A Novel Turbo-aided Rocket-augmented Ramj et Combined Cycle Engine Concept LING Wenhui WEI Baoxi LUO Chunqin LI Tinghe GANG Qiang

Beijing Power Machinery Research Institute, Science and Technology on Scramjet Laboratory, Beijing 100074, China

Abstact： A novel multi-mode propulsion system is proposed for potential application to hypersonic planes. A unique feature of this concept is the use of mature turbo engine combined with RBCC. Its configuration is similar to TBCC, but the tunnel is replaced by RBCC. turbo engine act as the primary thruster for Mach numbers in the range of about 0-2, and RBCC for Mach numbers of 2-6+. The obvious advantage is the high special impulse performance and high thrust-to-weight ability. But not like TBCC, it eliminates the need of advanced turbo engine which can work at Mach numbers of 0-4. It uses RBCC to resolve the relay problem from turbo mode to Ramjet mode. And with a suitable use of air augmented rocket, this new combined engine can provide high thrust-to-weight performance and extremely raise the stable combustion ability of ramjet mode at low pressure conditions. This allows the hypersonic plane to obtain the subsonic and hypersonic long-range cruise ability, the high maneuvering ability at hypersonic flight condition and high ceiling altitude hypersonic cruise ability. Preliminary performance estimates suggest that thrust and are comparable or superior to existing TBCC designs. keywrods: aerospace aircraft combined engine

1 Introduction As an ideal propulsion device for the wide range flight of future near space aircraft and aerospace vehicle, the combined engine is integrated with two or more types of engines through the structure or thermodynamic cycle. Compared with the single type of engine, it provides the advantages of wide working range, high average specific impulse, as well as flexible and convenient operation by playing the technical advantages of different engines within the scope of their respective work. At present, a series of combined engines, including Turbo-Based Combined Cycle (TBCC), Rocket-Based Combined Cycle (RBCC), Air Turbo Ramjet (ATR), Synergistic Air Breathing (SABRE) and TRIJET, have been largely invested and deeply investigated in world wide.

Air flow Low speed nozzle Low speed inlet High speed inlet Dual-combustion High speed nozzle mode ramjet

Compressor Air duct Igniter Spindle

Air

Generator Plenum Turbo Mixing Cooling Nozzle channel

Fig.1 Type of combined engine 2 State-of-the-art and trend analysis on typical combined engine 2.1 State-of-the-art （1）TBCC As a thermal cycle combination of turbo engine and ram engine, TBCC is divided into two types, namely series and parallel[1,2], as shown in Fig.2. Providing a higher specific impulse, TBCC is more suitable for long-distance cruise flight. However, TBCC has the disadvantages of relatively low thrust at hypersonic velocities, weak acceleration and maneuvering capability, difficult mode transition between turbo and ramjet, and strong dependence on the advance breakthroughs in high speed turbo technology.

Diffuser Duct valve Fuel injector Flame holder

Jet engine

(a)Series TBCC engine

jet engine

Air flow Low speed nozzle Low speed inlet High speed inlet Dual-combustion High speed nozzle mode ramjet (b)Parallel TBCC engine Fig. 2 Schematic diagram of TBCC engine

The United States is far ahead in the study of TBCC engine. The Ma3 TBCC was firstly adopted by the SR-71. In recent years, key technology research of the Ma6 TBCC is in sustainable development. Recently proposed SR-72 aircraft plan will gradually access the engineering application after 2030. At present, high speed turbo technology has yet to break, and it is very difficult to achieve the effective transition between the turbo mode and ramjet mode. This may be the largest limitation on the development of the Ma6+ TBCC. （2）RBCC engine RBCC is integrated with the ramjet engine and rocket engine through the thermal cycle and structure integration[3,4], as shown in Fig. 3. Be able to operate in the wide range of Ma0-25, H=0-orbit altitude in theory, RBCC can provide the power for the single stage access to orbit, as well as the first and second stages of two stage access to orbit. However, its application is restricted by the relatively low specific impulse and thrust in the low speed ejector mode.

