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Microwave Rockets

Posted by: Sigurd - Sun Dec 04, 2005 2:57 am
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Microwave Rockets 
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Post    Posted on: Fri Dec 09, 2005 2:20 am
nihiladrem wrote:
I will be very interested to see what you and Jordin can come up with.
You as in me? ( :shock: looks behind self). Actually I didn't really think much about the added propellant needed for escape. I guess I was just assuming that with isp of 800 I could take it for granted that sufficient propellant could be carried. I was thinking the microwave tracking and beam spreading at 2,500 km would be the main problems. I may need to reconsider.


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Post    Posted on: Fri Dec 09, 2005 3:36 am
Sorry Peter, my mistakes - I meant that last for Kevin. Much more confusingly I don't believe Jordin is working with Kevin on the heat-exchanger, but he is involved with very similar things for Laser launch. Sometimes when I see my own replies it takes me a long time to untangle what is going on - and as a rule there are mistakes :(
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I may need to reconsider

No, you're right - beam spreading and tracking will be the difficulty no matter what, the lower acceleration available just makes those things worse (becuase it increases the distance at which energy must be added)...


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Post    Posted on: Fri Dec 09, 2005 4:27 am
We're comparing microwave and laser beam facilities from the point of view of someone who needs to decide what to buy. Of course it's cocktail money compared to stuff like EELV, so I say buy both.

In any case, microwaves diverge much more than lasers (which can be a good thing as well), so my ascent trajectory is short, high acceleration. If you really must launch people over a ~ 2000 km trajectory and you want to do it from the ground with one facility then you'd use a laser.

Good rules of thumb: $24K/m^2 for microwave aperture area and $2M/MW for gyrotrons.


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Post    Posted on: Sun Dec 11, 2005 3:58 am
Kevin Parkin wrote:
I expect it would be difficult to form a hotspot in this sort of configuration.

Some more thoughts on this. Hot-spots in nuclear thermal systems are not wholly derived from non-uniformity of the heat-generation itself but because of coupling between the temperature variations and the local flow. When more than one flow path is available, in a slightly warmer region, mass flow rate is generally reduced as flow has to go physically faster thus reducing the ease with which heat is transported away. In your tube configuration this affect could result in lower-flowing channels undergoing thermal runaway. In developing a complete system, you would need to demonstrate that thermal and flow differences do not tend to diverge in a multi-tube system. In nuclear systems I believe Doppler broadening helps limit this affect, something similar would happen with near-identical electrical heating elements working in parallel. Hotter elements generate less heat so things can remain stable. In the absence of similar affects, you may be forced to use additional pressure drop, most likely by deliberately choking the hydrogen flow entering each passage somewhere upstream of the major heating. (maybe you already intend to do this?) There may be the risk that a system only optimised for the highest possible power performance in a single tube might simply not be adequately stable under real conditions, however the extra pump performance needed to choke the flow to each tube need not be very great.


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Post Laminar flow instability   Posted on: Sun Dec 11, 2005 4:54 am
Agreed. Typically this type of flow behavior occurs in the laminar systems when the temperature is raised by more than about three times the inlet temperature, and is known as laminar flow instability.

Bussard shows in his book on nuclear rockets that single channels with turbulent flow are unconditionally stable, however I too wonder about the possibility of strange multichannel effects. In the worst case we can indeed do as you suggest, and I can imagine countless other tricks we can play...

In any case, it will be interesting to see if this occurs.


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Post    Posted on: Sun Dec 11, 2005 8:55 am
If the tubes do not sense each others output pressure the requirement for passive stability in a large tube array is just that each individual tube is stable against tube temperature perturbations at the design point. Since Bussard says such channels are stable this case perhaps isn't interesting.

If they do in fact sense each others output pressure things are more interesting, but if you can make a one dimensional model of the tubes, simulating perturbations with just a few tubes should be sufficient. Heat conduction between adjacent tubes can in all likelyhood be neglected.

Just given the low mass of the heat-exchanger and the very large heat flux, the time constant for any active system to stabilize flow needs to be very low. No idea what the pressure loss in a LH2 cavitating venturi would work out at, but it's probably incompatible with use of a turbo-pump.

