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Microwave beam a way forward?
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MirariNefas
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PostPosted: Wed Nov 21, 2007 6:16 pm 
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Does anyone have any comments on the following link which geos into some detail about experiments:


They seem to have lifted 3 grams with 1 MW of power. That sucks. At a maximum of 346 N/MW, it appear that they will need 28.4 MW just to hold one tonne against gravity, without accelerating it. At longer ranges efficiency will only decrease. Also, where it says "the phased array technology allows us to realize a single large-diameter coherent beam", take note of that. They really mean it when they say "large-diameter".

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As long as you have a system that can lift say 250kg loads, then that is fine as long as the system can do multiple launches in quick succession


You may find yourself more interested in gun-type launchers then. They can do that without the rectenna. Extremely high g-forces, of course, but most small loads can be built to withstand that.

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(that is one of the issues I have with the SFVA - are we sure it can given all the safety issues?)


Sure.
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Terraformer
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PostPosted: Sat Nov 24, 2007 11:40 am 
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Boil some water in my microwave oven? Would that wreck it?
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louis
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PostPosted: Sat Nov 24, 2007 12:32 pm 
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I presume Mirari is saying you will find that the steam stops microwaves penetrating...but I don't know. Is that why you are normally told to keep a cover on microwave meals? So escaping steam doesn't affect the efficiency of the microwaves?

Anyway, I have certainly read that microwave beam transfer of electricity can pass through clouds. I think Mirari is arguing about particular frequencies.

I'm no scientist or engineer as you can tell but I like to work from first principles and it seems to me if you were able to devise a system where you didn't carry the fuel or power source (the electricity) and you didn't carry the propellant (water/steam) then you would be well ahead of the game. However, I'll settle for the microwave beam.

3 grams for 1 MW may be pathetic but this is an infant technology. Is Mirari saying that is the limit of the technology? I wouldn't think so.
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MirariNefas
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PostPosted: Sat Nov 24, 2007 4:12 pm 
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Boil some water in my microwave oven? Would that wreck it?


No. It would just boil.

Haven't you ever boiled some water in your microwave? For hot chocolate, or tea?

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I presume Mirari is saying you will find that the steam stops microwaves penetrating...


Yes. Why does water get hot in a microwave? Why does it become steam? Because the water is absorbing the energy. Getting hot is the same as preventing penetration.

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Anyway, I have certainly read that microwave beam transfer of electricity can pass through clouds. I think Mirari is arguing about particular frequencies.


Yes.

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Is Mirari saying that is the limit of the technology?


No, but I am saying that they'll have trouble even matching this when it comes to distances of hundreds of kilometers.
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MirariNefas
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PostPosted: Fri Dec 07, 2007 8:49 pm 
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I posted this article under the space fountain thread, and I'll post it again here because he reviews a few different flight methods, including microwave/laser. https: e-reports-ext llnl gov pdf 193539 pdf
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MirariNefas
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PostPosted: Sat Dec 08, 2007 12:31 am 
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Here's part of the transcript of the power beaming portion (not actually much mention of microwaves specifically, but close enough):

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Nuclear energy can be efficiently stored on a rocket, but as we have seen, it is not easily used. We have already discussed the limitations of chemical onboard energy storage, so let's now consider a rocket whose propellant is heated by offboard energy. Although microwaves and electrons have also been proposed as energy carriers, lasers are superior, permitting long distance source-vehicle propagation with efficient and compact energy reception at the ship. A number of methods have been discussed for using laser energy to power rockets; I'll discuss one of these, an Avco proposal for direct launch from Earth into high energy orbits. Their rocket is two meters wide and four meters high, carries 5.4 Mg of water as propellant, and orbits a 1 Mg payload in a 0.5 Mg package. The water is hit by 160 laser pulses per second; each one vaporizes a thin layer, and then ignites a detonation wave in the blown-off material. This method of converting laser energy into thrust is only 44% efficient, but achieves an 8 km/s exhaust without a nozzle or optics, requiring only a 70 cm expansion skirt. A 1 GW average power CO2 laser is used, delivering energy to the rocket for 340 seconds, out to a slant range of 1000 km.

