Date: Fri, 30 Sep 1988 16:42-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: "~/st/lists/stdigest" Subject: Space-tech Digest #9 [ Sorry, I sent out two number 7's. I never said I could count. --Marc ] Contents: Steve Abrams Satellites & stuff Joe Beckenbach Information source for rad-hard parts Steve Abrams Tether propulsion Michael C. Matthews Space Tethers Bibliography (VERY LONG) Marc Ringuette Ion propulsion / solar mirror ------------------------------------------------------------ Date: Tue, 27 Sep 88 23:07:52 CDT From: sedspace@doc.cc.utexas.edu (405986289 abrams) Posted-Date: Tue, 27 Sep 88 23:07:52 CDT To: space-tech@cs.cmu.edu Marc, Sorry for the long time delay...family problems and homework. Besides, I had to keep adding more comments...I'm goiung to stop trying to catch up in order and primarily answer current stuff... I don't mind your delaying the sail post a while... >In order to be visible, would the satellite have to deploy a sheet of >white plastic or something? Do you know how big it has to be to be >visible? Why do we want it to be visible? For PR value, I guess? If a solar sail is used, it should make the image large enough to see with a telescope for sighting laser or radio communications. >RTG's: they're incredibly massive. A 100kg RTG can produce maybe 300 >watts of power (within a factor of 2, these are the numbers we're >working with for our Mars Rover project). In other words, they suck. Ugh! They *do* suck...I hadn't any idea...that certainly makes the solar power aspect more viable... >Solar power: why do you say there isn't enough solar power to be had? >The alternatives are far worse, so I suggest using a solar cell array to >produce no more than a few tens of watts, and using a very limited power >budget. Less than a square meter should be enough, shouldn't it? It seems >like by far the best way. Despite their low efficiencies, I guess that you're right in suggesting solar cells. However, if we are to do any orbit-pumping, we will need more than 'tens of watts' (unless we opt for the old mechanical 'tilting the sail'). If we just put a can up and leave it there, obviously the power requirements are minimal. If we plan to sail-pump the orbit via thermomodulation, we'll probably need a couple of hundred kilowatts. This many solar cells would really expand the cost. We may be able to 'spiderweb' the sail film (similar to the use of nitinol frames - here, however, we just need one NiTi frame around the outside) with something that is highly thermal-absorbing that can heat the film by simple conduction of solar heat. >Have you given thought to how to package a solar sail so that it can >withstand a few gravities, and then be deployed? That sounds difficult. >Assembly-in-orbit may be necessary, which would make the project hugely >more difficult. Unfortunately, for significant missions, in-orbit assembly is a must. For this coffee- can, it isn't. The sail can be *neatly folded* (to minimize wrinkling) and affixed to a shape-memory-alloy deployment frame that can be disconnected after deployment. If we aren't worried about wrinkling (although this can triple trip time; also, conducting polymers wouldn't wrinkle nearly as much), we can "spread" the sail out flat, grab the center and lift letting the sail drape down. A slight twist to "cylindrize" it (like an umbrella) and slide a telescoping cylinder down its length. Launch the cylinder to highly elliptical (e>0.5) orbit with apogee around 10,000 km. Spin the cylinder and slowly retract the telescoping part; as the sail emerges, centrifugal force deploys it. If, when we first spin the cylinder, we spin a similar, more massive cylinder (sans sail) in the opposite direction, we can couple the two to de-spin the deployed sail. The time of deployment is heavily dependent upon the tensile strength of the film and, hence, the sail thickness. Deployment during the "apogee" part of the orbit should allow plenty of time. >Why do you think of using laser