To obtain a sample that is representative, it is necessary to avoid impurities in the sample chamber. This is achieved by LOX cleaning each of the sub-assemblies in the sample area, LOX cleaning the final assembly, and then finally evacuating the sample chamber prior to flight. Both to avoid impurities and to avoid losing any of the sample, it is necessary to have positive seals on the entire sample area. Static and dynamic seals for this type of cryogenic application are commercially available. A standard parachute recovery system is used to return the sample chamber part of the rocket to earth. A fitting can be attached to the sample chamber to withdraw the sample and place it in another container for transportation to the laboratory, or the liquid hydrogen that has been boiled off can be replaced and the heat exchanger section of the rocket itself used as the container. As the hydrogen boils off, it must be vented in such a way that it does not cause excessive back pressure, which would raise the boiling temperature; that excessive liquid hydrogen is not lost; and that there is no back flow of air into the chamber, which might produce an explosive mixture of gases, This must be achieved at various combinations of acceleration, deceleration, rocket attitude, spin rate, liquid level, increasing and decreasing external pressure. If the missile is given a spin velocity, its rotary motion can be used as a means of separating the hydrogen vapor so it can be vented without carry-over or loss of liquid hydrogen in the vent system, During filling and acceleration venting is from the front of the hydrogen chamber through a vent line and out through a normally open valve. Burnout occurs before sampling; so during deceleration and sampling, venting is from the back of the chamber through a vent line which passes first to the front of the chamber and then back through the normally open valve and then out. This arrangement prevents loss of liquid hydrogen. These vents are located in the center of the chamberso that they will receive the hydrogen vapor which has been separated from the liquid by centrifugal force, normally open valve is closed. After sampling and before descent of the rocket the Venting is then through a relief valve in parallel with the normally open valve. This arrangement prevents back flow of air into the chamber on descent and prevents excessive loss of liquid hydrogen after landing. To avoid excessive loss of hydrogen due to heat transfer through the walls of the rocket, it is necessary to provide insulation, The weight of a vacuum chamber made this method of insulating unfeasible. blown polyurethene is the best solution. Freon At liquid hydrogen temperature the freon will condense, leaving a near vacuum in the cells of the polyurethene; so good insulation is achieved with minimum weight. Other areas in which particular problems exist are in the nose cone ejection prior to sampling, in the mechanism for opening and closing the door; fabrication of the heat exchanger, and providing for the contraction of the cooled parts. An over-all factor which must be considered in all aspects of the design is weight, which must be at an absolute minimum to achieve sampling at the high altitudes of interest. In determining the performance capabilities of the sampling system, several factors must be considered, These are heat exchanger and sample chamber capacity; the maximum altitude achievable with a given weight, thrust, etc. ; the point of the trajectory where the angle of attack is such that the inlet shock is lost; and the point where the vapor pressure of the noncondensibles in the sample equals the recovery pressure in the sample chamber. Any one of these factors can limit the capabilities of the system; and each must be optimized, considering its effect on the other factors, to achieve maximum performance of this system.