Interdisciplinary Applied Mathematics

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Jb_ (М) у


7-1 V M )


go


1/2



(Isp)L



and form the ratio



(^sp),FM


lu



n(Y 1)



1 1/2



4y


This ratio is equal to 0.474 for 7 =1.4 and 0.562 for 7 = 1.67.


This analysis has motivated Ketsdever et al. (1998) to design and optimize a free-molecule microresistojet (FMMR); see Figure 6.32. The FMMR is based on the transfer of energy into the propellant gas through molecular collisions with thin-film heated elements. The inlet conditions into the expansion slot correspond to a Knudsen number of approximately one. It was found in (Ketsdever et al., 1998), that the intrinsic Isp of the FMMR is about 60% of the intrinsic efficiency of a typical micronozzle. However, the proposed FMMR operates at very low stagnation pressures (50 to 500 Pa), and from the systems-efficiency point of view this may be advantageous, since, for example, the common problem of microvalve leakage is avoided. An FMMR has been fabricated, and heat transfer and total power input data have been obtained and presented in (Ketsdever et al., 2000a). For highly rarefied flows, small expansion ratios are preferable to avoid large shear losses. Different propellants can be used in FMMR, and the above equation shows that Isp is highest for helium, followed by ammonia and water vapor and then by argon. Specifically, DSMC results for argon reported in (Ketsdever et ah, 1998), show that a thrust of | mN is obtained at Isp    «    45    sec.    In the    experiments    presented    in (Ketsdever    et    al.,    1998),


argon was used as propellant, and the plenum pressure was 50 Pa. The thrust chip was fabricated using DRIE with 40 expansion slots of w = 100 p,m.

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