ElfredaCyania
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This is my first post here. Since the topic doesn’t quite belongs to any other category, I hope this is the right place.
If you are familiar with secret projects then you might have see nuclear ramjet (famously the Pluto/SLAM), large high supersonic aircraft (XB-70 and SSTs), and large subsonic nuclear-powered aircraft with long loitering time (like X-6). These designs have a decent amount of research and resources, but I’m working on a project that combines all of them—a large high-alt high-supersonic aircraft cruising for extended periods using nuclear ramjet, and this is where I stopped seeing any related information—extended period changes everything. I tried to figure out what technologies are necessary and here are some findings.
First, nuclear ramjet can certainly work at higher altitudes, Pluto missile itself will cruise at FL350 at Mach 4 before entering the target zone. For high supersonic speed, the most obvious problem is removing heat, at this speed aerodynamic heating is the main source of heat. Typical Mach 3 planes like SR-71 and XB-70 use their jet fuel as heat sinks and eject the heat out of the aircraft. Some of them include more complex mechanisms like using heat-absorbing reactions (endothermic fuel) in addition to pure phase changes (ie from liquid to gas). Pluto's TORY II engine followed this method by emitting the fissile material to dissipate heat (btw this makes it a funny variant of a fission-fragment rocket). For detailed information see 11.2 in this document from PDF pg. 285.
However for long flight and multiple flights, the integrity of the fuel rods are critical, TORY-II’s disposable design would not be feasible, we need what nuclear engineering call fuel cladding. Currently, the two major ways of fuel cladding are cylindrical and TRISO. To put it simply, they are about wrapping the nuclear fuel rod in cylinders or spheres. TORY-II’s fuel rod was cylindrically clad by graphite and BeO, primarily for their neutronic characteristics rather than fuel containment as both materials are brittle even without radiation. Modern power plant reactor rods mostly work underwater (ie PWRs), and it’s vastly different from the environment of a nuclear ramjet. Most research on modern ramjet-like nuclear propulsion is on spacecraft (NTR), not aircraft—again, they have very different working conditions like hydrogen (in NERVA, pg 3). In a word, no one knows what would a modern nuclear ramjet’s fuel rod look like.
For a no-emission nuclear ramjet, TORY-II’s heat dissipation method is not feasible since there’s no fuel to be ejected. The onboard heat management system would have to deal with all the heat generated by aerodynamics and engines, transporting heat through the frame and dissipating the heat via exhaust, convection or radiation. According to this research, convection and chemical reaction in atmosphere roughly equals to radiation and conduction inside the aircraft, and conduction is negligible, leaving the major way as black body radiation. Temperature and the emissivity of the skin are the determining factors of heat dissipation rate via radiation, as discussed in this paper in the introduction.
Fig 1. Heat flow at a sharp leading edge protected by a passive thermal protection system. Energy is transferred into the body via conduction and/or transferred back to the environment by radiation. (source)
Fig 2.Wall temperature of a leading edge surface versus radiated heat flux. A surface with higher emissivity is heated less than a surface with lower emissivity at any given heat flux. (source)
Take the Blackbird, Blackbird’s skin has an ε of 0.98 and an average temperature of 260~315C (source varies). Using equations 1 and 2 from the above paper, the Blackbird has a radiation heat flux of somewhere between 4000 to 6500W/m^2, if we assume the usable radiation surface is 400 m^2, its total usable radiation rate would be 1.6-2.5MW, which is the total usable capacity of radiation heat dissipation. If no material is ejected from the plane, this would cap the maximum speed. It’s very hard to obtain the power of aerodynamic heating, if we assume it has an L/D ratio of 5 and a weight of 40 tons at 950 m/s cruising, then it has a drag power of 75MW, but this number includes both shockwave and heating, and it’s almost impossible to get rid of the shockwave without running simulations.
Prolonged supersonic speed is likely to raise coolant temperature high, this is where high-temperature heat-transfer fluid comes into play. These solutions include silicone oil (commonly used in spacecraft), glycol/water and radical ones like liquid NaK (which is kind of insane in aircraft). And of course, the problem is there’s no precedence for such a system being used on large aircraft. It looks plausible to me for nuclear ramjet aircraft to carry a large amount of incombustible coolant to a quantity comparable to what conventional plane will carry fuel. The coolant could be ejected for better cooling, but it would cost more and reduce loitering time.
In a scenario involving an open reactor (like Pluto/SLAM’s) or for the sake of less radioactive pollution, there is a problem with how to take off without using the ramjets. This is a mature field, we have solutions like JATO, electric ducted fans or simply using turbojets, although the latter two are likely to make things much more complicated.
Aerodynamics would be much less of a problem. Aerodynamics are the same regardless of propulsion, the only difference would be how you handle intake air.
These are roughly what I’ve been looking into so far, any input would be useful, especially on new methods or problems that I’ve missed, as they would be critical before I start building the model and simulations.
Note: The project is building a hypothetical aircraft so it’s less about a definite result but more about the concrete science involved in the process. I’m focusing on what technology can make it fly, not why it won’t work or further development like radiation shielding for the crew, mission profile, budget and cost etc.
