Hypergol Sensor Using Passive Wireless SAW Devices

This proposal describes the preliminary development of surface acoustic wave (SAW) based hypergolic fuel sensors for NASA application to distributed wireless leak detection systems. SAW devices are a platform technology for passive wireless sensing of numerous possible measurands. ASR&D and its collaborators have demonstrated passive wireless sensors using SAW devices, for applications including temperature sensing, cryogenic liquid level sensing, hydrogen sensors, and humidity sensors under NASA SBIR and STTR funding. The proposed hypergolic fuel sensors will use SAW devices combined with chemically selective film elements to explore the possibility of producing sensitive hydrazine (HZ, MMH, and DMH), and nitrogen tetroxide sensors capable of detecting low ppb concentrations over a range of ambient conditions. This research will utilize the results obtained in ASR&D's nanocluster Palladium (Pd) film and coded SAW sensor and wireless interrogation system research, and existing hypergol sensing technologies. The proposed films should experience large conductivity changes due to interactions with the hypergolic chemicals being detected, producing measurable changes in SAW device performance, as seen in ASR&D's hydrogen sensors. During the Phase I project, issues including formation of the chemically selective films on piezoelectric substrates, optimization of these films, and sensor performance for different device types will be investigated. Successful completion of the proposed Phase I activities will establish the technical feasibility of producing the proposed sensors, evaluate the potential performance capabilities of optimized sensors, and define the additional work necessary to effect device implementation. Assuming the results of Phase I are positive, Phase II could result in development of multiple uniquely identifiable, wirelessly interrogable hydrazine and nitrogen tetroxide sensors.

Hydrazine [HZ], along with its methyl-substituted derivatives (including monomethylhydrazine [MMH] and dimethylhydrazine [DMH]) are flammable, toxic compounds that are also suspected carcinogens. Permissible exposure limits, as recommended by the American Conference of Government Industrial Hygienists, are 10ppb. Nitrogen tetroxide is a strong oxidizing agent, and has a threshold limit of 3 ppm. In order to minimize potential exposure of personnel and to facilitate cleanup, it is essential to rapidly identify and localize accidental releases of these materials in both vapor and liquid form. Sensor systems used for this purpose must be capable of detecting levels of these compounds at concentrations far enough below regulatory limits to trigger alarms before regulatory exposure limits have been met. Thus, detection at low ppb levels is desirable for hydrazine, and high ppb levels for nitrogen tetroxide. The proposed sensors have the potential of providing rapid, real-time monitoring for these chemicals, at levels low enough to enable alarm system operation. Such sensors would be useful in any facility that manufactures, stores, transports, or uses these compounds. The world market for hydrazine and its organic derivatives is growing, with applications in space, defense, and civilian arenas. Availability of a cost-effective monitoring technology for these compounds could enhance regulatory compliance and safety industry-wide.

Compounds in the hydrazine family and nitrogen tetroxide, which are hypergolic when used together, are used as rocket propellants in NASA, Air Force, and civilian spacecraft. Until recently, the Shuttle's APU and HPU systems used such fuels,as do both mono- and bi-propellant propulsion systems. The continued use of these compounds to fuel rockets in the next generation launch vehicles under development at NASA is likely. However, these compounds are hazardous, and human exposure or atmospheric release may present serious health, safety and environmental risks. Hence adequate leak detection technology is essential for safe use of these materials. The proposed hypergol sensors will be developed to work with the SAW multi-sensor interrogation system being developed by ASR&D. This would provide a multi-sensor system to be used by NASA for distributed real-time hypergolic fuel leak detection. The passive wireless nature of these sensors will allow remote monitoring, with power only required at interrogation system nodes, where sensor ID and signal processing occurs. The processed data can then be sent back to a central reporting station using standard wireless communication protocols. Small size, low cost, RFID capability, and rapid reversible responses make this sensor technology potentially applicable for personnel monitoring and similar applications.

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