Cavities generate delayed radar returns. But because the standard scattering models are overwhelmed by the complexity of the problem, no predictive physical model of cavity scattering has been available to describe what is observed. The delayed return is caused by the slow disgorgement of the radar energy captured by the cavity. For some cavities, and especially for the liquid-fueled rocket engine, the length of the extended return is many times the maximum dimension of the cavity. Since this

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... plies that the radar energy undergoes a great many internal reflections before it reemerges, it suggests treating the captured energy within the cavity as a well-mixed thermal radiation field. This suggestion leads to a simple scattering model for the well-mixed lossless thermal cavity and to a straightforward extension to accommodate the nozzle on the engine. The model gives, independently of both polarization and frequency, the formula (T = 2AF(0 i )F(0 s )cos0 i cos0 s for the narrowband bistatic RCS of the rocket-engine's extended signature, where 9 t and 0 S are, respectively, the angles between the engine axis and the lines of sight to the transmitting and receiving radars, A is the area of the nozzle throat, and F(9) is the nozzle gain factor. For a cavity with no nozzle, F(6) = 1. The concept also produces a somewhat longer formula defining the wideband extended radar signature. It predicts, independently of incidence or scattering angles, polarization or frequency, a decay rate of 2. MIL* dB/m. (L* is the characteristic chamber length defined in rocket engineering as the ratio of the chamber volume to the throat area.) A second component of the scattered energy, the nonextended return, is the energy reflected by the nozzle without entering the combustion chamber. Added noncoherently to the energy disgorged by the combustion chamber, it gives the total signature. The report presents the derivation of these formulas and then, by way of illustration, applies the model to a hypothetical rocket engine. It also includes predictions of the decay rates for a number of rocket engines, both foreign and domestic. Remarkably, the decay rates of six different high-thrust rocket engines all lie within a narrow 2-to-l range, whereas their thrusts cover a 180-to-1 range. The energy capture and release processes of the cavity are so simple to model that the overall model accuracy would seem to depend wholly on the assumption that the cavity energy is well mixed-a reasonably safe assumption for a rocket engine, as well as for other highly cluttered cavities. However, the report includes no comparison with measured data. in This page intentionally left blank. ACKNOWLEDGMENTS