With reusability accepted as a means of reducing operating costs, the size of the initial investment (research and development) is likely to determine the choice for the next generation boosters. High volume utilisation lifting bodies propelled by LH/LOX rockets in a vertical take-off mode are shown to be superior to several other concepts. This is largely due to the low manufactured weight without undue complexity or use of exotic materials, leading to low R&D and low unit cost.
Even lower costs can be shown for a modular concept (MUSTARD) in which basically identical lifting bodies units are utilised as both boosters and spacecraft. The concept is shown to be feasible, and progress on some aspects of the associated structural analysis is described.