The rocket analogue of a flywheel is a very heavy rocket, whose large inertia would smooth the changes in its velocity due to propulsion pulses.
Nevertheless, this solution is undesirable, because it would limit the acceleration of the rocket to small values.
The right solution is that mentioned by the other poster, i.e. to have a large number of pulsed rocket engines, whose pulses should have distinct phases. In this case there should be pairs of engines with synchronized pulses and symmetric positions, to avoid the rotation of the rocket, because the engines can no longer be aligned with its center of inertia.
An Orion drive actually scales pretty nicely. As I recall, a city 20 miles wide planted on top of a big metal plate was just about the sweet spot. Mostly this works because it is easy to make a bomb with more yield without making the bomb larger.
I have this amusing vision of a rocket ship that resembles a Slinky™ dog, the rear propulsion section connected to the habitable front by an exaggerated spring.
More likely you’d have the engines flying out in front on tethers, so like a skier behind a thermonuclear ski boat. Tethers would weigh a lot less than springs.
You could also have fully electromagnetic “springs” using powerful superconducting magnets.
I was going to just say springs, but this is more accurate. Orion (the original nuclear rocket) already had this. I'm not sure if the complexity of the multi-chamber pulsed plasma would make dealing with the vibrations more challenging though.
This article doesn’t mention it but this sounds like the fissile material also undergoes fission either as plasma or as part of what makes it plasma, making this like an “inboard” externally driven version of Project Orion (the bomb pogo stick idea, which on paper is workable).
This, Orion, and the nuclear salt water rocket seem like three actual “torch ship” engines that could theoretically be built with current technology. Nothing as loony as the Epstein Drive from The Expanse or the antimatter rockets from Avatar but able to get around the solar system way faster than any chemical rocket.
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.
> 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.
The hydrogen having a higher mean velocity doesn't mean it goes anywhere. When the uranium and moderator are initially vaporized the iron shell is still intact (there isn't enough time for the heat to conduct to it nor for the protons from the ionized hydrogen to escape) so everything is still contained within the projectile at that point. There are control drums that act both to moderate neutrons while the projectile is in motion and to keep the barrel subcritical when the projectile is clear of the section. In the paper they report that K_eff peaks right before the uranium is vaporized and then drops off rapidly.
> there's no discussion of the neutron economy inside the barrel.
The paper models the projectile and barrel as a single system. Power generation in the barrel is approximately 1/25th that in the projectile.
> 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.
The meat of the paper is generating and containing a high speed plasma projectile that just happens to be generated by fission. It's not trying to do anything innovative on the nuclear side beyond throwing a weird (and for non-space applications irrelevant) configuration into standard simulation software.
Just to be very clear, are you saying this paper looks good to you? You are not seeing any red flags?
Like, for example, the fact that the 7th entry of Table 2 contains 0% Uranium, but still has a K_eff of 1.095. What does that even mean?
Or like the following sentence on page 15:
> The MCNP6 modeling and calculation of the moderating ability of the fuel projectile that evaluated the uses of Lithium-6, Lithium-7, Boron-10, Boron-11, Carbon-12, and H2O
Why would you even consider Boron-10, which is one of the strongest neutron poisons? Maybe it's a mistake, right? Well, 3 of the rows in Table 1 contain Boron-10, and one (the ninth row) is the "champion" at the amount of energy production. A funny champion, considering that again it does not include any Uranium.
As for the iron not vaporizing, I suspect you are basing your conclusion on this statement:
> Because the projectile heats up so fast, there is no time for conductive heating of the iron. Since all the naturally occurring isotopes of iron have a thermal neutron absorption cross section less than 2.5 barns, direct neutron heating will not dominate. Therefore, it is highly probable that the iron sheath around the bullet will remain a solid far deeper into the barrel.
That "highly probably" is a bit vague. What exactly happens when the iron sheath disintegrates? Will it change phase instantaneously in its entirety, or from the inside to the outside. And if it's the second, will shrapnel hit the barrel? If It does, it's kind of a problem, isn't it?
> Power generation in the barrel is approximately 1/25th that in the projectile.
Does this mean that after firing 25 projectiles (i.e. after 25 seconds) the power generation in the barrel is equal to the power that is sufficient to vaporize the projectile? You could say it's not a biggie, because the barrel is much more massive, but it's finite. What happens after 250 seconds, or 2500 seconds? And more importantly, what happens to the neutron economy in the whole system after a good amount of the uranium in the barrel undergoes fission under this neutron bombardment? Are we going to have Xenon poisoning at some point?
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.
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.
