Comparison of hydrogen and hydrocarbon-fueled scramjet engines for orbital insertion

This paper compares the performance of the scramjet-powered stages of two different launch systems: one using a hydrogen-fueled scramjet stage and one using a hydrocarbon-fueled scramjet stage. The two launch systems are optimized with respect to payload delivery capability and then compared, assuming a fixed launch mass. A rocket-powered booster is used to achieve the required scramjet ignition conditions, and a rocket-powered orbital stage is used to accelerate the payload from scramjet shutdown to low Earth orbit. The trajectory simulator includes a full spheroidal, rotating Earth model, a fourth-order gravitation model, and an MSISE93 atm model. A gradient projection optimization routine is used to achieve an optimal solution using a set of time-referenced vertical accelerations as optimization parameters. Hypersonic engine performance is determined using a quasi-one-dimensional scramjet model. Results show that a hydrogen-powered scramjet launch system outperforms a hydrocarbon-powered system, due to its higher specific impulse and peak Mach number. Although payload mass fractions are shown to be favorable, the high structural requirements of the scramjet imply that reusability is a key characteristic to make them financially viable. Trajectories were found to be dominated by their lift requirements, which outweighed any performance advantage for hydrocarbon fuels in terms of their better storage capability.