Fig. 3 Schematic diagram of RBCC engine In the USA’s 2030 air breathing propulsion technology roadmap, RBCC engine is considered as one of the most potential candidates for space access. However, due to the strong innovation, difficulties in achieving the ejector thrust gain and mode transition, as well as slower than expected development on the key technologies of Ma4-8 scramjet, present technical maturity of RBCC stays at 4-5 Levels. Based on the NASA’s technology roadmap, the flight demonstration test of RBCC will be conducted between 2020-2025 to validate the mode transition technology. （3）ATR engine ATR engine is an combination of aero-engine and rocket engine/ramjet engine. Its main characteristic is that the turbo for driving the fan is driven by the gas from the gas generator or heated expanding hydrogen. Be able to operate in the range of Ma=0~5, and H=0~25km. The ATR engine can act as the first stage aircraft of the earth-to-orbit transportation system. However, it has the disadvantage of lower specific impulse than the ramjet engine at high Mach numbers. According to the gas source driving the turbo, the ATR engine can be divided into Air Turbo Ramjet Gas Generator (ATRGG) and Air Turbo Ramjet Expansion (ATREX). Fig.4 shows the operational principle. Compressor Air duct Igniter Combustor Spindle

Air

Generator Plenum Turbo Mixing Cooling Nozzle channel

(a)ATRGG engine

(b)ATREX engine Fig.4 Air Turbo Rocket/Ramjet Engine Since 1932, the concept scheme of ATRGG engine has been presented by the USA. Some ground tests have been conducted on the basis of tactical missile. After partial flight tests in 1990s, no clear development plan is introduced. In Japan, the investigation on the ATREX engine has begun since 1986. In the early 1990s, large amount of prototype ground tests and pre-cooler scheme investigations have been conducted. In the early 21 century, the technology research was directed to the pre-cool vortex jet engine. （4）SABRE engine Combining the technologies of turbo, rocket and ramjet, the SABRE engine provides two operating modes, namely air-breathing mode and rocket mode, as shown in Fig.5. In air-breathing mode, the maximum operating Mach number can reach 5.5 at which the engine is transited to the rocket mode.[8,9] The SABRE engine can operate in full range, and achieve a higher specific impulse than the rocket engine. Simultaneously, the light weight air pre-cool technology can greatly decrease the weight of the engine. However, the operating range of the air-breathing mode is relatively narrow, resulting in a limited enhance on the rocket. The restriction of adopting the low-temperature hydrogen fuel leads to a large aircraft, which is not convenient enough.

Fig.5 Schematic diagram of SABRE engine[9] The UK Skylon R & D company claimed that their power plan has achieved a great progress. They plans to conduct the whole machine test of the SABRE engine in 2019. The flight tests of the Skylon aircraft will be conducted after 2022, and the commercial operation will be put into in 2024. （5）TRIJET and T/RBCC engine TRIJET engine is powered by turbo engine, ejecting ramjet engine, and dual-mode ramjet, as shown in 7. It has three channels in which respective engine is installed [10,11]. The TRIJET engine can operate in the range of Ma=0-6+ and H=0-30km. By adopting the mature turbo engine, the stable transition between the turbo engine and dual-mode ramjet can be well realized through the ejecting ramjet.

Fig. 6 Schematic diagram of TRIJET engine TRIJET is presented by Aerojet Corporation and is the second generation of the Pyrojet engine. The Pyrojet engine is composed of two high speed Ma4 turbo engines and a dual-mode ramjet. Restricted by the development of the high speed turbo engine, the channel of one turbo is changed to be that of an ejecting ramjet, while that of the other turbo only adopts the mature Ma2.5 turbo engine. The ejecting ramjet ensure the stable transition between the turbo engine and dual-mode ramjet. The TRIJET engine can achieve the hypersonic flight in the absence of Ma4 turbo engines. However, it has complex system and large structure, which may lead to a lower thrust-weight ratio. At the same time, Aerojet Corporation presents a turbine-and-rocket-based conbined-cycle engine, which consists of an air inlet, a common noozle, a turbine, a DMRJ and rockets.

Fig. 7 Schematic diagram of TRIJET engine 2.2 Development trend and Revelation In a word, the development of combined power technology has the following main features: （1）In TBCC, there is a thrust trap between the turbo mode and ramjet mode. “The turbo cannot go up; while the ramjet cannot come down”. This is one of main technical problems which restricts the development of the Ma0-6 parallel TBCC engine. （2）The RBCC can act as the lower stage or upper stage of two stage access to orbit with different separation Mach number; The RBCC has an inherent disadvantage of low specific impulse in the low speed ejecting mode. This problem is hard to overcome in short time, and can be avoided using the airborne, turbo, and ejection. （3）Due to the low performance and complex structure of ATRGG and ATREX engines, the related investigations have been reported rarely in recent years. （4）With the sustainable development of turbo, rocket and ramjet, the "TRI-combination" power based on these technologies has become the hot research direction in horizontal takeoff and landing near space and aerospace vehicle power. Several countries have combined their own technical basis and advantages, and actively explored new "TRI-combination" engine program. The related concept schemes, such as SABRE and TRIJET, have been validated in varying degrees. After more than two years of in-depth demonstration, Beijing Power Machinery Research Institute creatively put forward a Turbo-aided Rocket-augmented Ramjet Combined Cycle Engine (TRRE), which provides a new thought for the development of the aerospace power.

