|Fig. 1: Schematic of an NTR being tested using the SAFE concept in the Nevada Testing Range.  (Courtesy of NASA.)|
With the success of the Mars rovers, the end of the Space shuttle, and SpaceX's resupply mission to the International Space Station (ISS), scientists and engineers are now looking further into the solar system for new challenges. One major problem with sending people on a mission to Mars is the long transit time. Extended time in space requires significant improvements in propulsion along with life support and electrical power systems.  Chemical rockets can only take individuals so far. NASA recently completed a study looking at a manned Mars mission in the post-2030 timeframe and found that nuclear thermal rockets (NTRs) beat their chemical counterparts. 
The first NTR program started in 1955 under the Los Alamos Scientific Laboratory (LASL), program name Rover, with the goal of developing a solid core nuclear rocket engine.  This program wanted to use a graphite nuclear fuel along with hydrogen coolant for the reactor that would then be expelled from the rocket to generate thrust. Graphite was chosen as the fuel source due to its neutronic characteristics and high operational temperatures.  However, the carbon hydrogen mixture is highly corrosive resulting in all fuel elements were coated in a protective coating to reduce corrosion rates. NTRs offered improved efficiency, lower mass to orbit, and higher thrust, resulting in a shorter mission, compared to their chemical counterparts.  These potential rewards resulted in significant testing being completed in the late 1950s and early 1960s.
Once the technology had validated these original hypotheses, the Nuclear Engine for Rocket Vehicle Applications (NERVA) program was launched to create a space qualified nuclear engine.  NERVA was widely successful with 23 reactors/engines built and tested. Some of the major accomplishments included operating the system for over an hour, conducting start-up and shutdown procedures, and reaching an ISP of 850, three times that of the kerosene engines on the Saturn V and twice that of the later LOX/hydrogen Space Shuttle Main Engine.  The ISP is the measure of efficiency for rocket engines with large values indicating a more efficient engine.
To conduct all of these tests after restrictions were put in place to limit the emission of radioactivity into the atmosphere, the NERVA engineers had to develop a way of cleaning the fission products out of the rocket exhaust. A "scrubbing" procedure was implemented where the exhaust from the motor passed through multiple refining schemes, including steam to remove particulates, silica gel to remove water and dissolved fission products, and cryogenically cooled charcoal bed to remove noble gases.  The flow rate, directly related to the thrust generated, of 1 kg/s for NERVA would not meet the mission requirements for any potential mission to Mars. Increasing the flow rate from the level of NERVA to a practical Mars mission requiring 20,000 lbf, approximately 10 kg/s, could require between $200-500 million dollars. 
|Fig. 2: Implementation of a NTR with twin LH2 powered turbopumps.  (Courtesy of NASA.)|
To reduce the cost of testing new NTRs, the Subsurface Active Filtering of Exhaust (SAFE) method was proposed. In the SAFE concept, the nuclear rocket test would utilize the methodology as nuclear weapon tests. The rocket would be sealed from above and fired downward into the sub-strata of the Nevada Test Site.  A general example of how the testing configuration would be implemented can be seen in Fig. 1.
From previous work on the subject, all fissionable products except the noble gases Xenon and Krypton will be trapped in the alluvium underneath the test range.  This would allow all the necessary preflight tests be completed for an NTR without the need for the expensive filtering process required for NERVA rockets. Using the existing methodology of scrubbing the exhaust, the testing costs were expected to be $46.5 million accounting for 30% management overhead, but the SAFE concept would reduce that cost to $3.9 million.  This difference in testing cost is substantial, but not only would this concept be cheaper, but also would allow the motor to be tested at different settings and for full duration compared to one power setting for the scrubbing procedure. The designers could then learn significantly more during the testing phase with the SAFE concept and the entire propulsion system would be more flexible to changes in payload and launch mass creep.
