Solid-core nuclear thermal rockets (NTRs) have the potential for specific impulse (ISP) a factor of 2 better than the best chemical propulsion can offer (NTRs can potentially offer up to 1000 seconds ISP). This added efficiency can potentially be used to decrease trip times, carry larger payloads, and/or decrease the number of heavy launches from Earth. A significantly shorter trip time is not practical for near-term Mars missions, so the benefit is generally accepted as additional (~2x) payload delivered per kg launched into Earth orbit. Therefore, the near-term benefit of an NTR is directly correlated with the cost per kg of Earth-to-orbit launches; the higher the cost of launching from Earth, the greater the attractiveness of NTP, and vice-versa.
When compared to Nuclear Electric Propulsion (NEP), the performance advantage of an NTR is higher thrust-to-weight, although with a much lower ISP. The technical advantage of an NTR over an NEP system is that the balance of plant is much simpler for an NTR. This feature also presents a major disadvantage, because all of the technical risk and development is within the reactor (i.e. nuclear in nature), as opposed to non-nuclear. Thus, NTR systems cannot utilize the development approach of using a simple reactor (with low nuclear technical risk), and transferring the development risk to non-nuclear technologies and system demonstrations.
In a solid core NTR, propellant/coolant flows through the reactor core, and thrust is created by expanding the propellant through a nozzle. Hydrogen is usually used as the propellant because the low atomic mass causes the greatest exhaust velocity (i.e. ISP); however, hydrogen is not a preferred reactor coolant because it is highly corrosive, dangerous to handle, and it requires cryogenic storage. The greatest challenge of an NTR is the extremely high temperature core to provide hydrogen exit temperatures of >2500 K. Materials must be found that have sufficient mechanical properties to support thermal and vibrational stresses at these temperatures while also withstanding possible erosion/corrosion with fast flowing hydrogen. There are numerous fuel forms that can be considered for NTRs, the most extensively studies and tested are the graphite-based fuels of the ROVER/NERVA programs. In more recent decades, there has been a good deal of research in cermet-based cores and pebble-bed cores.
The component technologies developed during the ROVER/NERVA programs were very impressive and fairly mature, and the existence of the previous work certainly makes the development of NTRs much more feasible than if the program had never taken place, but this does not mean that a modern day development program will be easy. The fact is that back in the NERVA program they were still testing and developing; and they were in some ways backtracking from system testing to special effects testing of fuel – because fuel erosion was still a major issue. There was no flight-system ready to fly (certainly not one that would be approved to fly in today’s environment); they were close, but the ability to get back to that point, and proceed from there would be a difficult task. There are 3 things missing from the old NERVA program that eliminate the potential for affordably developing a NERVA-based NTR.
1) Physical Infrastructure. The manufacturing and testing infrastructure that existed at the end of the NERVA program makes any nuclear development capability that exists within DOE or NASA today look miniscule.
2) Knowledge and Brainpower. There were hundreds of design, manufacturing, fabrication, testing and system experts working on NERVA in the ‘70s. Not only are most of these people gone, there has been relatively little knowledge transfer since this was before the era of digital record keeping, and use of the technology has not been continuous. Back then, nuclear development was more art and elbow-grease, as compared to science and CPUs; they built, tested and tweaked things and used their ingenuity to make things work. There are many examples of people trying to reproduce products (materials, parts, systems) developed 50 years ago that fail, likely because there were some steps in the process that workers did not record, or simply took for granted (but they would not be intuitive to someone starting from scratch today).
3) Political Environment. This issue is more than who is in power in Washington, D.C., at any particular moment. Unfortunately, an anti-nuclear bias is firmly entrenched in the US and the world. The bias is generally based on an uninformed fear of the dangers of radiation, which unfortunately is perpetuated by the media because exploiting this fear gets ratings (e.g. Fukushima, where there were no deaths and likely no future deaths caused by radiation, but the nuclear disaster got more press and caused more panic than the killing of ~20,000 people by the quake/tsunami). These fears have made the cost and risk of getting approval to run a nuclear-powered reactor test at least an order of magnitude higher than back in the 1970s. A program that relies on a nuclear-powered demonstration might have a chance in today’s environment (i.e., a nuclear test that you are confident will work), but one that requires iterative nuclear tests is generally unthinkable without a huge long term cost commitment (on the order of billions of dollars per year for a decade). In addition to development, launch approval is another factor where politics, public perception can kill a program. Nuclear material safeguards are a significant issue for all space reactors (except for a small class of low-performance reactors that could reduce the safeguards category). The difficulty in creating an acceptable nuclear and launch accident safety case is a function of system power and the drive for low mass, and an NTR is much more difficult than a modest power reactor in this respect. Another final programmatic factor that makes NTR development extremely difficult is time - a realistic program will take more than a decade (at least 2x the development time of a simple power system). Given the realities that occur in Washington D.C., a program that takes over 5 years is risky and a program that takes over 10 years is essentially a non-starter (even the incredibly ambitious Apollo program went from conception to landing a man on the moon is 9 years).
If all 3 of the above issues could be reset back to where they were in the early 1970s then an iterative nuclear test program could be feasible within a realistic NASA program. However, even if #1 and #2 were still available, #3 makes the costs and programmatic risks unacceptable to any program other than a national-mandate program like Apollo. The fact is that an adequate human exploration program can be performed with chemical propulsion, so it is unlikely that a nationally mandated NTR program would occur in the foreseeable future.
In conclusion, a nuclear thermal rocket would have significant benefit to NASA (albeit not as much as other fission systems), but it does not warrant more than a small fraction of near-term funding until an entry-level fission system is established in space.
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