Subject: Space-tech Digest #80 Contents: Jim Van Zandt Re: Satellite repair robots Henry Spencer Re: Satellite repair robots Paul Dietz Buckyballs and Ion Engines Phil Fraering Re: Buckyballs and Ion Engines Paul Dietz Re: Buckyballs and Ion Engines Paul Dietz Re: Buckyballs and Ion Engines Gordon Pusch Buckyball synthesis Paul Dietz Re: Buckyball synthesis Edward Wright Re: Buckyball synthesis Edward Wright Re: Buckyballs and Ion Engines Phil Fraering Re: Buckyballs and Ion Engines Henry Spencer Re: Buckyballs and Ion Engines Gordon Pusch Re: Buckyballs and Ion Engines ------------------------------------------------------------ To: space-tech@cs.cmu.edu Subject: RE>Repair Bots Date: Wed, 01 May 91 09:20:28 EDT From: jrv@sdimax2.mitre.org Michael Wallis writes... > And yes, you WILL need rad-hardened electronic componants. GEO is well > outside the Van Allen belts... ^^^^^^^ ...which means that radiation hardened components are needed only for long duration missions. There's an interesting chart in "Fundamentals of Astrodynamics" by Bate, Mueller, and White (figure 3.1-1 on page 153 in my edition) showing satellite lifetimes as a function of orbit altitudes for circular orbits. Satellites below about 150 mi are short lived due to atmospheric drag. Satellites between about 300 mi and 7000 mi are within the Van Allen radiation belts and have to tolerate a lot of radiation. In a geostationary orbit, the only limit he shows is due to meteoriods. The repair bot would have to survive one pass through the Van Allen belts, of course, but that takes less than six hours even for Hohmann transfers and requires hardness of the same order as a human. - Jim Van Zandt ------------------------------ From: henry@zoo.toronto.edu Date: Wed, 1 May 91 18:53:05 EDT To: space-tech@cs.cmu.edu Subject: Satellite repair robots Cc: space-investors@cs.cmu.edu Minor technical comments on repair robots (Marc Ringuette passed some of the space-investors stuff on to space-tech; I don't get s-i)... >Good point. A critical part of each mission is thoroughly mapping the >satellite blueprints, designing the torque-holds accordingly, and testing >out the holds on earth-based models of the satellite and robot, perhaps in a >swimming pool for boyancy micrograv simulation... One must beware of making this *too* critical, i.e. of designing the repair mission on the assumption of accurate information. Of the four US in-space repairs done to date, three have found that the bird was not actually shaped the way the blueprints said. The Solar Max repair nearly failed as a result, and blueprint:hardware discrepancies were the whole problem with GRO. A practical repair robot must stress versatility in equipment and procedures, so that it can cope with surprises. > ... The robot will have to be designed with > electronics standard for GEO satellites, not earth-based robots... Possibly worse. Especially if the repair robot is to be reusable, it may need much *more* radiation resistance than GEO satellites normally have, because it will be making more trips through the inner belt. The GEO birds are usually designed on the assumption that apogee-motor firing will not be unduly delayed, so only a few passes through the inner belt will occur. (Note, e.g., the initial concern that Hipparcos might die quickly when it turned out to be stranded in transfer orbit.) > First, launch it into an orbit slightly above geosync, using an appropriate > vehicle. ... Equip it with a cold-nitrogen > system for maneuvering; it's much simpler than hydrazine combinations, > and less likely to contaminate the target... You will need both (and I have seen spacecraft designs with both). The specific impulse of cold-gas thrusters is *terrible*; you will run out of gas very quickly if you use them for orbit changes. They are the best choice for sliding up close to a satellite, but you're going to want a less fuel-hungry system for all but the final approach, especially if you're trying to keep a reusable bird on station for a long time. