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I'm interested to hear more about Josh's idea to use sodium-seeded propellent heated by beamed power from a sodium vapour lamp. It appears to give a high Isp (and potentially, high thrust) in a system that could be used in space without needing to deal with the fallout from launching a nuclear reactor.
Of course, there are a lot of questions that need to be answered, it being just a hypothetical, back of the envelope system at the moment. Such as, how narrow can the beam be focused, how will the energy be transferred to the propellent, what intensity will the beam be...
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
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I've discussed this a bit in a previous thread. Interestingly, it was a thread in which you participated in. The post in question is here. I guess the real idea may have been missed because I discussed the receiving end in more depth than the transmitting end.
Actually I think higher Isps [than 500 s] are reasonable, if we can solve the power issue for the ground beam. I've suggested that this could be done with Sodium Vapor lamps, which produce monochromatic light much more cheaply than lasers.
Logically, if you're using sodium vapor lamps you want to use sodium vapor in your rocket engine because you can guarantee an identical emission and absorption spectrum. An approximate equation for the exhaust velocity of a rocket engine is given on the Wikipedia page for a De Laval nozzle. It's pretty accurate, giving a calculated Isp of the SSME of 384 s at sea level compared to 363 s in reality. Using a molar mass of 23 g/mol for Sodium and a Ratio of Specific Heats of 5/3 (Assumed the same as for Mercury Vapor and Noble Gases, I calculate that an Isp of 1000 s can be achieved using Sodium at 65,000 K.
Now, this seems like an obscenely high temperature, and it kind of is. But i's not as bad as it sounds. Because you don't need a heat exchanger the high temperature sodium doesn't need to touch the walls. In fact, if you control your beam properly you could probably keep the material along the chamber walls to 2000 K or less.
Now, if you transition to Water seeded with Salt (Assumed average molecular weight 20, ratio of specific heats 1.2), the required temperature falls to 23,000 K. However, because water would certainly break into its constituents at this temperature, you're probably looking more at 11,000 (The molar mass goes way down, but the ratio of specific heats heads towards the 5/3 for monatomic gases).If you use seeded Ammonia instead, you can get down towards 9,000 K.
There are some interesting trades going on here.
The basic idea is that beamed propulsion means that you have to transmit energy over long distances (hundreds or thousands of kilometers), in a focused beam. Because lasers produce monochromatic light, they're easy to focus and will respond to any distortions in the same way. However, lasers aren't the only things that produce monochromatic light. A sodium vapor lamp would as well (low pressure sodium; at higher pressures sodium exhibits pressure broadening of its spectrum), at the spectral lines of 589.1 and 589.6 nm, neither of which is strongly absorbed in the Earth's atmosphere. Light sources in general have good power densities, and sodium vapor can certainly be counted among that. I figure that putting a large sodium vapor lamp at the focal point of a parabolic concentrator will produce monochromatic light travelling perpendicular to the directrix of that parabolic concentrator. From there, proper placement of mirrors can give an arbitrarily large concentration ratio in a pretty much arbitrary direction. Further, because the light is monochromatic, dielectric mirrors can be used which are highly effective reflectors, having an efficiency over 99.9%.
While megawatt class sodium vapor lamps do not exist today, and I don't know how difficult it would be to scale up, the technology is simple and I would hazard a guess that it wouldn't be too bad. The biggest advantages here are that this is very much a known technology, that sodium vapor lamps are more efficient than lasers at turning electrical energy into luminous energy, and that they are much cheaper per watt generated.
-Josh
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Is it an in-space only propulsion?
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Well, it was proposed in the context of ground launch... I'm thinking that it could be used in space as well, to launch vehicles on quick trips - maybe even on Luna, since you're pretty much going in a straight line from the surface to L1.
What sort of experiment would be needed to be done to verify the system? Using sodium-doped hydrogen in a small rocket on a stand, heating by off the shelf sodium vapour lamps?
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
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That's where I would start. Even just using a low pressure sodium vapor lamp to boil water would be a good start, to see how tightly you can actually focus the beam of light produced by the lamp. That may be somewhat tricky, because I think a lot of sodium vapor lamps come with diffusers that would make it nearly impossible to get a good focus on the light they produce.
-Josh
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Hmmm, so I need a sodium vapour lamp without a diffuser, some salt water, and a thermometer. That shouldn't be hard to acquire.
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
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Certainly not. You also need a parabolic concentrating mirrors and a couple of secondary mirrors to focus the light (I'd certainly help design a mirror geometry if you would like), and probably some black powder, since as you know through the use of common sense, Saltwater does not absorb yellow light.
-Josh
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Terraformer, this topic goes back to 2014.... you were encouraging JoshNH4H to develop his ideas a bit. but the topic ran out of vapour at #7. I thing the whole question deserves a revisit.
My search this evening was to see if there were any topics that contain the word sodium, and this was the only one that showed up.
(th)
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