Analysis of the maximum flight Mach number of hydrocarbon-fueled scramjet engines under the flight cruising constraint and the combustor cooling requirement

Scramjet is the optimal propulsion system for hypersonic vehicles, and it is important to study its maximum flight Mach number. Upper limit and its influence factors of the flight Mach number of dual-mode scramjet used in hypersonic vehicles are investigated in this paper. Scramjet performance analysis model with the constant-area heating process, cooling requirement analysis model of the combustor and cruise model of the scramjet-powered hypersonic vehicle are established. Specific impulse of dual-mode scramjet engine with hydrocarbon fuel is similar to that of liquid oxygen kerosene rocket engine when the freestream Mach number is above 10. The ratio of hypersonic vehicle reference area to scramjet inlet capture area is introduced to establish the size relationship between the scramjet engine and the hypersonic vehicle. Considering the balance of thrust and drag, smaller ratio of hypersonic vehicle reference area to scramjet inlet capture area leads to higher flight Mach number that hypersonic vehicle can cruise. When the angle of attack is 10 degrees, the ratio of hypersonic vehicle reference area to scramjet capture area should be less than 5.5 to ensure that the vehicle can work at Mach 8. The maximum length to diameter ratio of the combustor is used to indicate active cooling requirements, and it is less than 5 under the cooling requirement when flight Mach number is above 9. Reducing the heat flux at the combustor wall and increasing the fuel heat sink is helpful to increase the allowable combustion chamber length, so as to increase the maximum flight Mach number of the scramjet engine on the premise of meeting thermal protection requirement.