Investigation of Boron Combustion for its Application in air Augmented Rockets

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During the past years different types of air-augmented rockets has been receiving increased emphasis. This presentation is related with the ducted type of air-augmented rocket for missions of long distance at low altitude. In the ducted rocket, ram-air is introduced into a secondary chamber where subsonic turbulent mixing and burning will occur with the fuel-rich exhaust of a primary chamber. A second nozzle is provided for acceleration of the product gases to supersonic velocity. Propellants with boron are attractive for air-augmentation because of their high density and high heat of reaction with oxygen. From the standpoint of volumetric energy release it is desirable to maximize the boron content in the propellant of the primary chamber (up to 70% by weight). However, the theoretical gain in specific impulse can, of course, only be attained with efficient boron combustion.

In a short review of specific literature it will be shown that the conditions for boron-particle ignition and efficient combustion are poor (in comparison to aluminium particles, for example) and not yet well understood. Therefore combustion studies of propellants heavily loaded with boron are necessary in order of developing air-augmented rockets with high combustion efficiency.

In the following results of an experimental research program will be presented which has been performed at the U.S. Naval Weapons Center to study the combustion behavior of boron propellants. An experimental apparatus will be described which permits the observation of the boron-combustion zone in the secondary chamber through quartz windows. During the tests the combustion efficiency can be controlled by c*-efficiency measurements and particle-sampling probes. The primary rocket motor is burning gaseous hydrogen-gaseous oxygen-boron mixture, so that the boron-particle temperature in the primary rocket can be easily varied by changing the hydrogen/oxygen mixture ratio.

By increasing the primary chamber temperature from 750 K to 2000 K there is a considerable change in the boron-combustion behavior (demonstrated by color cinephotography) and an increase of the overall reaction efficiency from 0.85 to 0.95. The same effect of increased combustion efficiency can be demonstrated by boron-particle sampling in the exhaust of the secondary chamber. These results show that the initial boron-particle temperature in the primary rocket should exceed a certain temperature before the particle makes contact with the air in the secondary chamber.

Considering a propellant with oxidizer, fuel, and boron, the temperature of the boron particles in the primary chamber has to be established by the oxidizer-fuel matrix if one assume that the boron does not start to burn violently in the primary motor (this assumption is well established for propellant heavily loaded with boron). Therefore the primary combustion temperature (computed with no boron reacting) becomes an important criterion for determining propellant compositions for air-augmented application. This requirement of high primary chamber temperature, of course, has to be optimized with other propellant and motor conditions. So an increase in primary chamber temperature is followed by a decrease in specific impulse. Results of a theoretical program will be shown in which the thermodynamic data of different propellants are computed in order to optimize the requirements of high primary chamber temperature and high specific impulse.