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Fri, 26 Apr 91 21:42:40 EDT From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: Buckyballs and Ion Engines I read with interest that some researchers are investigating "buckyballs" (buckminsterfullerenes, soccer-ball shaped molecules consisting of 60 carbon atoms) for use as reaction mass in ion engines. Recently, a simple and potentially low cost means of making buckyballs was discovered; Smalley is projecting the cost should drop to a few dollars per pound. The reason why you might want to use C60 in an ion engine can be seen from the physics of these engines. In an ion engine, a material is ionized, and positive ions are accelerated across a gap between two grids. The current is limited by space charge effects. The ion space charge limit is: 5.402E-8 ( v / M )^(1/2) E^(3/2) d^-2 amps/cm^2 where v is the valence of the ion, M the mass of the ion in AMU, E the potential (in volts) across the gap, and d the gap (in cm). At constant exhaust velocity and electrode spacing, the thrust density of an ion engine scales as (M/v)^2. An ion engine can therefore be made much more compact if it accelerates ion with a large mass/charge ratio. (Ultimately, the voltage required would be too large, however, and the electrode spacing d would have to be increased.) Also, for some applications (for example, earth orbital transfer) one would like exhaust velocities lower than are practical with existing ion engines (which are around 30 km/s). Higher mass ions could do this. C60 has a mass of 720 AMU, vs. ~130 for cesium or xenon. The space charge limited thrust density would be about 30 times higher. The advantage of C60 over other molecules would be that C60 appears to be very rugged, yet easily ionized, so it should be possible to ionize it without generating large numbers of low mass fragments, which would degrade engine efficiency (ion engines are most efficient when all the ions have about the same mass/charge ratio). I read that buckyball ions accelerated to 15,000 mph have been observed to survive impact with metal surfaces, which illustrates how resilient they are. Higher fullerenes (C70, etc.) have also been made and purified, and may make even better fuels. Also, large fullerenes could be made containing multiple heavy atoms trapped inside. Larger current densities in ion beams could also be useful in spacebased neutral beam weapons, in heavy ion beam driven inertial confinement fusion, in ion beam milling and other industrial processes involving carbon beams (diamond deposition, perhaps). Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Sat, 27 Apr 91 11:00:19 -0500 From: Fraering Philip To: dietz@cs.rochester.edu, space-tech@cs.cmu.edu Subject: Re: Buckyballs and Ion Engines Paul, you mentioned a "Smalley" in your post. Could you give a more complete reference? Phil Fraering dlbres10@pc.usl.edu ------------------------------ To: Fraering Philip Cc: dietz@cs.rochester.edu, space-tech@cs.cmu.edu, gwh@ocf.Berkeley.EDU Subject: Re: Buckyballs and Ion Engines Date: Mon, 29 Apr 91 09:37:55 -0400 From: dietz@cs.rochester.edu Paul, you mentioned a "Smalley" in your post. Could you give a more complete reference? Richard E. Smalley at Rice. He was quoted in the April 20 (?) issue of Science News as saying that they will eventually be able to make fullerenes by the "truckload", for a few dollars per pound. I believe this was a report on the previous week's ACS meeting in Atlanta. Chemical and Engineering News (April 22, pages 8-9) have more on fullerenes, but do not mention application to Ion engines. Whetten (?) at UCLA reportedly did the experiments on accelerating C60 ions into targets. According to other press reports I've since read, the targets were graphite and silicon and the ions were accelerated to 17,000 mph, not 15,000. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Mon, 29 Apr 91 22:49:45 EDT From: dietz@cs.rochester.edu To: fermat!r@la.tis.com Subject: Re: ion engines & fullerenes Cc: space-tech@cs.cmu.edu : You mentioned singly ionized C60 as a good propellant for ion engines. It seems that Hg2 would also be a good candidate. A more fundamental question: Why must it be a molecule? We can make cluster ions; or small drops of water. Millikan's experiment to measure the electron charge used tiny oil droplets. Why not singly ionized golf balls? There's obviously some other constraint I'm missing. But the size of the propellant quantum seems infinitely variable. It's not clear that C60 provides any new flexibility. (Perhaps reply to space-tech? I can't be the only person with this question.) Many molecules are possible. However, remember that we want to ionize them, and fragile molecules will tend to fragment. If the ion exhaust contains fragments of wildly different mass/charge ratios the efficiency of the engine will be degraded. C60 has the advantage that (1) the charged entities are nearly identical in mass (unlike oil droplets or other atomic clusters, say), and (2) each atom is bonded to at least three others, so they're hard to break up. As for mesoscopic objects: this has been considered, although the particles are not