Development and optimization of a multibumper design model for spacecraft protective structures

The development and optimization of a design model for multibumper spacecraft protective structures to defeat orbital space debris is presented. The Marshall Space Flight Center (MSFC) Materials and Processes (M&P) Laboratory Hypervelocity Impact Database is first filtered to experiments comprising metallic configurations without multilayered insulation present and for projectile velocities exceeding 2.5 km/sec. This filtering results in 337 single, double, and triple bumper hypervelocity impact experiments. Regression variables of interest include projectile diameter, density, velocity, and impact angle, bumper standoff distances, bumper densities and thicknesses, wall density and thickness, and number of bumpers. The dependent regression variable is the total number of plate penetrations, beginning with the wall and continuing through the witness plates. A unique intrinsically linear regression form, which accounts for the number of bumpers employed and invokes a posynomial (polynomial with positive coefficients, positive valued independent variables, and real valued exponents) form, is chosen based on a comparison of various regression forms using correlation coefficient and F-statistic as measures of effectiveness. The least squares regression is performed followed by an ANOVA, tests of the correlation and F value, and graphical examination of residuals. Regression results indicate that statistically significant least squares is possible using the chosen form on the MSFC M&P database with small residual effects. Generic nonlinear regression forms are also investigated.

The resulting regression model is next used in the formulation of a nonlinear optimization program. This program is devised to minimize the protective structures areal density subject to a limitation on total standoff distance between the first bumper and the wall. The decision variables of interest are the optimal values of the areal densities of the bumpers and wall, as well as the optimal individual standoff distances. The problem is solved using the dual transformation of geometric programming. The optimal independent variables and minimum system areal density are solved for analytically in terms of the systemic parameters. A sensitivity analysis to these parameters is then performed. Additionally, the optimal number of bumpers is evaluated in this sensitivity study. The most significant results from a hypervelocity impact standpoint are that additional hypervelocity impact tests and analyses should be performed to support understanding of multiple bumper, large particle diameter, large separation, large particle mass density, various particle impact angles, and spallation phenomenologies. Additionally, more emphasis should be placed on understanding the transition regions between particle shatter, melt, and vaporization, while less emphasis should be placed on small velocity differences within these regions. Major protective structures design results indicate that for Space Station Freedom impact scenarios of interest, and within the limitations of the regressed hypervelocity impact database, at most four metallic bumpers are optimal. In particular, a transition region from optimal number of bumpers of 2 to 3 (and 3 to 4) has been identified for particle diameters in the 0.25–0.5 cm (and 1 to 1.25 cm) range. An interesting transition region from 3 to 4 optimal number of bumpers has been discovered for standoff distances between 10 and 15 cm. Furthermore, the optimal protective structures design sensitivity to impact angle is very low. Finally, the results of this investigation indicate that this combination of regression form and resulting optimization approach is useful in identifying protective structures design trends for spacecraft subject to hypervelocity impact environments.     

Microcutting of steel,copper and zinc: A comparative study

The microcutting of copper, zinc and steel was performed in the speed range 5–55 m s^?1 using a single pyramidal diamond point with an apex angle of 90°. It was found that harder materials may plastically deform more than softer materials during cutting owing to the difference in crystal structure. Cutting efficiency is also governed by the bonding energy and dislocation dynamics within the workpiece.     

Comparing cytosolic expression to peroxisomal targeting of the chimeric L1/L2 (ChiΔH-L2) gene from human papillomavirus type 16 in the methylotrophic yeasts Pichia pastoris and Hansenula polymorpha

The chimeric ChiΔH‐L2 gene from human papillomavirus type 16, consisting of structural proteins L1 and L2, was successfully expressed in the cytosol of both Pichia pastoris and Hansenula polymorpha during methanol induction. In addition, a novel approach was employed whereby ChiΔH‐L2 was targeted to the peroxisome using peroxisomal targeting sequence 1 (PTS1) to compare ChiΔH‐L2 yields in the peroxisome vs the cytosol. The ChiΔH‐L2 gene was yeast‐optimized and cloned into plasmids aimed at genomic integration. Levels of intracellular ChiΔH‐L2 accumulation in the cytosol were highest in P. pastoris KM71 strain KMChiΔH‐L2 (1.43 mg/l), compared to the maximum production level of 0.72 mg/l obtained with H. polymorpha. ChiΔH‐L2 targeting to the peroxisome was successful; however, it appeared to negatively affect ChiΔH‐L2 production in both P. pastoris and H. polymorpha. Copyright © 2012 John Wiley & Sons, Ltd.     

