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Post by airc777 on Feb 27, 2019 0:27:42 GMT
Could you use a laser as your heat source instead of a nuclear core or a resistor so you aren't constrained by the cores melting point? Then you could use like, tantalum hafnium carbide nozzles or something.
Edit: Answered my own question with a three second google search. Laser Thermal Rocket has it's own sub section under Laser Propulsion on Wikipedia.
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Post by tepidbread on Feb 27, 2019 6:34:11 GMT
I wonder if the benefits of the extra exhaust velocity would adequately compensate the added weight from the laser system and the extra inefficiencies that such a system would possess. Might be a good alternative to MPDs if you care less about weight and efficiency and want more thrust, yet still want more specific impulse than a NTR.
I wonder if fuel could be heated simply by passing current through it resistivity rather than utilizing a resistor. Similar to an MPD but not taking advantage of the lorentz force and just heating propellant resistivity. Perhaps this already exists. A resistojet where the propellant is the resistor.
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Post by airc777 on Feb 27, 2019 15:25:50 GMT
I wonder if the benefits of the extra exhaust velocity would adequately compensate the added weight from the laser system and the extra inefficiencies that such a system would possess. Might be a good alternative to MPDs if you care less about weight and efficiency and want more thrust, yet still want more specific impulse than a NTR. I wonder if fuel could be heated simply by passing current through it resistivity rather than utilizing a resistor. Similar to an MPD but not taking advantage of the lorentz force and just heating propellant resistivity. Perhaps this already exists. A resistojet where the propellant is the resistor. Possibly, but you'd still be constrained by the temperature tolerance of the conductor you'd use to arch the power through your fuel resistor, although it might be a lot more mass efficient to not have a solid resistor.
Laser thermals would probably be a lot less mass efficient because you'd still need radiator area to cool the lasing medium, and they might not be higher temperature if your focusing lens has to contain the chamber temperature and pressure.
Magnetically contained laser thermals could potentially make stupidly high temperatures though, if you wanted you could do things like have a solid tungsten rod as your reaction mass and feed it into the beam path to make 5,930*C boiling tungsten exhaust. Or osmium if you wanted the densest possible fuel storage. Or iron if you wanted something more abundant.
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Post by bigbombr on Feb 27, 2019 17:24:10 GMT
Most laser thermal rockets are envisioned as having the laser seperate from the platform the engine is mounted on. See laser launch. Allows for drones with impressive delta-v and acceleration without any fancy torchdrives. You just need a mothership with a big honking laser.
Having a laser thermal drive with the laser on board is not as interesting: it's like a less efficient compromise between a resistojet and a MPDT.
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Post by AtomHeartDragon on Feb 27, 2019 17:58:45 GMT
Could you use a laser as your heat source instead of a nuclear core or a resistor so you aren't constrained by the cores melting point? Then you could use like, tantalum hafnium carbide nozzles or something. Edit: Answered my own question with a three second google search. Laser Thermal Rocket has it's own sub section under Laser Propulsion on Wikipedia. Given that any sort of electrically powered heating elements boils down to a race to the bottom (achieve zero efficiency by converting all the input into waste heat), I don't think laser is going to make any difference unless it is external to the craft (along with power source) or unless you can achieve stupidly high temperatures and manage to outMPDT MPDTs in exhaust velocity or achieve similar velocities but much better thrust (both likely including external laser as part of a bundle deal, since you don't want that performance wasted on moving huge powerplant around, and you may settle for worse performance if it gives you huge mass cuts).
I wonder if fuel could be heated simply by passing current through it resistivity rather than utilizing a resistor. Similar to an MPD but not taking advantage of the lorentz force and just heating propellant resistivity. Perhaps this already exists. A resistojet where the propellant is the resistor. That's called an arcjet. I wonder if the benefits of the extra exhaust velocity would adequately compensate the added weight from the laser system and the extra inefficiencies that such a system would possess. Might be a good alternative to MPDs if you care less about weight and efficiency and want more thrust, yet still want more specific impulse than a NTR. I wonder if fuel could be heated simply by passing current through it resistivity rather than utilizing a resistor. Similar to an MPD but not taking advantage of the lorentz force and just heating propellant resistivity. Perhaps this already exists. A resistojet where the propellant is the resistor. Possibly, but you'd still be constrained by the temperature tolerance of the conductor you'd use to arch the power through your fuel resistor, although it might be a lot more mass efficient to not have a solid resistor.
Laser thermals would probably be a lot less mass efficient because you'd still need radiator area to cool the lasing medium, and they might not be higher temperature if your focusing lens has to contain the chamber temperature and pressure. Magnetically contained laser thermals could potentially make stupidly high temperatures though, if you wanted you could do things like have a solid tungsten rod as your reaction mass and feed it into the beam path to make 5,930*C boiling tungsten exhaust. Or osmium if you wanted the densest possible fuel storage. Or iron if you wanted something more abundant.
Or soot slurried in LH2/hydrocarbon/emulsified in water. Because if you reach such sweet temperatures, why squander them on shoving heavy atoms around instead of maximizing exhaust velocity? Rocket propulsion is about maximizing some combination of exhaust velocity and mass flow, not vaporizing the most vaporization-unfriendly (and thus impressive) stuff you can find.
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Post by airc777 on Feb 28, 2019 0:33:37 GMT
Given that any sort of electrically powered heating elements boils down to a race to the bottom (achieve zero efficiency by converting all the input into waste heat), I don't think laser is going to make any difference unless it is external to the craft (along with power source) or unless you can achieve stupidly high temperatures and manage to outMPDT MPDTs in exhaust velocity or achieve similar velocities but much better thrust (both likely including external laser as part of a bundle deal, since you don't want that performance wasted on moving huge powerplant around, and you may settle for worse performance if it gives you huge mass cuts). It's still worth toying around with in the context of dabbling in alternatives to NTRs and MPDTs for applications where you already have very big power plants, like terawatt laserstars.
Or soot slurried in LH2/hydrocarbon/emulsified in water. Because if you reach such sweet temperatures, why squander them on shoving heavy atoms around instead of maximizing exhaust velocity? Rocket propulsion is about maximizing some combination of exhaust velocity and mass flow, not vaporizing the most vaporization-unfriendly (and thus impressive) stuff you can find. Yeah, sure. I was just pointing out that the list of all possibilities of things that you could build would also include tiny spacecraft with stupidly dense fuels. I originally started looking at these as an alternative to get around the safe use temperature of fuel rod hot cores, but with both a laser heat source and magnetic containment you aren't constrained by any material tolerances and you could hypothetically use tungsten vapor as a propellant, as well as more optimized things.
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