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National Advisory Committee for Aeronautics, Technical Notes - Propellant Vaporization as a Criterion for Rocket Engine Design; Calculations of Chamber Length to Vaporize a Single n-Heptane Drop

Calculations based on droplet-evaporation theory show that for a
given combustor length the percent of fuel mass vaporized can be in-
creased by decreasing the fuel-drop size and the initial drop velocity, .
or by increasing chamber pressure, final gas velocity, and initial fuel a
temperature. The analytical results of this study were correlated to give
a single curve of percent of fuel evaporated as a function of the chamber
length and the factors involving these parameters. The calculated results
agree with experimental results if the mass-mean—drop diameters for
various injectors are assumed to be about 100 to 200 microns.

The large number of different phenomena that can have a fundamental
role in the combustion within a rocket engine makes it difficult to
determine and study the controlling processes. Some of these phenomena,
as given in references 1 and 2, are atomization, heating, vaporization,
liquid and gaseous diffusion, combustion of droplets, and liquid- or gas-
phase reaction. Several papers (refs. 1 to 4) have presented the current
concepts used in designing rocket engines and the similarity parameters
used in scaling.

These current approaches have been based on flow and chemical—
reaction theories. The significance of vaporization was neglected or,
at best, included in the analysis by a single dimensionless group. To
demonstrate the importance of vaporization, a model was used which assumed
that vaporization of the least volatile prOpellant was the rate—controlling
step in rocket-engine combustion. Calculations were made to determine
the vaporization rate and also a history of the fuel vaporized as a
function of the chamber length. many engine parameters were varied to
show how such parameters affected the chamber length required to vaporize
a given percentage of the least volatile propellant.