Re-usable single stage to orbit launch vehicles promise to reduce the cost of access to space, but their success will be particularly reliant on accurate modelling of their aero-thermodynamic characteristics. Non-equilibrium effects due to the rarefaction of the gas in the atmosphere are important at the very high altitudes at which lifting R-SSTO configurations will experience their greatest thermal load during re-entry. Current limitations in modelling the behaviour of the gas and hence in capturing these effects have a strong impact on the accuracy with which the thermal and aerodynamic loading on the surface of the vehicle can be predicted during this design-critical flight regime. The problem is most apparent in the presence of strong shock interactions, and this is likely to exacerbate the problem of aerodynamic characterisation of re-usable single stage to orbit vehicles, especially given design pressures towards increased geometric complexity compared to historical spacecraft designs, and hence the complexity of the shock structures that the vehicle will produce in high-speed flight. The development of this class of vehicles will thus very likely be paced by the development of the specialised modelling tools that will be required to account fully for the properties of the gas at the high speeds and altitudes that are characteristic of their re-entry into the atmosphere of the earth.
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