Dicumarol

Thesis (M.D.)--Boston Universit

Novel numerical optimisation of the Hohmann Spiral Transfer

As the revenue of commercial spacecraft platforms is generated by its payload, of which the capacity is maximised when fuel-mass is minimised, there is great interest in ensuring the fuel required for the trajectory to deliver the satellite to its working orbit is minimum. This paper presents an optimisation study of a novel orbit transfer, recently introduced by the authors through an analytical analysis, known as the Hohmann Spiral Transfer . The transfer is analogous to the bi-elliptic transfer but incorporating high and low-thrust propulsion. This paper has shown that substantial fuel mass savings are possible when utilizing the HST. For a transfer to Geostationary Earth Orbit it is shown that a fuel mass saving of approximately 320 kg (~ 5 - 10% of mwet ) is possible for a wet mass of 3000-6000 kg – whilst satisfying a time constraint of 90 days. Several trends in the gathered data are also identified that determine when the HST with high or low-thrust plane change should be used to offer the greatest fuel mass benefit

In the soft-to-hard technical spectrum: Where is software engineering?

In the computer journals and tabloids, there have been a plethora of articles written about the software engineering field. But while advocates of the need for an engineering approach to software development, it is impressive how many authors have treated the subject of software engineering without adequately addressing the fundamentals of what engineering as a discipline consists of. A discussion is presented of the various related facets of this issue in a logical framework to advance the thesis that the software development process is necessarily an engineering process. The purpose is to examine more of the details of the issue of whether or not the design and development of software for digital computer processing systems should be both viewed and treated as a legitimate field of professional engineering. Also, the type of academic and professional level education programs that would be required to support a software engineering discipline is examined

Hohmann spiral transfer with inclination change performed by low-thrust system

This paper investigates the Hohmann Spiral Transfer (HST), an orbit transfer method previously developed by the authors incorporating both high and low-thrust propulsion systems, using the low-thrust system to perform an inclination change as well as orbit transfer. The HST is similar to the bi-elliptic transfer as the high-thrust system is first used to propel the spacecraft beyond the target where it is used again to circularize at an intermediate orbit. The low-thrust system is then activated and, while maintaining this orbit altitude, used to change the orbit inclination to suit the mission specification. The low-thrust system is then used again to reduce the spacecraft altitude by spiraling in-toward the target orbit. An analytical analysis of the HST utilizing the low-thrust system for the inclination change is performed which allows a critical specific impulse ratio to be derived determining the point at which the HST consumes the same amount of fuel as the Hohmann transfer. A critical ratio is found for both a circular and elliptical initial orbit. These equations are validated by a numerical approach before being compared to the HST utilizing the high-thrust system to perform the inclination change. An additional critical ratio comparing the HST utilizing the low-thrust system for the inclination change with its high-thrust counterpart is derived and by using these three critical ratios together, it can be determined when each transfer offers the lowest fuel mass consumption. Initial analyses have shown the HST utilizing low-thrust inclination change to offer the greatest benefit at low R2 (R2 - R1) and large AI (AI > 30º). A novel numerical optimization process which could be used to optimize the trajectory is also introduced

Resist? Welcome? Co-opt? Ignore? The pressures and possibilities of the REF and impact

The increased focus on impact in research evaluation represents a range of possibilities and pressures to those academics whose work is being assessed. For some it offers an opportunity to progress social justice causes and engage in participatory, bottom-up research approaches with less powerful groups; while to others it is further evidence of the managerial audit culture that is corrupting universities, trammelling academic freedom, and which must be resisted. Robert MacDonald considers both perspectives and suggests that even if the REF is an example of increased governmental control, it might yet provide space to engage in a positive, progressive politics of research

A novel approach to hybrid propulsion transfers

This paper introduces a hybrid propulsion transfer termed a Hohmann Spiral, incorporating low and high-thrust technologies, analogous to the high-thrust bi-elliptic transfer. To understand this transfer fully it is compared to a standard high thrust Hohmann and a bi-elliptic transfer. Two critical specific impulse ratios are derived independent of time that determine the point this novel transfer consumes the exact amount of fuel as the two compared transfer types. It is found that these ratios are valid for both a circular and elliptical starting orbit so long as the apogee of the elliptical orbit coincides with the target orbit radius. An expression representing the fuel mass fraction is derived dependent of time in order to allow a bound solution space. The final part of this paper investigates two orbit transfer case studies, one is a Geostationary Transfer Orbit to Geostationary Earth Orbit based on the Alphabus platform specification and the other is from Low Earth Orbit to an orbit near the Moon. It is found the thrust required to complete the former transfer in a specified duration of 90 days exceeds current technology and as such provides a technology requirement for future spacecraft. It is found however, for spacecraft of significantly smaller mass, in the region of 1000kg, compared to Alphabus (Max. mass at Launch =8100kg), the transfer consumes the same fuel mass as a standard high-thrust Hohmann transfer with realistic low-thrust propulsion values (150mN, 300mN and 450mN) within the set duration of 90 days. In addition, it is shown that utilising uprated thrusters (210mN, 420mN and 630mN) a fuel mass saving can be made. This could provide a potential transfer alternative for future smaller spacecraft. The second case study is bound to a maximum thrust of 150mN, but the mission duration is not specified to highlight the variation. It is found that the HST offers fuel mass savings of roughly 5% compared to a standard high-thrust transfer and approximately 1.5% compared to a bi-elliptic transfer for different scenarios

