Recent tests of the US Air Force’s most recent hypersonic development program, ‘Project Mayhem’ creates a vital avenue for the development of hypersonic aircraft, both military and civilian. The project, under the purview of the Air Force Research Laboratory (AFRL) is designed as a hypersonic missile – larger than the Air-launched Hypersonic Weapon (ARRW) also under development by the Air Force.
Whilst details on the Mayhem project are scant, its capabilities would be transformative for hypersonic aircraft engines. Existing models of hypersonic aircraft, such as the recently showcased Hermeus project ‘Quarterhorse’, uses a turbine-based combined cycle which they claim is capable of be launched using a conventional General Electric J85 turbojet which pivots to a separate hypersonic engine when it approaches hypersonicspeed. The existing demonstrator displayed by the company, does not show how it will achieve that without accumulating significant drag or weight.
The potential of viable multi-cycle designs is hugely significant for both weapons and aircraft concepts, and unlike existing hypersonic projects – it could produce a vessel that is capable of hypersonic flight and could land afterwards, making it reusable. Whilst this development would be more significant for hypersonic aircraft, it would also have utility for hypersonic missiles – providing them with the capability to adjust between speeds depending on mission requirements.
Flying at hypersonic speeds creates further problems not just with standard aviation fuel solid-fuel rocket systems face challenges in high altitudes where hypersonics typically operate and research is being conducted into advanced fuels to provide wider flammability limits as well as longer ranges. European tests of hypersonic aircraft have used hydrogen as fuel, this is because liquid hydrogen is not as combustible as hydrogen reducing the chances of explosion or fire, but this will incur significant costs compared to traditional fuel.
Multi-cycle engines are an area of increasing investment, with Rolls-Royce working with Reaction engines for combined cycle engines for use on a space-launch ship and DAPRA also has a multi-cycle program under development, titled the Advanced Full Range Engine (AFRE) program. The development of Mayhem has transformative capabilities for hypersonic flight and will likely provide existing programs with the technology needed for flight.