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

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

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

Combined high and low-thrust geostationary orbit insertion with radiation constraint

The sequential use of an electric propulsion system is considered in combination with a high-thrust propulsion system for application to the propellant-optimal Geostationary Orbit insertion problem, whilst considering both temporal and radiation flux constraints. Such usage is found to offer a combined propellant mass saving when compared with an equivalent high-thrust only transfer. This propellant mass saving is seen to increase as the allowable transfer duration is increased, and as the thrust from the low-thrust system is increased, assuming constant specific impulse. It was found that the required plane change maneuver is most propellant-efficiently performed by the high-thrust system. The propellant optimal trajectory incurs a significantly increased electron flux when compared to an equivalent high-thrust only transfer. However, the electron flux can be reduced to a similar order of magnitude by increasing the high-thrust propellant consumption, whilst still delivering an improved mass fraction

Incidence of physician-diagnosed osteoarthritis among active duty United States military service members

The article of record as published may be found at http://dx.DOI.org/10.1002/art.30498To examine the incidence of osteoarthritis and the influence of demographic and occupational factors associated with this condition among active duty US service members between 1999 and 2008

Distal thoracic aorta as inflow for the treatment of chronic mesenteric ischemia

Purpose: Mesenteric revascularization for chronic mesenteric ischemia (CMI) traditionally involves antegrade or retrograde bypass graft originating from the supraceliac or infrarenal aorta. The distal thoracic aorta (DTA) may provide a better inflow source than the abdominal aorta. The purpose of this study was to evaluate the results with the DTA used as inflow for the surgical treatment of CMI. Methods: All patients undergoing mesenteric revascularization for CMI with grafts originating from the DTA were identified from 1990 to 1999. A ninth interspace thoracoretroperitoneal incision was used for exposure, and distal aortic flow was maintained by use of a partial occlusion clamp. Results: Eighteen consecutive patients with CMI underwent mesenteric bypass grafting with the DTA used as inflow. All patients were admitted with chronic abdominal pain or weight loss, with two (12%) requiring urgent revascularization because of acute exacerbation of chronic symptoms. Fourteen (78%) patients had both celiac and superior mesenteric artery bypass grafts placed, and three (17%) patients had superior mesenteric artery grafts alone. There was one (6%) perioperative death and three (17%) major complications. There was no kidney failure, mesenteric infarction, or spinal cord ischemia. The life-table survival rate was 89%, 89%, and 76% at 1, 3, and 5 years, respectively. All 18 patients remained symptom free and required no additional procedures to assist patency. There was no evidence of graft stenosis or occlusion (100% patency) for those grafts evaluated objectively during the mean follow-up of 34.8 months (range, 1-97 months). Conclusions: Antegrade mesenteric revascularization with the DTA used as inflow is associated with low morbidity and mortality rates. Furthermore, it provides excellent midterm patency and survival results and should be considered as a primary approach for reconstruction of patients with CMI. (J Vasc Surg 2001;33:281-8.

Cancer in World Trade Center responders: Findings from multiple cohorts and options for future study

Background Three longitudinal studies of cancer incidence in varied populations of World Trade Center responders have been conducted. Methods We compared the design and results of the three studies. Results Separate analyses of these cohorts revealed excess cancer incidence in responders for all cancers combined and for cancers of the thyroid and prostate. Methodological dissimilarities included recruitment strategies, source of cohort members, demographic characteristics, overlap between cohorts, assessment of WTC and other occupational exposures and confounders, methods and duration of follow-up, approaches for statistical analysis, and latency analyses. Conclusions The presence of three cohorts strengthens the effort of identifying and quantifying the cancer risk; the heterogeneity in design might increase sensitivity to the identification of cancers potentially associated with exposure. The presence and magnitude of an increased cancer risk remains to be fully elucidated. Continued long-term follow up with minimal longitudinal dropout is crucial to achieve this goal

A Cannabinoid CB 1 Receptor-Positive Allosteric Modulator Reduces Neuropathic Pain in the Mouse with No Psychoactive Effects

Peer reviewedPostprin

Prospective screening for postoperative deep venous thrombosis in patients undergoing infrainguinal revascularization

AbstractPurpose: The incidence of deep venous thrombosis (DVT) in patients undergoing infrainguinal bypass graft procedures has not been well documented, and the need for routine prophylaxis remains controversial. The purpose of this study was to prospectively evaluate the risk of postoperative DVT complicating infrainguinal revascularization. Methods: Seventy-four patients undergoing infrainguinal bypass graft procedures during a 12-month period were prospectively screened for DVT. Bilateral lower extremity venous duplex scan imaging was performed preoperatively and within 1 week and 6 weeks, postoperatively. Routine DVT prophylaxis was not used, with anticoagulation reserved for specific indications. Results: Of the 74 patients screened, three patients (4.1%) had DVT identified on preoperative venous duplex scan imaging and were excluded from the study. Of the remaining 71 patients enrolled, only two patients (2.8%) had postoperative DVT. Postoperative DVT was ipsilateral to the bypass graft extremity in both patients, with involvement of the peroneal vein in one patient and the femoral vein in the other. Although routine prophylaxis was not used, 18 of these patients (25%) were anticoagulated for other indications, with DVT occurring in one patient (5.6%). Of the remaining 53 patients who did not receive postoperative anticoagulation, only one patient (1.8%) had DVT. Conclusions: According to this prospective study, the risk of postoperative DVT in patients undergoing infrainguinal revascularization is low. Routine prophylaxis is not recommended, with postoperative anticoagulation reserved for specific indications. (J Vasc Surg 2000;32:669-75.