An extension and numerical analysis of the Hohmann spiral transfer

This paper extends previous work on the Hohmann Transfer Spiral (HST) by introducing a plane change into the analysis. An analytical expression determining the critical specific impulse incorporating a plane change is derived for both a circular and elliptical initial orbit. This expression determines the point at which the HST is equivalent in terms of fuel mass fraction to the compared Hohmann transfer. The expression assumes that the inclination change is performed by the high-thrust system. The numerical approach uses a blending method coupled with optimised weighting constants to deliver a locally optimal low-thrust trajectory. By comparing the analytical and numerical approaches, it is shown that the analytical can deliver a good estimation of the HST characteristics so long as little orbit eccentricity control is required. In the cases where orbit eccentricity control is required, the numerical approach should be used. A case study from an inclined Geostationary Transfer Orbit, equivalent to a high-latitude launch site, to Geostationary Earth Orbit has shown that the HST can offer a fuel mass saving approximately 5% of the launch mass. This equates to the mass penalty associated with this high-latitude launch site and therefore mimics the advantages of a low-latitude launch site at the expense of a longer transfer duration

Basic Living Expenses for the Canadian Elderly

Our research undertakes to determine the basic living expenses required by Canadian seniors living in different circumstances in terms of age, gender, city of residence, household size, homeowner or renter, means of transportation and health status. The paper develops required expenses for food, shelter, health care, transportation and miscellaneous. The research identifies the typical expenses of seniors in each of these categories. Using 2001 as our base year, we follow the US Elder Standard to build an elderly threshold for Halifax, Montreal, Toronto, Calgary and Vancouver. The research is unique because it is the first Canadian study of absolute basic living expenses tailored to seniors, rather than simply to adults in general. This information is important to seniors, prospective retirees, financial planners, policy makers and actuaries in assessing the minimum level of income required in retirement and the adequacy of savings and income security programs. Our conclusions suggest that individual circumstances, rather than age, are the primary drivers in determining the cost of these basic expenses. Seniors are a diverse group, particularly with respect to health, so it is important that seniors and financial planners do not blindly rely on a fixed replacement ratio or universal level of income when projecting the level of finances needed to retire. This research enables the reader to determine the threshold that is suited to a senior’s general circumstances.Retirement Income Adequacy; Absolute Measure; Elder Standard; Canadian Data

The Canadian Elder Standard - Pricing the Cost of Basic Needs for the Canadian Elderly

We determined the after-tax income required to fi nance basic needs for Canadian elders living with different circumstances in terms of age, gender, city of residence, household size, homeowner or renter status, means of transportation, and health status. Using 2001 as our base year, we priced the typical expenses for food, shelter, medical, transportation, miscellaneous basic living items and home-based long-term care for elders living in fi ve Canadian cities. This is the fi rst Canadian study of basic living expenses tailored to elders instead of adults in general, prepared on an absolute rather than a relative basis. We also accounted for an individual’s unique life circumstances and established the varying effect that they have on the cost of basic expenses, particularly for home care. We found that the maximum Guaranteed Income Supple ment and Old Age Security benefi t did not meet the cost of basic needs for an elder living in poor circumstances.Canadian seniors, poverty measure, economic security, aging-in-place, cost-of-living, absolute measure, home care

Growing Farm Size and the Distribution of Farm Payments

Crop production is shifting to much larger farms. Since government commodity payments reflect production volumes for program commodities, payments are also shifting to larger farms. In turn, the operators of very large farms have substantially higher household incomes than other farm households, and as a result government commodity payments are also shifting to much higher-income households. Since the changes in farm structure appear to be ongoing, commodity payments will likely, under current policies, continue to shift to higher income households. This brief uses 2003 Agricultural Resource Management Survey (ARMS) data to detail the shifts.Farm structure, commodity programs, farm payments, farm household income, farm income, farm program payments, ERS, USDA, Agricultural and Food Policy, Industrial Organization,