Ignition Characteristics of Boron Particles in Ducted Rockets

Abstract

Ducted rockets, one kind of ramjets, are superior to the typical solid rocket motors in specific fuel consumption. Ducted rockets need large thrust since they are operated in supersonic. Using boron as fuel is effective approach to improve thrust Boron has a so high heat of combustion that it is possible improve thrust However it is difficult to ignite and burn boron in the combustor because boron has a very high melting point, and is able to be resident in the combustor very short time. In this study, the measure of ignition delay time of boron particles, the analysis of ignition and combustion characteristics of boron containing gas generators were evaluated. amjets are one type of air-breathing engine1. During flight, air is compressed by shock waves, and flows into the engine. Fuel is mixed with the air and burns. Oxidizer generally accounts for 70-80 % of the propellant in a typical rocket motor. A ramjet uses oxygen in the atmosphere as oxidizer, so oxidizer in the propellant is replaced by fuel. Since it improves specific fuel consumption, ramjets are able to fly fast and long time in the atmosphere. This ramjet concept had been applied to the solid rocket, and a concept of the ducted rocket was born. Figure 1 shows fundamental structures of a ducted rocket. The ducted rocket consists of air inlets, a primary combustor, a gas generator, a primary node, a secondly combustor, and a secondary nozzle. The gas generator is a fuel-rich propellant. In the primary combustor the gas generator burns and generates hot fuel-rich combustion products (fuel gas). During a fight supersonic air is compressed with shock waves in the air inlets, slowed down to subsonic, and recovered its pressure and temperature. The primary combustor products mix with the compressed air and bum in the secondary combustor. Combustion gas is exhausted through the secondary nozzle, and generates thrust. The gas turbines are also one type of air-breathing engine. They have mechanical compressor. Even though altitude and speed change, compressor inhales air and makes air-fuel ratio optimum. The compressed airflow is stable, so gas turbines keep stable combustion. A ramjet, however, compresses air with shock waves. Therefore its flight conditions change airflow conditions. It successively changes air-fuel ratio in the secondary combustor. It is difficult to optimize air-fuel ratio and to keep stable combustion.