This paper provides an overview of the most critical combustion instability in the development of liquid propellant rocket engines, focusing primarily on coaxial injector rocket engines using liquid hydrogen and liquid oxygen as propellants, which is also used in the rocket engines of Japanese H3 Rocket. The historical development, recent technological trends, and future directions of this technology are examined from the author's perspective.

Flame dynamics via interaction of twin buoyancy-controlled flickering flames has been studied and harmonics oscillation behaviors observed in the present system have been examined experimentally. Twin identical methane-air diffusion flames located at the same height formed over round-jet are used for the present purpose and their distance is carefully controlled to observe either in-phase and anti-phase harmonics oscillation modes. Methane ejected speed is slow enough to stay as buoyancy-controlled flame exhibiting the flickering motion, especially focused on the mode transition condition and its controllability. Flickering frequency is monitored using the fine thermos couple(s) located near the flame base. Schlieren image and direct movie/photo of flames are taken for further discussion. Using variable jet speed and burner diameter, it is successfully found the feasible scaling law to summarize the mode transition behavior. With varying the distance in time at prescribed frequency under well-controlled condition, “death mode (flickering is totally suppressed)” of interacting flames can appear even with twin-flame interaction system, which has first ever reported. This evidence clearly implies that there is system delay time to appear the instability driven by buoyancy and even controlling the viscous layer associated with the flame in time works effectively to suppress the further growth of the instability. Through the present attempts, it is revealed the possibility to control the appearance of harmonics oscillation mode induced by buoyancy, which may not be valuable in engineering application yet it is valuable in combustion science.

This paper shows the effects of sound interference with combustion, with some examples of previous studies. In the early days of research, sound was treated as an object to be suppressed because it destabilizes the combustion field. Since sound promotes mixing, some studies have actively used sound to suppress harmful emissions. Research was also conducted to improve combustion locally and effectively using high-frequency sound. In recent years, the use of high-frequency sound was specifically proposed to improve combustion in gasoline engines and as a fire extinguishing method. We would like to introduce several topics related to the interference between combustion and sound that have been studied from a wide range of perspectives and examine their effectiveness.

In the context of accelerating research and development of low-emission combustors, the significance of combustion instability has been increasingly recognized as a major technical challenge. Combustion instability, characterized by thermoacoustic phenomena with large amplitude pressure fluctuations, can lead to fatigue failure and damage of engine components. Consequently, keeping pressure fluctuations below the required levels is a crucial requirement in engine development. Hence, the suppression and avoidance of combustion instability have long been one of the major themes that have captivated the combustion community. This paper presents two recent research cases on combustion instability conducted by Japan Aerospace Exploration Agency. The first study delves into the phenomenological understanding of combustion instability in premixed hydrogen flames in low-swirl combustors. The second study focuses on developing technology for the early detection of combustion instability precursors under realistic engine conditions in low-emission aeroengine combustors. In both cases, “low-emission combustors” is a critical keyword, and these themes are gaining increasing attention in the global combustion community.

This review explores the combustion characteristics of aluminum (Al) dust clouds, which are gaining prominence as potential energy carriers. The combustion behavior of Al dust is influenced by factors such as dust concentration, particle size, and oxygen concentration. Despite extensive research, the intricate mechanisms underlying Al dust combustion remain partially elusive. Addressing this gap requires further experimental investigations and numerical analyses, including microgravity experiments, to unravel the precise flame propagation mechanisms. By advancing our understanding, we can enhance safety practices and optimize energy utilization. Additionally, comparative studies with other combustible dusts and practical industrial applications will contribute to harnessing Al dust as an efficient energy carrier.

Hypergolic ionic liquids (HILs), spontaneous ignition upon contact with an oxidizer, are expected to be non-toxic rocket fuels replacing hydrazine. Designing and tuning ILs while understanding reaction mechanisms at the molecular level is mandatory for their applications. This study comprehensively explored a set of reactions for an anion [N(CN)₂]⁻ or [BH₂(CN)₂]⁻, reported to be a component of HILs, with HNO₃ using the quantum chemical Artificial Force Induced Reaction (AFIR) method. For both cases, electrostatic interactions between HNO₃ and anions via protons have triggered the reactions. However, the pathways after the formation of hydrogen-bonded complexes were found to be quite different from each other despite the similarity of the anion structures. Specifically, nucleophilic attack by NO₃⁻ was observed for [N(CN)₂]⁻, but not for [BH₂(CN)₂]⁻. These differences were explained by the natural resonance theory (NRT); that is, the charge distributions of the anions governed the reaction pathways. A set of energy diagrams and electronic structure analyses is necessary to understand hypergolicity.