Laser Launch Vs. Space Elevator?

[eniac]

"I mentioned the petawatt and exowatt lasers just to point out that we can make high-powered lasers to spec without too much trouble."<br /><br />I bow before your superior experience, but I do seem to remember that peta- and exowatt power levels only occur in pico- and femtosecond pulses, which tend to occur in laboratory bench level systems which are quite irrelevant to the high continuous power (pulsed or not) lasers we are talking about here. <br /><br />The Myrabo experiment was based on ablation or air ionization (I forget which), so it must use a pulsed laser, but it also requires high continuous power, i.e. a lot of pulses over a longish time. Other proposals, such as by Kare ( http://www.niac.usra.edu/files/library/meetings/fellows/mar04/897Kare.pdf ) use a thermal collector and rocket engine, better suited for continuous wave lasers. I am not sure which one we are talking about, but if the Myrabo approach had worked so well and was scalable, why would they have stopped so apruptly after the first real test? <div class="Discussion_UserSignature"> </div>

 

[danhezee]

Required technology for the space elevator<br />*material strong and light enough to use as the ribbon that currently doesn’t exist but is being researched.<br />*some sort of dampening system that doesn’t exist Afaik isn’t being researched.<br />*lasers that don’t exist but are being researched.<br /><br />Required for laser launch<br />*lasers that don’t exist but are being researched. <br /><br />I have said this b4 on another thread about the space elevator. I was at the X-Prize Cup in 2006 when the Space Elevator Games was held in Las Cruses, New Mexico. I watched the ribbon shake violently back and forth when there was no wind at ground level but at the top there were some apparently strong winds. I didn’t go to the 2007 games in Utah but if I remember correctly I read on www.Spaceward.org blog (they are the ones hosting the event), they had wind troubles, too. For 2008 Spaceward wants to make a 1km high balloon structure for the competitors to climb. I imagine they will have more wind problems than they can possibly cope with. You won’t be able to simply unroll a ribbon from GEO and strap it down to a concrete pillar and start climbing. You are going to have to have an active system to dampen the oscillation, which will require energy. The atmosphere is too dynamic; IMHO the elevator will never work on Earth.<br /> <div class="Discussion_UserSignature"> </div>

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>I do seem to remember that peta- and exowatt power levels only occur in pico- and femtosecond pulses, which tend to occur in laboratory bench level systems which are quite irrelevant to the high continuous power (pulsed or not) lasers we are talking about here.<p><hr /></p></p></blockquote>It is true that the pulses from peta and exowatt lasers are in extremely short pulses. Considering that petawatt pulses have 16 times the energy used by all the homes in the U.S. combined during the same period of time, all of it focused on a spot the size of a grain of sand, it is obvious that we could not have such beams left on very long at a time. However, it is relevant that we can create 1kJ beam pulses of varying lengths of time, as our engineering requirements dictate. And, the Nova ignition research facility was not your ordinary lab bench! <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>The Myrabo experiment was based on ablation or air ionization (I forget which),<p><hr /></p></p></blockquote>I'm fairly certain it was based on heating the air (it looked in the video like ionization was taking place). <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>so it must use a pulsed laser, but it also requires high continuous power, i.e. a lot of pulses over a longish time.<p><hr /></p></p></blockquote>It used a 10 kW CO2 laser producing 25 pulses per second (pps) of IR light, with each pulse lasting for 18 microseconds (us), giving a 45% duty cycle. The laser used was capable of faster repetition rates and longer pulses. <br /><br />Back when I was getting my laser degree, the gas dynamic laser was considered obsolete, having been replaced by better chemical lasers (the ancestors of current chemical lasers like COIL). Even then, the gas dynamic lasers were capable of producing megawatt CW beams. <br /><br />The main reason we don't have very many 100 MW, long duty cycle lasers is that we don't have a lot of demand fo

 

[richalex]

Here is a link that might be of interest: <br /><br />Laser Focus World: Lasers focus on the extraterrestrial<br /><br />The article describes the history of research into using lasers to propel spacecraft since 1965, and the potential of using lasers to supply power to spacecraft.

