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There are conflicting design trades associated with the amplitude of the pressure drop. A low pressure drop is desirable to minimize feed system mass and pumping power, since there is less resistance associated with forcing the liquid through the orifice. High pressure drops, on the other hand, are often used to increase a rocket engine’s resistance to combustion instabilities and to enhance atomization of the liquids, thereby improving performance. The pressure drops help prevent against the propagation of any disturbances upstream of the injectors from inducing oscillations in the combustion, which is why they assist with reducing instability. They enhance atomization of the fuel and oxidizer because greater pressure drops increase the velocity (and therefore momentum) of the fluids, thereby With regards to these conflicting design goals, a good rule of thumb is that the injection flow resistance should be 20% of the chamber pressure. For example, the Helios system is designed to achieve a chamber pressure of 500 psi, suggesting the pressure drop across the injectors should be 100 psi.
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