The total pressure drop in the fuel system is 45.3 + 64.9 + 11.4 = 121.6 psi, and in the oxidizer system it is 75.0 + 14.2 = 89.2 psi.
The tank pressures required to obtain the desired flows are calculated by adding the chamber pressure to these pressure drops; that is, (p)0 = 306 + 89.2 = 395.2 psi and (p)f = 306+ 121.6 = 427.6 psi. To equalize the tank pressures so that a single gas pressure regulator can be used, an additional pressure loss must be introduced into the oxygen system. The correction to this simple pressurized liquid propellant system is accomplished by means of an orifice, which must be placed in the propellant piping between the oxidizer tank and the thrust chamber. Allowing 10 psi for regulator functioning, the pressure drop in a calibration orifice will be Ap = 427.6 - 395.2 + 10 = 42.4 psi. The regulator setting should be adjusted to give a regulated downstream pressure of 427.6 psi under flow conditions. The orifice area (assume Cd = 0.60 for a sharp-edged orifice) can be obtained from Eq. 8-2, but corrected with a g0 for English units.
~ Cds/2gp&p ~ 0.60V2 x 32.2 x 1.14 x 62.4 x 42.4 x 144 = 0.581 in.2 (or 0.738 in. diameter)
A set of balancing equations can be assembled into a computer program to assist in the calibration of engines. It can also include some of the system's dynamic analogies that enable proper calibration and adjustment of transient performance of the engine as during start. There is a trend to require tighter tolerances on rocket engine parameters (such as thrust, mixture ratio, or specific impulse), and therefore the measurements, calibrations, and adjustments are also being performed to much tighter tolerances than was customary 25 years ago.
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