China Reveals Key Test Progress On Hypersonic Combined-Cycle Engine
Chinese engineers say ambitious turbo-aided rocket and scramjet are on track for 2025 flight tests
Apr 10, 2017
| Aviation Week & Space Technology
Hyper Hybrid
Chinese engineers will test a prototype combined-cycle hypersonic engine later this year that they hope will pave the way for the first demonstration flight of a full-scale propulsion system by 2025. If successful, the engine could be the first of its type in the world to power a hypersonic vehicle or the first stage of a two-stage-to-orbit spaceplane.
Combined-cycle systems have long been studied as a potential means to access to space and long-range hypersonic vehicles because they use both air-breathing and rocket engines to enable aircraft-like operations from a standing start to cover a wide range of speeds and altitudes. Such systems also take advantage of using atmospheric oxygen for fuel.
Various turbine, rocket and ramjet combinations have been studied in the West for decades, but it seems that a new Chinese-developed variation on this theme—the turbo-aided rocket-augmented ram/scramjet engine (TRRE)—appears to be closest to becoming the first practical combined-cycle propulsion system. Developers at the Beijing Power Machinery Research Institute say the engine will have sufficient capability to power horizontal-takeoff-and-landing hypersonic “near-space reconnaissance-and-strike” vehicles, two-stage-to-orbit and even the single-stage-to-orbit vehicles.
New Hypersonic Power Option
Turbo-aided rocket-augmented ram/scramjet combined cycle (TRRE) set for free jet testing this year
Concept combines three main propulsion systems—turbine, rockets, ram/scramjets—in just two main ducts
Capable of operations from zero to Mach 6+, with targeted top speed in Mach 10 range
Targeted at near space reconnaissance and strike platform vehicles, two-stage and single-stage-to-orbit vehicles
Although similar to several earlier combined-cycle concepts, including the Trijet proposed by
Rocketdyne in 2008, the TRRE incorporates the three main propulsion systems in just two main ducts. The TRRE consists of a turbine, liquid oxygen/kerosene-liquid-fueled rockets and a kerosene-fueled ram/scramjet combined with a common inlet and exhaust and is designed to operate from a standing start to Mach 6+. The turbine, which operates from take-off to Mach 2, is housed in an upper low-speed duct, while the ramjet and rockets are located in the lower high-speed duct. Two rockets are mounted in the duct; an aft-mounted rocket for transonic acceleration and mode transition, and a main rocket mounted farther forward in the duct for flame stabilization during acceleration through to scramjet transition at Mach 6.
Updating test progress on the TRRE at the AIAA/China Academy of Engineering International Space Planes and Hypersonic Systems conference in Xiamen, Wei Baoxi of the Beijing Power Machinery Research Institute says simulations and experiments over the past two years have “validated its comprehensive advantages for acceleration, cruise, mobility and other aspects. The results show that the TRRE engine can reconcile the demands of high thrust at lower Mach numbers and high specific impulse at a Mach number of 6.0.”
For a typical cycle, the TRRE would operate in the turbine mode for takeoff with both ejector rockets in the high-speed duct, or channel, augmenting thrust to overcome transonic drag. Around Mach 2, the low-speed duct is closed and the engine transitions to using the ramjet and rocket/ramjets in the high-speed duct. From Mach 3 to Mach 6, the engine operates in ram mode and rocket ram mode using both the high-speed inlet and the forward-mounted ejector rocket in tandem. The engine enters scramjet mode with the activation of the rocket/ramjet compound combustion chamber at Mach 6.