Expression of unique chimeric human papilloma virus type 16 (HPV‐16) L1‐L2 proteins in Pichia pastoris and Hansenula polymorpha

Cervical cancer is ranked the fourth most common cancer in women worldwide. Despite two prophylactic vaccines being commercially available, they are unaffordable for most women in developing countries. We compared the optimized expression of monomers of the unique HPV type 16 L1‐L2 chimeric protein (SAF) in two yeast strains of Pichia pastoris, KM71 (Mut^s) and GS115 (Mut⁺), with Hansenula polymorpha NCYC 495 to determine the preferred host in bioreactors. SAF was uniquely created by replacing the h4 helix of the HPV‐16 capsid L1 protein with an L2 peptide. Two different feeding strategies in fed‐batch cultures of P. pastoris Mut^s were evaluated: a predetermined feed rate vs. feeding based on the oxygen consumption by maintaining constant dissolved oxygen levels (DO stat). All cultures showed a significant increase in biomass when methanol was fed using the DO stat method. In P. pastoris the SAF concentrations were higher in the Mut^s strains than in the Mut⁺ strains. However, H. polymorpha produced the highest level of SAF at 132.10 mg L^?1 culture while P. pastoris Mut^s only produced 23.61 mg L^?1. H. polymorpha showed greater potential for the expression of HPV‐16 L1/L2 chimeric proteins despite the track record of P. pastoris as a high‐level producer of heterologous proteins.     

Development of a strategy for the management of PBMR spent fuel in South Africa

South Africa is planning to expand its nuclear power generating capacity by deploying a number of pressurized-water reactors and pebble-bed modular reactors. It can be expected that this program will impact on the current and planned spent fuel and radioactive waste management systems in South Africa. This paper proposes an approach to develop a strategy for the management of PBMR spent fuel that would form an integral part of the overall national radwaste management system. The approach is expected to provide a conceptual spent fuel management strategy and will also highlight areas that need to be further developed, thus providing guidance for basic technology development.     

Microcutting of steel using pyramidal diamonds with different apex angles

The microcutting of steel with pyramidal diamond points was performed in the speed range 5–55 m s^?1. The apex angles of the diamonds were 90°, 110°, 130°, 150° and 170°. It was found that the material removal mechanism for the diamonds with apex angles 150° and 170° differed from the mechanism for the sharper diamond points. The latter points showed a standard chip formation cycle whereas the two blunter points probably removed material by an extrusion mechanism.     

Genetic characterization of HIV type 1 from migrant workers in three South African gold mines

The phylogenetic relationships between 44 HIV-1 isolates from 43 infected subjects employed by three adjacent South African gold mines were investigated. The patients were migrant workers originating from rural areas of South Africa and the neighboring countries of Lesotho, Botswana, Swaziland, and Mozambique. Proviral HIV-1 DNA was subtyped using a heteroduplex mobility assay (HMA) based on the 700-bp V3-V5 region of the env gene. DNA sequence analysis was used to confirm the subtype designation and to determine phylogenetic relationships between isolates. All 44 HIV-1 isolates were identified as env subtype C using both HMA and phylogenetic analysis. These isolates did not show a distinct phylogenetic relatedness based on the geographic origins of the migrant workers or show close homology to other subtype C sequences from southern Africa or India. However, five clusters of closely related sequences were identified, mainly involving miners of disparate geographic origins, suggesting possible epidemiological linkage in these few cases. The characteristic tetrapeptide sequence, GPGQ, at the tip of the V3 loop of subtype C viruses was conserved in the predicted amino acid sequences of most isolates. The heterogeneity of HIV-1 sequences among migrant workers in a mining cohort suggests multiple introductions of HIV-1 subtype C into this population that are not apparently linked to the geographic origins of the patients.