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The Forefront of Space Science

Hybrid Rocket Will Go for the Next Generation of Space Transportation
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Two major achievements of space engineering are the rocket and the satellite. Needless to say, the former is a technology to transport materials and people to outer space while the latter is a technology to provide various utilities from orbit in outer space. The Institute of Space and Astronautical Science (ISAS) initiated the research and development of rockets in the 1950s. Following the successful launch of the first Japanese satellite in 1970, ISAS had evolved its M (μ) Rocket and launched a number of scientific satellites. These R&D results have been incorporated in the rockets and satellites under development including the Epsilon Launch Vehicle, reusable sounding rockets, the Mercury explorer BepiColombo, and the X-ray astronomical satellite ASTRO-H.

In this article, I would like to introduce the hybrid-rocket research being implemented by a working group (HRrWG) founded by researchers from universities and ISAS in order to respond the future space transportation and social needs.

Advantages of Hybrid Rocket

The advantages of the hybrid rocket are thought to be safety, high-performance, environmental friendliness, functionality including controllability of combustion cutoff/reignition/thrust throttling, and low-cost. Below I would like to discuss why the hybrid rocket has such advantages.

Fig. 1 shows the main components of the hybrid rocket engine: it carries fuel as a solid and oxidizer as a liquid, so the fuel and the oxidizer never intermingle accidentally. Thus, the rocket is safe without risk of explosion. Since the fuel is not explosive material, it is easy to handle storage, production, and operation of the rocket, and the maintenance cost can be minimized. Oxygen, hydrogen peroxide, nitrous oxide, etc. are used for the oxidizer while hydrocarbon-based polymers such as hydroxyl-terminated polybutadiene, polyethylene, polypropylene, wax and PMMA (polymethylmethacrylate) are used for the fuel. Most of these materials are widely used in daily life, so they are inexpensive unlike specially made solid fuel.

Figure 1
Figure 1. Configuration of hybrid rocket engine

When these ingredients are burnt at an appropriate blend ratio, a specific impulse over 10% higher than that of solid propellant is obtained. Thus, we can build a high-performance rocket that has high thrust generated per unit mass of exhaust gas. Moreover, when hydrocarbon and oxygen react, for example, there are no hazardous byproducts such as chloride compounds. The engine is therefore environmentally friendly. On the other hand, thrust control, shutoff and reignition of the engine are possible by regulating oxidizer flow, eventually allowing us to build a high-function rocket.

Boundary Layer Combustion Is a Key to Hybrid Rocket

During engine operation, the liquid oxidizer is supplied by high-pressure gas or pump to the combustion chamber filled with the solid fuel. Usually, the fuel does not ignite by itself, so it is ignited by heat of the separately installed igniter. After ignition, flames are generated on the surface of the solid fuel. The fuel decomposes or melts by the heat to generate fuel gas. At the same time, the liquid oxidizer also evaporates by the heat to become oxidizing gas. In the space where these two gases convect and diffuse to be properly blended, a chemical reaction occurs to produce a flame known as the diffusion flame. Once the flame appears, its heat in turn drives the gasification of the fuel and oxidizer. Thus, the physicochemical chain reaction keeps fuel burning. Usually, the oxidizer is fed to flow along the fuel surface. The diffusion flame emerges in the “boundary layerEformed by the oxidizer flow’s adherence to the solid fuel’s surface. The phenomenon is called “boundary-layer combustion.EFig. 2 summarizes the physicochemical phenomena occurring within it. The figure shows that it is a very complicated process where a variety of phenomena interact with each other including chemical reactions, material phase changes, and material and heat transportation. The technology of the hybrid rocket depends on how well the boundary-layer combustion is handled.

Figure 2
Figure 2. Boundary-layer combustion

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