Frontier Research Institute for Interdisciplinary Sciences
Tohoku University

FRIS Interviews #16

FRIS Interviews#16

  • 齋藤 勇士Yuji Saito

    Assistant Professor

    Materials and Energy

    Research Fields

    Space Propulsion Engineering, Combustion Engineering

    Main Research Topics

    Investigation of basic phenomena of micro-diffusion flame

    Development of hybrid rocket space propulsion system

    Development of space propulsion system using metal/water hybrid combustion

    Data-driven sparse sensing

齋藤 勇士

Professor Saito lives by the motto, “differentiate before you discuss cosmology.” Keeping the big picture in mind, he always values fundamental research at hand.

Deeply intrigued by the enigmas of the universe, he is driven by the ambition to develop the world’s highest performance hybrid thruster.

Could you tell us why you chose the National Institute of Technology, Numazu College after graduating from junior high school?


My older brother had a big influence on me. He graduated from a technical high school and now works at Yamaha Motor, making motorcycles. He's always had a passion for DIY and creating new things, and that passion has gradually rubbed off on me. I have vivid memories of spending time soldering with him during my childhood, and the smell of soldering still lingers in my mind. Even though we now live in different cities, we stay in touch through messaging apps. Recently, we had a wonderful reunion during the Golden Week. By the way, Hamamatsu, our hometown, is famous for its delicious eel dishes. I grew up near Lake Hamana, where eels are abundant, and my uncle used to bring fresh eels home. This has made me a big fan of eel dishes. I still savor these delicious dishes whenever I visit my parents.

What were you like as a child?


Since kindergarten, I've always been captivated by the universe. I would spend hours gazing at the sky, wondering about what lay beyond. Initially, my interest was focused on the weather, but as I grew older, my fascination shifted towards space. The desire to explore this vast and unknown world only grew stronger with time. When I enrolled in Numazu College, I had the opportunity to engage in stimulating discussions about the universe with my friends. Although Hamamatsu and Numazu are both in Shizuoka Prefecture, they are about 150km apart, making commuting difficult. That's why I chose to live in a dormitory. Those five years in the dormitory allowed me to make incredible friendships. We would have secret conversations about the universe late at night, long after the lights were out. It's a cherished memory that I still hold dear to this day.

Do you have any interesting stories about those late-night conversations?


My motto is “differentiate before you discuss cosmology.” During my time at Numazu College, we would have lively discussions about space in the dormitory. However, I noticed that we were so engrossed in our fascination with space that we were neglecting our studies. It dawned on me that simply talking about our dreams wasn't enough. Around the same time, one of our friends had to retake a calculus exam. It was a wake-up call for me, and that's when I came up with my motto: “differentiate before you discuss cosmology.” This motto serves as a reminder that I can't just talk about my dreams; I need to focus on the tasks at hand and get them done. Although space engineering is seen as a fancy discipline, even to this day, I always keep this motto in mind, understanding that true success comes from having a solid foundation of knowledge.

In your second year at Numazu College, there were some incredible achievements in the field of physics. Professor Emeritus late Yoichiro Nambu from the University of Chicago, Professor Emeritus Makoto Kobayashi from The High Energy Accelerator Research Organization, and Professor Emeritus Toshihide Masukawa from the University of Kyoto were awarded the Nobel Prize in Physics for their groundbreaking research on the theory of particles. I heard that this remarkable accomplishment further fueled your interest in physics.


Absolutely. It had a profound impact on me. I have immense respect for their achievements, and it felt like something truly groundbreaking was happening. I remember I was really into astrophysics. I would often burst into offices of physics teachers to ask questions and learn from them.

Why did you decide to apply to Hokkaido University's School of Engineering?


I chose Hokkaido University because I had a strong desire to conduct research on rockets under Professor Harunori Nagata (currently a professor of the Laboratory of Space Systems, Division of Mechanical and Aerospace Engineering, Graduate School of Engineering of Hokkaido University), a renowned figure in aerospace engineering. During my time at Numazu College, I visited his lab and had a conversation with him. That experience left a lasting impression on me and motivated me to pursue further education under his guidance. We have remained in touch ever since.

Moving from Shizuoka Prefecture to Hokkaido must have been quite a change.


Definitely. I’m not a big fan of cold weather, and I always felt like my body wasn't functioning at its best during the winter. Even my family would ask if I was doing okay. However, my experience living in the dorms during my time at Numazu College helped me navigate daily life in Hokkaido without too many difficulties.

Can you tell us more about your research at Hokkaido University?


