Return-path: X-Andrew-Authenticated-as: 7997;andrew.cmu.edu;Ted Anderson Received: from hogtown.andrew.cmu.edu via trymail for +dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl@andrew.cmu.edu (->+dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl) (->ota+space.digests) ID ; Wed, 15 May 91 02:23:17 -0400 (EDT) Message-ID: Precedence: junk Reply-To: space+@Andrew.CMU.EDU From: space-request+@Andrew.CMU.EDU To: space+@Andrew.CMU.EDU Date: Wed, 15 May 91 02:23:12 -0400 (EDT) Subject: SPACE Digest V13 #553 SPACE Digest Volume 13 : Issue 553 Today's Topics: Re: Laser launchers Re: Honking at cyclists... Re: Ethics of Terraforming (was Re: Terraforming Venus) Re: Honking at cyclists... Administrivia: Submissions to the SPACE Digest/sci.space should be mailed to space+@andrew.cmu.edu. Other mail, esp. [un]subscription requests, should be sent to space-request+@andrew.cmu.edu, or, if urgent, to tm2b+@andrew.cmu.edu ---------------------------------------------------------------------- Date: 13 May 91 17:10:56 GMT From: cis.ohio-state.edu!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!emory!wa4mei!ke4zv!gary@tut.cis.ohio-state.edu (Gary Coffman) Subject: Re: Laser launchers In article <41687@fmsrl7.UUCP> wreck@fmsrl7.UUCP (Ron Carter) writes: >In article <2777@ke4zv.UUCP>, gary@ke4zv.UUCP (Gary Coffman) writes: >>Sorry, but a laser launch system must carry "reaction mass" to be heated >>by the laser beam. This *dead weight* produces no useful energy input. >>It's mass and tankage mass must still be carried aloft by energy input > ^^^^^^^^^^^^^^^^ >>from the laser beam alone. > >Current concepts have eliminated the tankage by carrying the propellant >as a solid (ice). This eliminates materials costs for the tankage and >gives a huge savings in manufacturing costs; freezers are very very >cheap to run and reproduce simple shapes (like slugs) quite well! This implies an extremely precise application of energy to the ice. An extremely small pointing error would cause off center heating of the block and off center thrust. I've heard this mentioned as a way of steering the vehicle, but such precise control seems well beyond state of the art as I understand it. Therefore I was considering a machined metal chamber to contain and direct the heated exhaust. This then implies tankage and pumps. If such precision is possible in the neccessarily high power beam, then all bets are off. Because of the previously mentioned problems with atmospheric blooming, I don't think such precise control is possible. >>Granted that at very low altitudes the >>atmosphere itself may be used as a working fluid, but above roughly >>5 km the atmosphere is not dense enough to carry the energy load and >>internal reaction mass must be carried. It makes little sense to carry >>*dead weight* when that same mass could release useful chemical energy. > >Only in the naive case. There are two points to consider: > >1.) Exhaust velocity. Laser propulsion can impart far > more energy to a gram of propellant than can chemical > reactions. This lowers the mass-ratio and the total > energy required. This implies that the beam power density must exceed the chemical energy released by a conventional rocket. Again I fall back on the contention that such high power density isn't practical through the atmosphere. >2.) Cost of carrying chemical fuels. They require tankage, > nozzles, pumps for liquids, heavy walls for solids, and > material and manufacturing for all of these. Laser > propulsion has the potential to eliminate them entirely. An ideal chemical rocket has a mass ratio of about 4, so eliminating the tanks, engines, etc will reduce the mass of the vehicle by less than 20% if we want any payload. This is significant, but not awe inspiring. >Are you at all capable of thinking beyond the naive case? I guess I could ask you the same question. You are making some mighty big assumptions that, in my opinion, are rather shakey. >>Using a space based laser is even worse in some respects than using a >>ground based laser. At least on the ground there is a large sink for >>the waste heat produced in the process of generating the beam. At least >>on the ground the massive tankage required for the laser reactants doesn't >>have to be heaved into space. >>... the mass required to be delivered to orbit to operate the >>laser far exceeds the mass of fuel needed to deliver the payload directly >>to orbit. > >Now you are assuming chemical lasers. Why would anyone carry a >chemical laser, which needs reactants which are used once and >exhausted, into space to launch things with? Why not excimer >lasers, or free-electron lasers, or solar-UV-pumped gas lasers? As I mentioned in a previous posting, cooling of any but gas dynamic lasers is a difficult problem on earth where a large heatsink is available. In the vaccum of space, I believe that getting rid of the waste heat will be much more difficult. Therefore I assumed that a space based laser would have to be of the gas dynamic type to avoid melting from it's own waste heat. If you have a plausible cooling method for such lasers, I'd love to hear it. >When you were knocking efficiency, you were talking about >electrically driven CO2 lasers on the ground (worst case, except >perhaps He-Ne lasers). Now that you're bashing space-based >lasers, you are talking about chemical reaction lasers (worst >possible choice). You can't have it both ways! Well yes you can. Different enviornments require different assumptions. On earth, electrically powered lasers make sense because there is no waste exhaust that needs to be dealt with. A large laser launch facility that launches often would have considerable enviornmental impact from the exhaust