Abstract
Introduction T HE accurate sensing of surface pressures on hypersonic ight vehicles presents formidable measurement challenges. The hostility of the sensing environment precludes intrusion into the ow, and measurements must be obtained via sizable lengths of small-diameter pneumatic tubing that connect the surface ports to the remotely located pressure transducers.An idealized pneumatic layout is depicted in Fig. 1. Further complicating the sensing problem is surface boundary layer heating,which causes sizable temperature gradientswithin the pneumatic tubing.Away from the stagnation region, local surface pressure levels,which must be sensed, are extremely low, often less than 0.002 atmospheres (0.2 KPa). The combination of very low pressure levels, small diameter pneumatic lines, and large temperature gradients makes molecular effects no longer negligible. The problem of predicting tube ow dynamics has been studied extensively. For nonrare ed conditions, Lamb,1 Iberall,2 Schuder and Binder, and Hougen et al., have developed closed-form frequencydomain solutions for simple tubing geometries and constant wall temperatures.Berg and Tijdeman and Tijdeman extended the analyses of Refs. 3 and 4 to develop a recursion formula for complexgeometriesthat consistof cascadesof tubesandvolumes.Parrot and Zorumski7 investigated the dynamic transmission of sound in a simple geometry tube subjected to very large temperature gradients. References 1–7 considered only continuum ow conditions; rare ed ow effects were not investigated. Tube ow for rare ed conditionsat high temperatureswith large temperaturegradientshas been investigated by Maxwell,8 von Knudsen,9 and Tompkins and Wheeler. These rare ed ow investigations,however, considered only steady- ow conditions. The dynamic in uence of rare ed ow phenomenaon pneumatic pressure sensing systems has not been generally well understood; consequently, research was initiated at the NASA Dryden Flight Research Center with a primary objective to develop a dynamic response model for pneumatic pressure sensing systems that is applicable to both continuumand rare ed ows.Whitmore et al.11 presented a detailedanalyticaldevelopmentand empiricalvalidationof one such model. This Note summarizes the information presented in that text.
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