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

Spacelab system analysis Marshall Avionics System Testbed (MAST)

A synopsis of the visits to avionics test facilities is presented. A list of recommendaions for the MAST facility is also included

Spacelab system analysis: The modified free access protocol: An access protocol for communication systems with periodic and Poisson traffic

The protocol definition and terminal hardware for the modified free access protocol, a communications protocol similar to Ethernet, are developed. A MFA protocol simulator and a CSMA/CD math model are also developed. The protocol is tailored to communication systems where the total traffic may be divided into scheduled traffic and Poisson traffic. The scheduled traffic should occur on a periodic basis but may occur after a given event such as a request for data from a large number of stations. The Poisson traffic will include alarms and other random traffic. The purpose of the protocol is to guarantee that scheduled packets will be delivered without collision. This is required in many control and data collection systems. The protocol uses standard Ethernet hardware and software requiring minimum modifications to an existing system. The modification to the protocol only affects the Ethernet transmission privileges and does not effect the Ethernet receiver

EXPLORATION OF THIOPHENE-BASED DONOR-DONOR AND DONOR-ACCEPTOR CONJUGATED OLIGOMERS

Conjugated polymers are one of the leading technologies in electronic materials research. In this work we investigate thiophene-based donor-donor (D-D) and donor-acceptor (D-A) conjugated oligomers for future application towards polymer solar cells (PSCs). Beginning from computational studies, our group targets specific monomers that show promise in both bandgap and solar cell efficiency. For this research, we have found a novel alkyl-nitrothiophene (2-bromo-5-cyano-3-hexyl-4-nitrothiophene, NT) monomer as well as a unique D-D combination of propylenedioxythiophene (ProDOT) and isothianapthene (ITN). In both of these cases, we found that we can tune the tetramer bandgap (in both D-D and D-A systems) simply by changing the order of monomers (rather than changing the identity of the monomers). Horner-Wadsworth-Emmons (HWE) was chosen to provide the ability to individually couple monomers, allowing us to synthesize sequenced oligomers. The synthesis of the novel alkyl-NT monomer was successful and progress towards additional monomers necessary for HWE coupling was made. A test Stille coupling towards the synthesis of the ProDOT-ITN oligomer was also attempted

Efficiency Quantification for Pulsed-source Digital Holographic Wavefront Sensing

The efficiencies of a digital holography (DH) system in the pulsed configuration and the off-axis image plane recording geometry are analyzed. First, the system efficiencies of an infrared-wavelength DH system in a homodyne-pulsed configuration are measured and compared to those of a visible-wavelength DH system in a homodyne-continuous-wave (CW) configuration. The total-system, excess-reference-noise, shot-noise-limit, and mixing efficiencies of the pulsed-source system were found to be consistent with those of the CW-source system. This indicated no new efficiencies were necessary to characterize pulsed-source systems when no temporal delay exists between the pulses. The consistency of efficiencies also showed infrared DH systems are viable but degraded due to infrared detector technology. A new efficiency, called the ambiguity efficiency, was introduced to account for the degradation in system performance as the temporal delay between the pulses increased. This novel efficiency was then experimentally verified. Second, a DH system in the heterodyne-pulsed configuration was characterized in terms of the total-system and ambiguity efficiencies. The efficiencies measured using a heterodyne-pulsed configuration were consistent with those measured using a homodyne-pulsed configuration. Therefore, there was no degradation in system performance by changing from a homodyne configuration to a heterodyne configuration. This will allow the effective range of pulsed-source DH systems to greatly increase. Third, the effect of spectrally broadening the source laser of a DH system in the heterodyne-pulsed configuration was analyzed. Experiments showed the ambiguity efficiency was not significantly affected by the degradation in temporal coherence. However, the total-system efficiency did change as a function of temporal coherence degradation

Rapid acute dose assessment using MCNP6

2017 Spring.Includes bibliographical references.Acute radiation doses due to physical contact with a high-activity radioactive source have proven to be an occupational hazard. Multiple radiation injuries have been reported due to manipulating a radioactive source with bare hands or by placing a radioactive source inside a shirt or pants pocket. An effort to reconstruct the radiation dose must be performed to properly assess and medically manage the potential biological effects from such doses. Using the reference computational phantoms defined by the International Commission on Radiological Protection (ICRP) and the Monte Carlo N-Particle transport code (MCNP6), dose rate coefficients are calculated to assess doses for common acute doses due to beta and photon radiation sources. The research investigates doses due to having a radioactive source in either a breast pocket or pants back pocket. The dose rate coefficients are calculated for discrete energies and can be used to interpolate for any given energy of photon or beta emission. The dose rate coefficients allow for quick calculation of whole-body dose, organ dose, and/or skin dose if the source, activity, and time of exposure are known. Doses are calculated with the dose rate coefficients and compared to results from the International Atomic Energy Agency (IAEA) reports from accidents that occurred in Gilan, Iran and Yanango, Peru. Skin and organ doses calculated with the dose rate coefficients appear to agree, but there is a large discrepancy when comparing whole-body doses assessed using biodosimetry and whole-body doses assessed using the dose rate coefficients

Population Density Measurements of the Excited States of an Optically Excited Argon Discharge Using Emission and Absorption Spectroscopy

The populations of the excited argon 3s23p54s (s5 only) and 3s23p54p configurations in a 10 W radio frequency discharge has been studied using optical emission and diode laser absorption spectroscopy. By optically bleaching the s5→p9 transition with a narrow band laser pump of about 10 W/cm2 at 811 nm, the p9 population was increased by about a factor of 2 at a pressure of 5 Torr. At higher pressure, collisional mixing to adjacent p-states limited the laser-increased p9 population to less than 10 percent. All other laser-induced p-state populations were minimally affected at low pressure and increased by about a factor of 4 at higher pressure. The low pressure absorption line shapes exhibited an inhomogeneous hole with band-integrated absorbance that scaled linearly with pump power. The s5 population was greater than 7×1011 cm-3 at a pressure of 5 Torr, and about 57 percent of this population was moved to the p9 state at 828 mW, or about 10 W/cm2. Implications for development of a higher power optically pumped rare gas laser are discussed