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Hi, happy new year to all!
I would like to use this very simple chemical-nuke hybrid rocket to power the spaceships of may next novel: I beg the engineers of this forum to tell me if it may work or not.
The engine is composed of many LOX-LH2 rockets, disposed around the rim of a truncated cone aerospike nozzle, which may also act as an entry shield, as a moderator/reflector and as a shadow shield, with an outer layer of tungsten, a layer of beryllium hydride and a beryllium neutron reflector, all LH2 regenaratively cooled.
The gas core is uranium exafluoride, stored in a gadolinium shielded tank.
During atmospheric operation the engine works like a normal LOX-LH2 rocket with a 6:1-5:1 oxidizer/fuel ratio.
When the spaceship is in space, the LOX/LH2 ratio is lowered to 1 or 0.5. Uranium exafluoride is puffed out from the truncated cone plug, become critical, reach 5000-7000°K and expands, forming a long spike that heat the plume like an afterburner, giving an exhaust velocity of 16 km/s or more.
During the burn, the core is confined by the plume. When the burn is finished, the core will flow out in the space, without contaminating the engine.
It may be very cheap because it has the simplicity of a chemical rocket, without the complexity of solid-core NTR control machinery.
May it be work?
Last edited by Quaoar (2014-12-26 06:10:16)
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Sounds like there would be a problem in terms of transferring the heat from the UF6 to the Hydrogen. Plus the UF6 would still probably fall into the atmosphere.
-Josh
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Sounds like there would be a problem in terms of transferring the heat from the UF6 to the Hydrogen. Plus the UF6 would still probably fall into the atmosphere.
The presence of water from oxygen in the exhaust gas can enhance heat transfer, if it not enough the plume can be seeded with carbon nanoparticles injected in the nozzle.
A layer of gas from gas generator turbines flows radially under the truncated cone base, avoiding direct contact with the plug core (it can be carbon seeded if necessary) and protecting it from melting.
Contamination is not a problem, because, in the story, Earth has space elevators and the rocket is supposed to connect Venus and Uranus cloud cities to orbit.
Last edited by Quaoar (2014-12-27 03:58:42)
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I'm still skeptical that you could get the heat transfer to work out properly, but I haven't done the calculations. If you want to do some and they show otherwise, then I suppose it's possible.
-Josh
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There was development of something similar: Gas Core Nuclear Thermal Rocket. Nuclear guys found the hotter they get exhaust, the higher Isp. To get exhaust hotter, the reactor had to be hotter. Limit is melting temperature of the reactor. Most solid core nuclear thermal rockets run as close as they can to melting, with a safety margin. But then some engineers designed an NTR to deliberately melt, raising the temperature. They came up with a few designs. Then they raised it further by running so hot the nuclear core would boil: gas core. This would start with UF6, then heat so much the compound would break down into uranium vapour and fluorine gas. They had to transfer heat to liquid hydrogen somehow, and best heat transfer is by direct contact. So they used technology from a uranium enrichment centrifuge. The idea was spin the uranium vapour in the combustion chamber of a rocket engine, creating a vortex. Inject liquid hydrogen from the top, down the centre of the vortex. Hydrogen would boil and heat, leaving the bottom as super heated gas. The throat was designed to separate the hydrogen gas from uranium vapour, so the uranium stays in the combustion chamber while hydrogen gas goes out through the throat into the exhaust bell cone. Design simulations show a tiny fraction of uranium will be lost with exhaust, but the vast majority will stay inside the engine. And it has high thrust, and extremely high Isp.
There are catches with this design. Splitting the atom produces two smaller atoms; they're called fission fragments. Media calls that high level nuclear waste. That's the extremely radioactive stuff; much more radioactive than uranium. All the fission fragments will be in exhaust. So you can't operate this within the Earth's atmosphere. In space, that radiation is like adding a teaspoon of water to the ocean. But you can't run it at all in atmosphere. This raises the question, how do we test it?
The second catch: how to you restart? In order to stop the engine, you have to dump the core out the exhaust cone. Then you need fresh UF6 to re-fill the reactor.
Last edited by RobertDyck (2014-12-30 12:55:36)
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I'm still skeptical that you could get the heat transfer to work out properly, but I haven't done the calculations. If you want to do some and they show otherwise, then I suppose it's possible.
I'm not able to do the calculation, but the problem of heat transfer was addressed by a group of Lewis Research Center in the 50-60. They know hydrogen is almost transparent to UV radiation up to 10000°K, as you correctly said, but they solved the problem using a mixture of solid particle seeded hydrogen.
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I think that maybe there's something we're all missing here, which is the fact that there seems to be no reason at all to build this rocket engine. An open cycle gas core design could have an Isp of 5000 s, but with the mixture radios being proposed chemical would be probably choose to 200 s. Don't bother with the chemical, just use the gas core.
-Josh
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I think that maybe there's something we're all missing here, which is the fact that there seems to be no reason at all to build this rocket engine. An open cycle gas core design could have an Isp of 5000 s, but with the mixture radios being proposed chemical would be probably choose to 200 s. Don't bother with the chemical, just use the gas core.
The idea was a very simple rocket: gas-core has a lot of trouble in avoiding the contact between core and chamber wall: having the core confined outside in the plume will result in a simpler and lighter engine, with less problem of cooling that can run all regeneratively cooled, avoiding heavy radiators.
The chemical rockets are cheep and are used without afterheater in low delta-V maneuver(2-4 km/s for change plane, course correction and low energy orbital transfer) to not waste very expensive uranium. Chemical rockets plus AH will be used only in high delta-V maneuver, like orbital ascend from a cloud city of Venus or Uranus, or to insert the ship in high energetic monotangent orbit to outer planets.
An alternative may be a solid core NTR with a gas core afterheater: solid core will have something like 10.5 km/s of exhaust velocity and can be used alone in high energetic orbit from inner to outer planets, with a magnetoshell for capture (all the planets has space habitats where it is possible to refuel, so spaceships don't need to bring propellant for the return trip) and the spike core afterheater will be used only to ascend orbit from a gas giant cloud city.
Last edited by Quaoar (2014-12-29 05:18:32)
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