Research and development on inlets for rocket based combined cycle engines
Introduction
A rocket based combined cycle (RBCC) engine effectively combines rocket engines of high thrust-to-weight ratio and an air-breathing engine of high specific impulse in a single flow passage, while exhibiting excellent performance over an extremely wide flight range [[1], [2], [3], [4], [5], [6]]. An RBCC engine can commonly take off from the ground and operate over the entire ballistic trajectory with various modes, including the ejector, ramjet, and scramjet mode. It is expected to be one of the most promising propulsion systems for reusable space transportation and hypersonic cruise vehicles [[7], [8], [9]].
For a ram-/scram-jet propulsion system without the rotating compressor/turbine components, the desired inflow and compression ratio for the combustor over the entire range are provided totally by the inlet in a controllable and reliable manner with minimum aerodynamic losses (i.e., maximum compression efficiency or minimum entropy increase) [[10], [11], [12]]. Owing to the wide range of flight conditions to be encountered in supersonic and hypersonic flight, the chosen configuration of the inlet must be able to provide flows for several interrelated (and frequently conflicting) design requirements simultaneously. Therefore, it is well known that the performance of the inlet plays a vital role in the overall performance of an airbreathing engine [13,14]. Any well-designed supersonic inlet should deliver good performance in the start, air capture, compression, low drag, total pressure recovery, back pressure resistance, and airflow quality in the entire operational domains [[15], [16], [17], [18], [19], [20]].
Specially, since the operation range of an RBCC engine is much wider than the conventional aero-engines, which covers static, subsonic, transonic, supersonic, and hypersonic flight regimes through multiple modes, the engine thus has much stricter requirements on its inlet. Although an RBCC inlet is usually designed for super/hyper-sonic speeds and does not have to be tuned only to the sub/trans-sonic regime, nevertheless, as a highly integrated propulsion system used over an extremely wide range of Mach numbers and altitudes, an RBCC engine is required to operate in multiple modes using the same inlet. Or rather, as an essential part of the engine, an RBCC inlet must work in the ejector mode, although the flight conditions are far off its design conditions, or even it is unstarting at most of time. Therefore, the design and control of an RBCC inlet are much more complicated. In a functional point of view, an RBCC inlet is mainly used as a flow passage for the air ejection at low speeds in the ejector mode, whilst as a component for efficient air capture and sufficient compression at high speeds in the ramjet and scramjet modes. Obviously, the requirements on the functions of the RBCC inlet vary significantly in different operational modes, and the design thus has very distinct or even contradictory principles. In addition, the airframe/engine integration structure is the mainstream configuration in most of the current flight vehicles using RBCC propulsion systems. Herein, the RBCC inlet not only affects the performance of the RBCC engine itself, but also significantly influences the aerodynamic shape design and the flight control of the entire vehicle. Therefore, an RBCC inlet has an important and even a dominating role in the overall design, aerodynamic configuration, flight attitude control, and engine performance of the RBCC-powered flight vehicle. To design an RBCC inlet with comprehensively excellent performance in an extremely broad flight range on the premise of good structural accessibility and maneuverability is an important goal that lots of researchers have been pursuing for decades.
