Realistic Designs A- M - Atomic Rockets. GCNR Spacecraft. Propulsion. NTR- GAS/open. Fueluranium- 2. Propellanthydrogen. Specific Impulse. Exhaust Velocity. Mass Flow. 0. 8 to 6. Thrust. 20,0. 00 to. NFixed Thrust. 22. NThrust Power. 0. GWInitial Accel. 0. GCNR Spacecraft. Mars Courier. Mission. Duration. Wet Mass. 95. 0,0. Dry Mass. 29. 0,0. Mass Ratio. 3. 2. Thrust. 15. 0,0. 00 NInitial Accel. Specific Impulse. Exhaust Velocity. That's only 2. 7 months. Using Hohmann trajectories a round- trip Mars mission will take 3. Mars- Terra launch window to open. The report starts off with the common complaint that most rocket propulsion is either high- thrust + low- specific- impulse or vice versa. The problem being that rocket designers want a high- thrust + high- specific- impulse engine. In other words they want a torchship. The closest thing they can find that is actually feasible is a Gas- Core Nuclear Thermal Rocket. Open- cycle of course, closed- cycle has only half the exhaust velocity. So what if it spews still- fissioning uranium in an exhaust plume of glowing radioactive death? The report examines the GCNTR's performance to see if it is a torch drive. It comes pretty close, actually. Changes in 1.6.0. The full internal version number for this update release is 1.6.0. The external version number is 6u10. My daily commute is about 30 slow, zombie shuffles between my bedroom and my home office. While I’m grateful to say I have the option to work wherever and whenever. The higher the specific impulse / exhaust velocity, the more waste heat the engine is going to deal with. They figure that a GCNTR can control waste heat with standard garden- variety regenerative cooling like any chemical rocket, but only up to a maximum of 3,0. Past that you are forced to install a dedicated heat radiator to prevent the engine from vaporizing. Otherwise the engine vaporizes, your spacecraft has no engine, and perhaps centuries from now your ship will come close enough so that space archaeologists can recover your mummified remains. As everybody knows, thermal rockets use a heat source to heat the propellant (usually hydrogen) so that its frantic jetting through the exhaust nozzle creates thrust. Solid- core nuclear thermal rockets (NTR) use solid nuclear reactors. They are limited to a specific impulse (Isp) of about 8. K. Any higher specific impulse raises the temperature high enough that the reactor starts to melt. And nobody likes an impromptu impression of the China Syndrome. If you want an Isp of 5,0. K! Also as everyone knows the gas- core NTR concept is the result of clever engineers thinking outside of the box and asking the question what if the reactor was already vaporized? Instead of solid nuclear fuel elements it uses a super hot ball of uranium vapor which is dense enough and surrounded with enough moderator (neutron reflector) that it still undergoes nuclear fission. The fission produces huges amounts of thermal radiation, which heats the hydrogen propellant. The fissioning uranium is like a nuclear . The reaction chamber directs a flow of propellant around the sun to be heated. Since this is using the concentrated energy of fission there is no real limit to the thermal energy generated (think nuclear weapons). Unfortunately there is a limit to the hydrogen propellant's ability to absorb heat. Any heat that the hydrogen fails to sop up will hit the engine walls. If this unabsorbed heat is more than the heat radiator can cope with, bye- bye engine. This puts the upper limit on the engine's Isp capability. Engine. Cavity Linergraphite +5% niobium. Moderatorberyllium oxide. Propellant. Presure. The outer layer is the pressure vessel (since both the propellant and uranium gas needs lots of pressure to make this thing work), a layer of beryllium oxide (Be. O) moderator (a neutron reflector to help the uranium undergo nuclear fission), and an inner porous slotted cavity liner that injects the cold propellant to be heated. In the center is the furious blue- hot atomic vortex of uranium plasma. Sadly, this structure does suffer from waste heat. Which is a problem but not a major one. Most of the thermal radiation is soaked up and removed by the propellant. Hydrogen propellant does not do zippity- doo- dah to soak up gammas and neutrons, all of it sails right through the propellant to hit the engine structure. Deep inside the engine structure, gamma- rays and neutrons are more penetrating than x- rays. This waste heat is managed by the engine heat radiator (and a bit managed by regenerative cooling, about as effectively as a 3- year- old helping Daddy wash the car). Most of the engine is the beryllium oxide moderator. It is designed to operate at 1,4. K, which is below the 1,7. K melting point of the Be. O but above the 1,1. K radiator temperature (otherwise the radiator will refuse to remove the heat). The hydrogen propellant is pumped into the engine at about 5. That's not good. To remedy this sad state of affairs, it is . This is done right before the propellant exits the porous cavity liner into the flood of heat from the nuclear vortex. The seeding absorbs all the thermal radiation and passes the heat to the propellant by conduction. The seeding material will be something like graphite, tungsten, or non- fissionable uranium 2. Around the exhaust nozzle the seeding concentration will have to be increased to 2. The cold 2. 