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p><br />Required technology for the space elevator<br />*material strong and light enough to use as the ribbon that currently doesn’t exist but is being researched.<br />*some sort of dampening system that doesn’t exist Afaik isn’t being researched.<br />*lasers that don’t exist but are being researched.<br /><br />Required for laser launch<br />*lasers that don’t exist but are being researched.<br /><p><hr /></p></p></blockquote><br />Whether we are talking Myrabo's flying saucers or Kare's light driven rocket, there are bound to be some unique problems which haven't been researched. After all, the only saucer we have is Myrabo's, and as far as I know noone has ever designed a rocket engine driven by outside heat.<br /><br />These should be included in your second list. <br /><br />That said, the first list is also missing a good number of items, most of which can be found discussed in Edward's 2003 NIAC report ( http://www.niac.usra.edu/files/studies/final_report/521Edwards.pdf )<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[billslugg]

<font color="yellow">a 10 kW CO2 laser producing 25 pulses per second (pps) of IR light, with each pulse lasting for 18 microseconds (us), giving a 45% duty cycle.</font><br /><br />25 x .18 x 10^-6 x 100 = .00045% <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>a 10 kW CO2 laser producing 25 pulses per second (pps) of IR light, with each pulse lasting for 18 microseconds (us), giving a 45% duty cycle.<br /><br />25 x .18 x 10^-6 x 100 = .00045%<p><hr /></p></p></blockquote><br /><br />If I could do math, I'd be an engineer! <br /><br />Well, that's an even better number for this argument, in that it demonstrates that we are not necessarily looking at "longish pulses" from the laser. The laser was off (not producing a beam) 99% of the time in the successful demonstration. That should be an easy rep rate to meet.

 

[billslugg]

99.99955% <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[eniac]

Actually, I think the math should be:<br /><br />25 * 18 * 10^-6 * 100 = 0.045 %<br /><br />There are two possibilities: <br /><br />1) The 10 kW refers to pulse power, in that case, the laser had a continuous power of 0.00045*10,000 = 4.5 W.<br /><br />2) The 10 kW refers to continuous power in which case the pulse power was 10,000/0.00045 ~ 22 MW.<br /><br />The latter must be what we are talking about, because 10 kW seems nowhere near strong enough to ionize air. The 100 MW needed for laser launch, incidentally, refers to continuous power, not pulse power.<br /> <div class="Discussion_UserSignature"> </div>

 

[billslugg]

Yes, I had a stray decimal point sneak in there. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>It is true that the pulses from peta and exowatt lasers are in extremely short pulses. Considering that petawatt pulses have 16 times the energy used by all the homes in the U.S. combined during the same period of time, all of it focused on a spot the size of a grain of sand, it is obvious that we could not have such beams left on very long at a time. However, it is relevant that we can create 1kJ beam pulses of varying lengths of time, as our engineering requirements dictate. And, the Nova ignition research facility was not your ordinary lab bench!<br /><p><hr /></p></p></blockquote><br /><br />From here: http://en.wikipedia.org/wiki/Nova_laser I gather that Nova was capable of generating 6 pulses a day, of 100 kJ infrared light. Because the pulses are so short, this is called "terawatt". However, the continuous power of this facility is quite minuscule, namely<br /><br />6 * 100,000 J/day = 6.9 W.<br /><br />About the power of a AA battery. It is a long way to 100 MW from there...<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[grdja]

Theoretical peak strength for carbon nanotubes remains far above one needed for a space elevator, and practical CNT implementations are advancing every year. And maybe we will someday be able to somehow synthesize diamondid fiber. <br /><br />Remember, you could build a space elevator out of tissue paper, though tapper ratio would be rather hight <img src="/images/icons/smile.gif" /> Nanotube materials have already far surpassed almost all other materials by tensile strength. Edwards concept selects and demands a material strong enough so that initial cable can be launched in realistic time period and on realistic cost. If planed strength is 100GPa and in 2020. there is a commercially available nanotube composite with 70GPa, well at such strengths you can and simply make the ellevator cable fatter in middle.<br /><br /><br />For right here and now, any simple BDB approach that ends cheap and working would be ideal and preferable to all other solutions. IMHO laser launch is way more out there than a space elevator.

