To overcome defects of conventional hybrid rockets such as low combustion efficiency and the O/F shift during the combustion, the authors have proposed a new form of hybrid rocket fuel. The fuel is a fibrous bed in which oxidizer gas flows. Stable diffusion flame appears at the exit surface. Previous researches show that sudden increase of the fuel regression rate occurs with the increase of ambient pressure. This sudden increase is attributed to the flame spreading between fuel fibers. To clarify the limit of fuel gap space the diffusion flame can spread into, experimental study was made. Critical gap space, which means the minimum gap space the diffusion flame can spread into, was obtained experimentally as a function of oxygen gas flow velocity and ambient pressure. Using this result, necessary conditions to realize a stable combustion with this new fuel form are shown.

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To overcome defects of conventional hybrid rockets such as the loss of specific impulse, which is caused by the O/F shift during the combustion, and the low combustion efficiency, the authors have proposed a new idea of design. The point of this idea, named “End-Burning Hybrid Rocket, ” is that oxidizer gas flows in the gap space of a porous solid fuel bed. Diffusion flame is formed at the end of the solid fuel bed. Experimental studies were made to clarify the basic combustion characteristics of the propellant. Results show that pressure exponent of the burning rate with the same equivalence ratio is approximately 0.85 and virtually independent with the equivalence ratio. Using this result, a designing method of End-Burning Hybrid Rocket Motor is shown. Finally, thrust and specific impulse is estimated as functions of oxidizer gas flow rates to investigate the throttling characteristics of the motor.

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Spectra of nitric oxide (NO) bands were measured for air plasmas in a free stream and in a channel nozzle of the NAL 750 kW arc-jet wind tunnel to obtain radiative characteristics and temperatures of the flows. Measured band intensity distributions were analyzed on the process model, in which electronically excited NO molecules are populated from the continuum state by the inverse pre-dissociation reaction N+O → NO (C²Π), followed by a population redistribution on molecular energy states due to collisions and radiation. Synthetic spectra by that model agreed well with measured ones. Temperature determination was also made using the spectral matching method, resulted in rotational temperatures of 680±30 K in the free stream and 2, 000±300 K in the channel nozzle flow. It was found that the vibrational temperature in the free stream was considerably lower than 1, 000 K and that the electronic excitation temperature was impossibly determined in principle.

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Repair with bonded fiber-reinforced polymer composite patches has been recognized as an efficient and cost-effective method to extend the service life of aging aircraft. In this paper, a two-dimensional three-layered finite element model consisting of layers of composite patch, adhesive and cracked structural panel is made to analyze the stresses in cracked rectangular structural panels repaired with a bonded circular composite patch. The composite patch and structural panel are modeled as separate layers which are covered with Mindlin plate elements, whereas the adhesive is modeled as a thin elastic layer. The modified crack closure method is used to obtain stress intensity factors at the crack tip. First, by calculating the stress intensity factors, the effects of the diameter and thickness of the circular composite patch on the patching efficiency are examined. The stress intensity factors are also calculated at various hygrothermal conditions and the effect of the environment on the patching efficiency is clarified. Finally, the diagram of patching efficiency is presented to evaluate easily the patching efficiency at hygrothermal conditions.

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