Vernier exhaust perturbations on radar and altimeter systems during a lunar landing

A PROCEDURE that has been proposed to accomplish a soft lunar landing "is to use a high thrust main engine stage to remove most of the approach velocity in an open loop fashion, and a low thrust vernier engine stage to control the spacecraft to a soft landing. The vernier stage employs terminal sensors to measure altitude [altimeter] and velocity [Doppler], throttling to control descent rate, and may employ body maneuvering to control horizontal velocity." It is the purpose of this paper to make an estimate of the interference that the vernier rocket exhaust may exert on the performance of the Doppler and altimeter systems of the lunar landing vehicle. The subject of rocket exhaust interference on missile guidance and communication has been under considerable study in the past. The most detailed studies on this topic are in the form of unpublished company reports and studies. We shall make use of the results and conclusions of some of these studies" to provide upper bound estimates of flame effects on the guidance of the lunar vehicle. The main results of these studies are as follows: 1) Molecular absorption and polarizability are insignificant in determining the electrical properties of the rocket exhaust; the free electrons play the dominant role. 2) The electron collision frequency is determined (and therefore predictable), in the main, by the presence of those molecules with permanent electric dipole moments. 3) The observed degree of ionization of liquid rocket exhausts at exit plane conditions is enormously larger than predicted by equilibrium theory. The ionization can be explained as due to the thermal ionization of low-ionization fuel impurities (e.g., sodium) in the combustion chamber followed by slow electron-ion recombination as the combustion products accelerate, expand, and cool through the thrust chamber. The possibility of chemi-ionization as the cause of the observed ionization is not ruled out, however. 4) Computer programs were developed to predict the spatial distribution of the exhaust gases (flow pattern) for vacuum operation of the rocket motors. 5) The exhaust gas flow patterns combined with knowledge of electron collision frequency and fractional ionization (considered frozen beyond the exit plane) allow prediction of the complex electrical conductivity throughout the exhaust flare. The propagation of electromagnetic waves (of not too large