Subject: Space-tech Digest #119 Contents: Propellants for Laser Rockets (3 msgs) Tether/Skyhook Materials & Anchors (14 msgs) ------------------------------------------------------------ Date: Sat, 25 Apr 92 20:52:36 -0400 From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: Propellants for Laser Rockets Following up a comment Henry Spencer made on sci.space, I was wondering what would make a good propellant for a laser thermal rocket (or, more generally, a thermal rocket). Hydrogen is an obvious choice for maximizing Isp, but has low density. Ammonia is perhaps a better choice, at least for the early stage of a launch. Ammonia cracks to hydrogen and nitrogen at high temperature. Nitrogen is resistant to dissociation; at 1 bar and 5000 K nitrogen is only about 3% dissociated (if I've computed the equilibrium coefficient correctly); hydrogen at that temperature and pressure is almost entirely dissociated. The H2/N2 mix produced by ammonia cracking has an average molecular weight of 8.5, dropping as the hydrogen dissociates. For N2 + 6 H, the average molecular weight of the exhaust is a mere 2.9. Another possibility is lithium. Liquid lithium can be stored without extreme difficulty (melting point is 180.5 C). The boiling point is high, 1342 C, which is either a blessing (the liquid makes an excellent coolant), or a curse (much energy is needed to vaporize it, and condensation in the exhaust can cause inefficiencies). Being a molten metal, liquid lithium could perhaps be pumped electromagnetically. Maybe an MHD generator/pump could be arranged so that one could have a pump-fed rocket with no moving parts? I understand alkali elements will dissolve to some extent in anhydrous ammonia, producing a conductive liquid (with solvated free electrons). So perhaps lithium-ammonia would be an interesting fuel. Or does this react eventually to form lithium amide and hydrogen? Lithium hydride could be interesting as a solid fuel, if it is sufficiently strong. Wasn't this mentioned as a possibility for the P4 concept? And, does lithium hydride dissolve in lithium, or does the hydrogen just go back to molecular form? Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: henry@zoo.toronto.edu Date: Sat, 25 Apr 92 22:57:59 EDT To: space-tech@cs.cmu.edu Subject: Re: Propellants for Laser Rockets >Hydrogen is an obvious choice for maximizing Isp, but has low density. Note that exhaust velocity (aka Isp) is not the whole story for Earth-to- orbit applications. Thrust also matters, because of gravity losses and the like. And here you run into a tradeoff, because varying the molecular weight affects exhaust velocity and thrust in opposite directions. (An intuitive way to grasp this is that a low-MW exhaust is also a low-density exhaust, thrust is exhaust velocity times mass flow, and the direct dependence of mass flow on density overpowers the square-root dependence of exhaust velocity on MW.) Note also that laser rockets potentially have high enough exhaust velocity that you don't need to make it your sole focus of optimization. The P4 system (payload, propellant, photons, period) doesn't really care too much how big the brick of propellant under the payload is, so exhaust velocity is not that big an issue unless it gets excessively low. You can afford to trade it off a bit to get higher thrust and other useful properties (such as convenient handling and benign exhaust products). >Lithium hydride could be interesting as a solid fuel, if it is >sufficiently strong. Wasn't this mentioned as a possibility for the >P4 concept? At one point it was Jordin Kare's favorite P4 fuel, if memory serves, although I think he ended up preferring Delrin plastic. He said at one point that the MW actually turned out not to matter that much to performance, so you'd select for other characteristics first. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ From: sequent!techbook.com!szabo@uunet.UU.NET (Nick Szabo) Subject: Re: Propellants for Laser Rockets To: dietz@cs.rochester.edu Date: Sat, 25 Apr 92 23:11:26 PDT Cc: space-tech@cs.cmu.edu On a related note, a recent paper in _Science_ reinterprets the Ceres IR spectrum as indicating ammonium saponite in addition to water-bearing phylosilicates on the surface. The wavelength was previously interpreted as water frost (a nearly identical wavelength), but at the indicated temperature of 231.5K water ice is not stable on the surface. Whether water and/or ammonia ice is to be found under the surface remains unknown. Ceres is a member of the most common carbonaceous asteroids, so this finding could have far-reaching consequences, and hints at a possible source of native ammonia for thermal rockets. Nick Szabo szabo@techbook.com ------------------------------ Date: Thu, 23 Apr 1992 21:41-EDT From: Donald.Lindsay@GANDALF.