BTW. Thanks for the info on laminar flow instability.


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Post    Posted on: Mon Dec 12, 2005 1:00 pm
Is there some reason that Hydrogen was chosen as the reaction mass? Wouldn't a higher density non-cryogenic fuel allow for a smaller less complex structure? Or is there something that makes Hydrogen the ideal reaction mass?


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Post    Posted on: Mon Dec 12, 2005 1:18 pm
Hydrogen has the lowest molecular mass of all elements, the physics of rockets are such that this means you can achieve the highest exhaust speed for a given chamber temperature with it. If you are limited by the temperature capability of your materials, nothing else comes close.


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Post Long term future for microwave launch?   Posted on: Thu Dec 22, 2005 11:28 am
Hi,
This idea has been a favorite of mine since I heard about the LightCraft project, but articles on that always seemed dated about 2000. Im glad to discover this is an ongoing area of investigation.
( Even us amateurs have our favorites in this very slow horserace;) )

Could I ask you to speculate what its future is?

One specific question is how the design would evolve if power was pretty much whatever you could ask for: multiple stations, stations in orbit, whatever. How slow and relaxing could reaching orbit theoretically become? would heating atmosphere for propellent become viable?


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Post    Posted on: Thu Dec 22, 2005 11:14 pm
Good questions.

Directed energy launch in some form or other is coming.

It may actually be Japan that does it first, as they are very advanced on the lightcraft approach. The trouble here is that it takes a unique type of double pulse laser before this can be seriously tested, and nobody has the money to build one (a few hundred million dollars). But if someone does, then I expect we would start launching nanosats the size of coke cans that way in the not-too-distant future.

On the heat exchanger side there will be hopefully be some firsts in the lab over the next 6 months, so the idea may gain some wider exposure at that point. After that the real work begins, and there is still much study to be done. Heat exchanger systems may happen with a laser or microwave source, depending on how the future cost studies come out. If successful, expect to see a fleet of reusable upper stages about the size of a car launching ~ 100 kg payloads into space every few minutes circa 2020.


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Post    Posted on: Fri Dec 23, 2005 12:56 am
Thanks for your reply!

It is amusing that the scale of money required appears about the same that which people keep investing in big dumb blockbuster movies that flop. I'll be thinking about that next time I rent one of them.

Looking forward to more updates!


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Post    Posted on: Fri Jan 06, 2006 9:39 am
This is my first look into this new forum.

There was the idea to use Perfluorcarbons if men are launched at accelerations around 19G. The thread Could Perfluorcarbons be a partial solution? is discussing that. All I could find out myself is that humans can breath Perfluorcarbons and that this is used to revive parts of the lung sometimes - but at the current state of using Perfluorcarbons for breathing humans can't be active. As long as they breath Perfluorcarbons they are condemned to passivity - in contrary to mice and rats. But it would be interesting to develop the method up to a point where humans can stay active while using it.

Another good thing here is the already mentioned similarity to the method to heat hydrogen by lasers as suggested by a NIAC study - Kevin, what about enhancing your PhD thesis to such a study? Can you imagine to work with them?



Dipl.-Volkswirt (bdvb) Augustin (Political Economist)


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Post    Posted on: Fri Jan 06, 2006 9:02 pm
Quote:
Another good thing here is the already mentioned similarity to the method to heat hydrogen by lasers as suggested by a NIAC study - Kevin, what about enhancing your PhD thesis to such a study? Can you imagine to work with them?


A very timely question! I've started a new thread 8):

New Paper: A Comparison of Laser and Microwave Approaches


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Post Update   Posted on: Fri Feb 17, 2006 12:52 am
Hey,

I am a visiting student at Caltech currently working on the microwave thermal rocket project and progress has been made during the past month. We have successfully completed two new cylindrical resonance cavities that do not leak radiation. We are able to heat up rods with 60% Alumina content however the highepr purity rods will not go to thermal run away. The cavity has a very strong electric field and a stable plasma forms before the rod becomes hot which eats up the microwave energy. We plan to modify the cavities to even out the field.
We have also hooked up a Nitrogen line to the rods to experiment with a "safe" gas before testing hydrogen.

Alex


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