The first thing to note is that the rocket is extremely simple; tasks such as steering and propellant injection are done from the ground, by tailoring the laser pulses. The use of pulsed energy relaxes temperature constraints, and allows attainment of high exhaust speeds while using very convenient propellants such as water. The amount of propellant carried and its exhaust velocity are chosen to optimize conversion of laser energy to payload speed; this leads to mass ratios of 4-5. It's striking that laser propelled rockets are much smaller than optimum designs using onboard energy, but require huge (GW) size lasers. This results from two relationships between the size of a rocket and the power required to propel it. One limit, usually the least stringent, is set by the Earth's need to overcome the Earth's gravity. The second limit follows from the need to deliver energy to the rocket while it is still within range of the laser. If we include the energy conversion inefficiency, and the structural mass function, we find an energy cost of about 340 kJ/g of payload, and for a 1000 km range, a need for 1 GW/Mg. This power requirement, coupled with the difficulty of building large lasers, drives laser-drive rocket schemes to small sizes. It's probably not cost effective to reenter, recover, and refurbish such small objects, so the Avco design focuses on simple, inexpensive expendable rockets.


I'll add another section in a bit.
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louis
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PostPosted: Sat Dec 08, 2007 6:12 am 
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Thanks for that Mirari - looks v. interesting. Unfortunately my understanding of what is being said is somewhat hampered as I don't know what Mg refers to. This can't be milligram, the only mg I know in the UK - surely not!

Can you help out?

I'm working out that the cost of the 5 minute laser burst is about $3 million. So it would be interesting to see what the minimum projected cost per Kg of payload is likely to be.

It's interesting that they do refer to water as the fuel/propellant (that argument was never fully resolved I think). Can lasers operate well in dense cloud? If so, my idea of extracting the water from clouds in humid environments (thus avoiding perhaps 60-90% of mass at take off) could come into its own. On the fact of it, it doesn't sound like it's using that much water - so could water be condensed out of the clouds at a fast enough rate through some fan intake system or similar? I notice on the Space X site the Falcon rocket seems to be condensing a load of water as it's rising.
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MirariNefas
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PostPosted: Sat Dec 08, 2007 8:22 pm 
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Mg refers to megagram, or one metric tonne. This is technically the proper SI unit to use for this level of mass, but most people use the tonne instead.

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I'm working out that the cost of the 5 minute laser burst is about $3 million.


Is that just for power? I think the laser itself would cost more, and I know it would need maintenance. Anyway, I suppose your minimum would be $3000/kg then, but this author makes more optimistic assumptions. I'll write up another paragraph or two.

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It's interesting that they do refer to water as the fuel/propellant (that argument was never fully resolved I think).


As far as I'm concerned it's clear. Microwaves are only valuable for high transmission efficiency, which direct vaporizing of water precludes. A laser system like this is much less efficient - notice the short range, and the thrust conversion efficiency of 44%. But it has its advantages in vehicle design.

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Can lasers operate well in dense cloud?


Can some lasers? I think so. Can this one? No. I'd bet that they'd operate this on a clear sunny day.

The laser vaporizes water. That's how it produces thrust. If you put water in the way, the laser will vaporize that. And less laser will reach the craft. You'd end up doing more harm than good.
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MirariNefas
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PostPosted: Sat Dec 08, 2007 9:03 pm 
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One second look, where did you get $3 million for five minutes of 1 GW laser? That's way in excess of power costs.