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Tether/Skyhook Materials The latest Discover reports that the dragline silk produced by spiders "would have to be nearly 50 miles long before it would break under its own weight". [It also stretches to 130%.] It was stated once, here, that Kevlar 29 had a "hanging length" of 200 KM. Is that the same measure? If so, then kevlar wins, and science fiction just lost a great plot opportunity :-) But while I have your attention: how are we doing on improving on Kevlar? There's something better called, ah, Spectra? Don ------------------------------ Date: Thu, 23 Apr 92 22:52:02 -0500 From: pgf@srl03.cacs.usl.edu (Phil G. Fraering) To: Donald.Lindsay@gandalf.cs.cmu.edu, space-tech@cs.cmu.edu Subject: Re: Tether/Skyhook Materials Just thought I'd mention: kevlar isn't some miracle substance; it does need to be cladded, for protection against corrosion and photochemical action, both of which can make it brittle.... Phil ------------------------------ From: sequent!techbook.com!szabo@uunet.UU.NET (Nick Szabo) Subject: Re: Tether/Skyhook Materials To: Donald.Lindsay@GANDALF.CS.CMU.EDU Date: Fri, 24 Apr 92 4:04:48 PDT Cc: space-tech@cs.cmu.edu X-Mailer: ELM [version 2.4dev PL32] > How are we doing on improving on Kevlar? There's something better > called, ah, Spectra? I'd be interested in seeing specs on Spectra, since from what I understand it is easier to manufacture than Kevlar. Amaco has a graphite epoxy called T-1000 that has a tensile strength of 6,900 MPa (as opposed to 2,800 MPa for Kevlar). Does anybody know how T-1000 is manufactured, or have references to same? Is T-1000 being used in any commercial or military products? Has the existence of buckytubes been confirmed? Nick Szabo szabo@techbook.com ------------------------------ Date: Fri, 24 Apr 92 16:24:14 -0400 From: Jon Leech To: space-tech@cs.cmu.edu Subject: Re: Tether/Skyhook Materials >Has the existence of buckytubes been confirmed? I wasn't aware there was any doubt about it. Richard Smalley talked here a few weeks ago and showed photomicrographs dating back a decade; people had created them but thought they were rolled up sheets rather than nested tubes. Jon __@/ ------------------------------ From: henry@zoo.toronto.edu Date: Fri, 24 Apr 92 16:41:22 EDT Subject: Re: Tether/Skyhook Materials To: space-tech@cs.cmu.edu >Has the existence of buckytubes been confirmed? It has been weakly disconfirmed, in fact. Some of the observations that were thought to indicate tubular fullerenes in fact appear to have been caused by larger spherical ones, if I recall correctly. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ From: henry@zoo.toronto.edu Date: Fri, 24 Apr 92 16:45:22 EDT Subject: Re: Tether/Skyhook Materials To: space-tech@cs.cmu.edu >Has the existence of buckytubes been confirmed? Sigh, cancel previous message... Why does my memory always come back just after I send the "if I recall correctly" message...? Some of the observations thought to indicate tubular fullerenes may have been caused by string-of-beads configurations of several ordinary buckminsterfullerene molecules in a row. At first glance this *looks* like a tube, but when you look more closely, it's not. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Mon, 27 Apr 92 08:52:46 CDT From: eder@hsvaic.boeing.com (Dani Eder) To: sequent!techbook.com!szabo@uunet.UU.NET Subject: Re: Tether/Skyhook Materials Cc: space-tech@cs.cmu.edu My understanding, which comes from having surveyed fiber suppliers last year (for tether materials), is that T-1000 is still classed as a developmental fiber (i.e. it is made in kg quantities). Hercules, which both makes carbon fiber and uses it in rocket motor cases and other high strength/weight projects, has a fiber called IM9, which is rated at 920,000psi (6,350MPa). It is also classed as developmental, but in their case that means they don't yet have enough customers to devote one of their main fiber production lines to it. A fiber production line has a capacity of 800,000 lb/year. They did tell me it runs $75/lb, compared to $20/lb for their AS4 fiber (580,000psi). I have been told by their technical staff that they can probably push the present carbon fiber making process up to 1,200,000 psi, if anyone had a real need. Developing a higher strength fiber costs money, so they would want to see a real customer out there. Most of their effort in the last few years has been towards higher stiffness fibers rather than higher strength, since stiffness is very importatnt in airplane construction. Also, with high strength fibers in hand, you can mix high strength & high stiffness fibers in a particular composite to tailor the properties to the specific application. As far as Spectra, the brochure from Allied Chemical lists it's strength as as 435,000psi, and it's density as 0.97. For reference, carbon fiber has a density of 1.8. This means that carbon fiber wins on strength/weight when it goes over 800,000psi. This is usually the critical factor in tether design. Other factors can affect your choice. Spectra is lousy at high temperature (it is a very high molecular weight polyethylene, but still a plastic). Carbon fiber is not very flexible. Neither is safe from atomic oxygen attack in low Earth orbits. So 'best' material depends on the specifics of your application. ------------------------------ Date: Mon, 27 Apr 92 10:08:01 CDT From: ssi!lfa@uunet.UU.NET (Louis F. Adornato) To: uunet!cs.cmu.edu!space-tech@uunet.UU.NET Subject: Re: Tether/Skyhook Materials What is the minimum tensile strength needed for a skyhook? I read somewhere that it worked out to something around the strength of a diamond crystal. Just how close is the current technology to supporting a skyhook? Of course, even if the material is available, there's