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Both the energy transfer and the normal and tangential force coefficients for scattering of a nearly monoenergetic beam of He atoms (incident energy Ei=63 meV) from a clean single crystal LiF(001) surface have been measured under wind tunnel conditions in the range of surface temperatures from 300 to 720 K. Expressions are derived for the differential reflection coefficient for a realistic He-LiF(001) potential with the inclusion of multiphonon processes, and these expressions are used to calculate the average energy and momentum transfers as well as their higher moments. The comparisons between theory and experiment indicate the presence of a moderate degree of surface roughness. The measured recovery temperature depends on incident beam angle and is significantly larger than expected for complete or partial accommodation, in good agreement with the theoretical calculations.

Pitot pressure and heat-transfer measurements have been made in plumes of 0.5-N (conical nozzle), 2-N, and 5-N (contoured nozzles) monopropellant hydrazine thrusters. The main objectives are to check the DFVLR simple plume model and to determine reliable model input values for real thrusters. The methods used for the heattransfer measurements (applying a sphere probe), the recovery temperature determination, and the evaluation of the plume quantities relevant for plume impingement calculations are outlined. The Pitot pressure measurements showed the existence of shock disturbances in the near plume flowfield of the contoured nozzles. Stagnation temperatures between 900 and 1350 K were deduced from the measured recovery temperatures. The corresponding molecular weight range was found to be between 11 and 14.5 and the most reasonable mean effective ratio of specific heats to be K = 1.4 ±0.03 for the expansion from the stagnation chamber to the continuum plume flow. This value is proposed for simple plume model calculations. The model heat-transfer results agree well with the experiments. Nomenclaturespecific heat at constant pressure and volume, respectively d = diameter F = thrust 7 sp = specific impulse 7 sp = F/m m -mass M = molecular weight Ma = Mach number p = pressure Pr = Prandtl number <2 = heat transfer r = radius r = recovery factor, Eq. (7) R = specific gas constant Re 2 = Reynolds number, Eq. (19) St = Stanton number, Eq. (6) t = time T = temperature u = velocity x = centerline distance from nozzle exit X l = dissociation degree of NH 3 d" = momentum thickness 6 E = nozzle exit angle AC = ratio of specific heats, K = C P /C V JLI = viscosity p = density Subscripts BL = boundary-layer (continuum) theory cond = conduction E = nozzle exit condition FM = free molecule K = in the vacuum chamber lim = limiting condition for Mo-oo loss = losses by radiation and conduction off, on = thruster not firing and firing, respectively r = at recovery condition rad = radiation s = sphere u = velocity w =at the wall, sphere probe condition 0 = stagnation condition 1 = freestream 2 = condition behind a normal shock wave Superscripts 1,2,... = number of iteration ( )* = nozzle throat ( ) =time derivative IntroductionI N a previous study, existing analytical plume flow models 1 " 3 were extended to deliver all flow quantities relevant to impingement calculations. Free-molecular plume flow was included by the definition of a freezing surface. 4 ' 5 In this DFVLR model, constant composition flow is assumed with mean constant gas properties. The angular plume flow description is most sensitive to changes in the ratio of specific heats of the exhaust gases.The present work is part of an extensive study of plume flow and impingement effects on spacecraft surfaces, that serves to test, verify, and improve the model by analyzing systematically the influence of the thruster nozzle geometry, nozzle boundary layer, and ratio of specific heats using pure gases. 6 This paper deals with experiments in real hydrazine plume flows from thr...

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