communications? Why not have a fixed >shape sail and use normal communications? Just use the sail for thrust >and for power generation (focusing the light on a solar array). It >makes everything much simpler, and you don't have to solve nearly so >many totally new communications problems. Please explain why you >suggest this. ... >What's your mental picture of the power generation here? Also, a detail - >I was assuming you'd be using a reflective PRZP and would have it concentrate >the light on the sunward side of the sail. Isn't that your plan? The reason I thought of lasers over radio was that the signal could be better spatially defined, so lots of power wouldn't be wasted and so that we could exert control from Earth without wreaking havoc with FCC regs. By the way, I include masers under this category. The wavelength would be chosen to minimize atmospheric absorption and, simultaneously, isn't a significant contributor to the solar irradiance. With laser- transmitters and CCD-receivers, we can keep the power requirements down to something schools could afford or, perchance, have lying around. How about this? Form the sail into a spherical "cap" with as great a radius of curvature as possible. There will be some losses in propulsive power due to non-normal angles of incidence, but this can be quantified. Put the solar cell array and CCD (with filters to remove a lot of extraneous radiation that isn't close to the wavelength of the laser/maser chosen) at the center of the "sphere"; sunlight is focused onto the solar cells. In addition, an amplitude-modulated laser signal can be extracted (like an AC signal superimposed on a DC line to control house lights, etc.) from the ambient, concentrated solar flux. We would be limited to signalling when the Earth is closer to the Sun than the sail during its orbit - so that the inside of the cap is point towards us. This avoids any problems with re-shaping the sail, and still allows for propulsion, power generation, and communications. Steve Abrams ------------------------------ Date: Wed, 28 Sep 88 08:43:02 -0700 From: Joe Beckenbach (mangler-in-training) To: space-tech@cs.cmu.edu Subject: Information source for rad-hard parts Hello again, folks. The guy at the Mars Observer Camera project, whom I have mentioned, is Scott Brylow (scott@moc.jpl.nasa.gov). If you have specific questions for him, ask him. [Though the results would be interesting to know on space-tech and possibly sci.space as well. I know I'd be interested in keeping track of such information as well.] Oh, and the MO will be doing the French-Russian balloon data relay through the MOC. Things will get quite heavy on the data-transmission; thank goodness there's going to be several very high compression and transmission modes available. The MOC team actually is pulled from various corners of Caltech and is under the auspices of Caltech. Just think: a crew of dedicated staffers and students, totalling less than 20 people at peak so far, should be able to get a Mars Observer instrument in place with a reasonable sum of money. Now, if only we can get an equivalent effort going with amateur/volunteer effort.... Joe Beckenbach joe@csvax.caltech.edu "Pardon me, buddy, but could you spare an orbital navigation computer?" ------------------------------ Date: Wed, 28 Sep 88 23:34:41 CDT From: sedspace@doc.cc.utexas.edu (405986289 abrams) To: space-tech@cs.cmu.edu Subject: Tether propulsion Does anyone know of any good references for the earlier suggestions to boost a satellite's orbit via a current-carrying tether? The area enclosed by the current-carrying tether would have to be pretty extensive in order to generate enough of a magnetic moment to interact with Earth's relatively weak magnetic field. Has anyone generated any numbers for this? I'm getting ready to sit down to do this (I wanted to earlier, but pre-Shuttle enthusiasm has kept my phone ringing), so I'll let ya' know. Oh yeah, what current levels can we reasonably expect