The cost-optimal size of future reusable launch vehicles

The paper answers the question, what is the optimum vehicle size – in terms of Low Earth Orbit (LEO) payload capability – for a future reusable launch vehicle? It is shown that there exists an optimum vehicle size that results in minimum specific transportation cost. The optimum vehicle size depends on the total annual cargo mass (LEO equivalent) envisaged, which defines at the same time the optimum number of launches per year (LpA). Based on the TRANSCOST-Model algorithms a wide range of vehicle sizes – from 20 to 100 Mg payload in LEO, as well as launch rates – from 2 to 100 per year – have been investigated. It is shown in a design chart how much the vehicle size as well as the launch rate are influencing the specific transportation cost. The comparison with actual ELVs (Expendable launch Vehicles) and Semi-Reusable Vehicles (a combination of a reusable first stage with an expendable second stage) shows that there exists only one economic solution for an essential reduction of space transportation cost: the Fully Reusable Vehicle Concept, with rocket propulsion and vertical take-off. The Single-Stage Configuration (SSTO) has the best economic potential: its feasibility is not only a matter of technology level but also of the vehicle size as such. Increasing the vehicle size (launch mass) reduces the technology requirements because the law of scale provides a better mass fraction and payload fraction – practically at no cost. The optimum vehicle design (after specification of the payload capability) requires a trade-off between lightweight (and more expensive) technology vs. more conventional (and cheaper) technology. It is shown that the use of more conventional technology and accepting a somewhat larger vehicle is the more cost-effective and less risky approach.