Ground testing of a Nuclear Thermal Rocket (NTR) is critical to its development and application. Ground testing allows longer duration than possible in flight, greater instrumentation and post test evaluation of all engine components. The NERVA/Rover engines were tested in Nevada in the open air. Open air testing is not contemplated for a modern NTR develop program. A closed test facility must contain any radioactive or other environmentally undesirable materials from entering the Biosphere. A closed facility also provides a more accurate simulation of the space environment by operating at a partial vacuum condition. The first drawback of a closed facility test of even the current small ~25KLbf NTR engines is the large volume of the exhaust. Even when cooled to ambient temperatures an hour long test would generate over a Billion cubic feet of hydrogen (>l,000 Good year Blimps). Before this exhaust can be released it must be cooled and filtered to remove any solid, liquid or gaseous contaminates. A facility to store the full amount of gas for later processing or a real time processing system capable of handling over 300,000 standard cu ft/second are likely to be very expensive. If the exhaust can be condensed, the volume that must be stored or consciously processed can be drastically reduced. Hydrogen must be cooled to about 20 degrees K to condense. This is not a practical solution but if the hydrogen is combusted within the facility, the resulting steam can readily be condensed to water. Aerojet has developed a thrust augmenting technique for an NTR called the Lox Augmented Nuclear Thermal Rocket (LANTR). This "oxygen afterburner" can be employed to increase the vehicle thrust by 100-300% by burning the hydrogen with oxygen in the NTR nozzle. Including the LANTR injectors in the ground tests would allow condensing up to 50% of the hydrogen. Subsequent injection of oxygen or air into the exhaust ducting can convert 90% of the hydrogen to water. This reduces the volume of gases that have to be treated by an order of magnitude allowing steady state treatment. Steve Howe of the Center of Space Nuclear Research (CSNR) has proposed an inbore test technique called SAFE (Subsurface Active Filtration of Exhaust) that injects the NTR exhaust into the poursous ground at the Nevada Test Site. This paper provides more details on the operation of the SAFE the concpet and subscale test validation options.
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