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DEAR SIR, I should like to add one or two points to the series of articles by Mr. Bielkowicz entitled “Evolution of Energy in Jet and Rocket Propulsion”. With reference to Tables I and III (March) mention is not made of the importance of the mixture density in choosing a rocket fuel. Although specific impulse is a major factor most rockets are designed for high speed flight involving large losses due to drag, and, as the fuel of a rocket is usually a large percentage of the total mass, it is important to use a fuel mixture with as high a density as possible. A convenient method of determining the relative merits of various fuel mixtures is by means of the “density impulse”. This is a figure obtained by multiplying the specific impulse by the relative density of the fuel mixture, this giving a measure of the impulse per unit volume. In Tables I and II are given the specific impulses, densities and density impulses for the fuels examined in the first article (Tables I and III).