When determining propulsion concepts to take astronauts to Mars or further out in the solar system, NTRs keep offering the best results. The main characteristics leading to this outcome include the higher specific impulse, discussed previously, increased tolerance to payload and mission results, lower initial mass to low Earth orbit, and additional thrust in space.  This last characteristic is very important because it allows mission designers the flexibility to consider fast transit missions with a longer surface stay. A fast transit is possible because NTRs can add a much larger delta-V, the measure used to determine what orbits are possible, for the same mass change compared to a chemical rocket.
NASA Glenn developed a "7-launch" mission plan to send astronauts to Mars using NTRs for the second stage of an Ares-V rocket compared to 9 Ares-V rockets needed when using a chemical second stage.  While the Ares-V rocket project has been discontinued in favor of the space launch system, this potential to reduce the number of launches by over 20 percent is valuable considering the cost of rocket launches not to mention the possibility of failure and delays in construction. A schematic of a hypothetical NTR using today's technology is shown in Fig. 2.
To construct an NTR, a compact fission reactor core is created containing 93% enriched Uranium U-235 fuel.  This reactor would generate hundreds of megawatts that would be used to heat the LH2 fuel and accelerate the entire vehicle. In the NERVA configuration, the LH2 will cool the different parts of the engine from the intense exhaust heat before powering the turbopumps.  Balancing the flow rate through the turbopumps with the reactor temperature allows the engines to operate in different power settings. The feasibility of the rockets has been proven through multiple tests consisting of 28 start-up and shutdown cycles along with over two hours of engine operation. [1,2] These results show that NTRs have the potential to meet all the mission requirements for a successful human deep space mission.
When considering potential roadblocks to near term usage of NTRs, three obstacles stand out: cost of implementation, accuracy of modelling, and containment in the event of potential rocket failure. Reducing testing costs using the SAFE concept will allow full flight rated NTRs to be tested on the ground before being flown. Combining this saving with the reduction in number of launch costs helps to make potential NTRs more feasible in a strictly cost sense compared to chemical rockets.
Before any testing of an NTR can be completed, better numerical models of the nuclear reaction should be developed. Initial work in this field has been done trying to validate against the Small Nuclear Rocket Engine (SNRE) from the Nuclear Engine Definition Study (NEDS).  These models allow designers the flexibility to test more exotic configurations trying to improve the end rocket. Once testing commences, additional data will become available that can be used to validate another level of NTRs used in the future.
Beyond just examining radical configurations, model improvement helps with the final problem area, containment of nuclear fuel. During the Kiwi testing, Los Alamos scientists considered the potential problem of inserting fuel too rapidly into the core of the nuclear reactor. These scientists found within 300 ft of the accident, the radiation would be fatal, inside a 1000 ft radius, windows would break from the shock and out to 10 miles, there were a few radiation "hot spots" up to a few mR/hr on the day of the test.  Outside of two miles, however, the scientists concluded, "a Kiwi-TNT type of excursion, without a fission-product inventory in the reactor, creates inconsequential hazards to personnel and property beyond about 2 miles."  This gives a potential guideline for the future to limit the potential hazard to personnel and property in the case of a failure.
Once the potential problems in the application of NTRs can be solved, many other enticing mission options beyond Mars exploration are available. One of these potential options is using NTRs within a larger vehicle of approximately 10,000 tons to travel to Jupiter's moon, Callisto.  In this study, the space vehicle would be launched without a propellant, but a near earth object, such as the moon or asteroid, would supply the propellant in the form of ice. During the 1960s, water was used as the coolant for reactor temperatures up to 1100 K.  Thus, the ice would be melted from the heat of the NTR and the liquid water would be used as the coolant and propellant for the rocket. Following this design, NTRs on spacecraft would allow much larger mission payloads with the ability to refuel at other sources of ice in the solar system. These opportunities for significant exploration make NTRs a high reward technology that should be investigated in the future of spaceflight.
© Andrew D. Wendorff. 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.
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