to generate? I know that it will depend upon the material as well as the cross-sectional area of the tether, but haven't any idea as to what is technologically feasible. Needless to say, I like this idea (if only because we can exert so much control over it)... Another question...to what potential (relative to the Earth) can we expect an extended metal surface to be charged by solar wind? Does anyone have access to satellite charging data? This additional charging adds yet another component of force to be considered... Steve ------------------------------ Date: Fri, 30 Sep 88 17:36:01 GMT From: matthews%asd.span@Sds.Sdsc.Edu (Michael C. Matthews) To: space-tech@cs.cmu.edu Subject: Space Tethers Bibliography (VERY LONG) Well, I'm glad to see that this discussion has finally turned to my area of expertise. In response to Steve Abrams' request, I'm posting this bibliography of tethered system applications. I'm working on a control system stability analysis for the Tethered Satellite System program, and have access to tools and documentation that would be useful to an amateur group trying trying to build a probe using this technology. I'll post more on this topic soon, but I'd rather not make this posting any longer than it already is. Probably the best introductory reference to the space applications of tethered systems of all types is the _Tethers in Space Handbook_, August 1986, put out by NASA Headquarters. I'm not sure exactly how to go about getting this book, but you could probably write Headquarters about it. The inside of the front cover reads: > This document is the product of support from many organizations and > individuals. General Research Corporation, under contract to NASA > Headquarters, compiled and prepared the final document. > > Sponsored by: National Aeronautics and Space Administration > NASA Headquarters > Code MT > Washington, DC 20546 > > Contract Monitor: Edward Brazill > > Task Monitor: Paul Penzo > > Contract Number NASW-3921 > and Title: Planning and Analysis of Advanced Programs > > Contractor: General Research Corporation > Space Systems Operations (McLean) > 7655 Old Springhouse Road > McLean, VA 22102 > > Project Manager: T.G. Reese > > Handbook > Coauthors: W.A. Baracat, C.L. Butner > If you write them, ask if they have a newer version of the Handbook, since there has been a great deal of good work in this area in the last two years, especially in support of the Tethered Satellite System program. The _Tethers in Space Handbook_ has an excellent section on the fundamentals of dynamics and astrodynamics of space tethers, a section on current programs in tethered satellites, a lengthy section on proposed tether applications, a very informative appendix with lots of good data on some of the proposed systems, and a compendious bibliography section (see below). ----------- The following is a partial listing of the references section of the _Tethers in Space Handbook_. Note that this listing is dated August 1986, and there have been many good papers published since then (many of them in support of the Tethered Satellite System program.) J.A. Carroll "Guidebook for Analysis of Tether Applications," Contract RH4-394049, Martin Marietta Corporation, March 1985. Available from Mail Code PS01, MSFC, NASA. J. Sisson, "Development Status of First Tethered Satellite System," NASA/Marshall Space Flight Center, Alabama, January 1986. M. Nolan, R. Hudson, J. Sisson, D. Crouch, M. Vignoli, "Shuttle Tethered Satellite Program," June 1984. E. Vallerani, F. Bevilacqua, F. Giani, "Tethered Satellite System Present Program and Future Applications," AAS 85-124. _Applications of Tethers in Space_, Vol. 1, Workshop Proceedings, Williamsburg, Virginia, June 15-17, 1983, NASA CP-2364, March 1985. _Applications of Tethers in Space_, Vol. 2, Workshop Proceedings, Williamsburg, Virginia, June 15-17, 1983, NASA CP-2365, March 1985 J. Pearson, "Anchored Lunar Satellites for Cislunar Transportation and Communication", _Journal of