Hydrogen hypersonic combined cycle propulsion: Advancements, challenges, and applications

In the progress toward net zero, two propulsion systems are attracting increasing interest, one based on energy stored in batteries, and the other based on energy stored in hydrogen. In between the applications where hydrogen energy storage has more advantages, there is hypersonic propulsion, which is the object of this perspective. Hypersonic vehicles are aircraft or missiles able to travel at speeds above Mach 5. There are four main types of hypersonic vehicles, spaceplanes, glide missiles, cruise missiles, and aircraft. While glide missiles generally do not need propulsion, cruise missiles, and aircraft are designed to sustain hypersonic speeds for extended periods. They use advanced propulsion systems, which include scramjets, relying on the forward motion of the vehicle to compress incoming air for combustion, and featuring supersonic combustion. While scramjets work well at hypersonic speeds, they must be integrated with other propulsion systems, such as ramjets, rockets, and turbojets, to move back and forth from zero speed. Spaceplanes have been designed so far by using only rockets or integrating rockets and turbojets for better operation in the atmosphere. Spaceplanes' operation in the atmosphere could also be further improved by using ram/scramjets. Point-to-point hypersonic travel for civil applications, which is the focus of this work, requires significant advantages in jet propulsion systems that have to work over different regimes, from subsonic to transonic to supersonic to hypersonic. The work reports on the challenges of hydrogen hypersonic combined cycle propulsion.