anywhere near golf ball sized. It might even be a good idea. However, the mass/charge ratio is extremely high, which means that very high voltages are needed. I bet there would be problems with parasitic currents from ions or electrons, which, even if small, could soak up most of the input power, as well as causing annoying hard radiation. Perhaps some sort of multistage linear accelerator could be used, tuned to objects of very high mass/charge -- assuming the mass/charge ratio could be precisely controlled. A macroparticle electrostatic rocket would be very useful if it could be fueled with extraterrestrial material, say finely ground lunar or asteroid rock. But maybe C60 could be made easily from asteroid carbon. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Tue, 30 Apr 1991 00:18 EST From: "GORDON D. PUSCH" <@BITNET.CC.CMU.EDU:PUSCHG@CRL.AECL.CA> Subject: Buckyball synthesis To: space-tech@CS.CMU.EDU X-VMS-To: SPACE-TECH X-VMS-Cc: PUSCHG As I understand it, the bulk-synthesis technique for fullerenes is ridiculously simple: starting with pure carbon, run a *very* high current carbon-arc in a helium atmosphere, or else blast graphite with a high-power laser in a helium atmosphere, and collect the resulting soot. With the right combination of power and helium pressure, a large (more than 50%) fraction of the soot will be fullerenes. To seperate out the fullerenes, mix the soot with benzene --- fullerenes are highly soluable in benzene, but the rest of the soot isn't. Distill off the benzene, and you're left with almost pure C_60, if you do everything right. The main bottleneck I see will be extracting pure carbon from carbonaceous asteroid material --- which, if I recall correctly, is almost a bad a witch's-brew as coal (Thoughts on this, Paul?). *If* pure carbon can be produced economically from C-type asteroids, I see no other problem besides supplying the necessary energy to making buckyballs by the tonne out in the belt --- or better yet, on an "Earth-grazer" :-). Gordon D. Pusch ! BITnet: TASCC A&D, Stn. 49a ! AECL, Chalk River Laboratories ! Phone: (613) 584-3311, X-4107 (off.) Chalk River, Ont. K0J 1J0 ! (613) 584-2368 (hm.) Canada ! ------------------------------ Date: Tue, 30 Apr 91 07:34:41 EDT From: dietz@cs.rochester.edu To: @BITNET.CC.CMU.EDU:PUSCHG@CRL.AECL.CA Subject: Re: Buckyball synthesis Cc: space-tech@cs.cmu.edu On the problem of extracting pure carbon from carbonaceous asteroids... There could be several ways of doing this. Carbon monoxide can be formed from the reaction of steam with carbonaceous material. CO can then be reacted to form carbon and CO2. Alternately, if mostly organic "gunk" can be extracted, it could be heated to high temperature. Most elements boil off, leaving behind purer carbon. On earth this is typically done in an arc furnace, but in space I imagine one could use a solar furnace, assuming some way of protecting the mirror could be found. The result would probably be contaminated with various reduced, refractory elements, like silicon, but presumably some level of contaminants would be ok, as they would be removed in the fullerene synthesis step. If the mass can be spun during heating, the molten contaminants could be separated by density. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Tue, 30 Apr 91 14:19:27 -0500 From: "Edward V. Wright" To: @BITNET.CC.CMU.EDU:PUSCHG@CRL.AECL.CA, space-tech@CS.CMU.EDU Subject: Re: Buckyball synthesis >*If* pure carbon can be produced economically from C-type asteroids, >I see no other problem besides supplying the necessary energy to making >buckyballs by the tonne out in the belt --- or better yet, on an >"Earth-grazer" :-). Helium losses might also be a problem, with the closest source of resupply being Earth or Jupiter. (And if you're going to be doing things like mining the atmosphere of Jupiter, you'd better count on nuclear propulsion systems, not ion engines! :-) ------------------------------ Date: Tue, 30 Apr 91 14:12:57 -0500 From: "Edward V. Wright" To: dietz@cs.rochester.edu, space-tech@cs.cmu.edu Subject: Re: Buckyballs and Ion Engines >Also, for >some applications (for example, earth orbital transfer) one would like >exhaust velocities lower than are practical with existing ion engines >(which are around 30 km/s). I don't quite understand this. I can see why orbital transfers might not *need* as high an exhaust velocity as planetary missions. But I thought, in general, that higher exhaust velocities were always preferable. Am I missing something? ------------------------------ Date: Tue, 30 Apr 91 14:42:01 -0500 From: Fraering