If you are familiar with secret projects then you might have see nuclear ramjet (famously the Pluto/SLAM), large high supersonic aircraft (XB-70 and SSTs), and large subsonic nuclear-powered aircraft with long loitering time (like X-6). These designs have a decent amount of research and resources, but I’m working on a project that combines all of them—a large high-alt high-supersonic aircraft cruising for extended periods using nuclear ramjet, and this is where I stopped seeing any related information—extended period changes everything. I tried to figure out what technologies are necessary and here are some findings.
First, nuclear ramjet can certainly work at higher altitudes, Pluto missile itself will cruise at FL350 at Mach 4 before entering the target zone. For high supersonic speed, the most obvious problem is removing heat, at this speed aerodynamic heating is the main source of heat. Typical Mach 3 planes like SR-71 and XB-70 use their jet fuel as heat sinks and eject the heat out of the aircraft. Some of them include more complex mechanisms like using heat-absorbing reactions (endothermic fuel) in addition to pure phase changes (ie from liquid to gas). Pluto's TORY II engine followed this method by emitting the fissile material to dissipate heat (btw this makes it a funny variant of a fission-fragment rocket). For detailed information see 11.2 in this document from PDF pg. 285.
However for long flight and multiple flights, the integrity of the fuel rods are critical, TORY-II’s disposable design would not be feasible, we need what nuclear engineering call fuel cladding. Currently, the two major ways of fuel cladding are cylindrical and TRISO. To put it simply, they are about wrapping the nuclear fuel rod in cylinders or spheres. TORY-II’s fuel rod was cylindrically clad by graphite and BeO, primarily for their neutronic characteristics rather than fuel containment as both materials are brittle even without radiation. Modern power plant reactor rods mostly work underwater (ie PWRs), and it’s vastly different from the environment of a nuclear ramjet. Most research on modern ramjet-like nuclear propulsion is on spacecraft (NTR), not aircraft—again, they have very different working conditions like hydrogen (in NERVA, pg 3). In a word, no one knows what would a modern nuclear ramjet’s fuel rod look like.
For a no-emission nuclear ramjet, TORY-II’s heat dissipation method is not feasible since there’s no fuel to be ejected. The onboard heat management system would have to deal with all the heat generated by aerodynamics and engines, transporting heat through the frame and dissipating the heat via exhaust, convection or radiation. According to this research, convection and chemical reaction in atmosphere roughly equals to radiation and conduction inside the aircraft, and conduction is negligible, leaving the major way as black body radiation. Temperature and the emissivity of the skin are the determining factors of heat dissipation rate via radiation, as discussed in this paper in the introduction.
Fig 1. Heat flow at a sharp leading edge protected by a passive thermal protection system. Energy is transferred into the body via conduction and/or transferred back to the environment by radiation. (source)
Fig 2.Wall temperature of a leading edge surface versus radiated heat flux. A surface with higher emissivity is heated less than a surface with lower emissivity at any given heat flux. (source)
Take the Blackbird, Blackbird’s skin has an ε of 0.98 and an average temperature of 260~315C (source varies). Using equations 1 and 2 from the above paper, the Blackbird has a radiation heat flux of somewhere between 4000 to 6500W/m^2, if we assume the usable radiation surface is 400 m^2, its total usable radiation rate would be 1.6-2.5MW, which is the total usable capacity of radiation heat dissipation. If no material is ejected from the plane, this would cap the maximum speed. It’s very hard to obtain the power of aerodynamic heating, if we assume it has an L/D ratio of 5 and a weight of 40 tons at 950 m/s cruising, then it has a drag power of 75MW, but this number includes both shockwave and heating, and it’s almost impossible to get rid of the shockwave without running simulations.
Prolonged supersonic speed is likely to raise coolant temperature high, this is where high-temperature heat-transfer fluid comes into play. These solutions include silicone oil (commonly used in spacecraft), glycol/water and radical ones like liquid NaK (which is kind of insane in aircraft). And of course, the problem is there’s no precedence for such a system being used on large aircraft. It looks plausible to me for nuclear ramjet aircraft to carry a large amount of incombustible coolant to a quantity comparable to what conventional plane will carry fuel. The coolant could be ejected for better cooling, but it would cost more and reduce loitering time.
In a scenario involving an open reactor (like Pluto/SLAM’s) or for the sake of less radioactive pollution, there is a problem with how to take off without using the ramjets. This is a mature field, we have solutions like JATO, electric ducted fans or simply using turbojets, although the latter two are likely to make things much more complicated.
Aerodynamics would be much less of a problem. Aerodynamics are the same regardless of propulsion, the only difference would be how you handle intake air.
These are roughly what I’ve been looking into so far, any input would be useful, especially on new methods or problems that I’ve missed, as they would be critical before I start building the model and simulations.
Note: The project is building a hypothetical aircraft so it’s less about a definite result but more about the concrete science involved in the process. I’m focusing on what technology can make it fly, not why it won’t work or further development like radiation shielding for the crew, mission profile, budget and cost etc.
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