Flow Characteristics in a Nozzleless Rocket

Let us consider a gas flow in a port of an internal-burning propellant. The cross-sectional area of the port is assumed to be constant throughout the port from the head-end to the rear-end, i. e., the port is one-dimensional along the flow direc-tion.I l The mass discharge rate from the rear-end of the port, m, is given by 

Since the specific heat raho of the combushon gas, y, lies between 1.2 and 1.4, the ratio of the discharge coefficients, Cd, j/Cd, is 0-80 for y = 1.2 and 0.78 for y = 1.4. This result indicates that the mass flow rate from the port exit is approximately 20 % lower than that from the isentropic nozzle exit used for a convenhonal rocket motor when the head-end pressure is equal to the chamber pressure in the rocket motor.[5] 

It should be noted that the gas flow process in the port is not isentropic because mass and heat addihons occur in the port. This implies that there is stagnation pressure loss and so the specific impulse is reduced for nozzleless rockets. When a convergent nozzle is attached to the rear end of port, the static pressure at the port exit, Pj, continues to decrease to the atmospheric pressure and the specific impulse of the nozzleless rocket motor is increased. The expansion process in a divergent nozzle is an isentropic process, as described in Section 1.2. 

Nozzleless rockets are very simplified and low-cost rockets because no nozzles are used. Their specific impulse is lower than that of conventional rockets even when the same mass of propellant is used. Normally, a convergent-divergent nozzle is used to expand the chamber pressure to the atmospheric pressirre through an isentropic change, which is the most effective process for converting pressure into propulsive thrust. The flow process without a nozzle increases entropy and there is stagnahon pressure loss. 

Hybrid rockets are intermediate between solid rockets and hquid rockets in terms of the nature of the combination of solid fuel and hquid oxidizer. Since the fuel and oxidizer components of a Hquid rocket are physically separated, two mechanical systems are needed to feed these components into the combustion chamber. On the other hand, a hybrid rocket uses a polymeric inert material as a fuel and a Hquid oxidizer, and so only one mechanical system is needed to feed this Hquid oxidizer into the combusHon chamber. 

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