Settled Cryogenic Propellant Transfer

Cryogenic propellant transfer can significantly benefit NASA s space exploration initiative. LMSSC parametric studies indicate that "Topping off" the Earth Departure Stage (EDS) in LEO with approx.20 mT of additional propellant using cryogenic propellant transfer increases the lunar delivered payload by 5 mT. Filling the EDS to capacity in LEO with 78 mT of propellants increases the delivered payload by 20 mT. Cryogenic propellant transfer is directly extensible to Mars exploration in that it provides propellant for the Mars Earth Departure stage and in-situ propellant utilization at Mars. To enable the significant performance increase provided by cryogenic propellant transfer, the reliability and robustness of the transfer process must be guaranteed. By utilizing low vehicle acceleration during the cryogenic transfer the operation is significantly simplified and enables the maximum use of existing, reliable, mature upper stage cryogenic-fluid-management (CFM) techniques. Due to settling, large-scale propellant transfer becomes an engineering effort, and not the technology development endeavor required with zero-gravity propellant transfer. The following key CFM technologies are all currently implemented by settling on both the Centaur and Delta IV upper stages: propellant acquisition, hardware chilldown, pressure control, and mass gauging. The key remaining technology, autonomous rendezvous and docking, is already in use by the Russians, and must be perfected for NASA whether the use of propellant transfer is utilized or not

Aspen Crown Dieback and Mortality on the Williams Ranger District, Kaibab National Forest, Arizona

Crown dieback and mortality of quaking or trembling aspen (Populus tremuloides) were extensive within pine-oak and mixed conifer forest types of the Williams Ranger District, Kaibab National Forest in northern Arizona. I collected data from 48 aspen sites to determine if predisposing site and stand factors and contributing damaging agents were associated with aspen crown dieback and mortality. Overstory aspen mortality averaged 50% by stems per hectare and 44% by basal area. Based upon univariate relationships, elevation was the most significant site factor related to both overstory aspen crown dieback (R2 = 0.15, P = 0.0069) and overstory aspen mortality (R2 = 0.24, P = 0.0004). The most significant stand factor related to crown dieback was live aspen density (R2 = 0.18, P = 0.0029), while percent conifer (R2 = 0.45, P \u3c 0.0001) was the most significant stand factor related to mortality. Canker diseases, wood-boring insects, and animal damages were common in the overstory size class. The significant damaging agents in relation to both overstory crown dieback and mortality were canker diseases (R2 = 0.13, P = 0.0123; R2 = 0.18, P = 0.0028, respectively) and wood-boring insects (R2 = 0.24, P = 0.0005; R2 = 0.56, P \u3c 0.0001, respectively). Sapling and tall sucker aspen mortality were high (\u3e 80 and 70%, respectively), while short sucker mortality was low (16%). Many sites did not have live aspen regeneration, therefore, sample sizes were low, and relationships were often inconclusive or weak. Animal damages and canker diseases were common in the sapling and tall sucker size classes. Only animal damages were common in the short sucker size class. Among damaging agents and regeneration size classes, the only significant univariate relationship found was between animal damages and short sucker aspen mortality (R2 = 0.15, P = 0.0198). Based on a negative exponential diameter distribution, there was lack of aspen recruitment in saplings and small diameter overstory stems. If high mortality and low recruitment continues, aspen stands will be replaced by conifer after larger, and presumably older, overstory aspen stems die. The multivariate relationships of overstory aspen crown dieback, overstory aspen mortality, and short sucker aspen mortality among site, stand, and damaging agent factors were explored using step-wise multiple regression. The significant multivariate associations with overstory aspen crown dieback were elevation (F1,44 = 16.38, P = 0.0002) and incidence of canker diseases (F1,44 = 15.02, P = 0.0004). The significant factors explaining the variation in overstory aspen mortality were forest type (F1,43 = 5.92, P = 0.0192), overstory percent conifer (F1,43 = 8.24, P = 0.0063), and incidence of canker diseases (F1,43 = 33.05, P \u3c 0.0001), and wood-boring insects (F1,43 = 33.29, P \u3c 0.0001). The significant factors explaining the variation in short sucker aspen mortality were slope (F1,31 = 4.90, P = 0.0344), short sucker percent conifer (F1,31 = 5.00, P = 0.0327), and incidence of animal damages (F1,31 = 6.85, P = 0.0136). According to previous research, ungulate herbivores contribute to aspen decline in northern Arizona by causing damage to aspen regeneration. Ungulate damages were common in all size classes (between 49 and 66%), but significant relationships were limited to short sucker aspen mortality. No data were collected from within ungulate exclosures in this study. Controlled experiments inside and outside of ungulate exclosures are needed to determine the impact of ungulates

Slow lifelong growth predisposes Populus tremuloides to tree mortality

Widespread dieback of aspen forests, sometimes called sudden aspen decline, has been observed throughout much of western North America, with the highest mortality rates in the southwestern United States. Recent aspen mortality has been linked to drought stress and elevated temperatures characteristic of conditions expected under climate change, but the role of individual aspen tree growth patterns in contributing to recent tree mortality is less well known. We used tree-ring data to investigate the relationship between an individual aspen tree\u27s lifetime growth patterns and mortality. Surviving aspen trees had consistently higher average growth rates for at least 100 years than dead trees. Contrary to observations from late successional species, slow initial growth rates were not associated with a longer lifespan in aspen. Aspen trees that died had slower lifetime growth and slower growth at various stages of their lives than those that survived. Differences in average diameter growth between live and dead trees were significant (? = 0.05) across all time periods tested. Our best logistical model of aspen mortality indicates that younger aspen trees with lower recent growth rates and higher frequencies of abrupt growth declines had an increased risk of mortality. Our findings highlight the need for species-specific mortality functions in forest succession models. Size-dependent mortality functions suitable for late successional species may not be appropriate for species with different life history strategies. For some early successional species, like aspen, slow growth at various stages of the tree\u27s life is associated with increased mortality risk

Warm, Dry Conditions Inhibit Aspen Growth, But Tree Growth and Size Predict Mortality Risk in the Southwestern United States

Widespread, rapid aspen (Populus tremuloides Michx.) mortality since the beginning of the 21st century, sometimes called sudden aspen decline (SAD), has been documented in many locations across North America, but it has been particularly pronounced in the southwestern United States. We investigated the relationship among aspen growth, mortality, and climate across three forest types in northern Arizona using cross-dated tree-ring samples from 126 live and 132 dead aspens. Aspen growth was negatively correlated with warm temperatures and positively associated with higher precipitation. Using survival analysis techniques to investigate the links between aspen mortality, tree traits, and climatic conditions, we found that tree traits played a larger role in mortality risk than climate factors. Trees with larger diameters, older trees, and trees with faster growth rates over the past 50 years had a reduced risk of mortality. Management actions aimed at maintaining the most vigorous, fastest growing aspen in the region could help mitigate the impacts of a warmer, drier future