the small matter of moving Sri Lanka to the equator... (actually, there's a not bad mountain just inland of the west coast of South America, very close to the equator, with what might be a decent harbor nearby - next time I'm near an atlas I'll dig up the name). Lou ------------------------------ Date: Mon, 27 Apr 92 12:18:48 PDT From: gwh@lurnix.COM (George William Herbert) To: space-tech@cs.cmu.edu Subject: Re: Tethers The much-quoted problem of the ground end of a tether really isn't one. Most people go ape about "it has to come down on land!", when building a at-sea platform (floating and anchored or even on legs) is trivial compared to building the tether itself. You can drop your tether down in the ocean quite easily, though power to the tether elevators then becomes a bit more difficult (though not impossible). -george william herbert gwh@lurnix.com gwh@ocf.berkeley.edu ------------------------------ From: henry@zoo.toronto.edu Date: Mon, 27 Apr 92 17:46:40 EDT To: space-tech@cs.cmu.edu Subject: Re: Tethers >You can drop your tether down in the ocean quite >easily, though power to the tether elevators then >becomes a bit more difficult (though not impossible). The right way to power beanstalk elevators is from above, in which case this becomes a non-issue. Just tie a power satellite to the free-fall point on the beanstalk. Note, though, that there is one reason to prefer a good solid anchor for a beanstalk: at most points along the equator, there will be small -- well, relatively small -- horizontal forces due to the lumpiness of Earth's gravitational field and outside perturbations, and something has to oppose them if you don't want the base wandering around. A floating base needs either anchors on the bottom or a lot of propulsive power. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Mon, 27 Apr 92 19:09:33 -0400 From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: Tethers I was wondering how much it would save if you could have the end of a tether moving in the atmosphere. Moravec's spinning tethers are one concept. Another would be to have a tether with one end in (sy) a 12 hour orbit. The lower end, at the equator, would be moving at about 1000 mph. Could this sawn-off tether be done with existing materials? Paul ------------------------------ Date: Mon, 27 Apr 92 19:49:55 EDT From: John Roberts Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. To: space-tech@cs.cmu.edu Subject: Re: Tethers Is a beanstalk the same as a space elevator? If so, I thought you told me a year or two ago that one big advantage of a fixed base is that it allows the payload going up and the payload going down to be unbalanced - the difference in angular momentum goes into speeding up or slowing down the rotation of the Earth, rather than tearing the elevator away from the planet or causing it to fall to the ground. John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ Date: Mon, 27 Apr 92 19:27:21 EDT From: Hans Moravec To: space-tech@cs.cmu.edu Subject: Re: Tethers Reply-To: moravec@cs.cmu.edu > Note, though, that there is one reason to prefer a good solid anchor for > a beanstalk: at most points along the equator, there will be small -- > well, relatively small -- horizontal forces due to the lumpiness of Earth's > gravitational field and outside perturbations, and something has to oppose > them if you don't want the base wandering around. A floating base needs > either anchors on the bottom or a lot of propulsive power. > > Henry Spencer at U of Toronto Zoology > henry@zoo.toronto.edu utzoo!henry A beanstalk without a really solid anchor isn't good for serious work at all. Without anchor, it has to be in orbital equilibrium, and so will sink and drift as soon as any extra payload is added, or accelerated. The far more useful alternative is to anchor it FIRMLY, and tension it to maximum design load by means of an orbital ballast somewhere significantly beyond geosynch. The ballast pulls outward centrifugally- the farther out, the smaller its mass need be (about 90,000 miles is nice potential wise - cable taper required is same there as at bottom - cable is fatter everywhere in between, fattest at geosynch). The forces exerted by payloads then simply subtract from the anchor force, and the whole system is pretty stable (except for stretch, coriolis, tides, etc.) hans moravec ------------------------------ Date: Mon, 27 Apr 1992 16:38:26 -0700 (PDT) From: David Cortesi Sender: David Cortesi Subject: Re: Tethers (moving base of) To: space-tech@cs.cmu.edu Content-Type: TEXT/US-ASCII; charset=US-ASCII On Mon, 27 Apr 92 17:46:40 EDT, uunet!zoo.toronto.edu!henry wrote: > Note, though, that there is one reason to prefer a good solid anchor for > a beanstalk: at most points along the equator, there will be small -- > well, relatively small -- horizontal forces due to the lumpiness of Earth's > gravitational field and outside perturbations, and something has to oppose > them if you don't want the base wandering around. Just what scale of motion should we picture here? If it's order 10 meters a minute, a floating base with NO anchor might be just the ticket. Just let it slosh around! Of course if it was gonna make like a big swizzle stick... ------------------------------ End of Space-tech Digest #120 *******************