For my research, I focused on developing a new type of end-burning hybrid thruster (rocket). This thruster uses a combination of solid fuel, like plastic, and liquid or gaseous oxidizers as propellants. The unique aspect was that I used stereolithography, a 3D printing technology, to create the rocket. The rocket engine consisted of 433 tiny ports, each 0.3mm in diameter, arranged in a circle of 38mm in diameter. These ports allowed oxygen to flow through for combustion. This concept was initially explored in the 2000s, but the traditional drilling method proved to be challenging and unreliable because drills are prone to breaking or penetrating. Right before I began my PhD program, the University of Tokyo sent a proposal to Professor Nagata about this rocket engine idea. He saw the potential and recommended using a 3D printer to tackle the project. His suggestion really inspired me, so I jumped at the chance and made it the main focus of my doctoral research.

After conducting around 200 combustion experiments, I successfully created the world's first end-burning hybrid thruster. Typically, labs at the university level conduct an average of 100 combustion experiments, but I went above and beyond. I spent long hours in the lab, working on experiments from early morning till late at night. While my peers took turns, I was the only one who stayed in the lab all day. Towards the end, I managed to complete each experiment in about an hour, but it was much more time-consuming in the beginning.

I also explored specific topics, such as how fire behaves under changing pressure, the thrust it produces, and how fire keeps its shape under the influence of fuel, known as flame-holding mechanisms.

“Standing on the Shoulders of Giants”
Always keeping gratitude to forerunners in mind

Can you tell me what you're currently researching?


Right now, I'm working with a startup called ElevationSpace Inc. from Tohoku University. We're developing hybrid thrusters for small satellites to safely re-enter the Earth's atmosphere. We aim to produce the best quality unit in this field. I believe the rockets we're studying are vital for deep space exploration of the Moon, Mars, and Saturn. The flames play a crucial role in the success of these missions. Currently, I'm focused on making sure that each flame behaves as planned in the 433 ports within a 38mm diameter.

It seems like fire behavior can change depending on the conditions.


Exactly. For example, if there are any errors in fuel production, it can lead to unexpected and unstable flame behavior and become a major issue. Moreover, since fire typically burns at temperatures around 3000 Kelvin, approximately 2727℃, the fuel itself reaches extremely high temperatures. This makes temperature control a challenging aspect of my research. So, I have to deal with two extremes in my work. On the one hand, I need to take a macro perspective to assess the overall scope of the rocket system. On the other hand, I have to zoom in on a single fire at a micro-scale.

Can you tell me more about how the collaboration with ElevationSpace came about?


When I joined FRIS in April 2021, Professor Toshinori Kuwahara, who is one of the co-founders of ElevationSpace and an associate professor at the Department of Aerospace Engineering, Graduate School of Engineering at Tohoku University, approached me. We had several conversations where we discussed our perspectives and aligned our visions. After considering the collective benefits of working together, we decided to collaborate. This research has also been recognized and supported by the New Energy and Industrial Technology Development Organization (NEDO) through their Intensive Support Program for Young Promising Researchers.

What is the key to making the hybrid thruster a reality?


One key aspect is understanding the combustion mechanisms of microdiffusion flames. To make practical use of this technology, we need to gather fundamental data on the combustion process, such as how it behaves under different conditions. This data will give us valuable insights into the overall performance of the rocket. Another crucial step is developing a method for analyzing this data. Rocket combustion experiment data has been accumulated and it has now become big data. However, we face a challenge when it comes to loading all this data onto a satellite's CPU. It would inevitably slow down the computational processing. To overcome this, I'm exploring a technique called sparse sensing. By using a relatively sparse representation of data, we can reduce sample complexity and efficiently capture the combustion state. In other words, it's about strategically sampling at a specific temporal or spatial rate to capture the whole picture with a single snapshot.

Ultimately, to have complete control over the rocket, we need to establish effective methods for analyzing combustion data. It's not just a matter of multitasking; it's more like juggling four or five different tasks at once.

What is the attraction of doing research?


For me, the attraction of research is like the concept of "standing on the shoulders of giants." This phrase, popularized by Isaac Newton but originally credited to Bernard of Chartres, captures the essence of research. By building upon the discoveries and work of those who came before us, we can make breakthroughs and uncover new findings. I approach my research with gratitude for the previous studies and researchers who have paved the way. While my contribution may be small in the grand scheme of collective learning, I aspire to leave my mark on the history of research, just as those before me have done.

The success of the hybrid thruster experiment, for example, is a testament to the efforts of many researchers who have delved into microscale combustion mechanisms and worked tirelessly to bring these ideas to life. I believe my achievements are a culmination of their collective efforts, and I should always hold gratitude and respect for those who have paved the way.

