In a Technical Note in the June Journal (p. 354), Dr. A. L. M. Grzedzielski discusses some aspects of the Matrix Force Method of Structural Analysis and in particular has raised objections to the concept of initial strains and its use by the present writers as an artifice in the treatment of cut-outs and modifications in structures. Dr. Grzedzielski asserts that the idea of initial strains is not valid if the flexibility matrix of the unassembled structural elements is other than purely diagonal. Unfortunately this assertion is just not true.

There have been numerous attempts to predict the stand—off distance of bow shock waves, ranging from the exact solutions of the inviscid flow equations by Mangier (1959) and Van Dyke (1959) to the semi—empirical treatment of Moeckel (1949) and Love (1957). Sinnott (1959) has recently proposed a single formula Δ/R=K cot θm where K is a numerical constant given as 0·77 for axisymmetric bodies and the symbols are defined in Fig. 1.

The effect of initial waviness on the wrinkling of faces in sandwich construction is studied. Formulae are derived to determine the failing stress when the faces wrinkle due to failure of the core in tension, compression or shear. The importance of core strength requirements in maintaining surface smoothness is noted. A comparison of theory with experiments is made, and the agreement between the two is found to be reasonably good.

A sandwich construction consists of two thin face layers of high-strength material and a thick core layer of lightweight material. The function of the core is twofold. Firstly, it increases the bending rigidity of the faces and second, it stabilises them so that they will not wrinkle until high stress is reached.

The problem of launching rockets to explore conditions in interplanetary space formed the subject matter of a previous note by the author. In this note further aspects of such exploration by probes are considered.

The orbits considered in the previous note were elliptic, and touched the orbit of the Earth at the conclusion of each journey around the Sun. For journeys within the Earth's orbit it is of interest to consider the situation in which the probe first enters one of these elliptic orbits and then, at the point of closest approach to the Sun, is placed in a circular orbit around the Sun.

In many cases of the flow through porous screens, one may consider it to be made up of a number of jets passing through the openings of the screen. These jets are separated by a series of wakes behind the solid parts of the screen. The majority of investigations on the flow through such screens have been concerned with the measurement of pressure drop and its variation with different flow conditions; it has been assumed that the pressure is discontinuous at the screen itself and that the pressure drop coefficient Δp/½ρυ2 provides sufficient information, where Δp is the pressure drop across the screen, ρ the density of the fluid, and υ the velocity of approach to the screen.