This paper investigates the cost­-benefit, payload performance and technical feasibility of green storable upper stage design concepts on their potential implementation with existing launchers. Since the demand for reliable, safe, and cost­-effective transport to space is growing, it is necessary to find sustain­able rocket propellant solutions. Green fuels that react hypergolic with hydrogen peroxide as oxidisers can provide distinct advantages in the performance, complexity and reliability when integrated into sys­tems with severe geometrical constraints, such as upper­ stage propulsion modules. This research aims to quantify the cost­-effectiveness of green storable upper stage concepts on the commercial market prospect. This is done through detailed research into design concepts operating on various storable propellant combinations. These various design concepts for different propellant combinations are described by the hypothetical storable upper stage called the “Prototype X”. As an oxidizer, concentrated hydrogen peroxide (HTP), combined with non-­carcinogenic, non­-toxic, and non­ corrosive green storable fuels, can provide reliable and environmentally friendly propulsion solutions. Moreover, green hypergolic storable propellants will lower the operational cost, reduce system complexity and are safer to procure, store and handle. These ”Prototype X” upper stage design concepts will be analysed and optimised on their cost­-per­-kg payload characteristic. Detailed mass and cost optimization analysis was performed for a green storable launcher upper­ stage concept. It was found that HTP together with screened storable fuels shows promising and acceptable performance characteristics in terms of specific impulse and density specific impulse. Im­plementation of these green storable propellants into the upper stage design results in dry mass re­ductions of over 20% compared to the conventional cryogenic hydrolox upper stage design. Although current ’green’ storable propellants can reduce the cost­-per­-flight of the upper stage by 8.9%, resulting from constructive cost reductions in development, manufacturing and (pre­launch) operation phases, their wet mass burden reduces the payload performance by at least 308.09%. This is mainly due to the reduced performance of current ’green’ storable propellants, compared to cryogenic propellants. Therefore, a cross­-over analysis was conducted for the “Prototype X” storable design concept. Here the focus was on finding the required performance of a hypothetical storable “Fuel X” that would make the storable concept economically attractive. Calculations suggest that further development in storable propellant engineering can improve the payload performance of the storable upper stage concept by +37.79% for the same wet mass, compared to the cryogenic design, while reducing the geometrical dimensions of the upper stage by roughly 33%. Storable ’green’ propellants significantly reduce com­plexity and add reliability and safety to the system, potentially improving the payload performance even further. Detailed analysis points out that a hybrid launch vehicle, comprised of a cryogenic first stage and a ’green’ storable upper stage, would be the most cost-­effective expendable launch vehicle in the West having a payload performance of 3.80 𝑘€/𝑘𝑔. Taking into account the added safety, higher reli­ability and reduced complexity of the storable systems, it is expected that the proposed hybrid launch vehicle (either expendable or reusable) will outperform all medium to heavy launch vehicles on the market today. These findings indicate that the green (hypergolic) storable upper stage can be a cost­-effective and reliable solution to future space flight applications.