Astronautical Sciences_, Vol. 27, No. 1., pp. 39-62, Jan-Mar 1979. "Study of Orbiting Constellations in Space," Contract RH4-394019, Martin Marietta, Smithsonian Astrophysical Observatory, December 1984. _Applications of Tethers in Space_, Workshop Proceedings, Volume 1, Venice, Italy, NASA CP2422, March 1986. _Applications of Tethers in Space_, Workshop Proceedings, Volume 2, Venice, Italy, NASA CP2422, March 1986. _Applications of Tethers in Space_, Workshop Proceedings, Executive Summary, Venice, Italy, NASA CP2422, March 1986. G. von Tiesenhausen, ed., "The Roles of Tethers on Space Station," NASA TM-86519, Marshall Space Flight Center, October 1985. M.D. Grossi, "Spaceborne Long Vertical Wire as a Self-Powered ULF/ELF Radiator," _IEEE Journal of Oceanic Engineering_, Vol. OE-9, No. 3, pp. 211-213, July 1984. P.A. Penzo "ELF/ULF Radio Wave Generation Using Tethers," _JPL Tethers in Space Studies Report_, 1986. G. von Tiesenhausen, ed., Tether Applications Concept Sheets, June 28, 1984. F. Bevilacqua, P. Merlina, and A. Anselmi, "The Science and Applications Tethered Platform (SATP) Project," Aeritalia Space Systems Group, Torino, Italy, Tether Applications in Space Workshop, Venice, Italy, October 15-17, 1985. J. Laue and F. Manarini, "The Tethered Retrievable Platform Concept and Utilization," IAF-82-13, 33rd IAF Congress, Paris, France, September-October 1982. S. Vetrella and A. Moccia, "A Tethered Satellite System as a New Remote Sensing Platform," University of Naples, Italy, Undated. "Tether Released Recovery," Final Report, NASA Contract NAS8-35096. "SATP Definition Study," Mid-Term Report, Aeritalia, TA-RP-AI-002, March 21, 1986. M.D. Grossi, "Theoretical Investigation of the Generation and Injection of Electromagnetic Waves in Space Plasma by Means of a Long Orbiting Tether," Final Report, Contract NAS8-33520, February 1981. "Selected Tether Applications in Space, Phase III," NASA Contract NAS8-36616. "Selected Tether Applications in Space, Phase III," NASA Contract NAS8-36617. G. Colombo, D.A. Arnold, M. Dobrowolny, M.D. Grossi, "Investigation of Electrodynamic Stabilization and Control of Long Orbiting Tethers," Contract NAS-33691, Interim Report, Smithsonian Astrophysical Observatory, March 1981. M. Martinez-Sanchez and S.A. Gavit, "Four Classes of Transportation Applications Using Space Tethers," Space Systems Laboratory, Massachusetts Institute of Technology in Contract with Marin Marietta, March 1984. M. Martinez-Sanchez, "The Use of Large Tethers for Payload Orbital Transfer," Massachusetts Institute of Technology, 1983. G. Colombo, "The Use of Tethers for Payload Orbital Transfer," NASA Contract NAS8-33691, Vol. II, March 1982. D. Stuart, "Tethered Rendezvous and Docking," Draper Labs, NASA Contract NAS8-36602. V. Chobotov, "Gravity-Gradient Excitation of a Rotating Cable- Counterweight Space Station in Orbit," _Journal of Applied Mechanics_, Vol. 30, pp. 547-554. A.C. Clarke, "The Space Elevator: 'Thought Experiment', or Key to the Universe?," _Adv. Earth Oriented Appl. Space Techn._, Vol. 1, pp. 39-48, 1981. H.L. Mayer, "Swarms: Optimum Aggregations of Spacecraft," Aerospace Corporation, ATR-80(7734)-1, February 29, 1980. "Tethered Orbital Refueling Study," Contract No. NAS9-17059, Martin Marietta. L.G. Napolitano and F. Bevilacqua, "Tethered Constellations, Their Utilization as Microgravity Platforms and Relevant Features," IAF-84-439. S. Bergamaschi, P. Merlina, "The Tethered Platform: A Tool for Space Science and Application," AIAA-86-0400, AIAA 24th Aerospace Sciences Meeting, Reno, Nevada, January 6-9, 1986. P.A. Penzo, "Tether for Mars Space Operation," JPL, 1984 Conference Paper, University of Colorado, Boulder, Colorado, July 10-14, 1984. P.A. Penzo and H.L. Mayer, "Tethers and Asteroids for Artificial Gravity Assist in the Solar System," JPL, AIAA Paper 84-2056, August 1984. P.A. Penzo, _JPL Tethers in Space Studies Report_, 1986. _Proceedings of Tether Applications in Space Program Review_, McLean, VA, General