Scramjet-powered vehicles with similar shapes and the same second stage trajectory to orbit are used to establish relationships between the vehicle volume, the vehicle launch mass and the payload mass. It is shown that increasing the fuel density or the payload density can increase the payload mass, while simultaneously decreasing the vehicle launch mass. The maximum payload mass without a volume constraint is found to be half the dry mass. Practical volume constraints are shown to reduce this fraction. To increase the scramjet thrust, additional propellant is often injected into the scramjet how. It is shown here that for a hydrogen-fuelled vehicle, replacing this additional hydrogen with neon or with a hydrogen-oxygen mixture can significantly increase the payload and decrease the launch mass. It is shown also, for less rigorous assumptions, that replacing the scramjet hydrogen fuel with a hydrocarbon fuel can have the same effect, such that a vehicle to place a given payload in orbit can be both smaller and less costly.
