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Hydrogen atoms were probed in the near-e eld plume of a 1.8-kW e ight-type arcjet thruster, using two-photon laser-induced e uorescence. The arcjet was operated on an N 2/H2 gas mix simulating hydrazine decomposition products. Thevelocity distributionsof Hatoms were used to obtain theirtranslational temperatures. Comparisons of the laser-induced e uorescence signal intensity, radial linewidth, and derived density distribution with direct simulation Monte Carlo results were in good agreement except near the nozzle exit plane. A relatively weak signal was observed near the exit plane, while the maximum signal was unexpectedly obtained 0.8 cm downstream. Fluorescence quenching, linewidth variation, and self-absorption effects are proposed to account for most of the discrepancy in signal intensity. The results improve the knowledge of near-e eld e ow parameters for the arcjet and ultimately enhance the accuracy and understanding of predicted plume impingement torques and other arcjet plume phenomenology on spacecraft.

Density, velocity, and temperature data were obtained in the near-e eld plume of a 1.8-kW hydrazine arcjet thruster, using the NH molecule as a probe. This was the e rst laser spectroscopic study on a e ight-type arcjet. The rotational temperature of NH was obtained in v = 0;1 of the electronic ground state and the vibrational population in v = 1;2. Rotational and vibrational temperatures were similar, and their variation along the thrust axis was minor. Line shapes as well as radial and axial proe les of the peak line intensity were obtained. Maximum laser-induced e uorescence signal was obtained on-axis 1.4 cm downstream from the exit plane, suggesting rapid NH production in the plume. Axial velocity components were determined at various plume locations. The power distribution of the available channels was found, on summation, to be in good agreement with arcjet input power. Direct simulation Monte Carlo predictions have been generated for comparison with much of the experimental data and to provide additional e owe eld information. Good agreement was achieved in most cases.

The nature and mechanisms of explosions caused by the contact of hypergolic liquid propellants were investigated in detail for several combinations of fuels and oxidizers. It is shown that the explosion phenomena observed can be classified into three categories. 1) In the case of N2H4/NTO, sudden gasification of a superheated liquid layer formed at the boundary of two liquids occurs spontaneously and a detonation-like reaction proceeds in the reactive mixture produced. 2) In the cases of MMH/NTO and UDMH/NTO, the sudden gasification is caused by the shock of a local ignition, and a turbulent-combustion reaction proceeds in the reactive mixture produced. 3) In the cases of hydrazine type fuels/FNA, the sudden gasification occurs spontaneously as in the case of N2H4/NTO, but it is not augmented by chemical reaction, and in these cases the observed explosion is weak. Information on the vapor layer, which is formed between reactive fuel droplet and pool liquid plays an important role for the occurrence of explosion, is also given, that is based on the high-speed motion picture records.