The drag coefficients for cylinders normal to the flow have been determined experimentally at Mach ∼2, Mach ∼4, and Mach ∼6 with Knudsen numbers extending from continuum conditions to free-molecule-flow conditions. The results indicate a smooth transition from inviscid values at low Knudsen numbers to free-molecule-flow predictions for diffuse reflection at high Knudsen number. Small departure theories which are applicable to the ``near free molecule flow'' regime are compared to the experimental data.

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The requirements imposed by investigations of flow and heat transfer characteristics of low density gases have been met by developing a precision manometer for measuring pressures in the range from approximately 0.05 mm Hg to 20 mm Hg. The instrument makes use of optical elements to allow the observer to view a clear magnified image of the meniscus. Precision lead screws and gearing control the movement of the optical viewing systems, and together with a mechanical differential indicate the differential height on a counter directly in thousandths of an inch. The instrument employing a low vapor pressure oil can be operated and read rapidly, and the reproducibility of its readings by different observers is half its least count, or 0.001 mm Hg.

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