By Douglas Barrie, Senior Fellow for Military Aerospace
According to early June reports, the China Aerospace Science and Technology Corporation’s Fourth Academy, which is responsible for propulsion technology, carried out flight tests of a solid-fuel, variable flow ramjet for air-to-air missile (AAM) applications. Confusingly, however, at least two of the reports claim that the AAM’s speed will be hypersonic, accepted generally to be in excess of Mach 5. Supersonic covers the speed regime from Mach 1 to 5.
Ramjet engines are most efficient at supersonic, not hypersonic speeds, where, at least with traditional design approaches, above Mach 5 they become increasingly inefficient. A ramjet operates by slowing the airflow into the engine from supersonic to subsonic speed for combustion; a scramjet, which is designed to operate efficiently at hypersonic speed, does not need to slow airflow into the engine to subsonic speed for combustion (hence the name, supersonic-combustion ramjet, or scramjet for short).
Furthermore, for air-to-air applications, it is arguable as to the overall merit of a Mach-5-plus flight, when weighed against the increased demands such a flight profile would imply, compared to a ramjet-powered weapon.
As yet, the only rocket/ramjet-powered air-to-air missile known to be in service, the European MBDA Meteor, likely has a cruise speed in excess of Mach 2.5 at low-to-medium altitude and above Mach 3 at higher altitudes. The considerable advantage conferred by a ramjet design, when compared to a solid-rocket-motor AAM, is that it offers a higher average cruise speed, greater maximum-engagement range, and the capacity to deal with manoeuvring targets at ranges where a solid-rocket-motor-powered missile would be defeated.
At speeds beginning above Mach 4, and certainly beyond Mach 5, such are the increased temperature demands that different, and more expensive, materials are required for key areas of the airframe, which would drive up the cost of the weapon and likely also complicate manufacturing.
China has been developing ramjet technology applicable to air-to-air weapons for well over a decade. Ground firings of a ramjet-motor-powered AAM airframe were carried out no later than 2007, and several different unofficial configurations have been shown in model form and in artist’s impressions.
This ramjet work forms part of broader Chinese AAM development, including the PL-10 imaging infrared within visual range missile, which has only recently entered service, and the active-radar-guided PL-12 medium-range AAM, which may well be the subject of upgrade work. The active-radar-guided PL-15 is a medium-to-long-range AAM that is yet to enter service, which appears intended for the Chengdu J-20 heavy combat aircraft and upgrades of the Chengdu J-10. Trials of an as yet publicly undesignated very-long-range AAM are also under way.
While the association of hypersonic propulsion with AAMs remains open to debate, China is working on hypersonic engines, as are France, India, Russia and the United States. Early applications of hypersonic propulsion would include high-speed cruise missiles for land or maritime attack.
This analysis originally featured on the Military Balance+, the new IISS online database that enables users in government, the armed forces and the private sector, as well as academia and the media, to make faster and better-informed decisions. The Military Balance+ allows users to customise, view, compare and download data instantly, anywhere, anytime.