Embracing Backcasting to Map out Solutions from Ideal Future

What are the core principles of your research?


The core principles of my research revolve around making a real-world impact and understanding the societal responsibilities that come with it. This perspective, stemming from my engineering background, has guided me throughout my academic journey, from Hokkaido University to my time as a project researcher at the University of Tokyo's Graduate School of Frontier Sciences. Both institutions emphasize a balanced approach to fundamental, applied, and mission-oriented research. Additionally, I often utilize the method of backcasting, which involves envisioning a desirable future and planning backward to the present. This approach is particularly effective for addressing challenges that may undergo significant shifts, as opposed to traditional forecasting that starts from the present and projects into the future.

It can be challenging to envision the future, but it's also exciting to imagine what could be possible.


When tackling significant challenges, backcasting can be a useful approach. However, it's important to lay a solid foundation through fundamental research. I always keep in mind that relying too much on backcasting can bring limitations. As my motto says, "differentiate before you discuss cosmology," we need to have a strong base before looking too far ahead. Consistency and diligence are keys, not just in research but in life as well.

How did you come across FRIS?


I got to know about FRIS through some acquaintances and was impressed by their dynamic and independent approach to research. Initially, I had a conversation with Professor Tomomi Tsunematsu, who leads the Tsunematsu Group at FRIS and specializes in techniques for putting mice to sleep. I also met Professor Takuya Mabuchi from the Institute of Fluid Science during my time as a project researcher at the University of Tokyo's Graduate School of Frontier Sciences. Then, in 2019, I was invited to join Tohoku University's Graduate School of Engineering as an assistant professor and officially became a member of FRIS in 2021. Engaging in large-scale research under the macro-chair system has its own charms, but freedom and autonomy I have as an assistant professor in FRIS really suit me.

What advantages do you find in being a member of FRIS?


There are several advantages to being part of FRIS, one of which is the open and collaborative environment. At FRIS, I have far less situations in which I need to go through extremely formal and polite emails to reach out a professor I’ve never met. Instead, you can simply meet them in person, have discussions, and get advice or solutions that you might not have thought of before. In my first year in 2021, due to COVID-19, I didn't have many face-to-face interactions with other FRIS’ members. However, once the pandemic eased, we organized a gathering, which I took charge of. We had a fantastic lunch and toured each other's labs. During this event, I approached Dr. Yuka Hatano, an expert in facial reconstruction, with a question about analyzing rocket fuel combustion methods. She promptly suggested using CT scans, and we immediately put the idea into action. I was impressed with how quickly collaborations came together. The people at FRIS are always ready to lend a hand when you're stuck with a problem. There is a strong sense of mutual support.

In universities in general, interdepartmental communication is so poor that makes sharing of equipment and machines almost impossible. However, at FRIS, sharing equipment, expertise, and techniques is incredibly easy. We can pursue research aligned with our interests, access the necessary technology and information, and collaborate to bridge any knowledge gaps. In my case, we successfully applied CT scans for rocket fuel analysis, which had positive ripple effects. I appreciate the direct communication with researchers from diverse fields and look forward to participating in more interdisciplinary research in the future.

What did you hope to find when you used CT scans on the rocket fuel?


I wanted to capture the shape of the fuel inside the rocket. In other words, I wanted to use non-destructive testing to examine the internal state of the fuel. After the CT scans, Dr. Hatano promptly shared the data with me. I was blown away by the level of detail—it displayed cross-sections of the fuel with 0.5mm intervals. There was even a video included, which was so vivid that it made me feel like I was a flickering flame. It was truly remarkable.

So, Professor Kaoru Maruta, the director of the Institute of Fluid Science at Tohoku University, is your mentor. How has Professor Maruta influenced your research?


I'm really grateful to have Professor Maruta as my mentor. He not only provides me with lab space for experiments but also eagerly helps me when I struggle with understanding the basics or difficult literature. Moreover, he encourages discussions and helps me connect with other colleagues at FRIS.

What kind of researcher do you hope to become in the future?


I aspire to make meaningful contributions to space development. To achieve this, I’m seeking to conduct my research with broad perspective. On this foundation, I approach my research with an open mind and actively share techniques, knowledge, and even the challenges I face. Furthermore, I aim to be a researcher who can always engage in open discussions and collaboration.

What are your thoughts on the future of FRIS?


I believe FRIS has always had a global perspective, continuously delve into interdisciplinary research, and reevaluate the role of young researchers both in Japan and worldwide. I strongly resonate with this principle and hope it remains a cornerstone of FRIS' philosophy. Personally, I also highly value a global outlook and am dedicated to exploring the potential of interdisciplinary research in the future.

(Interview conducted in May 2022)