for the Next Generation Space Telescope (NGST)

The NASA's Next Generation Space Telescope (NGST), a 6 m to 8 m diameter device, is described. The NGST will be radiatively cooled to 30 K to 60 K in order to carry out extremely deep exposures at near infrared wavelengths, for 5 years to 10 years lifetime. Initial contamination control design options include strict materials selection and baking out of both hardware down to the component level, minimizing or eliminating the exposure to the optical telescope assembly to sunlight or earth albedo during deployment and early in-orbit operations.

Wooldridge, Eve M.↗

NASA NTRS1998

Contamination Control Considerations for the Next Generation Space Telescope (NGST)

The NASA Space Science Program, in its ongoing mission to study the universe, has begun planning for a telescope that will carry on the Hubble Space Telescope's exploration. This telescope, the 'Next Generation Space Telescope' (NGST), will be 6-8 meters in diameter, will be radiatively cooled to 30-60 Kelvin in order to enable extremely deep exposures at near infrared wavelengths, and will operate for a lifetime of 5-10 years. The requirement will be to measure wavelengths from 1-5 microns, with a goal to measure wavelengths from 0.6-30 microns. As such, NGST will present a new contamination control challenge. The Goddard Space Flight Center (GSFC) performed one of three preliminary feasibility studies for the NGST, presenting a telescope with an 8 meter, deployable primary mirror and a deployable secondary mirror. The telescope would be radiatively cooled, with the optical telescope assembly (OTA) and the science instrument module (SIM) isolated from the warmer spacecraft support module (SSM). The OTA and the SIM would also be shielded from sunlight with an enormous, inflatable sun-shield. The GSFC telescope was designed for launch on an Atlas HAS, which would require launching the telescope in a stowed configuration, with the SSM, antennae, sun-shield, primary mirror 'petals', and secondary mirror deployed once on-orbit. The launch configuration and deployment scenario of an exposed telescope measuring near infrared and cooled to 30-60 K are the factors presenting contamination hazards to the NGST mission. Preliminary science requirements established are: less than 20% reflectance decrease on optical surfaces over the wavelength range, and less than 0.3% obscuration of optical surfaces. In order to meet these requirements, NGST must be built and launched with careful attention to contamination control. Initial contamination control design options include strict selecting of materials and baking out of hardware down to the component level, minimizing or eliminating exposure of the OTA to sunlight or earth albedo during deployment and early on-orbit operations, cleaning of the primary and secondary mirrors at the launch site, cleaning of the launch vehicle fairing, locating thrusters and vents on the warm side of the sun shield only, and the possibility of including a deployable cover if that is shown to be necessary.

Wooldridge, Eve M.↗

NASA NTRS1998

Initial Evaluation of Space Environmental Effects on the NGST Sunshield

The "Next Generation Space Telescope" (NGST), the follow-on telescope to the Hubble Space Telescope, will carry on exploration of the early universe with a primary mirror 6-8 meters in diameter optimized to operate in the infrared. The mirror and its instruments will perform extremely deep exposures at near infra-red wavelengths (0.5-30 microns), and will operate for 5-10 years. In order to achieve the requirements, cryogenic temperatures between 30-60 Kelvin must be maintained on the telescope (OTA) and in the science module (SIM). A primary feature for passive cooling in the designs presented is that of an enormous, light-weight deployable sunshield. As a result, issues of contamination from the sunshield and space environmental effects on the sunshield itself present a critical matter: if the sunshield becomes a source of contamination, or if environmental effects damage the sunshield, the NGST mission could be compromised or could fail completely. A molecular redistribution analysis has been performed on the Goddard Space Flight Center (GSFC) design for NGST. The analysis revealed that because the shield will initially cool down faster than the OTA, the shield would not be a significant source of molecular contamination during the cooling phase. However, if the shield were ever to warm up, it would be a very large source of molecular contamination. The sunshield itself is susceptible to degradation from an external source of contamination: the space environment at L2 or at 1 x 3 AU. It is therefore necessary to design the sunshield to withstand the space environment. Thin films and coatings on the sunshield have been evaluated and testing has begun so that a suitable film and/or coating can be chosen or developed for the NGST mission. The evaluation and test results will be presented.