Philip To: ewright@mozart.convex.com Cc: dietz@cs.rochester.edu, space-tech@CS.CMU.EDU Subject: Buckyballs and Ion Engines >>Also, for >>some applications (for example, earth orbital transfer) one would like >>exhaust velocities lower than are practical with existing ion engines >>(which are around 30 km/s). >I don't quite understand this. I can see why orbital transfers >might not *need* as high an exhaust velocity as planetary missions. >But I thought, in general, that higher exhaust velocities were >always preferable. Am I missing something? Because the increace in momentum imparted to the propellant, and therefore to the spacecraft, is linearly dependent on the momentum, wheras the energy required is dependent on the square of the momentum. In short, by halfing the exhaust velocity, the power needed is cut by a factor of four. Phil Fraering dlbres10@pc.usl.edu Joke going around: "How many country music singers does it take to change a light bulb? Four. One to change the bulb, and three to sing about the old one." ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 30 Apr 91 16:14:51 EDT To: "Edward V. Wright" Cc: dietz@cs.rochester.edu, space-tech@cs.cmu.edu Subject: Re: Buckyballs and Ion Engines > But I thought, in general, that higher exhaust velocities were > always preferable. Am I missing something? Higher exhaust velocity is always better but it has a price. In particular, if your energy supply is limited -- often the case with ion engines -- then sacrificing exhaust velocity can give you higher thrust, which can be valuable when working in strong gravitational fields. If your acceleration from your engines is not at least of the same order of magnitude as the local acceleration of gravity, very inefficient trajectories tend to result. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Wed, 1 May 1991 17:42 EST From: "GORDON D. PUSCH" <@BITNET.CC.CMU.EDU:PUSCHG@CRL.AECL.CA> Subject: Buckyballs, Ion engines, and Isp To: space-tech@cs.cmu.edu X-VMS-To: SPACE-TECH X-VMS-Cc: PUSCHG >>Also, for >>some applications (for example, earth orbital transfer) one would like >>exhaust velocities lower than are practical with existing ion engines >>(which are around 30 km/s). > >I don't quite understand this. I can see why orbital transfers >might not *need* as high an exhaust velocity as planetary missions. >But I thought, in general, that higher exhaust velocities were >always preferable. Am I missing something? Despite the infatuation often exhibited with high Isp's (after all, "more is better" *is* part of our culture... ;-), a good "rule of thumb" for *chemical* engines is that the exhaust velocity V_ex (= g*I_sp) should be comparable to the total mission delta-V. This usually argued to be because when delta-V = V_ex, the total fuel energy per unit payload mass is minimized. The logic here is a little fuzzy, since it's been a *looooong* time since the fuel was the most expensive component of the launch-vehicle, but it's still a fair rule of thumb. Now for chemical fuel, the mass-specific energy is fixed, so V_ex is a property of the fuel itself; the only way to increase mission velocity is to either increase the mass-ratio --- and since M_initial / M_final = exp( delta-V / V_ex ), you lose *REAL* fast this way --- or to go to a fuel with a higher I_sp. For non-chemical propulsion, the mass and energy sources are decoupled, and one could in principle choose whatever V_ex one wanted. However, as Phil Fraering pointed out, doubling V_ex *quadruples* the power requirement --- and for a given power source, mass is roughly proportional to power, so the same trade-off still arises: to minimize vehicle cost, V_ex should be on the order of delta-V. Amusingly, Bob Forward has calculated that even for antimatter this rule still holds, regardless of delta-V: to minimize the amount of antimatter required, the mass-ratio should be about 4, so one should choose V_ex to be about delta-V / 1.4, approximately. What changes is the M/AM mix: only micrograms of AM per tonne reaction mass for ground-to-orbit or earth-escape missions, up to about a 50-50 mix for relativistic delta-V's (about c/2; beyond that, even M/AM requires rediculous mass-ratios). Forward calculates that M/AM becomes competitive with H2/O2 when the cost of antimatter drops to about 10 megabucks( US! :-) per microgram ... Gordon D. Pusch ! BITnet: TASCC A&D, Stn. 49a ! AECL, Chalk River Laboratories ! Phone: (613) 584-3311, X-4107 (off.) Chalk River, Ont. K0J 1J0 ! (613) 584-2368 (hm.) Canada ! ------------------------------ End of Space-tech Digest #80 *******************