3 Concept scheme and technical challenges of TRRE 3.1 Concept scheme As a kind of air-breathing combined cycle engine, the TRRE is highly integrated with the turbo, rocket, and ramjet through the structure integration, thermal cycle and working process. By using the oxygen in the atmosphere adequately, the TRRE can realize the comprehensive optimization of thermodynamic cycle efficiency, structure volume and weight. Aided by the auxiliary function of the turbo, the TRRE provides a low fuel consumption rate at low speeds, and is able to realize the horizontal takeoff and landing, as well as low-speed cruise flight like the airplane. Aided by the rocket thrust enhancement, it can improve acceleration and maneuvering, expand the boundary of the stable operation, realize the smooth mode transition, and solve the "thrust trap" in the relay of the turbo and ramjet. Because the Ramjet/Scramjet provides a low fuel consumption rate at high speeds, the TRRE can perform a long time hypersonic cruise. The concept scheme of the TRRE is base on the technology development. Aiming at Ma0-6+, present scheme adopts available Ma2 mature turbo engine and high-low speed channel parallel structure. The work cycle and schematic diagram of TRRE can be seen in Figs. 8 and 9, respectively. Fig. 10 shows the performance envelope of the specific impulse and thrust weight ratio of the TRRE fueled by hydrocarbon. The TRRE provides a high averaged specific impulse. Its maximum thrust weight ratio can reach 12-15. The wide thrust adjustment range helps the TRRE be adapted to a variety of mission requirements. Fuel Inlet Combustor Afterburner Nozzle

` ` Compressor Turbo(Ma2 scalar) Gas LO 2 generator Fuel

Air Fuel Main Combustor Gas Ma0-6+ Ejector rocket Fuel LO2 Fig. 8 Work cycle of recent TRRE plan

Low speed channel

Integrated inlet Ejector rocket High speed channel Integrated nozzle

Fig. 9 Schematic diagram of TRRE

Fig.10 Schematic diagram of specific impulse and thrust weight ratio of TRRE (hydrocarbon) Fig.10 shows the operating process of the TRRE. In the range of Ma=0-2, the engine operates in the turbo mode; in the range of Ma2=2.5, the mode transition is accomplished, and then the hybrid rocket-ramjet combustor starts work. Because this plan is based on the turbo engine with mature technology, the feasibility validation of key technologies can be accomplished before 2020. The engineering transformation ability will be provided between 2025 and 2030.

Ma：0-2 Turbo mode

Ma：2-2.5 Relay mode

Ma：2.5-6 Ramjet/ Rocket-Ramjet mode Ma：6-8 Scramjet/ Rocket-Scramjet mode

Ma：>8 Rocket mode

Fig. 11 Work process of TRRE In future (approximate 2025), with the technical breakthrough of the scramjet with much higher Mach number (Ma10+), the working range of the TRRE can be extended to Ma0-10+ (hydrogen); with the technical breakthrough of the high speed turbo, the working range of the low speed turbo can be extended to Ma4. Consequently, high specific impulse can be achieved in a wider range. After 2030, with the technical breakthrough of the light air pre-cooling and detonation combustion, the TRRE can extend the turbo mode to higher Mach numbers by adopting the pre-cooling technology; the application of detonation combustion can further improve the thermal cycle coupling degree of three powers. These two advanced technologies can effectively enhance the cycle efficiency, reduce the structure weight, which helps to realize the single stage access to orbit. Figs. 12 and 13 show the work cycle of the TRRE. Fuel Inlet Combustor Afterburner Nozzle

` ` Compressor Turbo(Ma4 scalar) Gas LO 2 generator Fuel

Air Fuel Main Combustor Gas Ma0-10+ Ejector rocket Fuel LO2 Fig. 12 Operation cycle of the medium term TRRE scheme

