Cruise Flight Duration of a Low Mach Number Ramjet

Abstract

The one-dimensional flow through a ramjet operating at Mach numbers between about 1.1 and 1.6 and with lean fuel-air ratios is analyzed and programmed for computation. The addition of routines utilizing empirical drag data, a model for estimating the engine mass, and a scaling technique makes it possible to calculate flight durations in cruise for a given total mass when the net thrust and cruise flight conditions are specified. The calculation of typical applications shows a limited improvement (measured in terms of flight duration) when Mach number is increased from 1.2 to 1.6. This is attributed to the use of a simple pitot intake and converging nozzle in the interest of simplicity and low cost. Increasing the altitude and using leaner fuel-air ratios, however, yield extremely good results. An increase in the flight duration by a factor of as much as five is found, assuming the very lean mixture strengths to have no detrimental effect on combustion efficiency. Since this assumption will not be valid in practice, a design which bypasses most of the excess air around the combustion chamber is considered. Comparable results are obtained. Nomenclature a = constants (subscripts 0-21) in Eq. (19) A = area, excess air in core engine c = specific heat C — coefficient D = diameter, drag / = fuel-air ratio F = thrust h = specific enthalpy K = scaling factor L = length m = mass flow rate M = Mach number p = pressure R = gas constant, radius Re = Reynolds number / = time T = temperature V = volume x = dimensionless groups (subscripts 1 and 2) in Eq. (19) y = dimensionless group in Eq. (19) Z = total excess air 7 = ratio of specific heats A = difference 77 = efficiency p = density Subscripts