0% seeded hydrogen will reduce the specific impulse a bit but it has to be done. The porous cavity liner (in some as yet to be defined manner) magically sets up flow patterns so that the propellant flows around the hot uranium and exits via the exhaust nozzle. Meanwhile miraculously the uranium is trapped in a stagnant cavity in the center so hideously radioactive fissioning uranium does not escape through said exhaust nozzle. Uranium escape not only exposes the crew to deadly radiation, it is also a criminal waste of uranium (that is, it lets get away uranium that is not contributing to the engine's thrust). The interior of the engine (cavity diameter) is 2. This gives a fuel- to- cavity radius ratio of 0. The idea is for the uranium sphere to be 4. However since hydrogen propellant is going to diffuse into the atomic vortex, the uranium sphere might be up to 5. This means the effective volume of pure uranium will be closer to 2. The uranium can be injected by pushing a very thin rod of solid uranium into the chamber. The uranium penetrates the Be. O moderator inside a tunnel lined with a cadmium oxide neutron poison, because otherwise there would be a nuclear explosion once the uranium was surrounded by Be. O. This is a bad thing. The engine was designed to have the nuclear reaction happen in the core of the chamber, not in the walls. As the uranium rod enters the chamber, the heat of the fission ball vaporizes the rod so the fresh uranium atoms can join the party. A problem is how to get the process started. At startup, there ain't no ball of fissioning uranium to heat up the rod. The report says that the engine will have to be started by first blowing in some hydrogen and somehow injecting some powered uranium metal into the stagnant cavity until it reaches critical mass. Sounds tricky to me. Figures 2a through 2c above are for a reactor of the following characteristics: Spherical geometry. Uranium- 2. 35 fuel. Beryllium- oxide (Be. O) moderator. Fuel- to- cavity radius ratio 0. Cavity liner thickness 0. Cavity liner graphite + 5% niobium. Figure 2a shows that the 2. U critical mass ranges from 1. Now for a given cavity diameter, you can reduce the critical mass required by adding more Be. O neutron reflector. This means the pressure inside the engine can be lowered, which means the mass of the pressure shell can be lowered. Alas the increased penalty mass of the Be. O moderator more than offsets the mass saving on the pressure shell.(Elsewhere the report notes an optimal thickness of Be. O to be 0. 4. 6 m, and a cavity diameter of 2. Eyeballing the graph implies a critial mass of 2. U.)Figure 2b shows that if the Be. O moderator thickness is fixed, increasing the cavity diameter will decrease the critical density (the curved line will be closer to the bottom of the graph). Not shown in the table is the unfortunate fact that increasing the cavity diameter also has the side effect of increasing the total Be. O weight.(Elsewhere the report notes an optimal thickness of Be. O to be 0. 4. 6 m, and a cavity diameter of 2. Eyeballing the graph implies a critial density of 1. If the uranium plasma ball has a volume of 3. However, since propellant seepage will make the sphere about 5. Which is a reasonably close eyeball value to a second eyeball value. I'm just playing number games with the graphs, do not put too much credence to these speculations on my part.)Figure 2c shows that there is an optimum Be. O moderator thickness which gives a minimum critical density for a given Be. O moderator weight. Why is there an optimum Be. O moderator thickness? If the Be. O is too thin there is excessive neutron leakage (the purpose of the Be. O moderator is to reflect escaping neutrons back into the fissioning uranium, basically kicking the out- of- bounds neutrons back into play). Excessive neutron leakage means the blasted cavity diameter will have to be extremely large to avoid very high critical densities. If the Be. O is too thick, the total Be. O weight becomes very large. Even though you can get away with smaller cavity diameters without the heartbreak of very high critical densities. Figure 2c is telling you that the optimum Be. O thickness is 0. So all the engine weight estimates below are assuming a Be. O thickness of 0. Experiments show that an effective fuel volume is about 2. The paper assumes the engine can accelerate at about 0. The idea is to get the maximum thermal radiation from the fissioning atomic fireball into the cold hydrogen propellant, and the minimum thermal energy escaping the hot hydrogen propellant (which reduces the specific impulse and scorches the heck out of the cavity wall). Figure 5b shows experimental data for tungsten- seeded hot hydrogen. It says that adding just a few percent by weight of tungsten will increase the thermal absorption cross section to between 2,0. Download Simple Patch to Enable Aero Glass Transparency and Personalization Features in Windows 7 Home Basic and Starter Editions. 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