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>Funding might have been a problem; I mentioned that his last test was subsidized by a man who was convicted of tax evasion a few years later. <br /><p><hr /></p></p></blockquote>Of course, funding is always the problem. However, most of the time the lack of funding can be traced directly to a lack of demonstrated success or potential, so I think my question about why nothing has happened in the last 7 years remains valid and unanswered. Sure there have been talks (to a homeschooling group, of all the forums in the world...), but nothing new in terms of substance, as far as I can tell. <br /><br />About the tax evasion, the same lack of progress that attracts no investors also attracts bad investors, I would think...<br /> <div class="Discussion_UserSignature"> </div>

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>most of the time the lack of funding can be traced directly to a lack of demonstrated success or potential<p><hr /></p></p></blockquote>Neither of which is the situation in this case. <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>so I think my question about why nothing has happened in the last 7 years remains valid and unanswered.<p><hr /></p></p></blockquote>The links I provided mention that some things have been happening over the last 7 years. They just aren't all public. <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>Sure there have been talks (to a homeschooling group, of all the forums in the world...)<p><hr /></p></p></blockquote>Just as I cannot be an engineer, you cannot be an editor. <br /><br />The talks were not *to* a homeschool group. A homeschool group merely *reported* on the talks. They did a good job. <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>but nothing new in terms of substance, as far as I can tell.<p><hr /></p></p></blockquote>I guess that $4 million and work with the Air Force that he has been doing counts for nothing? <br /><br />You might be interested, as I am, in the reports found on NASA's NTRS:<br /><br />NTRS: Myrabo<br /><br />I see that Myrabo has been busy these last few years.

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>Theoretical peak strength for carbon nanotubes remains far above one needed for a space elevator<p><hr /></p></p></blockquote>That's nice. Can you cite anyone who has produced more than an inch-long nanotube cable? We need 15k MILES of cable.

 

[kelvinzero]

Just throwing this into the mix.<br /><br />http://science.nasa.gov/headlines/y2000/ast07sep_1.htm :<br /><font color="yellow">Any potential material may be characterized by the taper factor -- the ratio between the cable's radius at geosynchronous altitude and at the Earth's surface. For steel the taper factor is tens of thousands -- clearly impossible. For diamond, the taper factor is 21.9 including a safety factor. Diamond is, however, brittle. Carbon nanotubes have a strength in tension similar to diamond, but bundles of these nanometer-scale radius tubes shouldn't propagate cracks nearly as well as the diamond tetrahedral lattice."</font><br /><br />I guess there are more recent links. That is just the first I found. 20 times the radius would be 400 times the area or mass per meter of cable.<br /><br />

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>You might be interested, as I am, in the reports found on NASA's NTRS:<br /><br />NTRS: Myrabo<br /><br />I see that Myrabo has been busy these last few years.<br /><p><hr /></p></p></blockquote>Indeed, so it appears. This looks promising. No flight tests, though, are there?<br /> <div class="Discussion_UserSignature"> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>Theoretical peak strength for carbon nanotubes remains far above one needed for a space elevator<p><hr /></p></p></blockquote><br />I believe there is still plenty of doubt left on that one.<br /><blockquote><font class="small">In reply to:</font><hr /><p>Remember, you could build a space elevator out of tissue paper<p><hr /></p></p></blockquote>Theoretically, perhaps. Practically, 50 GPa at low density (1-1.5) is the limit below which a space elevator becomes uneconomical, because it will weigh much more than it can lift.<br /><blockquote><font class="small">In reply to:</font><hr /><p>there is a commercially available nanotube composite with 70GPa<p><hr /></p></p></blockquote>No, most certainly not. Sometimes people confuse strength and modulus, which have the same units. Modulus is >20 times larger than strength, generally. The two are related through the maximum elongation before breakage, which tends to be in the single digit percent range. Strength currently maxes out around 7-10 GPa, using polyethylene and other polymers, not nanotubes. Carbon nanotubes have a modulus of about 1,000 GPa, which puts a strength of 50 GPa well within reach.<br /><br />There are a few other possibilities, my favorite being linear carbon, or polyyne, which should be stronger than nanotubes and less susceptible to defects. It is not stable under atmospheric conditions, but should be in space. Then there is boron nitride nanotubes, which are much like carbon nanotubes, but stickier and more resistant to defects.<br /> <div class="Discussion_UserSignature"> </div>