Wooldridge, Eve M.↗

NASA NTRS2001

Effects of Manufacturing and Deployment on Thin Films for the NGTS Sunshade

The Next Generation Space Telescope (NGST) is being developed as an advanced astronomical observatory. The NGST proposes to utilize several thin film membrane layers to create a shield for protection of the telescope from solar thermal energy and stray light, The shield will take the form of a polygon, approximately 15 x 30 m, with individual membrane layers positioned so that they do not come in contact with one another. The membrane shield will be deployed and supported by a series of booms, which will be packed into a small volume for launch. Finally, the shield will be deployed on orbit. Several film materials are being considered for the membrane shield, including CPI, Kapton E, Kapton RN, and Upilex. Each of these polyimide materials was tested to determine their durability over the 10-year mission. New facets of materials testing have been introduced in this study to develop performance data with greater realism to actual use, particularly that of degradation from packing, launch and deployment processing. Materials were exposed to handling that simulated the life of the materials from manufacture through deployment with standardized fixtures and then exposed to a simulated, L2, 10-year radiation environment. Mechanical and thermal radiative properties were measured before and after each phase of testing. This paper summarizes the program and test results.

Wooldridge, Eve M.↗

NASA NTRS2004

Evolutionary Models of Cold, Magnetized, Interstellar Clouds

We modeled the long-term and small-scale evolution of molecular clouds using direct 2D and 3D magnetohydrodynamic (MHD) simulations. This work followed up on previous research by our group under auspices of the ATP in which we studied the energetics of turbulent, magnetized clouds and their internal structure on intermediate scales. Our new work focused on both global and smallscale aspects of the evolution of turbulent, magnetized clouds, and in particular studied the response of turbulent proto-cloud material to passage through the Galactic spiral potential, and the dynamical collapse of turbulent, magnetized (supercritical) clouds into fragments to initiate the formation of a stellar cluster. Technical advances under this program include developing an adaptive-mesh MHD code as a successor to ZEUS (ATHENA) in order to follow cloud fragmentation, developing a shearing-sheet MHD code which includes self-gravity and externally-imposed gravity to follow the evolution of clouds in the Galactic potential, and developing radiative transfer models to evaluate the internal ionization of clumpy clouds exposed to external photoionizing UV and CR radiation. Gammie's work at UIUC focused on the radiative transfer aspects of this program.

Gammie, Charles F.↗

NASA NTRS2005

Crosslinking Amine-Modified Silica Aerogels with Epoxies: Mechanically Strong Lightweight Porous Materials

The mesoporous surfaces of TMOS-derived silica aerogels have been modified with amines by co-polymerization of TMOS with APTES. The amine sites have become anchors for crosslinking the nanoparticles of the skeletal backbone of the aerogel by attachment of di-, tri and tetra-functional epoxies. The resulting conformal coatings increase the density of the native aerogels by a factor of 2-3 but the strength of the resulting materials may increase by more than two orders of magnitude. Processing variables such as amount of APTES used to make the gels, the epoxy type and concentration used for crosslinking, as well as the crosslinking temperature and time were varied according to a multivariable design-of-experiments (DOE) model. It was found that while elastic modulus follows a similar trend with density, maximum strength is attained neither at the maximum density nor at the highest concentration of -NH2 groups, suggesting surface saturation effects. Aerogels crosslinked with the tri-functional epoxide always show improved strength compared with aerogels crosslinked with the other two epoxides under identical conditions. Solid C-13 NMR studies show residual unreacted epoxides, which condense with ne another by heating crosslinked aerogels at 150 C.

Meador, Mary Ann B.↗

NASA NTRS2004

Mechanically Strong Aerogels Formed by Templated Growth of Polymer Cross- Linkers on Inorganic Nanoparticles

In the search for materials with better mechanical, thermal, and electrical properties, it is becoming evident that oftentimes dispersing ceramic nanoparticles in plastics improves performance. Along these lines, chemical bonding (both covalent and noncovalent) between a filler and a polymer improves their compatibility, and thus enhances certain properties of the polymeric matrix above and beyond what is accomplished by simple doping with the filler. When a similarly sized dopant and matrix are used, elementary building blocks may also have certain distinct advantages (e.g., in catalysis). In this context, researchers at the NASA Glenn Research Center reasoned that in the extreme case, where the dopant and the matrix (e.g., a filler and a polymer) are not only sized similarly, but their relative amounts are comparable, the relative roles of the dopant and matrix can be reversed. Then, if the "filler," or a certain form thereof, possesses desirable properties of its own, such properties could be magnified by cross-linking with a polymer. We at Glenn have identified silica as such a filler in its lowest-density form, namely the silica aerogel.

Leventis, Nicholas↗

NASA NTRS2001

Effects of Vacuum Ultraviolet Radiation on Thin Polyimide Films Evaluated

NASA anticipates launching the Next Generation Space Telescope (NGST) mission-- whose purpose is to examine the origins of our universe by making measurements in the infrared portion of the spectrum--in 2009. So that the telescope can operate at very low temperatures (less than 100 K), a halo orbit about the second Lagrangian point (L2) is being considered because it is far from Earth and its reflected sunlight. The Sun-Earth L2 point is located 1.53 10(exp 6) km from the Earth in the direction away from the Sun. This mission presents new challenges in many areas of technology, including the development of a multilayer insulation sunshield for the telescope. This sunshield is required to be large (proposed dimensions of approximately 33 by 14 m), storable, deployable, and lightweight. In addition, its polymer film layers must be seamable, foldable, and resistant to tearing and creep, with low outgassing. The sunshield must maintain its structural integrity and its Sun-facing side must maintain a low solar absorptance to thermal emittance ratio (alpha/epsilon) over the planned 10-yr mission duration including over 80,000 hr facing constant sunlight.

