|Fig. 1: The NERVA nuclear thermal rocket design. (Source: Wikimedia Commons)|
In the past decade, NASA has begun testing the viability of nuclear thermal rocket fuels at the Marshall Space Flight Center in Huntsville, Alabama using a simulator called the Nuclear Thermal Rocket Element Environmental Simulator, or NTREES.  The appeals of such a technology are manifold. Primarily, nuclear thermal rockets (NTRs) provide a promising avenue to manned interplanetary spaceflight; William Enrich of NASA writes that "Missions to Mars will almost certainly require propulsion systems with performance levels exceeding that of today's best chemical engines."  Fortunately, the specific impulses expected of NTR engines are nearly twice that of the best performing chemical lox/hydrogen engines today; specific impulse is the unit used to measure the efficiency of a rocket, such that a rocket with higher specific impulse will require a lower propellant flow rate to generate a particular thrust, thus allowing for higher efficiency and consequently increased thrust. [1,2] Thus, NTRs could prove an auspicious technology in realizing further space travel and exploration.
NTRs utilize a nuclear reactor, which generates energy through the fission of an atom of typically uranium or plutonium, to heat hydrogen propellant that is then expelled as exhaust through a nozzle to produce thrust. The first such nuclear engines were constructed by the Nuclear Engine for Rocket Vehicle Application (NERVA) program, a joint effort between the U.S. Atomic Energy Commission and NASA. The program was terminated in the 1970s despite highly promising test results that measured thrusts of up to 250,000 pounds and specific impulses of up to 850 seconds, which far surpasses the specific impulse of the best modern chemical rockets at 450 seconds. [1,3] In part, the program was ended due to safety and environmental concerns, as the NERVA testing was conducted in the Nevada desert and released the hot hydrogen propellant exhaust directly into the atmosphere.  The modern regulatory environment would likely make such testing impossible. Due to the promising nature of the technology, NASA researchers identified the need to test NTRs without releasing the exhaust while also being in compliance with nuclear testing treaties and accordingly constructed NTREES.
NTREES allows researchers to simulate and test the effects of nuclear thermal rocket fuels using non-nuclear materials.  The simulator is able to accurately model the environment (excluding radiation) that a nuclear thermal rocket would undergo in spaceflight. The conditions expected in such a rocket are modeled by inductively heating the test articles mounted in the simulator to produce the anticipated temperatures and heat fluxes that would be caused by nuclear fission while also being exposed to flowing hydrogen. To perform these tests, the lab has been modified to allow the gaseous hydrogen flow required and to produce up to 50kW of inductive heating, with the possibility to expand the facility to reach up to several megawatts.  The use of electrical inductive heating allows the facility to avoid the problems associated with fission product contamination and radiation; the tests performed in NTREES provide researchers with most of the relevant data about the test element's operational qualities without the cost and long-term planning required for nuclear testing.
The simulator has four main components: the vacuum/pressure chamber, the exhaust treatment system, and the data acquisition system. The vacuum/pressure chamber is a 3 meter long water-cooled vessel capable of reaching a maximum operating pressure of 6.9 MPa, although during ASME qualification testing the chamber was able to withstand pressures up to 9.9 MPa.  The chamber has 21 sapphire viewports to accommodate pyrometers that allow for remote measurement of the temperature distributions in the test element. The exhaust treatment system (ETS) is responsible for cooling the effluent stream of hydrogen and nitrogen as it leaves the chamber. The ETS first cools the heated hydrogen from the chamber by mixing it with large quantities of nitrogen. The gas then enters the water to gas heat exchanger where the gas is further cooled with liquid water, which is vaporized and exits the system through a filter and valve. The induction heater uses electromagnetic induction to heat the test element by sending a high-frequency alternating current through an electromagnet, which generates electric eddy currents inside the conductor element. Finally, the data acquisition system is comprised of pressure sensors, temperature sensors, flow sensors, thermocouples, hydrogen detectors, pyrometers, and a mass spectrometer.  Each of these components provides valuable information about the characteristics of the test element under the conditions expected in NTRs.
Perhaps the greatest advantage of NTREES is its ease and safety compared to directly testing NTR fuels. Whereas NTR tests are costly, time-consuming, and hazardous, the tests performed at NTREES allow researchers to quickly acquire vital knowledge about the performance characteristics of potential fuels and geometries in representative environments. The simulator is also remarkably safe, due to the system controller that independently shuts of GH2 flow in the event of: detection of a hydrogen leak or an excessive hydrogen level in the chamber; loss of nitrogen purge pressure or flow; test article rupture; possible failure in any safety monitoring system. 
Ultimately, NTREES offers a promising opportunity to explore the technical challenges and overall viability of nuclear thermal rocket fuels; a crucial step in realizing the technology. In order for an NTRs to come to fruition, it is vital to show that such rockets could be safe and effective, and NTREES provides concrete evidence to that end.
© Zachary Meza. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 W. J. Emrich Jr., R. P . Moran and J. B. Pearson, "Nuclear Thermal Rocket Element Environmental Simulator (NTREES) Upgrade Activities," American Institute of Aerenoautics and Astronautics, AIAA 2012-4307, 30 Jul 12.
 G. P. Sutton and O. Biblarz, Rocket Propulsion Elements, 8th Ed. (Wiley, 2010).
 W. H. Robbins and H. B. Finger, "A Historical Perspective of the NERVA Nuclear Rocket Engine Technology Program," U. S. National Aeronautics and Space Administration, Report No. 187154/AIAA-91-3451, July 1991.