 

[eniac]

This is what Wikipedia has to say about polyyne, aka carbyne:<br /><br /><i>Carbyne, or polyyne, is also another name for Linear Acetylenic Carbon [5] (LAC) the carbon allotrope that has the chemical structure [6] -(C:::C)n- . Carbon in this modification is linear with sp orbital hybridisation, and is a polymer with alternating single and triple bonds. This type of carbyne is of considerable interest to nanotechnology as its Young's modulus is forty times that of the hardest known material - diamond [7].</i><br /><br />A strength forty times that of diamond would certainly be sufficient, although I do not find that particular number believable. Anyway, good for a lead in case carbon nanotubes do not live up to their early promise.<br /> <div class="Discussion_UserSignature"> </div>

 

[grdja]

Eniac, you misunderstood me. I said, that if hypothetically, in year 2020 there was a 70GPa fiber and previous designs called for 100GPa for ideal case, someone would cash out and do it with immediately available material.

 

[j05h]

<i>> Theoretical peak strength for carbon nanotubes remains far above one needed for a space elevator, .... Remember, you could build a space elevator out of tissue paper, though tapper ratio would be rather hight</i><br /><br />Theory is a lot different than practice, especially with such new materials. The problem with nanotubes/fullerenes is not their individual strenght, but getting them to stick together. I'm pretty sure that no matter how much tissue paper you put at the top, that a space elevator made of wood pulp is going to snap under it's own mass.<br /><br /><i>> For right here and now, any simple BDB approach that ends cheap and working would be ideal and preferable to all other solutions. IMHO laser launch is way more out there than a space elevator.</i><br /><br />I'll take cheaper BDBs, too. I want to keep comparing laser launch and space elevators because either's deployment would be such a tremendous leverage. Why is laser launch more out-there than elevators? Myrabo has worked out the dynamics of the launch vehicle and the lasers are within reach, plus his team has actually demonstrated it works at lab-level. We're still trying to make nanotubes in normal lengths, never mind 15,000 miles worth. I was at the XPrize in '06 and it was cool watching the climbers, but the elevator technology has a long road to travel. What actual issues do you see with laser-launch? Why is it more out there? <br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[nexium]

The space elevator climber will likely use a laser for propulsion. One estimate was 12 meters in diameter adaptive optics produced minimum spot size of 25 centimeters at 100 kilometers range at power level of several megawatts at 840 nano meters = near infrared. Last I heard a prototype laser was under construction. If successful, this should be adequit for both the elevator and small space craft launch. For the space craft a second stage with at least a little delta v is needed to orbit Earth or orbit the Sun. I'll edit this, if I discover my numbers are in error. Neil

 

[eniac]

Laser requirements are very different for the SE and the LightCraft. SE Lasers are continuous, optical wavelength, and 1-2 MW for a 20,000 kg load. LightCraft lasers are pulsed, any wavelength, and 100 MW (continuous) for a 100 kg load. <br /><br />The apparent large ( /> 10,000-fold) disadvantage in MW per kg payload for the LightCraft is at least partly offset by much faster launch times. <br /> <div class="Discussion_UserSignature"> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>One estimate was 31 meters in diameter adaptive optics produced minimum spot size of 1.25 meters at 50,000 kilometers range at power level of several hundred megawatts.<br /><p><hr /></p></p></blockquote>All of these numbers are different from what I remember. I think mirror size is supposed to be less than 10 meters (31 would be extremely challenging), spot size many meters and (laser) power 1-2 MW. The 100 MW you mention perhaps refers to electrical input power, needed because of the generally poor efficiency of lasers.<br /> <div class="Discussion_UserSignature"> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>Eniac, you misunderstood me. I said, that if hypothetically, in year 2020 there was a 70GPa fiber and previous designs called for 100GPa for ideal case, someone would cash out and do it with immediately available material.<p><hr /></p></p></blockquote><br />Sorry, of course you are right. The misplaced period behind the "if ..." part of your sentence threw me off. <div class="Discussion_UserSignature"> </div>