of a gas dynamic laser alone, not to mention effects caused by the beam. A large gas dynamic launch laser would produce as much exhaust, probably more, than a comparable power chemical rocket engine. Meanwhile a space based laser has the problem of disposing of waste heat efficiently. Really huge radiators capable of disposing of gigawatts would be required for all but a pass through gas dynamic laser. >Take your straw-man engineering proposals and compost them, please. Take your naive dream laser launchers and look at the dirty little engineering details. >>The higher exhaust velocity for a given heat input that light molecules >>like H2 give you is extremely valuable in increasing performance. > >You can't get any significant exhaust velocity out of H2 without >supplying energy to it externally; you can't react H2 with H2. >If you burn it with O2, you've got H2O. Laser-launch systems >will probably use (surprise!) H2O! Ah, but if they use straight H2, and because they supply the energy externally they can, they would get better efficiency. >>The blooming problem is more than just a defocusing of the beam that >>can be solved by adaptive optics. At the beam power levels required, >>the atmosphere is turned to a superheated plasma.... > >Pray tell what those power levels are, and why we have to have >these high power levels anywhere except in the immediate vicinity >of the back end of the target. Tell me why we can't just plug >in this power density limit as a design criterion for the optical >system. We know several things about the beam power levels. 1. The power level will have to be at least as high as the power level produced by a chemical rocket lifting the same mass. Gigawatts. 2. The power level must be high enough to turn enough ice to explosive plasma to give the same thrust level as a chemical rocket. Gigawatts. 3. A beam power level high enough to turn ice to plasma will also be high enough to turn air to plasma. 4. For a highly dispersed beam source (multiple lasers in a widely spaced ground array) the geometry of the situation shows that somewhere below the apex of the converging beams the power density reaches the plasma point of air. 5. As air pressure decreases, the power needed to ionize what air remains decreases. From 4 we can determine that beam guidance will be sufficiently disrupted that your block of ice won't receive the precision heating necessary to avoid off center thrust. From 5 we can say that problems will occur at lower beam power levels as the vehicle gets higher (until it gets *really* high anyway). From 1 and 2 we can say that we're talking about incredible beam power densities, in the gigawatts per square meter range. From 3 we can feel sure that the beam *will* bloom. If we restrict beam power levels to less than 1 megawatt per square meter, we are talking at least a thousand beams for a 1 gigawatt launcher. And as they converge, some point, at least a meter from the target, beam power density approaches the full power of a single beam system. During the early phases of launch, the beams will be converging at nearly right angles to the vehicle. This seems to preclude precise steering of your ice vehicles. >>A one square inch kelvar tether to geosync orbit would weigh 45.2 million >>pounds. I don't have the handbook here at the terminal, but kelvar doesn't >>approach 45 million pounds per square inch tensile strength by orders of >>magnitude. It couldn't support itself, much less a usable payload. > >More oversimplifying. This ignores reduced weight as altitude >increases, tapering and improvement in materials. To quote >one of the experts (lightly editted): > >-From: andrew.cmu.edu!hm02+ (Hans P. Moravec) >-Newsgroups: sci.physics >-Subject: Orbital elevators (was Ringworld) >-Message-Id: >-Date: 21 Nov 90 04:05:39 GMT >- >[material deleted] >- For Earth the taper with Kevlar would be 10^11, and the cable could >-hoist only about one trillionth of itys own mass -- which I admit makes >-it impractical! But the required taper is exponential in the weight to >-strength ratio of the material, and a material only five times better than > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ >-Kevlar (within the measured strength/weights of single crystals of various > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ >-substances) would bring the taper ratio to 160 and the mass fraction to > ^^^^^^^^^^^ >-1/2000. > >So much for "physical impossibility". It is just something that >materials science can't quite do today. It is quite possible. Ok, I concede this one. I hadn't considered a tapered cable. Still, growing a single crystaline strand, WITH A TAPER, that would reach to geosync, sounds rather unlikely for the simple reason that crystals like to determine their own shape. I still think this is a fundamental rather than developmental problem. Note that we are talking about beanstalks here and not dynamic tethers. They have another set of practical problems as launcher replacements. Gary ------------------------------ Date: 14 May 91 00:16:36 GMT From: cis.ohio-state.edu!zaphod.mps.ohio-state.edu!samsung!noose.ecn.purdue.edu!en.ecn.purdue.edu!irvine@tut.cis.ohio-state.edu (/dev/null) Subject: Re: Honking at cyclists... In article <1991May13.180909.16448@watdragon.waterloo.edu>, jdnicoll@watyew.uwaterloo.ca (James Davis Nicoll) writes: > > > So, what happens at the core of human settlement when the core > regions have been stripped of useable resources, and new resources from the > outlying regions can't be shipped back fast enough to suit demand? There > is that nasty speed of light restriction on travel, not to mention how > expensive shipping objects is likely to be over interstellar