Abstract
Characteristic temperature responses of simulated hydrazine leakage at ground and on-orbit conditions were investigated to prove a leak-detection concept for the Space Shuttle hydrazine auxiliary power unit (APU) application. Leakage rates of 0.5, 2.0, and 4.0 cc/min were tested. The hydrazine fuel used for testing met Space Shuttle propellant specification SE-S-0073. The ground condition was performed in a gaseous nitrogen (GN2) atmosphere at approximately 86.2 kPa (12.5 psia) to simulate the orbiter aft compartment prior to launch. The simulated on-orbit pressure was achieved by evacuating a vacuum chamber to its ultimate vacuum pressure of approximately 1.33 x 10 Pa (1 x 10 torr). During the APU operation, surfaces near locations where leak detection might be implemented reach elevated temperatures. The effect of this heat on leak detection also was examined. Test temperatures were approximately 65.6°C (150°F) for simulated ground conditions, and 121.1°C (250°F) for simulated on-orbit conditions. This leak-detection concept using evaporative cooling was able to generate distinctive leak characteristic curves for each leak rate. Although exact quantification of a leak would not be possible, qualitative determination of the leak amount appeared achievable. The tests demonstrated potential for use of this evaporative cooling sensor in a vacuum environment where conventional leak sensors do not work effectively. INTRODUCTION The top goals and objectives of the Space Shuttle Program (SSP) are to improve safety and reduce loss of vehicle risk (Criticality 1 risk). At the request of NASA, Boeing proposed a hydrazine leak-detection system development study. The effort of this study generated an evaporative-cooling leak-detection concept. Currently, there is no dedicated system to detect a hazardous fluid leak in the aft compartment. APU hydrazine leakage is a Criticality 1 risk. The NASA Safety and Mission Assurance (S&MA) Office predicted vehicle loss as the result of a hydrazine leak is 1 in 1400 per the APU risk model. Commercial hydrazine leak sensors are available, but they are designed for a minute leak for personal hazard monitoring. However, the APU requires a leak detector for a “gross” leak. In other words, commercial hydrazine sensors are too sensitive for APU applications. In addition, there is no commercial sensor available for use in a space environment. This led to the development of the APU hydrazine leak-detection system. The objective of the study presented in this report was to analyze, design, and test a hydrazine leakdetection system prototype for an on-orbit application. The development testing included water and hydrazine as working fluids. Water was used to identify potential test apparatus problems and obtain a baseline response because of its well-published physical data. In addition, water is safer to handle than hydrazine if there are any test modifications required. Two types of sensors were tested in this program. The first leak-detection concept is based on evaporative cooling as a result of an instantaneous reduction of the fluid surface pressure when an on-orbit leak occurs. A sudden pressure drop at the surface causes the fluid to be in a superheated state, which causes rapid boil-off. This rapid boil-off produces evaporative cooling at the surface. The effect of the surface evaporative cooling produces a characteristic temperature drop as a result of heat removal by conduction. The leak-detection system relies on sensing the characteristic temperature drop during a leak when the fluid is exposed to space vacuum. The characteristic temperature drop indicates that there is a potential leak. The implementation of the leak-detection system requires prior knowledge of the
Published Version
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