Dever, Joyce A.↗

NASA NTRS2004

Synthesis, Characterization, and Optical Properties of a Cyano-Functionalized 4,5,9,10-tetraaryl-l,6-dioxapyrene

5,10-Di(4-cyanophenyl)-4,9-di(4-methylphenyl)-1,6-dioxapyrene ( CN-diox), a symmetrically substituted 4,5,9,10-tetraaryldioxapyrene, was synthesized in seven steps from 1,4-dihydroxynaphthalene. The synthetic methodology incorporated a base-catalyzed ring-closure process followed by dehydration to introduce the first tetraaryl- 1,6-dioxapyrene. Crystal structure and electrochemical analysis were performed to directly compare the properties of CN-diox to previously reported dioxapyrene derivatives, specifically 1,6-dioxapyrene (Diox) and 4,9-diethyl-2,7-dimethyl- 1,6-dioxapyrene (Alkyl-diox). Optical spectroscopy studies were performed to evaluate the potential of the 1,6-dioxapyrenes as fluorescent probes. CN-diox revealed a broad absorption centered near 450 nm (epsilon = 31,900/M/cm) in THF with a corresponding fluorescence at 619 nm (Phi(sub f) = 0.011). This was in sharp contrast to both Diox and Alkyl-diox which displayed broad absorption bands near 400 nm (epsilon approx. 5,000-10,000/M/cm) in THF with corresponding fluorescence near 500 nm (Phi(sub f) = 0.059 and 0.082 for Diox and Alkyl-diox, respectively). The luminescence of CN-diox was found to be solvatochromic (lambda(sub max) = 619 nm-644 nm) with single exponential lifetimes of less than 1.3 ns. Neither Diox nor Alkyl-diox showed solvatochromic properties.

Tyson, Daniel S.↗

NASA NTRS2004

Epoxy Crosslinked Silica Aerogels (X-Aerogels)

NASA is interested in the development of strong lightweight materials for the dual role of thermal insulator and structural component for space vehicles; freeing more weight for useful payloads. Aerogels are very-low density materials (0.010 to 0.5 g/cc) that, due to high porosity (meso- and microporosity), can be, depending on the chemical nature of the network, ideal thermal insulators (thermal conductivity approx. 15 mW/mK). However, aerogels are extremely fragile. For practical application of aerogels, one must increase strength without compromising the physical properties attributed to low density. This has been achieved by templated growth of an epoxy polymer layer that crosslinks the "pearl necklace" network of nanoparticles: the framework of a typical silica aerogel. The requirement for conformal accumulation of the epoxy crosslinker is reaction both with the surface of silica and with itself. After cross-linking, the strength of a typical aerogel monolith increases by a factor of 200, in the expense of only a 2-fold increase in density. Strength is increased further by coupling residual unreacted epoxides with diamine.

fabrizio, Eve↗

NASA NTRS2006

Chemical, Physical, and Mechanical Characterization of Isocyanate Cross-linked Amine-Modified Silica Aerogels

We describe a new mechanically strong lightweight porous composite material obtained by encapsulating the skeletal framework of amine-modified silica aerogels with polyurea. The conformal polymer coating preserves the mesoporous structure of the underlying silica framework and the thermal conductivity remains low at 0.041 plus or minus 0.001 W m(sup -1 K(sup -1). The potential of the new cross-linked silica aerogels for load-carrying applications was determined through characterization of their mechanical behavior under compression, three-point bending, and dynamic mechanical analysis (DMA). A primary glass transition temperature of 130 C was identified through DMA. At room temperature, results indicate a hyperfoam behavior where in compression cross-linked aerogels are linearly elastic under small strains (less than 4%) and then exhibit yield behavior (until 40% strain), followed by densification and inelastic hardening. At room temperature the compressive Young's modulus and the Poisson's ratio were determined to be 129 plus or minus 8 MPa and 0.18, respectively, while the strain at ultimate failure is 77% and the average specific compressive stress at ultimate failure is 3.89 x 10(exp 5) N m kg(sup -1). The specific flexural strength is 2.16 x 10(exp 4) N m kg(sup -1). Effects on the compressive behavior of strain rate and low temperature were also evaluated.

Katti, Atul↗