The hotshot and shock tunnels still fall short of the reservoir pressures and temperatures required to duplicate reentry from orbital trajectories. Impossible as it seems, the gas flows in these intermittent devices generally follow the laws of a steady thermodynamic process. To duplicate more closely reentry ambient conditions and to avoid even more stringent reservoir conditions, it is necessary to turn to a process that adds energy to the flow after it has been expanded to a supersonic state, thus averting the necessity of containing the full-flow energy at stagnation. The expansion tube, developed by Langley scientists, does this.
The expansion tube process begins like that of a shock tube, with the rupture of a high-pressure diaphragm separating the driver gas from the test gas.
The resulting shock wave, which proceeds through the test gas, encounters a second, low-pressure diaphragm, which ruptures on contact. Because of the very low pressure of the accelerating gas in the third chamber, the test gas is expanded, cooled, and accelerated in a downstream direction, thus greatly increasing its flow energy. Thus the test gas has been processed first by a shock wave, which heats and accelerates it, and then by an expansion wave, which cools and further accelerates it. The test gas arrives at the test section at a relatively low static temperature and pressure but moving at very high speed-close to typical reentry speeds of 25 000 feet per second. For perhaps 200 to 300 microseconds the model is engulfed by a test flow moving as fast as a reentry vehicle but still rather rarefied and cool, just like the upper atmosphere. Instrument response on the order of a microsecond permits acquisition of valuable data. Different gases or mixtures of gases may be used in the test chamber, thus simulating various planetary atmospheres.