distances. > I suppose it'd be like fungus; colonise a region, breed like hell until > the local resources are gone, and send out spores to repeat the process. > Exactly! Its not impossible, just expensive to ship goods. Rome in the days of the Empire relied on Egyptian grain ships that took months to reach the hungry populace of non-self sufficient Rome. But in our case months are turned into DECADES here. These shipments would last longer than many governments and companies. And I doubt that colonists would be really interested in shipping off their produce and things to be consumed by 'the core' in exchange for 50 year old consumer goods! I think resource management will be the key to solar exploration and colonizatiopn, rather than the 19th century planeary 'rape' that is implied. There will be very little trade, probably limited to unique local products. Well, unless really cheap cargo ships are made... Even in the core, technology will develop that will renew resources that are now considered 'non-renewable.' Population will be limited by the cost of resources in a bizarre supply/demand like curve (very Malthusian.....). > Ignoring C, what do you do when you've 'eaten' the universe? > It only takes millenia of growth at our current rate to convert the > universe into human flesh, Despite my personal disagreement with how long it will take to 'eat' the universe. We can renew resources. Higher science and technology that expansion will require will (hopefully) tech us a lot about efficient use of resources as well as how to renew the so-called 'non-renewable' resources. Also, I don't think that the WHOLE universe will ever be turned into 'human flesh' - I don't like the gloom and doom of Malthus, but his theories have some validity. When resources become scarcer, people will not have as many kids because the expense of resources, population drops until it becomes cheaper, essentially a damped harmonic of growth when resources are scarce. > birth rate will drop, if our industrial growth continues at the current > rate, we'll 'eat' the Universe industrially in a few millenia. As far > as I know, the Universe is finite, so there are limits (which can be very > large, depending on your assumptions about how efficiently we can use the > Universe) to growth. All things considered - no one knows how big the universe is, or how in 5000 years we can 'eat' it? Only 5000 years? (few thousand = 5000 :) ) 100 billion stars in a galaxy, with billions of galaxies. A googol-plex of stars for STARTERS, and only 5000 years. Our galaxy is 100,000 light years across. If we could turn material into human flesh as fast as the speed of light in all directions, it would take 100,000 years for our galaxy! 200 + million years for the nearest next galaxy, etc! I think you might be off by a factor of 1000 or even 10000 for our galaxy alone. :) > > I suspect that the current conditions are akin to a phase > change, and population after the current industrial revolution will > be somewhat steady, but much higher than prior to the IR. > I think that poulation will expand in the exponential fashion that it has until everyone has TV and 1000 channels ( TV-the most effective birth control! :) ), or resources dry up. Then, as costs of resources (food, etc) rise, people will not increase in the numbers they have and population will steady off. There is also a trend that favors resource renewal rather than industrial age plundering. I think we are growing out of our 'throw-away society' phase. (At least I hope so!) -- +-----------------------------------------------------------------------+ | Society of Philosophers, Luminaries, | Brent L. Irvine | | and Other Professional Thinking People..... | Only my own ramblings | +-----------------------------------------------------------------------+ ------------------------------ Date: 13 May 91 18:16:45 GMT From: voder!pyramid!lstowell@ucbvax.Berkeley.EDU (Lon Stowell) Subject: Re: Ethics of Terraforming (was Re: Terraforming Venus) In article dlbres10@pc.usl.edu (Fraering Philip) writes: > >Uh, perhaps the face shouldn't be discussed on this newsgroup. You are implying that "space" enthusiasts have absolutely no sense of humor? I resent the implication. >I don't mean to imply that the face is or is not an artifact, it's >just that at this stage of the game, discussion of the face is >pointless, and blanket statements about the face such as "Does anyone Interesting statement, given that it is unlikely that anyone has taken a fairly good look at it. Who knows what the doggone thing is.....and that is PRECISELY the point! Frankly, I would much rather take a good look at some core bores from Mt. Olympus. >elxe think it sorta looks like Maggie Thatcher?" could end up >putting you in a situation where you're stepping on a lot of >people's blue suede shoes oops, I mean toes.... :-) Thank you, I'll take that as a compliment. Sometimes toes NEED stepping on. ------------------------------ Date: 14 May 91 03:39:24 GMT From: cis.ohio-state.edu!zaphod.mps.ohio-state.edu!wuarchive!rex!rouge!dlbres10@tut.cis.ohio-state.edu (Fraering Philip) Subject: Re: Honking at cyclists... Actually, everyone else out there knows we have potential to use up the entire universe. That's why very shortly the Earth will be destroyed to make way for an interstellar bypass. -- Phil Fraering || Usenet (?):dlbres10@pc.usl.edu || YellNet: 318/365-5418 ''It hardly mattered now; it was, in fact, a fine and enviable madness, this delusion that all questions have answers, and nothing is beyond the reach of a strong left arm.`` - Larry Niven and Jerry Pournelle, _The Mote in God's Eye_ ------------------------------ End of SPACE Digest V13 #553 *******************