Research Corporation, July 1985. V.B. Braginski and K.S. Thorne, "Skyhook Gravitational Wave Detector," Moscow State University, Moscow, USSR, and Caltech, 1985. B. Bertotti, R. Catenacci, M. Dobrowolny, "Resonant Detection of Gravitational Waves by Means of Long Tethers in Space," Technical Note (Progress Report), Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, March 1977. R.V. Statchnik, D.Y. Gezari, "SAMSI: An Orbiting Spatial Interferometer for Micro-Arcsecond Astronomical Observations," Proc. Colloquium "Kilometric Optical Arrays in Space," Cargese (Corsica), October 23-25, 1984 (ESA SP-226, April 1985). G.J. Corso, "A Proposal to Use an Upper Atmosphere Satellite Tethered to the Space Shuttle for the Collection of Micro-Meteoric Material," _Journal of the British Interplanetary Society_, Vol. 36, pp. 403-408, 1983. James E. McCoy, "PMG Reference System Designs for Power & Propulsion," abstract. Unknown, "Utilization of the External Tanks of the STS," draft of results from workshop held at the University of California, San Diego, August 23-27, 1982. "Preliminary Feasibility Study of the External Tank (ET) Deorbit by a Tether System," Martin Marietta Memo 83-SES-665, May 24, 1983. M.C. Contella, "Tethered Deorbit of the External Tank," Johnson Space Center, April 24, 1984. J.A. Carroll, "Tethers and External Tanks: Enhancing the Capabilities of the Space Transportation System," Research and Consulting Services, La Jolla, California, December 20, 1982. L. Bright, "Saturn Ring-Rendezvous Mission Utilizing a Tethered Sub-Satellite," JPL, Memorandum 312/84.8-938, May 7, 1984. "Tether Assisted Penetrators for Comet/Asteroid Sample Return," by Paul A. Penzo (JPL); peper submitted for 1986 AIAA/AAS Astrodynamics Conference. H. Alfven, "Spacecraft Propulsion: New Methods," _Science_, Vol. 176, pp.167-168, April 14, 1972. K. Kroll, Presentation Package for the NASA Tether Working Group Meeting at Marshall Space Flight Center, February 1986. ----------- In addition, I have included the Electrodynamics section of the Selected Bibliography: H. Alfven, "Spacecraft Propulsion: New Methods," _Science_, Vol. 176, pp.167-168, April 14, 1972. J. Anderson, D. Arnold, G. Colombo, M. Grossi, and L. Kirshner, "Orbiting Tether's Electrodynamic Interactions," final report on NAS5-25077, Smithsonian Astrophysical Observatory, April 1979. D.A. Arnold and M. Dobrowolny, "Transmission Line Model of the Interactions of a Long Metal Wire with the Ionosphere," submitted to _Radio Science_, 1979. D.A. Arnold and M.D. Grossi, "Natural Damping in the Electro- dynamic Tether," Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, January 24, 1983. D.A. Arnold, "General Equations of Motion," Appendix A of "Investigation of Electrodynamic Stabilization and Control of Long Orbiting Tethers," Interim Report, Smithsonian Astrophysical Observatory, March 1981. P.M. Banks, P.R. Williamson and K.L. Oyama, "Shuttle Orbiter Tethered Subsatellite for Exploring and Tapping Space Plasmas," _Astronautics and Aeronautics_, February 1981. B. Bertotti, R. Catenacci, M. Dobrowolny, "Resonant Detection of Gravitational Waves by Means of Long Tethers in Space," Technical Note (Progress Report), Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, March 1977. V.B. Braginski and K.S. Thorne, "Skyhook Gravitational Wave Detector," Moscow State University, Moscow, USSR, and Caltech, 1985. C. Chu and R Gross, "Alfven Waves and Induction Drag on Long Cylindrical Satellites," _AIAA Journal_, Vol. 4, p. 2209, 1966. G. Colombo, D.A. Arnold, M. Dobrowolny, M.D. Grossi, "Investigation of Electrodynamic Stabilization and Control of Long Orbiting Tethers," Contract NAS-33691, Interim Report, Smithsonian Astrophysical Observatory, March 1981. M. Dobrowolny, "Wave and Particle Phenomena Induced by an Electrodynamic Tether," Smithsonian Astrophysical Observatory Special Report #388, November 1979. M. Dobrowolny, G. Colombo, M.D. Grossi, "Electrodynamics of Long Conducting