The principle of the ram-jet or athodyd has been understood for a number of years. It has been applied in many ways, the most prominent being the flying bomb and, rather less spectacularly, the ducted radiator. The principle is to. convert the velocity energy of a high speed air stream to pressure energy by means of a suitable duct, to apply heat energy to the compressed air, and then to expand this heated air in the form of a high velocity jet. The advantage of such a mechanism is an increase in thrust given by the product of the mass flow of air, and the difference in velocities between entry and exit; the disadvantages are, the necessity to project the duct at high forward speed before any thrust is obtained and, except at very high speeds indeed, the low thermal efficiency.