5 ms·

Interesting concept, but plenty of red flags. The idea is to have achieve fission criticality in a projectile of uranium fired inside a gun of uranium. The sys

Interesting concept, but plenty of red flags. The idea is to have achieve fission criticality in a projectile of uranium fired inside a gun of uranium. The system is designed so that the gun is always subcritical, while the projectile is supercritical, because there's some moderator inside the projectile. The authors claim they simulated the neutron economy of the system using the MCNS6 software, a soft created by the Los Alamos National Lab [1]. The idea is certainly cool, and I wish so much for it to be feasible. But the paper has so many red flags. The main red flag is this: according to the paper, the projectile completely vaporizes when it is 11 centimeters inside the gun. The effective multiplicative factor K_eff (the ratio of the number of neutrons in a new generation vs the old) is calculated to be about 1.1. Anything above 1 is super-critical, meaning the number of fission events increases exponentially with each generation. The paper mentions that they used the software MCNP to estimate this factor, but it does not mention how they treated the change of phase. When the projectile reaches the temperature they describe (11600 K), the mean velocity of the hydrogen ions is about 16 times higher than the mean velocity of the uranium atoms, so basically all the hydrogen is gone. The amount of moderator just went from a certain number to zero. What exactly happened to K_eff? It must go down, but how much? More precisely, the K_eff must be a function of how much the vaporized projectile has expanded, and how little of the original hydrogen is still around. Nothing like this is hinted. My point is that the computation is highly complex, but the way the paper presents it reads like a high school science project. Another big red flag is this: there's no discussion of the neutron economy inside the barrel. We are just told the barrel is subcritical, but even so, it is subject to a huge bombardment of neutrons from the supercritical projectile. Those neutrons will trigger fission events in the barrel. If you shoot 1 projectile per second, this is bound to have quite a bit of an effect, quite soon. Fission is enormously energetic. And yet another red flag: why was this paper published in Acta Astronautica, and not in a nuclear physics of nuclear engineering journal? The application is indeed to rockets, but the meat of the paper is the nuclear fission. One would be much more comfortable to hear the paper was peer-reviewed by nuclear scientists. As a point of reference, the latest issue of this journal [2] has 49 articles, and only one is nuclear related. That one has the title "Direct fusion drive based on centrifugal mirror confinement". In other words, "pie in the astronautical sky". Unfortunately, this pulsed plasma rocket sounds about as feasible as the direct fusion drive rocket. [1] https://mcnp.lanl.gov/ https://mcnp.lanl.gov/ [2] https://www.sciencedirect.com/journal/acta-astronautica/vol/219/suppl/C https://www.sciencedirect.com/journal/acta-astronautica/vol/...

kristianp 3y ago
You're clearly knowledgeable in this subject, is there a nuclear propulsion design that stands out to you as the most feasible? I like fission fragment rockets(1) because they seem simple and apparently have very high efficiency, but I'm no expert. (1) https://en.wikipedia.org/wiki/Fission-fragment_rocket https://en.wikipedia.org/wiki/Fission-fragment_rocket
- credit_guy 3y ago
  There are lots of designs for nuclear rockets, and while I read about them, I did not spend time to poke them and see if there are any obvious issues. The problem with nuclear designs is that things that work on paper don't necessarily work in practice, as Admiral Rickover is known to have ranted once. Given that, to me the most promising 2 nuclear designs are the nuclear thermal rocket design, and the Orion design. Here's why. The Rover/NERVA program [1] is underappreciated. In terms of scientific and engineering achievement it rivals the Manhattan project, while having a fraction of its budget. Just to get a sense of the distance between theory and practice: the idea of a nuclear thermal rocket is to push hydrogen through a nuclear core. It gets in cold, it comes out hot, and voila, you have a nice rocket engine. What could be simpler? There are a few problems. The first is the scale. Here's a quote about the nuclear engine Phoebus 2A [2]: > This was followed by a test of the larger Phoebus 2A. A preliminary low power (2,000 MW) run was conducted on 8 June 1968, then a full power run on 26 June. The engine was operated for 32 minutes, 12.5 minutes of which was above 4,000 MW, and a peak power of 4,082 MW was reached. At this point the chamber temperature was 2,256 K (1,983 °C), and total flow rate was 118.8 kilograms per second (262 lb/s). For comparison a full-size nuclear AP1000 reactor like the one that was just started at Vogtle produces about 3.6 GW-thermal, so less than the 4 GWt mentioned here. Such a reactor circulates about 20 tons of water per second through the core. Somehow this rocket engine is able to extract more power using 50 times less coolant by mass, and from a core that literally fits on the bed of a small truck. The vibrations and temperatures inside this core were tremendous. In various tests parts of the fuel rods ruptured, sometimes the hydrogen would catch fire, sometimes valves would break. All these annoying engineering challenges had to be overcome. But eventually they were overcome. That's the important thing we know about the thermal nuclear engine: we know it can be done, because it was done. Some people may complain that the ISP from these engines topped at 900 seconds. Considering the technological readiness of this technology, I think this is nothing to sneeze at. There are good reasons to believe with this technology we can reach 1000 seconds, and maybe a bit higher. The second technology on my list is the Orion project. It was never implemented, but my heuristics are like this: 1. Freeman Dyson was, in the common understanding of the word, a genius. It is true that he did not have a PhD, but aside from that, as a scientific mind, he was probably the equal of Feynman. 2. The thing that makes the spaceship move, the nuclear bombs, are a very mature technology. Pairing that with a pusher plate remains to be validated, but it's highly likely to work. The pusher plate idea was tested with conventional bombs, and we have no particular reason to think it wouldn't work if you increase the yield of the bomb. Of course, the thing that goes against project Orion is the fact that we live in the real world, and in this world nuclear bombs are a problem. You don't want to start ferrying thousands of nukes to space without thinking twice. But if we can figure out the non-proliferation aspect of the project Orion, I think it's the most likely configuration to enable us to do deep space travel. [1] https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA [2] https://en.wikipedia.org/wiki/Project_Rover#Phoebus https://en.wikipedia.org/wiki/Project_Rover#Phoebus