Tethers in the Near-Earth Environment," Smithsonian Astrophysical Observatory, _Reports in Geoastronomy_, No. 3, October 1976. S.D. Drell, H.M. Foley and M.A. Ruderman, "Drag and Propulsion of Large Satellites in the Ionosphere: An Alfven Propulsion Engine in Space," _Journal of Geophysics Res._, Vol. 70, No. 13, pp. 3131-3145, July 1965. P.M. Finnegan, "A Preliminary Look at Using a Tethered Wire to Produce Power on a Space Station," NASA LeRC, May 10, 1983. T.B. Garber, "A Preliminary Investigation of the Motion of a Long, Fexible Wire in Orbit," Rand Report RM-2705-ARPA, March 23, 1961. M.D. Grossi, "Spaceborne Long Vertical Wire as a Self-Powered ULF/ELF Radiator," _IEEE Journal of Oceanic Engineering_, Vol. OE-9, No. 3, pp. 211-213, July 1984. M.D. Grossi, "Theoretical Investigation of the Generation and Injection of Electromagnetic Waves in Space Plasma by Means of a Long Orbiting Tether," Final Report, Contract NAS8-33520, February 1981. M.D. Grossi, "On the Feasibility of Electric Power Generation and Electromagnetic Wave Injection by Electrodynamic Tethers," Tech. Note TP 83-003, Smithsonian Astrophysical Observatory, January 1983. M.D. Grossi, "A ULF Dipole Antenna on a Spaceborne Platform of the PPEPL Class," Report on NASA Contract NAS8-28203, Smithsonian Astrophysical Observatory, May 1973. M.D. Grossi, "Engineering Study of the Electrodynamic Tether as A Spaceborne Generator of Electric Power," NASA Contract NAS8-35497, Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, April 1984. R. Guidici, "Electrodynamic Tether for Power on Propulsion Design Considerations," NASA MSFC-PD, March 20, 1984. R.W.P. King, "The Thin Wire Antenna Embedded in a Magneto-ionic Plasma," Harvard University, 1980. J.E. McCoy, "Electrodynamic Tether Applications - Massive Tether Dynamics Study," General Status Review, NASA JSC-SN 3, May 3, 1984. J.E. McCoy, "Plasma Motor/Generator - Proof of Function Experiment," NASA JSC-SN3, July 1, 1984. J.E. McCoy, "Plasma Motor/Generator - Electrodynamic Tether Applications in Space," NASA JSC-SN3, June 13, 1984. R.D. Moore, "The Geomagnetic Thruster--A High Performance 'Alfven Wave' Propulsion System Utilizing Plasma Contacts," AIAA Paper No. 66-257. J. Pearson, "The Orbital Tower, A Spacecraft Launcher Using the Earth's Rotational Energy," _ACTA ASTRONAUTICA_, Vol. 2, pp. 785-799, Pergamom, 1975. P.A. Penzo "ELF/ULF Radio Wave Generation Using Tethers," _JPL Tethers in Space Studies Report_, 1986. Wei-Yuan Tang, "Comparison of Three Kinds of Possible Power Generators as Space Shuttle Power Extension Package," Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, December 31, 1981. W.B. Thompson, "Electrodynamics of a Conducting Tether," Final Report to Martin Marietta Aerospace Corporation, Research Contract RH3-393855, California Space Institute and Department of Physics, University of California, San Diego, December 1983. P.R. Williamson and P.M. Banks, "The Tethered Baloon Current Generator: A Space Shuttle Tethered Subsatellite for Plasma Studies and Power Generation," Final Report, NOAA Contract No. 03-5-022-60, January 1976. P.R. Williamson, et al., "Measurements of Vehicle Potential Using a Mother-Daughter Tethered Rocket," Utah State University, NASA Grant NSG-6027, 1982. P.R. Williamson, P.M. Banks, and K. Oyama, "The Electrodynamic Tether," Utah State University, Logan, Utah, NASA Contract NAS5-23837, May 1978. ULF/ELF Antenna, NASA Contract NAG-551. Unknown, "Report of the Plasma Physics and Environmental Perturbation Laboratory Working Groups, "Program Development Contract NASA TM X-64856, March 1974. ---------------------------------------------------------------------------- Disclaimer: I only work | Mike Matthews for Lockheed, I don't | Lockheed Engineering & Sciences Company, Inc. speak for them. I don't | Avionics Systems Department speak for NASA either, | Flight Control Systems Section for that matter. Any | Tether Dynamics Group opinions expressed here | Houston, Texas are mine