I found something which states that the first 500 kilowatts of energy usage are typically charged at 5.8 cents per kilowatt hour to end consumers like us (big companies probably may be charged less, or a lot less if they have their own power generation),

So, that's $58 thousand per gigawatt hour, and $16.11 per gigawatt second. That's about $5477.77 per tonne of payload, and $54.78 per kg. A lot of space articles predict that power can be bought at a bulk rate of 2 cents per kilowatt hour (I don't know why and this may be out of date considering current oil prices, but we'll assume they want to build their own nuclear plants or something). That gives us $18.89 per kilogram. This article assumes even less than that, maybe factoring in some potential future price decreases as technology develops.
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MirariNefas
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PostPosted: Sat Dec 08, 2007 9:34 pm 
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The transport cost is governed by three components: the laser, the electrical energy, and the expendable rocket. Let's (reasonably) assume a 20% efficient laser; hence we need 1.7 MJ/g, which costs us 9 $/kg. There is no experience with lasers of GW size, but they probably cost more per watt than power plants; if we assume a 2 $/W capital cost, we must add 3 $/kg to price. The rocket cost is critical; a unit price of even $10,000 drives our transport cost to 22 $/kg. How can we reduce this price by the factor of 2-4 required to make large-scale colonization feasible? Two avenues exist for reducing the rocket portion of the cost. These rockets are roughly the size of a small car, and for Tg/yr transport are built in similar quantities. But they are much simpler machines than a car, so it's plausible to assume a unit cost of $2,000. Another route to cost reduction involves scaling up the vehicle size. This should reduce the structure-to-payload mass ratio, and may even make reuse economical. In order to deliver 1 Tg/yr, we require 10 GW of laser capability. Large lasers are most readily built in small modules; by grouping them in 1 or 2 sites instead of 10, we can lift larger unit payloads. Reductions in the energy cost will not be as dramatic. By using a non-constant exhaust speed, and more efficient energy-thrust conversion, we can cut the needed energy to 200 GJ/Mg of photons, ie 1,000 GJ/Mg of electricity. This improves the overall conversion efficiency to 6%, but energy costs still dominate; there are two ways to reduce them further. The first is to find a more efficient laser. A free electron laser should give twice the electrical efficiency, and can be operated at a shorter wavelength. This allows longer range transmission, which permits us to use bigger payloads per launch. The other option is to only thrust into low orbit, and use some other more efficient scheme to reach escape. By reducing the energy that must be transferred while within the range of the laser, this option permits us to raise the unit payload size by a factor of two. Laser propulsion costs, assuming cheap expendable rockets, but staying with CO2 lasers, should be reduced to 10 $/kg for escape, and 6 for low orbit. Whether or not laser propulsion is suitable for large-scale space colonization purposes will depend upon how closely we can attain the assumed values of laser efficiency and cost.

Escaping from the Earth's surface with rockets is a difficult task; the transport costs for the preceding systems are reduced if they only lift payload to low orbit rather than all the way out of the terrestrial gravity well. For laser propulsion the savings is a factor of two, while for H2/O2 rockets it's about 4-fold. We want rocket systems that will carry payload from orbit to escape, but which don't impose these cost penalties.
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PostPosted: Sat Dec 08, 2007 11:08 pm 
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Mirari -

Sorry - for some reason I'd confused my per watt and per KW prices per hour! I was working on 4 cents as the starting point, but happy to accept your figures. My apologies for the diversion.

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"Laser propulsion costs, assuming cheap expendable rockets, but staying with CO2 lasers, should be reduced to 10 $/kg for escape, and 6 for low orbit. Whether or not laser propulsion is suitable for large-scale space colonization purposes will depend upon how closely we can attain the assumed values of laser efficiency and cost."

Is my maths wrong again, or are these incredibly low figures? Don't we currently work in the $10,000 to $20,000 per Kg range for chemical rocket launches, as opposed to $16 per kg!

Even if these figures are over=optimistic by a factor of 1000, it will still be a tremendous advance.

I certainly think if we need to look for alternatives to chemical rocket production then this is a priority. Even if capital costs of the lasers are in the billions, they are going to be a lot cheaper than the space fountain approach.

