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Post by matterbeam on Apr 20, 2019 1:25:17 GMT
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Post by AtomHeartDragon on Apr 20, 2019 13:48:14 GMT
Laser reactors would make an interesting kind of components for CDE - less loses overall than laser + power TEFR, but not able to provide power for other subsystems. I wonder about one thing - could a laser reactor be used in an NTR of a kind to yield better Isp with solid core? The reactor would be cooled directly by flowing propellant (no radiators necessary), which would than be injected into the chamber and heated by laser to temperatures far exceeding the tolerances of solid materials.
Maybe it could make "bring your own power" laser thermal worthwhile?
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Post by matterbeam on Apr 21, 2019 0:07:59 GMT
Laser reactors would make an interesting kind of components for CDE - less loses overall than laser + power TEFR, but not able to provide power for other subsystems. I wonder about one thing - could a laser reactor be used in an NTR of a kind to yield better Isp with solid core? The reactor would be cooled directly by flowing propellant (no radiators necessary), which would than be injected into the chamber and heated by laser to temperatures far exceeding the tolerances of solid materials.
Maybe it could make "bring your own power" laser thermal worthwhile?
Technically, a large fraction of the output that we consider as turning into waste heat can easily be sent through a generator or a rocket engine to be exploited. For example, the coolant loop that heats up the blackbody emitter to 3000K could also be used to heat up a Brayton cycle turbine for power generation. Using a fraction of a nuclear reactor's output to generate a laser and then using that laser to further heat up a propellant is perfectly possible. You would get a bit more Isp. However, due to thermodynamics, you will get less overall power. Let me give you an example. Imagine a 1GW NTR sending out 1000s Isp exhaust with 203.8kN thrust. The mass flow is 20.78kg/s. We use half of that power to generate a laser with a (generous) 20% efficiency. You get 500MW of exhaust with 1000s Isp and 101.9kN thrust. The mass flow is 10.39kg/s. You then use your 100 MW laser to further heat up the exhaust. It should reach an Isp of 1200s with the same thrust. The rocket engine's total power is only 600MW but you've increased your Isp by 20%.
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Post by AtomHeartDragon on Apr 21, 2019 12:00:57 GMT
Laser reactors would make an interesting kind of components for CDE - less loses overall than laser + power TEFR, but not able to provide power for other subsystems. I wonder about one thing - could a laser reactor be used in an NTR of a kind to yield better Isp with solid core? The reactor would be cooled directly by flowing propellant (no radiators necessary), which would than be injected into the chamber and heated by laser to temperatures far exceeding the tolerances of solid materials.
Maybe it could make "bring your own power" laser thermal worthwhile?
Technically, a large fraction of the output that we consider as turning into waste heat can easily be sent through a generator or a rocket engine to be exploited. For example, the coolant loop that heats up the blackbody emitter to 3000K could also be used to heat up a Brayton cycle turbine for power generation. Using a fraction of a nuclear reactor's output to generate a laser and then using that laser to further heat up a propellant is perfectly possible. You would get a bit more Isp. However, due to thermodynamics, you will get less overall power. Let me give you an example. Imagine a 1GW NTR sending out 1000s Isp exhaust with 203.8kN thrust. The mass flow is 20.78kg/s. We use half of that power to generate a laser with a (generous) 20% efficiency. You get 500MW of exhaust with 1000s Isp and 101.9kN thrust. The mass flow is 10.39kg/s. You then use your 100 MW laser to further heat up the exhaust. It should reach an Isp of 1200s with the same thrust. The rocket engine's total power is only 600MW but you've increased your Isp by 20%. I was thinking of cooling the reactor with propellant flow so all the waste heat ends up in the exhaust anyway. When you think about it, heating things up with laser is just your run of the mill way of circumventing thermodynamics by avoiding equilibrium, same as with pulsed NTR and whatnot.
The remaining questions would be: - Can you actually manage to increase Isp in this setup? (TBH, I really hate the notion of specific impulse - why not use something less Earth-centric, like exhaust velocity?)
- Will your entire setup end up being some horribly massive Rube Goldberg contraption defeating the whole point, like with normal laser thermal with on-board laser?
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Post by matterbeam on Apr 22, 2019 1:41:11 GMT
Technically, a large fraction of the output that we consider as turning into waste heat can easily be sent through a generator or a rocket engine to be exploited. For example, the coolant loop that heats up the blackbody emitter to 3000K could also be used to heat up a Brayton cycle turbine for power generation. Using a fraction of a nuclear reactor's output to generate a laser and then using that laser to further heat up a propellant is perfectly possible. You would get a bit more Isp. However, due to thermodynamics, you will get less overall power. Let me give you an example. Imagine a 1GW NTR sending out 1000s Isp exhaust with 203.8kN thrust. The mass flow is 20.78kg/s. We use half of that power to generate a laser with a (generous) 20% efficiency. You get 500MW of exhaust with 1000s Isp and 101.9kN thrust. The mass flow is 10.39kg/s. You then use your 100 MW laser to further heat up the exhaust. It should reach an Isp of 1200s with the same thrust. The rocket engine's total power is only 600MW but you've increased your Isp by 20%. I was thinking of cooling the reactor with propellant flow so all the waste heat ends up in the exhaust anyway. When you think about it, heating things up with laser is just your run of the mill way of circumventing thermodynamics by avoiding equilibrium, same as with pulsed NTR and whatnot.
The remaining questions would be: - Can you actually manage to increase Isp in this setup? (TBH, I really hate the notion of specific impulse - why not use something less Earth-centric, like exhaust velocity?)
- Will your entire setup end up being some horribly massive Rube Goldberg contraption defeating the whole point, like with normal laser thermal with on-board laser?
You're quite right. Any laser set-up, whether a reactor-laser or a more conventional nuclear electric laser, is trading efficiency for exhaust velocity. There exists a balance point where the propellant saved from having a higher exhaust velocity becomes greater than the increased weight of the equipment needed to boost it above normal. Since deltaV requires exponential increases in mass ratio with any change to exhaust velocity, it is generally a good thing to do. A 20km/s mission requires a mass ratio of 5.3 with 12km/s exhaust velocity compared to 7.4 with 10km/s exhaust velocity, for example. You are saving when you extra equipment weighs less than (7.4-5.3) = 2.1 times your original mass.
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