Optimisation of Hypersonic Vehicles for Airbreathing Propulsion

Thomas Jazra (2010). Optimisation of Hypersonic Vehicles for Airbreathing Propulsion PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

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Author Thomas Jazra
Thesis Title Optimisation of Hypersonic Vehicles for Airbreathing Propulsion
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2010-11
Thesis type PhD Thesis
Supervisor Michael K. Smart
David J. Mee
Total pages 213
Total colour pages 15
Total black and white pages 198
Subjects 09 Engineering
Abstract/Summary A significant proportion of current and near-term space applications have masses of only a few hundred kilograms. Orbiting those is associated with stringent economical burdens in light of the typically low payload mass fractions (<1 %) of launch vehicles with small gross mass. After decades of maturation, rocket-only launch systems are operated close to theoretical limits and the potential for efficiency improvement is almost depleted. Therefore, next-generation launch vehicles are envisaged to include airbreathing propulsion, which is thought to allow for greater system efficiency. Smart and Tetlow (2009) examined the use of a three-stage rocket-scramjet-rocket system for transporting payloads of approximately 100 kg to LEO. This configuration combined conventional first and third rocket stages with a winged-cone type second-stage vehicle powered by three Mach 6-12 scramjet engines. Preliminary analysis of the three-stage vehicle indicated a scramjet shutdown at Mach 11.6, and a larger payload mass fraction to orbit than current rocket-only systems of similar scale. The present work numerically explores the preliminary findings of Smart and Tetlow (2009) in more depth, with a focus on the scramjet-powered second stage. In particular, the influence on the stage design and performance of a refined approach to the modelling of the vehicle, and the effects of varying the mass (+/-25 %), the freestream dynamic pressure during the acceleration period (25, 50, 100 kPa), and the scale of the vehicle (-25 %) are investigated. To this end, a numerical optimisation system for hypersonic airbreathers was developed. It combines the calculation of the external aerodynamics of the second-stage vehicle, the internal scramjet flowpath and the ascent trajectory during powered flight, and also includes a model to estimate the gross mass. Varying vehicle mass, freestream dynamic pressure and vehicle scale, six separate cases in which the vehicle design is optimised for maximum Mach number upon burnout of the scramjet engines are discussed. It is demonstrated that the final Mach number predicted by Smart and Tetlow (2009) is somewhat high, and that more realistic modelling of the airbreathing second stage limits its performance to M = 11.1 even in the most competitive design case considered. This is due to the high aerodynamic drag on the vehicle that exceeds the installed engine thrust at greater Mach numbers. In all six cases, the importance of incorporating the ascent trajectory into the optimisation methodology of the second stage shows; the trajectory is of relevance to both vehicle design and performance. In particular, an increasing thrust-minus-drag balance of the vehicle with increasing freestream dynamic pressure is determined. Consequently, the best performance at scramjet shutdown across the six design cases is achieved at q∞ = 100 kPa. In contrast, the influence of the mass of the accelerator is not as straightforward. The three vehicles optimised assuming different specific vehicle masses yield similar final performances (M = 10.8 to 11.0), while the heaviest design achieves the greatest average acceleration. This counterintuitive result is due to the specification of a constant q∞-level during the acceleration period, for which the heaviest design requires higher angles of attack. These allow more thrust to be generated by the engines, more than compensating for the loss in acceleration due to the additional vehicle mass. Ultimately, the influence of the competing effects of a higher final Mach number and a larger mass of the second stage must be evaluated in the context of a complete launch system rather than the second stage in isolation. The results obtained from downsizing the vehicle suggest that the average density increases with decreasing gross mass, which is attributed to the lower mass proportion of LH2 fuel in the smaller designs. However, a larger number of sample vehicles will be required in the future to investigate this trend in more depth.
Keyword scramjet
hypersonic vehicle design
multidisciplinary design optimisation
rocket-scramjet-rocket launch architecture
scramjet-assisted space access
Additional Notes colour pages: 3, 14, 18, 41, 42, 43, 46, 47, 61, 64, 66, 70, 71, 108, 122 no A3 pages no landscape pages

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Created: Fri, 01 Jul 2011, 07:00:53 EST by Mr Thomas Jazra on behalf of Library - Information Access Service