Since the development of the German V-2 rocket in the middle of World War II, the design of the liquid-fuel rocket engine has largely followed a simple template. The propellant and oxidant are mixed in the combustion chamber to produce a hot gas mixture under high pressure. It is very hoped that the back of the rocket can be omitted to generate thrust.
However, the Japan Aerospace Exploration Agency (JAXA) Recently successfully tested a different type of rocket called a rotary detonation engineThe engine relies on a completely different method of combustion in order to generate more thrust with less fuel. We will delve into how it works, and how the Japanese test foreshadows the future of this technology.
Deflagration and explosion
Humans like to burn fuel to do useful work. So far, whether it is a steam engine, a gasoline engine, or a rocket engine, all these technologies have one thing in common: they all rely on fuel burned in deflagration. Ever since we started sitting around a campfire, we are all familiar with this easy-to-control slow burning method.
However, there are potential efficiency gains Replaced by burning fuel in an explosionThis is a shock wave generated by combustion, which propagates faster than the speed of sound, thereby further rapidly propagating the knocking reaction, and starts with a huge pressure increase. The main advantage of burning fuel in this way is that more energy can be obtained from the huge pressure increase. Therefore, by releasing more energy from the same amount of fuel, an engine based on the knocking process can theoretically be more energy efficient.
However, there are several problems with operating the engine in a knock-based cycle. It may be difficult to maintain a continuous detonation response. In addition, the large peaks of temperature and pressure during the explosion and the associated shock waves can easily damage or destroy parts made of very hard materials. So far, engineers in many fields have been trying to tame and control the explosion process so that it can be used successfully.
The rotary detonation engine consists of a combustion chamber with an annular structure. In this ring, fuel and oxidizer are injected and ignited by detonating the mixture. The purpose is to make the shock wave of this explosion propagate around the annular combustion chamber, causing further explosions in a continuous cycle.
It turns out that it is difficult to make this concept work. Although the concept was originally developed at the University of Michigan in the 1950s, it was only in the last few years that engineers successfully demonstrated a continuously operating rotary detonation engine.A team from the University of Central Florida In 2020, the hydrogen-oxygen fuel engine was demonstrated, In the test, a thrust of up to 200 lbf (890 N) was generated. This feat is achieved by carefully adjusting the size of the jet of propellant so that the mixture is just right for a controlled explosion. If the mixing is wrong, the fuel will burn in a slower deflagration, with no benefit to thrust or efficiency.
Field test in Japan
Unlike the experiment at the University of Central Florida, the Japanese effort involved the launch of a real rocket. The test uses a standard sounding rocket with a conventional engine to launch the test payload hundreds of kilometers above the earth. The second stage of the rocket is equipped with a rotating detonation engine. The mission was carried out using the sounding rocket S-520-31, Launched from JAXA Uchinoura Space Center on July 27, 2021.
The second stage was successfully launched and ran for 6 seconds, during which time it produced 112 lbf (500 N) of thrust, equivalent to 56% of the Florida team’s ground demonstrator. The data collected from the experiment confirmed that the engine was operating as expected, burning its fuel under knocking conditions.
In view of the successful demonstration of flight hardware, JAXA hopes to put the technology into practical application within five years. Constructed in cooperation with a team from Nagoya University, hoping to further develop the technology to create more efficient spacecraft in the future. It can be applied in various fields, from the first and second stage rocket engines, to deep space missions to make full use of limited fuel resources.
In the past few years, this technology has made great strides. With multiple independent groups now showcasing working engines, it shakes the “impossible” title that has been associated with the concept of rotating detonation for half a century. Obviously, a lot of engineering is needed to build a practical engine with better performance than existing designs. However, with recent progress in this field, there are now promising sparks telling us that it can be done.