Historically, a few overviews or reviews on the RBCC propulsion system have been provided in the open literature and reports. The developments of RBCC engines from the late 1950s to 1989, as well as the results of representative research were summarized by Foster and Escher in 1989 [21]. In 1998, Daines and Segal presented a review on the combined rocket and airbreathing propulsion systems for earth-launch applications [22]. Selected technical issues involved with these engines were generally discussed, including the airframe/engine integration, flow passage design for multi-mode operation, fuel selection, mixing enhancement and afterburning in the ejector mode, thermal choking, and flameholding, etc. Fry reviewed the worldwide development of ramjet propulsion since the early 1900s in 2004 [23]. Wherein, the generation and development of some RBCC propulsion systems were presented, such as the Ejector Ramjet (ERJ) engine (a precursor of today's RBCC family of engines), Integrated System Test of an Airbreathing Rocket (ISTAR), etc. Hiraiwa, Ueda, Masuya, et al. delivered the research progress of Kakuda Space Center (KSC) in the Japan Aerospace Exploration Agency (JAXA) on the design, aerodynamics, supersonic mixing and combustion, structure and materials for RBCC engines continuously in 2006 [24], 2008 [25] and 2013 [26]. In 2007, Liu et al. [27] introduced the fundamental concepts and principles of RBCC, and summarized the research progress of RBCC engines in different countries, including the U.S., Japan, Europe and China, etc. Huang et al. [28] delivered a survey on the mode transition of several combined cycle engines in 2014, including the typical ejector-to-ramjet mode transition and ramjet-to-scramjet mode transition concerning RBCC engines. Later in 2015, Cui et al. [29] reviewed the research progress of combustion organization of RBCC engines, in the aspects of fuel injection scheme, combustion mode, as well as thermal throat adjustment. In 2019, Zhang et al. [30] summarized the worldwide progress in the overall layout of different types of RBCC engines, in terms of rectangular and axisymmetric configuration engines. Also in 2019, Wang et al. [31] presented the systematical progress and achievements of the RBCC engine research in Northwestern Polytechnical University during the past two decades, with coverage of the theoretical analysis and principle verification, component design and test, and small-scaled prototype integration and verification, etc.
Based on the existing reviews, it is found that the RBCC inlet is recognized as a crucial component to an RBCC engine and the variable geometry RBCC inlet technology is considered as an inherent core technology. However, there have been few dedicated reviews on the inlets for RBCC engines so far. It is of instructive significance for deep understanding of the distinctive operational characteristics of the RBCC inlets and further optimizing the designs and applications of RBCC engines to comprehensively study, summarize, and analyze the research progress of RBCC inlets.
This paper presents a detailed review and analysis on the research and development of inlets for RBCC engines. Section 2 gives an overview of the RBCC inlet research progress in different historical periods of RBCC development in different countries. Key technical and scientific issues concerning RBCC inlets are summarized in Section 3. Following in Section 4, several typical RBCC inlet applications and the corresponding research are reviewed and analyzed to provide clear and systematical ideas as well as appropriate methods through the entire work for the relevant researchers. Finally, suggestions on the further RBCC inlet research are proposed in Section 5 based on the analysis of the research progress.
Section snippets
RBCC inlet progress overview in different development periods of RBCC
The concept of RBCC has been put forward in the late 1950s and has been developing continuously. In different historical periods, many countries have conducted different degrees of research on RBCC and have made lots of remarkable achievements. It can be found from various open literature and reports that RBCC has already been studied for nearly 70 years. Accordingly, we divide the RBCC development roadmap into three major stages, including the proof-of-concept (POC) period, the rapid
Key technical and scientific issues of RBCC inlets
Based on the literature review and analysis in Section 2, this section sorts out some key technical and scientific issues that are unique to the RBCC inlets.
Typical RBCC inlets
This section sorts out the primary features of some typical RBCC inlets developed in different historical periods, as well as the corresponding research progress of these inlets. To be mentioned, this section only focuses on the RBCC inlets in the full sense, and does not cover the more sophisticated combinations of turbojet, rocket and scramjet engines in some open literature, such as the Trijet and TRRE, etc.
Conclusions and research recommendations
An RBCC inlet commonly operates in an extremely wide flight range through multiple modes, covering static, subsonic, transonic, supersonic, and hypersonic speeds. It plays a vital role in the RBCC propulsion system and the entire flight vehicle. This paper comprehensively and systematically reviews the research progress on inlets for RBCC engines in different countries and different historical periods of the RBCC development. Distinctive operational characteristics of RBCC inlets and key
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research work is supported by the National Natural Science Foundation of China through grant 51976171 and 51606156.
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