If this can hoist one tonne loads, seems to me we are in business. It is simply a question of designing the payloads so that they can virtually self assemble in space. And I have never been a fan of mega projects anyway.

Of course in terms of a Mars mission, you would probably still need a chemical rocket to get back. It seems unlikely we could take the laser infrastructure with us.
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MirariNefas
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PostPosted: Sun Dec 09, 2007 3:20 am 
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Is my maths wrong again, or are these incredibly low figures? Don't we currently work in the $10,000 to $20,000 per Kg range for chemical rocket launches, as opposed to $16 per kg!

Even if these figures are over=optimistic by a factor of 1000, it will still be a tremendous advance.


Given his energy assumptions I think he's off by an order of magnitude, and maybe a bit more for payoff of development, but yeah, cheaper than current systems by a long shot nonetheless.

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Even if capital costs of the lasers are in the billions, they are going to be a lot cheaper than the space fountain approach.


Agreed. But the space fountain approach would ultimately bring prices down even lower, even for large payloads and humans. And as the author said, we have no real experience with such large laser systems. There's a lot of assumptions being made, and to assume such strides in efficiency and the payoff of capital costs, we're talking about a lot of systems capable of launching a lot of payload per year (the author is specifically looking at systems that'll handle 1 Tg/yr, or a thousand <edit - sorry, a million> launches of this thing a year). That's still quite a hefty investment, and may be more like tens of billions in infrastructure and development costs for a lesser use system.

Even fairly small governments can spare ten billion, so I wouldn't mind putting some money into laser launch development. But I think that in the long run, there's no real contest.

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Of course in terms of a Mars mission, you would probably still need a chemical rocket to get back. It seems unlikely we could take the laser infrastructure with us.


Yeah, but if space access were cheap enough than within a few missions I'd bet that you could set up a space elevator if you really wanted to. It's much easier to do that on Mars. Although, the lack of an atmosphere would make a laser system more efficient on Mars too.


I'd like to mention that later on in this article, the author ultimately suggests working on laser systems for LEO access and bolo momentum exchangers for escape systems as the post-shuttle development push (though, prophetically, he predicted that the standard attitude toward space would just lead to minor upgrades on chemical rocket systems, as we're seeing in development now). He does this because the technical hurdles and investment levels are lower than for a dynamically supported system like his Starbridge, and the increase in space traffic would cause demand for such large-scale systems.

I don't like this because I'm impatient and I want a system which can take me to space for the price of airfare before I'm an old man, but an intermediate system can open up more tourism, asteroid mining, or solar power satellites. After that, the free market would do its thing eventually.
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MirariNefas
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PostPosted: Tue Mar 04, 2008 4:26 pm 
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From wikipedia:

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In the 1980's researchers at NASA worked on the potential use of lasers for space-to-space power beaming, focussing primarily on the development of a solar-powered laser. In 1989 it was suggested that power could also be usefully beamed by laser from Earth to space. In 1991 the SELENE project (SpacE Laser ENErgy) was begun, which included the study of laser power beaming for supplying power to a lunar base.

In 1988 the use of an Earth-based laser to power an electric thruster for space propulsion was proposed by Grant Logan, with technical details worked out in 1989. His proposal was a bit optimistic about technology (he proposed using diamond solar cells operating at a six-hundred degrees to convert ultraviolet laser light, a technology that has yet to be demonstrated even in the laboratory, at a wavelength that will not easily transmit through the Earth's atmosphere). His ideas, with the technology scaled down to be possible with more practical, nearer-term technology, were adapted.

The SELENE program was a serious research effort for about two years, but the cost of taking the concept to operational status was quite high and the official project was ended in 1993, before reaching the goal of demonstrating the technology in space. However, some research is was still continuing. There was some hope that an array for a laser-powered aircraft demonstration might be developed.[19]
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PostPosted: Mon Nov 21, 2016 3:34 am 
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Im considering a piston drive from Superlative. Among the benefits, I hope to stretch cleaning cycles way out.
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