OLOx2idizer Fuel Gas generator Oxidizer Fuel Compressor Turbo Air Fuel Gas Detonative Pre-cooling Inlet combustion Main Combustor Ma0-10+ Ejector rocket Nozzle Fuel Oxidizer

图 13 Operation cycle of the forward TRRE scheme

The TRRE can realize the horizontal takeoff and landing, and fully utilize the oxygen in the atmosphere. It provides higher average specific impulse and stronger maneuverability. By utilizing the mature turbo technology, present dual-channel scheme can effectively solve two technical bottlenecks, namely the thrust gain in the ejecting mode of the RBCC and high speed turbo of the TBCC, which is more feasible in practical engineering. High efficient and organic structure integration put more demand on the material and control, which promotes the development of the engine and related majors. In a word, the TRRE has significant advantages in the aspects of innovation, practical feasibility, technical drive, and extension. 3.2 Technical challenges of TRRE The core technical difficulty of the TRRE is how to combine three types of engines effectively. Due to the obvious difference of three types of engines on the thermal cycle, it is very difficult to not only make each cycle operates under the optimal state of efficiency, but also achieves the structural integration and loss of weight. It is required to focus on the following six key technologies: （1）Integrated technology In the wide working range, it is necessary to take into account the acceleration and cruise performance of the aircraft simultaneously. This makes the working state of the engine change dramatically, and the engine components be not easy to match. The matching of the engine components directly affects the engine performance, and even determines whether the engine can work normally. During the mode transition, it is very difficult to adjust the flow rate and thrust of two channels, and the smooth transition of the thrust is a key technology. （2）Shared and adjustable intake and exhaust technology Due to the wide working range, it is very difficult to match the flow rates of high and low speed channels. The inlet starting and regulation law are complex. The flow distortion in the low speed channel is relatively large, and the flow separation is easy to occur in local region. In the processes of intake regulation and mode transition, the matching between flow rate and pressure, stable operation, as well as continuous thrust should be ensured. （3）Wide range, high efficiency and low resistance combustion technology To take account of the engine performances at high and low Maher numbers, it is required to realize multiple combustion modes (such as ejecting mode , subsonic mode and supersonic mode) as well as smooth mode transition in the same channel. It is very difficult to realize and regulate the thermal throat in each combustion mode, which is the key to achieve high efficient combustion. Besides, the ignition is difficult to occur at low speeds, while the aerodynamic loss is large at high speeds. Consequently, it is required to master the ignition and flame stabilization technology in a wide range. （4）Structural integration ejecting rocket technology under variable working conditions Restricted by the structural integration and limited spatial scale, the ejecting rocket is required to be installed in the ramjet channel, and ensure the high efficient mixing and combustion simultaneously. In the wide working range, the rocket engine should not only provide the ability to regulate the flow rate and mixing ratio. but also combine the fuel supply. The heat management of the active cooling should be guaranteed by the rocket cycle system, which has a significant design difficult. （5）Channel regulation and multi-variable control technology When multi-mode works cooperatively, it requires large amounts of controlling parameters for adjusting the injection pressure and flow rate of the propellants in a wide range, regulating the channel, and conducting multiple start. Both the high coupling degree of controlling parameter and complex controlling function put forward a very high demand on the controlling system of the engine. （6）Reusable lightweight materials and heat management technology The large heat region puts forward a higher requirement on the fuel heat sink. Local heat flux of the ejecting rocket is in the order of 10MW/m2, Multi-adjustable structure is faced with high temperature dynamic sealing problem. Consequently, it is very difficult to perform the heat management. The overall performance puts forward an extremely high demand on the lightweight structure, which is hard to be satisfied by present structure of light metal materials. It is also very difficult to realize the active cooling structure lightweight technology. 4 Concluding remarks The TRRE has the ability to work in the in-near-beyond space, horizontally take off and landing, make full use of the oxygen in the atmosphere, and provide high average specific impulse and strong maneuverability. By combining the mature turbo technology with RBCC in parallel configuration, recent TRRE plan not only avoid the low specific impulse of the RBCC in the ejecting mode, but also solve the “thrust trap” during the relay process between turbo and ramjet. It provides the ability of horizontally taking off and landing, as well as Ma0-6+ cruise flight. In future, after the technical mature of the Ma4 high speed turbo, Ma10 scramjet, and lightweight pre-cooling, the TRRE scheme can be optimized by further improving the cycle efficiency and structural integration.

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