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How Do Rocket Engines Work?
Published in Travis S. Taylor, Introduction to Rocket Science and Engineering, 2017
In some engines, no igniter is needed, such as in the Space Shuttle orbital maneuvering system (OMS) thrusters. Those smaller rocket engines implement a single engine based on the Apollo Service Module’s Service Propulsion System. The engine uses monomethylhydrazine (MMH) for fuel and nitrogen tetroxide (N2O4) for oxidizer. When the two propellants are mixed, they are volatile enough to spark the reaction without an external ignition source. A self-starting reaction like this is called hypergolic. The advantages of using hypergolic systems are fairly obvious. The mechanical systems are much less complex. The combustion rate of a hypergolic engine can be controlled by two flow control valves: one to control the fuel and one the oxidizer. Another advantage to hypergolic propellants is that large explosive quantities can’t gather in one place. This is because the two compounds are volatile with each other, and, as they come into contact, they start to burn. A disadvantage of hypergolic systems is that they typically have a significantly lower Isp than nonhypergolic ones.
Chemical Mechanism of MMH/NTO and Simulation in a Small Liquid Rocket Engine
Published in Combustion Science and Technology, 2019
Lingyun Hou, Pengfei Fu, Yantao Ba
Computational fluid dynamics (CFD) is becoming as important as experimental approaches in clarifying combustion processes inside a rocket engine. Of the several sub-models for CFD, the chemical mechanism of propellants is crucial in terms of both the computational cost and simulation accuracy. Monomethylhydrazine (MMH) and nitrogen tetroxide (NTO) have been widely used as bipropellants for rocket engines because of their high energy content, excellent physical properties, and chemical reactivity (Schmidt, 2001; Sutton, 2003). However, the reaction mechanism between MMH and NTO has not been studied thoroughly for two reasons. First, MMH is highly poisonous. Second, both MMH and NTO are active chemicals. It is impossible to obtain premixed MMH/NTO samples at room temperature, even under a highly diluted condition (Bassin et al., 1994). There is little experimental chemical kinetic information available on the hypergolic MMH/NTO combination.