alone and are, | MATTHEWS%ASD.SPAN@STAR.STANFORD.EDU therefore, Truth. | matthews@cup.portal.com ------------------------------ Date: Fri, 30 Sep 1988 16:29-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Ion propulsion / solar mirror I've been thinking some more about an innovative but possible satellite project. Ion Propulsion <--- My favorite! ============== I like ion propulsion. A small, simple mechanism can convert electricity into thrust at more or less any Isp you want (so you pick about 3000sec so you don't need much reaction mass but the power budget still isn't huge). I think an ion propulsion experiment would be a super payload for a small satellite. It's simple enough to be feasible, but of course an actual design that can survive launch and work robustly is no easy trick. Even with only a square meter or so of solar cells (a couple of kg, giving maybe 20 to 50 watts), you can get reasonable accelerations. According to my figures, you can get a 5 kg package moving 1000 m/s in about a month, given ion propulsion and 50 watts of power, at 50% efficiency. These are very rough numbers, but even a minimal power budget can still do real live electric propulsion. Solar sails =========== It seems clear to me that a solar sail that would give effective propulsion is just too thin and fragile to be sent up from earth. It may be possible to use a more robust material, figure out a deployment scheme, and make small adjustments to an orbit. I don't think trying that kind of deployment from a tiny package is practical. Mirror for solar power ====================== How about getting far more power per kilogram by shipping up a deployable mirror (similar to a small solar sail, but with less stringent requirements)? Perhaps Steve's idea of using a Phase Reversal Zone Plate (PRZP) to focus the light would be the most effective. The tricky problems would probably be (a) the deployment mechanism, and (b) solar cells that won't overheat in that much sunlight. What is a PRZP? =============== I mentioned PRZP's in the middle of one of my massive postings earlier, but it was probably opaque to most of you. My officemate's Optics text filled me in on how it works. It's based on diffraction. Diffraction around an opaque object gives rings, as light waves of different phases reinforce each other or cancel out. But instead of an opaque object, what if you put a set of concentric, opaque rings such that waves which reinforce the bright spot behind the rings are let through, and those which would cancel it are blocked. This is a Fresnel zone plate. In a Phase Reversal Zone Plate, the alternate rings are just retarded by half a wavelength so they also reinforce the bright spot, doubling the effect. When you work out the size of the rings, there's a big spot in the center and smaller rings to the outside. Here's a cross section: (( ( ( ( ( ) ) ) ) )) The configuration would be, a flat plate with PRZP rings on it, and at the focal point, a bunch of solar cells. The focal point won't be exact because sunlight certainly isn't monochromatic; I haven't worked out the magnification factors you could expect. Steve will probably roast me for my mangling of the concepts. One question, though, for Steve: does a PRZP work with reflection also (putting stripes on a mirror)? I presume it does, but... =============== So what do you think of sending up an ion propulsion setup, and having a little satellite that can boost itself into a higher orbit? It would be fun to send it farther, but communications could become much trickier. Can we handle it? ------------------------------------------------------------------------- | Marc Ringuette | mnr@cs.cmu.edu | "Use the Force, Luke." | | CMU Computer Science | 412-268-3728 | [watch this space for other | | Pittsburgh, PA 15213 | | quotes from great literature] | ------------------------------------------------------------------------- ------------------------------ End of Space-tech Digest #9 *******************