Aircraft or rocket?
HIKARI focused on propulsion systems that require very specialized knowledge owned by only a few entities in the world. Concepts involving one single engine as well as combinations of different engine types have been studied, along with the respective tanks required for each option. This topic also included an assessment of noise at take-off resulting from the options studied.
Subsonic combustion ramjets (operating up to Mach 5) have been largely studied since decades and several countries have developed and operated operational military systems (USA, Russia, France, Germany).
Regarding dual mode ramjets or scramjet technology, there have been ground firing tests (for example France already demonstrated a dual mode ramjet operating from Mach 2 to Mach 7+), and even flight tests of scale models equipped with an engine launched from a rocket or an airplane to test the behaviour at specific Mach numbers.
Within the EU LAPCATII project, some theoretical and experimental studies are being performed with respect to the design of two high-speed hydrogen-fed vehicle concepts, both equipped with dual-mode air breathing propulsive systems. The goal was to demonstrate the feasibility of aerodynamic and propulsive efficiencies allowing long-haul range. That’s why HIKARI focused on two different topics:
- Engines and propulsion architecture: HIKARI contributed to the further comprehension of two options that are being investigated in other projects: Single Engine, and Rocket within a multiple engines concept (such as the ZEHST one in Europe with turbofan, rocket and ramjet technology). In both cases, HIKARI proposed to get a step further the work already on-going with the existing projects. In the case of single engine, an assessment of noise at take-off has also been performed. As for rocket engine, this technology is very mature for space applications, but deserves to be revisited when addressing point-to-point transport of paying passengers. Indeed embedded safety and reusability / ease of operations became a must, and impact to rocket engine design was carefully crafted as such.
- Tanks: two kinds of activities have been conducted: one which is system design oriented, aiming at providing parameters driving efficiency of storing capability of cryogenic tanks, and the other one being more technology oriented, focusing on technology of tank wall.
Future aviation, be it subsonic or supersonic, relied on cryogenic fuels such as methane or hydrogen due to availability or environmental consideration. On a more specific topic regarding rocket related technologies, a safety level and operational capabilities have been achieved, compliant with aeronautic transport standards. Additionally HIKARI project explored strategies to decrease the noise footprint experienced around airports, in particular during take-off and amplified its knowledge on expected noise emission of high speed type of aircraft and researched noise abatement procedures.
PCTJ Mach 4 Wind-tunnel Test
Two propulsion types were specifically considered: the Pre-Cooled Turbojet (PCTJ) and the reusable rocket engine. The fuel system to feed the engines, and especially the tank characteristics were also analyzed for two fuel types: liquid hydrogen and liquid methane. Additionally, the noise of an aircraft propelled by this multi-cycle engine has been simulated in the airport environment. First results, prior to any noise reduction procedure or engine design optimization, indicate noise levels inferior to those of the Concorde. Specific emphasis should be given in the coming steps to prepare the noise regulation applicable to high-speed aircraft and to adjust the design and procedures to minimize the noise impact in the airport vicinity.