kristianp 3y ago

You're clearly knowledgeable in this subject, is there a nuclear propulsion design that stands out to you as the most feasible? I like fission fragment rockets(1) because they seem simple and apparently have very high efficiency, but I'm no expert. (1) https://en.wikipedia.org/wiki/Fission-fragment_rocket https://en.wikipedia.org/wiki/Fission-fragment_rocket

credit_guy 3y ago
There are lots of designs for nuclear rockets, and while I read about them, I did not spend time to poke them and see if there are any obvious issues. The problem with nuclear designs is that things that work on paper don't necessarily work in practice, as Admiral Rickover is known to have ranted once. Given that, to me the most promising 2 nuclear designs are the nuclear thermal rocket design, and the Orion design. Here's why. The Rover/NERVA program [1] is underappreciated. In terms of scientific and engineering achievement it rivals the Manhattan project, while having a fraction of its budget. Just to get a sense of the distance between theory and practice: the idea of a nuclear thermal rocket is to push hydrogen through a nuclear core. It gets in cold, it comes out hot, and voila, you have a nice rocket engine. What could be simpler? There are a few problems. The first is the scale. Here's a quote about the nuclear engine Phoebus 2A [2]: > This was followed by a test of the larger Phoebus 2A. A preliminary low power (2,000 MW) run was conducted on 8 June 1968, then a full power run on 26 June. The engine was operated for 32 minutes, 12.5 minutes of which was above 4,000 MW, and a peak power of 4,082 MW was reached. At this point the chamber temperature was 2,256 K (1,983 °C), and total flow rate was 118.8 kilograms per second (262 lb/s). For comparison a full-size nuclear AP1000 reactor like the one that was just started at Vogtle produces about 3.6 GW-thermal, so less than the 4 GWt mentioned here. Such a reactor circulates about 20 tons of water per second through the core. Somehow this rocket engine is able to extract more power using 50 times less coolant by mass, and from a core that literally fits on the bed of a small truck. The vibrations and temperatures inside this core were tremendous. In various tests parts of the fuel rods ruptured, sometimes the hydrogen would catch fire, sometimes valves would break. All these annoying engineering challenges had to be overcome. But eventually they were overcome. That's the important thing we know about the thermal nuclear engine: we know it can be done, because it was done. Some people may complain that the ISP from these engines topped at 900 seconds. Considering the technological readiness of this technology, I think this is nothing to sneeze at. There are good reasons to believe with this technology we can reach 1000 seconds, and maybe a bit higher. The second technology on my list is the Orion project. It was never implemented, but my heuristics are like this: 1. Freeman Dyson was, in the common understanding of the word, a genius. It is true that he did not have a PhD, but aside from that, as a scientific mind, he was probably the equal of Feynman. 2. The thing that makes the spaceship move, the nuclear bombs, are a very mature technology. Pairing that with a pusher plate remains to be validated, but it's highly likely to work. The pusher plate idea was tested with conventional bombs, and we have no particular reason to think it wouldn't work if you increase the yield of the bomb. Of course, the thing that goes against project Orion is the fact that we live in the real world, and in this world nuclear bombs are a problem. You don't want to start ferrying thousands of nukes to space without thinking twice. But if we can figure out the non-proliferation aspect of the project Orion, I think it's the most likely configuration to enable us to do deep space travel. [1] https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA [2] https://en.wikipedia.org/wiki/Project_Rover#Phoebus https://en.wikipedia.org/wiki/Project_Rover#Phoebus