The TRRE combined-cycle system integrates a high-speed turbine, rockets and ramjets in an “over-under” two-duct configuration. Credit: Beijing Power Machinery Research Institute
“The main advantage of the TRRE is that it can solve the problems of an RBCC at low thrust and low speed by using the turbine engine for takeoff and landing as well as low-speed flight,” says Baoxi. “The second advantage is that with the rocket engine it solves the problem of the TBCC transition thrust ‘pinch,’ and it can also achieve a high specific thrust from Mach 3 to Mach 10. If integrated well, it will provide smooth mode transition and solve the thrust gap between the turbine and ramjet as well as provide a wide range of thrust capability between subsonic, supersonic and hypersonic conditions. It will also be good for acceleration and maneuvering. The configuration will also enhance the stability of engine operation under extreme conditions using the combustion and steady flame effect of the rocket gas jet. Using these, we can expand the boundaries of stable operation,” he adds.
Numerical test results of the TRRE prototype show it can “operate in the full flight envelope of Mach 0-6+ and have demonstrated the integrated high- and low-speed channels work cooperatively,” says Baoxi. “They also show reliable power-mode transition and the feasibility of the rocket/ramjet working in cooperation in the high-speed channel over an extremely wide speed range between Mach 1.5 and 7.”
In 2016, developers completed inlet and nozzle wind-tunnel experiments as well as direct-connect test rig evaluations of power-mode transitions at Mach 1.8. Testing in the direct-connect rig was also performed to assess steady state performance between Mach 2 and 6. “The results verified the design methods of the TRRE inlet, nozzle and combustor. And the thrust performance obtained by the power mode transition experiments show the engine can achieve a reliable shift from the turbine mode to the rocket-ramjet mode,” says Baoxi. “When the scale effect is taken into account, thrust at the power mode shift state can reach around 16,000 lb. [8 tons] for an engine with the capture area of 1 m2, which basically meets the requirement of the vehicle design,” he adds.
One of the biggest milestones for the program will occur later this year, when developers plan to conduct free jet tests of the engine for the first time. The work will evaluate the TRRE through power-mode transitions and steady state operation at Mach 2-6 and forms the heart of the first development phase, which is focused on proving core technologies and overall operations. During this phase, which runs through 2020, Baoxi says: “We plan to adopt a small turbine for the prototype to verify the working principle.”
Baoxi indicates that the turbine for the ground prototype will be an off-the-shelf, low-bypass engine which is capable of around Mach 0.8. However, he adds that the engine will be adapted through unspecified means to represent conditions at Mach 1.8, which is the lowest mode transition speed already tested in the direct connect rig. “So it can be used to validate our operating principle,” he notes.
For the follow-on flying demonstrator, Baoxi says the turbine will likely be based on the WS-15, a super-cruising turbofan under development by Xian Aero Engine Corp. for later production versions of the twin-engine Chengdu J-20 stealth fighter. However, even though the initial batch of J-20s entered service early this year with the People’s Liberation Air Force, they are believed to be powered by an interim variant of the Russian-made Saturn AL-31 rather than the WS-15. An official quoted on the website China Military Online on March 13 commented that although WS-15 development is proceeding well, overall progress for production readiness has been hampered by quality control issues with relatively recently developed areas of advanced engine technology for China, specifically single-crystal superalloy turbine blades and powder metallurgy superalloy turbine disks.
It is unclear if the targeted thrust of the WS-15 (believed to be more than 40,000 lb. when installed in the J-20) is suited to the transition Mach numbers aimed at for the flying demonstrator planned for the second development phase in the 2020-25 time frame. “Before 2025, an in-service mature turbine engine will be adopted to form the engineering program and support completion of the small horizontal-takeoff-and-landing free-flight test vehicle,” says Baoxi, who confirms the aircraft will conduct the tests from a runway rather than being air-dropped from a carrier aircraft.
Phase three, running from 2025-30, will focus on development and integration of an advanced high-speed turbine engine into the TRRE. Program success will also hinge on parallel breakthroughs in “the operation of the scramjet at higher Mach numbers, particularly in technology areas such as the adjustable combustion chamber ramjet suitable for a wide range of work,” says Baoxi. In addition, development of a high-efficiency precooling system will be required. Preliminary work to support this is underway at various sites in China. Once combined with these enhancements, he adds, “the operating range of the TRRE engine can be further expanded.”