The Propulsion group at Dryden Flight Research Center is performing analyses of the effectiveness of chemical-species sensors for detecting and assessing leakage. The group is also studying relationships between (1) rates at which high-energy species leak from the propellant-feed systems of advanced aircraft and spacecraft and (2) the rates of leakage of inert species substituted for the high-energy species in the same systems. Several commercial sensors and a prototype noncommercial sensor have been and continue to be subjected to tests of their ability to detect their respective chemical species accurately and quickly. Analysis of the data from these tests is expected to determine whether it is possible to develop instruments capable of detecting propellant leaks over a wide range of temperature and pressure. If suitable, the instruments can be employed in various flight experiments in which there are requirements for such leak-detection capability. Analysis of data from inert- and high-energy-propellant leak testing will help in scaling leakage from gaseous propellant feed systems without the need to use high-energy propellants during subsequent leak tests.

These Plots Represent Responses of oxygen and hydrogen sensors tested in the program described in the text.
Commercial-off-the-shelf (COTS) oxygen sensors that were intended for automotive and medical uses have proven quite effective for monitoring oxygenated environments. Calibrations with a 1 percent oxygen gas have demonstrated uncertainty of 0.1 percent at pressure altitudes up to 50 kft (≈15 km). The top part of the figure presents results of an uncertainty analysis of one these sensors. The middle part of the figure presents results of a dynamic test in which a calibrated fully compensated (that is, pressure-and-temperature-compensated) oxygen sensor was operated at a rate of ascent of 60 kft/min (≈0.3 km/s) from 18 kft (≈5.5 km) to 105 kft(≈32 km) and found to be accurate to within 0.7 percent. The lower part of figure presents results of a test in which a prototype sensor that measures changes in the speed of sound was found to afford a capability for measuring concentrations of hydrogen mixed into nitrogen.

Calibration of the COTS and the fully compensated oxygen sensors have been finished and these sensors have been integrated into the Hyper-X vehicle (an experimental hydrogen-fueled hypersonic airplane) for flight testing. Validation testing of these sensors was scheduled to begin at or about the time of reporting the information for this article. Calibration and dynamic tests of a fully compensated hydrogen sensor were also scheduled to begin about a month earlier. It was expected that if the results of the tests of the hydrogen sensor show that requirements are satisfied, then this sensor would also be integrated into the Hyper-X vehicle. Also at the time of reporting:

  • Negotiations to procure another prototype speed-of-sound sensor for characterization and calibration were under way; and
  • At the University of California at Los Angeles, there was research in progress to determine a correlation between the rates of leakage of gaseous hydrogen and gaseous helium.

This work was done by Neal Hass of Dryden Flight Research Center. For more information, contact the Dryden Commercial Technology Office at (661) 276-3689.