minimize frictional losses, the flow must be diffused as rapidly as possible. However, a practical limit is reached when the angle of divergence causes the boundary layer to separate from the wall. Secondly, the back pressure will rise if the rate of condensation is less than the inlet flow rate. Sufficient heat transfer surface must be provided to condense all of the air entering the inlet, at a vapor pressure lower than the recovered total pressure at the diffuser exit. The internal volume of the sample chamber must be sufficiently large to prevent excessive pressure buildup after air collection, assuming the sample is heated before it is recovered. This volume must also be adequate to prevent the vapor pressure of the noncondensible gases present in trace quantities from rising to a value equal to the recovery pressure in the sample chamber. The design of the heat exchanger to meet the requirements for cooling, condensing, and freezing the air sample is influenced by two basic criteria, The first is the maximum rate at which heat can be transferred from the air to the hydrogen, which occurs at the maximum mass rate of air flow into the heat exchanger. second is the total heat which must be absorbed during the entire air sampling period. exchanger indicated that the extended surfaces on the air side were required. The Analysis of the heat A "bottle brush" type of surface, whereby many spines of small diameter wire were arranged in a spiral cross flow with each end contacting the inner wall of the heat exchanger tubes, was selected. This greatly extended the surface exposed to the air flow with only a small increase in weight per unit length. analysis, However, two factors could not be readily determined by First, the reduction in spine surface effectiveness due to partial surface contact between the ends of the spine and the inner wall of the tube. Second, the increase in pressure drop due to cross flow over the spines, These factors were determined experimentally. The heat exchanger must also have sufficient surface and passage area to provide for uniform deposit of frozen air. The deposition of frost on the exchanger surface at a particular region increases its thermal resis- tance at that point. Subsequent frost deposition will migrate to a region having less frost accumulation, This is a stable process resulting in a uniform deposition of frost, The liquid hydrogen used for condensing and freezing the air sample will not normally condense such gases in the atmosphere as hydrogen or helium. These gases, together with some neon, which has a boiling point only slightly above that of hydrogen, must be considered at the higher altitude limits of the collection range, since no further air can enter if the vapor pressure within the heat exchanger is equal to the recovery pressure, The altitude at which this becomes a limitation can be analyzed for any particular set of conditions. There will probably be considerable adsorption and consequent entrapment of the relatively small proportion of these gases on both the metallic and air frost surfaces, which will reduce the vapor pressure buildup that is calculated. The cooling and freezing of the air sample as it enters the heat exchanger depends upon the satisfactory transfer of the heat into the liquid hydrogen heat sink, surfaces of the tubes should be continuously wetted. For minimum surface requirements all liquid hydrogen By using a flooded type heat exchanger, the maximum rate of heat transfer occurs at the initiation of the sampling period when the maximum amount of liquid hydrogen is available and, therefore, maximum wetted surface available. 132